Environmental and fisheries influences on fish stock recruitment in the Baltic Sea (FAIR CT 98 3959)

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the Baltic Sea (FAIR CT 98 3959)

STOck REcruitment in the Baltic

Institute of Marine Sciences, Kiel, Germany (Coordinator) Danish Institute for Fisheries Research, Charlottenlund, Denmark

Finnish Game and Fisheries Research Institute, Helsinki, Finland Gotland University College, Ar Research Station, Gotland, Sweden

Baltic Sea Research Institute, Warnemuende, Germany

Federal Research Centre for Fisheries, Institute for Baltic Sea Fishery, Rostock, Germany

Final Consolidated Report Part I

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Environmental and fisheries influence on fish stock recruitment in the Baltic Sea (STORE)

Final Consolidated Report

Type of the contract: Shared-cost research project

Total cost: 1,350,957 ECU EC contribution: 1,010,000 ECU Commencement Date: 01.01.1999 Duration: 36 month Completion date: 30.06.2002

EC contact: EUROPEAN COMMISSION

Directorate-General XIV

Research Unit – JII 9, 6/11 Rue de la Loi 200

B-1049 Brussels/Belgium

Coordinator: Institute of Marine Science, Kiel (IFM) Department of Fisheries Biology

Düsternbroker Weg 20

24105 Kiel, Germany

Participant no 1: Institute of Marine Sciences, Kiel (IFM),Germany, legal status: Contractor

Participant no 1a: Sea Fisheries Institute, Gdynia (MIR), Poland, legal status: Subcontractor

Participant no 1b: Atlantic Scientific Research Institute of Marine Fisheries and Oceanography, Kaliningrad (AtlantNIRO), Russia,

legal status: Subcontractor

Participant no 1c: Hans Harald Hinrichsen, Lindau (H3), Germany, legal status: Subcontractor

Participant no 1d: Institute for Hydrobiology and Fisheries Research, Hamburg (IHF), Germany, legal status: Subcontractor

Participant no 2: Dansih Institute for Fisheries Research, Charlottenlund, Denmark, legal status: Contractor

Participant no 2a: Water Quality Institute, Hørsholm, Denmark Legal status: Subcontractor

Participant no 3: Finnish Game and Fisheries Research Institute, Helsinki (FGFRI), Finland, legal status: Contractor

Participant no 3a: Finnish Institute for Marine Research, Helsinki (FIMIR), Finland, legal status: Subcontractor

Participant no 3b: Estonian Marine Institute, Tallinn (EMI), Estonia, legal status: Subcontractor

Participant no. 4: Gotland University College, Ar Research Station, Gotland (GUC), Sweden,

legal status: Contractor

Participant no. 5: Baltic Sea Research Institute Warnemuende (IOW), Germany,

Participant no. 5a: Latvian Fisheries Research Institute, Riga (LATFRI), Latvia,

legal status: Subcontractor

Participant no. 6: Federal Research Centre for Fisheries (BFA)

Institute for Baltic Sea Fishery, Rostock (IOR), Germany,

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Executive summary ...1

Organisations involved...1

Background of the project ...2

Objectives...3

Research tasks...3

Achievements and Conclusions ...5

1. Viable egg production for Baltic cod and sprat ...5

1.1 Estimate how the spatial distribution and stock composition of cod and sprat vary temporally and seasonally in relation to stock size and environmental conditions...5

1.2 Evaluate the temporal and spatial development of gonadal maturation in relation to sex, length/age and condition...8

1.3 Examine the viability of sex products spawned in the various spawning areas in relation to parental age/size, condition and contamination by toxic substances ...11

1.4 Develop models of population egg production and characteristics (e.g. viability, size, buoyancy) and evaluate ways to incorporate variability in adult growth and condition...14

1.5 Estimate the reliability of spawning stock biomass as an unbiased index of viable egg production, and evaluate the uncertainty of present and historical viable egg production... ...16

2. Hydrographic factors influencing the developmental success of cod and sprat eggs and early larvae...18

2.1 Resolve spatial and temporal distribution of successful spawning of cod and sprat...18

2.2 Perform experiments on survival of cod and sprat eggs and early larvae in relation to hydrographic factors ...22

2.3 Resolve reproduction volume of cod and sprat...24

3. Identification of abiotic and biotic processes influencing the feeding environment, growth, distribution and survival of larval/juvenile cod and sprat...27

3.1 Resolution of environmental processes increasing survival success of young of the year cod and sprat...27

3.2 Develop inter and intra-annual time series of variations in physical oceanographic processes influencing the feeding environment and distribution of the young of the year pelagic stages ...31

4. Modelling the influences of hydrographic/biological processes on the survival, distribution and growth of fish early life stages ...33

4.1 Development, application and evaluation of Individual-Based Models (IBM) of drift and feeding of larval fish ...33

5. Prey/predator interactions and their impact on the dynamics of cod and sprat populations ...39

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planktivorous fish...39 5.2 Extend the time series of predation mortalities and recruitment of cod and

sprat as well as estimates of spawning stock sizes, structures and fishing

mortalities ...40 5.3 Develop area dis-aggregated estimates of recruitment, related juvenile

predation mortalities as well as spawning stock sizes and structures ...42 6. Model the combined effects of environmental variability and fishery on cod and

sprat recruitment and evaluate the sensitivity and applicability of critical stock

limits and biological reference points for fisheries management ...44 6.1 Identify and describe causal relationships influencing recruitment and

develop simulation models ...44 6.2 Enhance short-term predictions ...52 6.3 Incorporate recruitment models into stochastic single- and multispecies

medium-term stock prediction models...54 6.4 Determine biological management reference points...60

1. Viable egg production for Baltic cod and sprat ... 64 1.1 Estimate how the spatial distribution and stock composition of cod and

sprat vary temporally and seasonally in relation to stock size and

environmental conditions ... 64 Introduction... 64

Fine- to meso-scale vertical/horizontal distribution of cod and sprat during

spawning periods in relation to environmental conditions ... 64 Interannual variability in meso-scale horizontal distribution patterns of cod

and sprat during in relation to environmental conditions ... 65 Interannual variability in large-scale horizontal distribution, abundance and

population structure in relation to hydrographic features... 66 Intraannual variability in the large-scale horizontal distribution of cod and

sprat ... 67 Effect of environmental conditions on catch rates from commercial fleets and

research surveys... 67 Materials and Methods ... 67

Fine- to meso-scale vertical/horizontal distribution of cod and sprat during

spawning periods in relation to environmental conditions ... 67 Interannual variability in meso-scale horizontal distribution patterns of cod

and sprat during in relation to environmental conditions ... 70 Interannual variability in large-scale horizontal distribution, abundance and

population structure in relation to hydrographic features... 74 Intraannual variability in the large-scale horizontal distribution of cod and

sprat ... 75 Results ... 76

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spawning periods in relation to environmental conditions ... 76

Interannual variability in meso-scale horizontal distribution patterns of cod and sprat during in relation to environmental conditions ... 80

Interannual variability in large-scale horizontal distribution, abundance and population structure in relation to hydrographic features... 82

Intraannual variability in the large-scale horizontal distribution of cod and sprat ... 84

Discussion ... 85

Fine- to meso-scale vertical/horizontal distribution of cod and sprat during spawning periods in relation to environmental conditions ... 85

Interannual variability in meso-scale horizontal distribution patterns of cod and sprat during in relation to environmental conditions ... 88

Interannual variability in large-scale horizontal distribution, abundance and population structure in relation to hydrographic features... 90

Intraannual variability in the horizontal distribution of cod and sprat ... 91

1.2 Evaluate the temporal and spatial development of gonadal maturation in relation to sex, length/age and condition ... 92

Introduction... 92

Maturity ogives, sex ratios and mean weights ... 93

Histological validation of macroscopic maturity scales ... 94

Timing of spawning ... 94

Material and Methods... 95

Maturity ogives, sex ratios and mean weights ... 95

Histological validation of macroscopic maturity scales ... 97

Timing of spawning ... 98

Results ...101

Maturity ogives, sex ratios and mean weights ...101

Histological validation of macroscopic maturity scales ...103

Timing of spawning ...115

Discussion ...119

Maturity ogives, sex ratios and mean weights ...119

Histological validation of macroscopic maturity scales ...121

Timing of spawning ...123

1.3 Examine the viability of sex products spawned in the various spawning areas in relation to parental age/size, condition and contamination by toxic substances...126

Introduction...126

Viability of sex products in relation to male and female age/size and condition...126

Contamination of sex products by toxicants ...127

Material and Methods...128

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Results ...133

Discussion ...137

1.4 Develop models of population egg production and characteristics (e.g. viability, size, buoyancy) and evaluate ways to incorporate variability in adult growth and condition ...144

Introduction...144

Otolith-based relationships between cod growth and individual fecundity ...144

Temporal variations in growth and condition of cod and sprat ...145

Cod fecundity and atresia ...145

Cod egg production models ...146

Batch fecundity and spawning frequency of sprat ...146

Results ...153

Discussion ...164

Temporal variations in growth and condition and fecundity of cod and sprat...164

1.5 Estimate the reliability of spawning stock biomass as an unbiased index of viable egg production, and evaluate the uncertainty of present and historical viable egg production estimates ...169

Introduction...169

Validation of cod potential egg production and sensitivity to variability in input parameters ...170

Estimation and validation of viable egg production estimates of cod ...170

Effects of variability in input parameters on sprat reproductive potential ...171

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parameters ...172

Results ...175

Discussion ...178

2 Hydrographic factors influencing the developmental success of cod and sprat eggs and early larvae...182

2.1 Resolve spatial and temporal distribution of successful spawning of cod and sprat ...183

Introduction...183

Abundance and horizontal distribution...183

Vertical distribution...184

Diurnal vertical migration pattern of sprat larvae ...185

Field based mortality estimates ...185

Modelling the vertical distribution of sprat eggs in the changing conditions of the Eastern Baltic ...191

Estimation of sprat spawning stock biomass in the Gotland Basin ...192

Sprat early life stage abundance in relation to varying hydrographic parameters in the Gdansk Deep ...194

Results and discussion ...195

Abundance and horizontal distribution...195

Vertical distribution...197

Diurnal vertical migration pattern of sprat larvae ...199

Field based mortality estimates ...200

Modelling the vertical distribution of sprat eggs in the changing conditions of the Eastern Baltic ...205

Estimation of sprat spawning stock biomass in the Gotland Basin ...207

Sprat early life stage abundance in relation to varying hydrographic parameters in the Gdansk Deep ...208

2.2 Perform experiments on survival of cod and sprat eggs and early larvae in relation to hydrographic factors...213

Introduction...213

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Experiments on cod ...214

Experiments on sprat ...215

Results ...217

Discussion ...221

2.3 Resolve reproductive volume of cod and sprat...223

Introduction...223

Baltic cod reproductive volume ...224

Baltic sprat reproductive volume...225

Results and discussion ...225

Baltic cod reproduction volume...225

Baltic sprat reproductive volume...229

3 Identification of abiotic and biotic processes influencing the feeding environment, growth, distribution and survival of larval/juvenile cod and sprat...231

3.1 Resolution of environmental processes increasing survival success of young of the year cod and sprat...231

The spatial and temporal variability in abundance and production of zooplankton available to larval cod and sprat relative to oceanographic regime ...236

Results ...238

Discussion...241

Resolution of larval food resource utilization ...244

Results ...246

Discussion...247

Resolution of variations in larval growth ...247

Results ...271

Discussion...284

Nutritional condition of sprat larvae...299

Results ...299

Discussion...301

3.2 Develop inter and intra-annual time series of variations in physical oceanographic processes influencing the feeding environment and distribution of the young of the year pelagic stages...302

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3-D Simulations of upwelling...303

Upwelling and turbulence...307

Satellite imagery...307

Results ...308

3-D Simulations of upwelling...308

Upwelling and turbulence...314

Satellite imagery...315

Discussion...315

4 Modelling the influences of hydrographic/biological processes on the survival, distribution and growth of fish early life stages ...319

Introduction...319

Advection of cod early life stages ...321

Bio-physical modelling of larval cod growth and survival...325

Identification of hatch and spawning locations and juvenile distribution of sprat ...329

Results ...330

Advection of cod early life stages ...330

Bio-physical modelling of larval cod growth and survival ...334

Identification of hatch and spawning locations and juvenile distribution of sprat ...338

Discussion ...338

5 Prey/predator interactions and their impact on the dynamics of cod and sprat populations ...345

5.1 Estimation of predation on early life stages of cod and sprat by planktivorous fish ...346

Introduction ...346

Impact of predation by herring and sprat on mortality of sprat and cod early life stages...348

Results ...350

Discussion...355

Prey selection of herring and sprat ...359

Material and method ...359

Results ...360

Discussion...361

Estimating Baltic sprat population sizes from egg production ...362

Results ...366

Discussion...369

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sprat as well as estimates of spawning stock sizes, structures and fishing

mortalities ...373

Introduction...373

Multispecies Virtual Population Analyses ...373

Results and Discussion...377

Variability in feeding intensity and diet composition of cod...389

5.3 Develop area dis-aggregated estimates of recruitment, related juvenile predation mortalities as well as spawning stock sizes and structures...397

Introduction...397

Reliability of spatially dis-aggregated MSVPA runs performed within the Baltic CORE project ...397

Material and methods...397

Results ...398

Discussion...404

Updated spatially dis-aggregated MSVPA runs...406

Results ...408

Discussion...412

6 Model the combined effects of environmental variability and fishery on cod and sprat recruitment and evaluate the sensitivity and applicability of critical stock limits and biological reference points for fisheries management ...415

6.1 Identify and describe causal relationships influencing recruitment and develop simulation models ...416

Area aggregated MSVPA...416

Introduction ...416

Area dis-aggregated MSVPA approach...417

Introduction ...417

Expansion of the MSVPA to account for changes in food intake and growth...424

Introduction ...424

Modelling...424

Discussion...425

Multispecies stock production model ...426

Introduction ...426

Results ...430

Discussion...431

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Introduction ...432

Results ...433

Discussion...433

Conceptual models of the combined effects of environmental variability and fishery on recruitment...433

Review of processes affecting recruitment ...433

Regime shift from a cod to a sprat dominated system...439

The potential impact of contamination ...440

Trends in eutrophication ...440

Carrying capacity for cod recruitment in the Baltic and other marine ecosystems ...442

Introduction ...442

Results ...443

Discussion...443

Decadal-century scale fluctuations in cod and sprat abundance in the Baltic Sea ...443

Introduction ...443

Results ...444

Discussion...445

Scientific knowledge of biological processes potentially useful in fish stock predictions ...445

Introduction ...445

From recruits to spawning stock ...447

From spawning stock to egg production ...452

From egg to larval production ...455

From larval production to recruits ...458

Bayesian approaches for estimating parameters of stock-recruitment relationships ...464

Introduction ...464

Results ...465

Discussion...465

Validation of coupled tropho- and hydrodynamic model of cod larval survival ...466

Introduction ...466

Results ...467

Discussion...467

Statistical recruitment models for cod and sprat ...468

Introduction ...468

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Impact of climate variability on recruitment of sprat...470

Introduction ...470

Results ...472

Discussion...474

Incorporation of environmental variability and spatial heterogeneity into sprat stock-recruitment relationships ...475

Introduction ...475

Results ...477

Discussion...478

Effective Reproductive Environment; an approach to include environmental effects in cod recruitment models ...480

Introduction ...480

Results ...488

Discussion...488

Validation of stock recruitment relationships considering environmental variability and spatial heterogeneity ...489

Introduction ...489

Results ...490

Discussion...490

Update of stock recruitment relationships for cod considering environmental variability and spatial heterogeneity...491

Introduction ...491

Results ...494

Discussion...497

Stock projection scenarios ...501

ARIMA model identification of future environmental conditions ...501

Identification of future environmental conditions from historic time series ...502

Fisheries management scenarios ...503

Eutrophication ...503

6.2 Enhance short-term predictions...506

Cod recruitment prediction from larval surveys...507

Introduction ...507

Results ...508

Discussion...509

Temperature sensitive recruitment models for sprat...510

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Results ...511

Discussion...511

Sensitivity of predictions for assumptions of mean weight at age for cod ...511

Introduction ...511

Results ...512

Discussion...512

6.3 Incorporate recruitment models into stochastic single- and multi-species medium-term stock prediction models...513

Medium-term projectionsfor Baltic cod incorporating environmental effects on recruitment ...515

Introduction ...515

Results ...517

Discussion...517

Medium- term predictions for sprat utilizing temperature sensitive stock recruitment relationships ...518

Introduction ...518

Results ...521

Discussion...521

Medium- term predictions for cod and sprat using statistical stock-recruitment models...522

Introduction ...522

Results ...523

Discussion...523

Multispecies medium- to long-term projections using 4M...524

Introduction ...524

Results ...524

Discussion...526

Medium-term projections using a multispecies production model ...526

Introduction ...526

Results ...528

Discussion...529

ECOSIM runs ...530

Introduction ...530

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Summary...531

6.4 Determine biological management reference points ...531

Reference points and ICES...532

Reference points and process information ...533

Biological reference points for central Baltic cod ...534

Biological reference points for Baltic sprat...537

Biological reference points in a multi-species context ...538

7 References ...545

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Executive Summary

Title

Environmental and fisheries influences on fish stock recruitment in the Baltic Sea

Organisations involved

Institute of Marine Sciences, Kiel (Coordinator)

Danish Institute for Fisheries Research, Charlottenlund & Hirtshals Finnish Game and Fisheries Research Institute, Helsinki

Gotland University College, Ar Research Station, Gotland Baltic Sea Research Institute, Warnemünde

Federal Research Centre for Fisheries, Institute for Baltic Sea Fisheries, Rostock Sea Fisheries Institute, Gdynia

Latvian Fisheries Research Institute, Riga AtlantNIRO, Kaliningrad

Institute for Hydrobiology and Fisheries Research, Hamburg Finnish Institute for Marine Research, Helsinki

Estonian Marine Institute, Tallinn

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Background of the project

One of the main objectives for fisheries management as agreed at the Intermediate Ministerial Meeting in Bergen in 1997 by the ministers responsible for the protection of the environment of the North Sea and for fisheries as well as the European Commission is “to achieve sustainable exploitation of the living marine resources, thereby securing a high yield of quality food”.

The precautionary approach in fisheries management and responsible fishing have played a central role in international conferences and agreements on sustainable exploitation of fish stocks in recent years (e.g. The Rio Declaration of the UN Conference on Environment and Development, FAO Code of Conduct for Responsible Fisheries, The United Nations Conference on Straddling Fish Stocks and Highly Migratory Fish Stocks). The implementation of the precautionary approach has been identified in these meetings as the central task for future fisheries management strategies.

An integral part of applying the precautionary approach is the determination of key stock reference points, with a priority set for limit biological reference points that result in medium- and long-term sustainability of the stock. A crucial factor in determining the biological reference points is the quantification of processes causing recruitment variability and uncertainties in stock and recruitment parameters. Presently, ICES Assessment Working Groups develop reference points based mainly on single species considerations, thus being susceptible to failure due to multispecies interactions (e.g. predator/prey) or fluctuating environmental conditions (e.g. deep water oxygen conditions in the Baltic).

A first prerequisite of accepted management strategies is that there is a quantifiable relationship between spawning stock (or some more precise measure of viable egg production) and recruitment. Such a relationship is in general very difficult to derive from given time series of stock and recruitment observations due to the large environmentally-induced variation in recruitment success. In practice, target values identifying recruitment-limiting stock sizes are generally poorly defined and are often taken simply as the lowest value of spawning stock size recorded for a specific stock. Depending on the available time series and observed range of variation in stock size, the perceived limit value can be very different, giving different implications for the long-term exploitation levels and sustainable yield. It is clear that the potential for reducing a fish stock to a range of recruitment-limiting stock sizes, or to commercial extinction, appears to be high if the stock is at a low level and environmental conditions are prohibitive for recruitment success.

In the case of cod in the Baltic, there is some evidence of a relationship between spawning stock biomass and recruitment. However, this relationship is sensitive to environmental conditions and trophic interactions.

For example, low oxygen concentrations at cod spawning sites and clupeid predation on cod eggs have both been shown to be important determinants of recruitment. The close coupling between cod and sprat, where sprat, the principal prey of adult cod influence the predators reproductive success via consumption of cod eggs at high sprat stock levels and possibly via reduced viable egg production at low sprat availability, necessitates the inclusion of this species in management initiatives on cod. Sprat and cod spawn in the same areas in the Baltic, with overlapping spawning periods, however, the spawning strategy and recruitment success of sprat is very different from cod. Typically sprat produces a series of year-classes below average and then one or two very abundant year classes. The limiting environmental factor for reproductive success of sprat appears to be temperature in the intermediate water layer which influences

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egg mortality. These processes are currently not considered in the management of Baltic cod, but we hypothesize that:

a) different environmental regimes are identifiable,

b) their effects on recruitment are quantifiable and as a result,

c) different fishing strategies based on these regimes may be justifiable.

If critical spawning stock sizes or threshold egg production values are not adapted to long-term variations in the environmental regime, especially the cod population in the Baltic may be at risk of recruitment failure due to unfavourable combinations of biotic and abiotic variables, although being above any minimum level showing still successful recruitment in the historical data set so far.

Objectives

The objectives of the research project are to:

1. Determine stock-recruitment relationships for Baltic cod and sprat in relation to key environmental factors influencing the production of viable spawn and the survival of early life history stages.

2. Improve short-term predictions of stock development by integrating recruitment estimates based on the present status of the stock and its biotic and abiotic environment.

3. Develop predictive recruitment models for medium- to long-term forecasts of stock development under different environmental and fishery scenarios.

4. Estimate biological management reference points, critical stock limits and target spawning stock sizes based on stock-recruitment relationships and stock development simulation models, and considering the precautionary approach for fisheries management.

By addressing these objectives, the project provides a contribution to the GLOBEC Regional Programmes:

“Cod and Climate Change” (ICES-CCC) and “Small Pelagic Fishes and Climate Change” (SPACC).

Research tasks

The key questions to be answered by the project are: how do environmental factors influence the stock- recruitment relationship for cod and sprat stocks in the Baltic and what are the implications of variations in these factors for the use of biological reference points and critical stock limits in the management of the fisheries. These questions will be addressed by following tasks:

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1) Evaluating the accuracy of the spawning stock biomass as a measure of viable egg production by:

a) determining the reproductive potential of the stocks in relation to size and structure of the spawning stocks, sex ratios, maturation processes and spatial distribution of the populations;

b) determining the viability of the produced eggs in relation to parental growth conditions and contamination with toxic substances.

2) Resolving the direct impact of hydrographic factors on the fertilization and developmental success of cod and sprat eggs and early larvae by field and laboratory experiments.

3) Identifying and describing the hydrographic, oceanographic and behavioural processes influencing growth, survival and distribution of young of the year cod and sprat and determining the component of the spawning stock contributing to recruitment through the examination of survivor characteristics.

4) Developing and employing combined drift and feeding models to ascertain the potential role that interannual hydrographic variability has on survival and growth of eggs and larvae.

5) Determining the impact of predation on early life stages of cod and sprat caused by clupeids.

Extending the time series of stock and recruitment estimates of cod and sprat utilizing MSVPA.

This includes a dis-aggregation of stock and recruitment estimates into different spawning areas characterized by specific environmental conditions.

6) Integrating the findings of the previous tasks into new recruitment models and thereby assess the utility of such models in management applications, e.g. simulation of medium- to long-term stock development scenarios under different environmental conditions and fishing activities, forecasts of annual recruitment ranges to be used as input in short-term stock predictions, determining Safe Biological Limits (SBL) and biological reference points and their sensitivity to environmental perturbations.

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Achievements and Conclusions

1. Viable egg production for Baltic cod and sprat

1.1 Estimate how the spatial distribution and stock composition of cod and sprat vary temporally and seasonally in relation to stock size and environmental conditions

Introduction

Cod and sprat aggregate in the deep Baltic basins to spawn, having largely overlapping spawning areas and spawning times. However, they show different spawning behaviour, vertical distribution and diurnal migration patterns. Sprat is a major prey of the top-predator cod, however, sprat also preys on early life stages of cod, both processes depending heavily on the fine- to meso-scale spatial and temporal predator/prey overlap.

Furthermore, hydrographic conditions conducive for cod and sprat egg survival and food supply for larval as well as juvenile and adult life stages may vary considerably not only between, but also within spawning areas. Thus, the distribution of the spawning effort within and between spawning areas appears to be of importance for the reproductive success.

This subtask utilizes research survey data to test the hypothesis that the small-scale vertical and meso- to large-scale horizontal distribution of cod and sprat vary with stock sizes and hydrographic conditions. The studies cover a description of the stock structure of cod and sprat populations and their vertical and horizontal distribution during spawning periods in relation to environmental conditions, specifically addressing:

a) within survey and within Sub-division distribution, to study the small- to mesocale hydrographic processes affecting the vertical and horizontal distribution,

b) intra-annual and within Sub-division distribution, to resolve the aggregation pattern in spawning season and areas,

c) intra-annual and between Sub-division distribution, to describe larger scale shifts in distribution between seasons in dependence of ambient environmental conditions considering fishing activities,

d) inter-annual and within Sub-division distribution, to determine the impact of changing environmental conditions on aggregation patterns within spawning areas, considering also stock structure and related maturation processes,

e) inter-annual and between Sub-division distribution and abundance characterizing varying population development and large scale shifts in the distribution of the stocks.

Results and Discussion

A depth stratified pelagic trawl survey conducted in the Bornholm Basin in early July 1999 confirmed that oxygen conditions in the bottom water limits the vertical distribution of cod, and enabled determination of a corresponding threshold value. At intermediate level of oxygen concentration no clear dependence of catch

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rates on oxygen conditions was detected. However, at higher oxygen concentrations catch rates declined sharply, which is related to the preference to higher salinities resulting in cod concentrations in and below the halocline characterised by salinities above 11 PSU. In contrast, for sprat no salinity preference was detectable within the ranges available. However, the reaction of sprat to low oxygen concentrations was very similar to cod, with occasionally higher catch rates at low oxygen saturation. An analysis of hydroacoustic data from May/June 1999 and 2001 revealed a corresponding oxygen tolerance for sprat of 1 ml/l oxygen concentration. During the trawl survey conducted in July 1999 as well as during summer hydroacoustic surveys (e.g. August 2001) sprat utilize the intermediate water as habitat, but in all these occasions the prevailing temperature in this water layer was above 4°C, while it is normally colder earlier in the year.

Hydroacoustic surveys in May/June 1999 and 2001 indicated an avoidance of this water layer at temperatures below 4°C. This corresponds to results from hydroacoustic surveys conducted 1992-2000 in eastern sprat spawning areas. Outside the basins, in areas where no halocline was present, sprat were distributed throughout the water column in larger, more patchy distributed shoals. The gonadal maturation stage did not affect the distribution of sprat, i.e. sprat encountered outside the main Bornholm Basin had in May/June 1999 on average the same gonadal maturation stage than inside the basin. In contrast, earlier observations within the CORE project showed that cod encountered outside the Bornholm Basin were less progressed in their gonadal maturation.

Based on results of the depths stratified trawl survey the distributional overlap volume of cod and its prey sprat and herring was estimated for the Bornholm Basin. Only a fraction of the sprat population vertically overlapped with the cod population. Sprat occurred in the intermediate water layer, in the halocline, and in the bottom water, while herring and cod occurred exclusively in the halocline and in the bottom water. Only parts of the sprat population were hence accessible for cod, and only a fraction of the sprat had access to cod eggs floating in and below the halocline. Cod-clupeid overlap volumes in summer appear to be determined by salinity stratification and oxygenation of the bottom water. Time series of corresponding hydrographic variables were used to estimate average habitat volumes and overlap indices for summer periods 1958-1999.

An increase in relative catch rates of cod in the Bornholm Basin throughout the spawning season, as determined from trawl surveys conducted in February to August 1996, with a concurrent decrease of catch rates in the Gdansk Deep and the Gotland Basin, suggests that fractions of the adult stock migrate from the Gotland and Gdansk Deep to the Bornholm Basin for spawning. Catch rates from surveys covering the Bornholm Basin during spawning times 1995-1999, showed a consistent pattern with similar proportional increases in CPUE as observed in 1996, despite the decline in stock density over time.

The seasonal development of age-group 2 catch rates in Latvian bottom trawl surveys conducted in Sub- division 28 indicates a migration of these juveniles from southern nursery areas in Sub-division 26 into the Gotland Basin. This has been confirmed by a comparative analysis of the distribution of age-group 1 and 2 as obtained by the international bottom trawl survey conducted in the 1. quarter. The analysis, in combination with drift modelling, has furthermore demonstrated, that high abundances of age-group 1 cod in southern Sub-division 26 may originate from successful spawning activity in the preceding year in the Bornholm Basin. In addition, the analysis of catch rates from the international trawl survey revealed substantial changes of the cod abundance and distribution with time, i.e. a depletion of the spawning stock in

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contrast, in Sub-division 25 the hydrographic conditions did allow a regular reproduction and the major part of the remaining spawning stock concentrated in the Bornholm Basin.

Concurrent to a decline in cod stock size since the early 1980s, the majority of the stock concentrated in western distribution areas. The international bottom trawl survey, applying an area stratified sampling scheme picked up the decline rather early and was also used in area aggregated or dis-aggregated form for tuning of multispecies models under Task 5 and validation of recruitment estimates from analytical models under Task 6. Catch rates from commercial fishing fleets, however, did not reflect the reduction in stock biomass very well and were consequently also not used as tuning indices in the assessment. But if adjusted to the habitat size of cod, catch rates of all commercial cutter fleets show a rather similar stock development than obtained from the international bottom trawl survey and the Extended Survivors Analysis (XSA) assessment.

Hydroacoustic/oceanographic surveys directed to sprat were carried out since 1978 in eastern areas of the Central Baltic (Sub-division 26 and 28). On basis of data from hydroacoustic surveys conducted in May-June and September/October 1992-2000 the distribution patterns of juvenile and adult sprat in these areas was investigated. The analysis of age-specific abundance data revealed intra-annual meso-scale (between statistical rectangles and depth strata) and inter-annual large-scale (between subdivisions and basins) changes in the horizontal distribution of sprat. The inter-annual large-scale changes are related to basin specific hydrography. The intra-annual variability in meso-scale horizontal distribution of recruits and spawning stock is related to seasonal spawning and feeding migration, but again influenced by the depth- specific hydrography.

In combination with results from earlier hydroacoustic surveys, it could be demonstrated, that in winter-spring sprat aggregations occur mainly in the western part of Sub-divisions 26 and 28, over deep waters of the Gotland Basin and Gdansk Deeps, concentrating within the halocline if temperatures in upper water layers decline below 3.5°C: In summer-fall sprat migrates eastwards closer to the coast and distributes within two layers - the warm surface water and below the intermediate water in the halocline with temperatures above 3.5˚C as long as the oxygen concentrations are above 1 ml/l. This vertical distribution pattern confirms results obtained in the Bornholm Basin. In both layers sprat aggregations were most dense where the suitable habitat layer thickness exceeds 30 m. Furthermore, the horizontal distribution is age dependent, with juvenile sprat being distributed closer to the coast than adult sprat.

Pelagic trawl surveys carried out in April, May/June and July 1988-1993 in the Bornholm Basin revealed a significant reduction in catch rates of sprat in July compared to earlier month, confirming that sprat leave the deeper part of the basin into shallower water areas after finalization of spawning activity. This corresponds also to relative distributions derived from hydroacoustic surveys conducted in Sub-division 25 in May/June in comparison to July/August throughout the 1980’s and confirms above described observation for the eastern Baltic.

Apart from these intra-annual fluctuations, the relative distribution of the adult sprat stock according to Sub- division as derived from the international hydroacoustic survey in autumn shows significant time trends with first increasing and then declining importance of Sub-division 24 and 25 (during the 1980’s). The relative abundance in Sub-division 26 was stable until mid 1980’s, but decreased to values well below 20% in most recent years as well. Relative abundance in Sub-division 27 increased to historic high values in most recent years, while the corresponding values in Sub-division 28 and also 29south were already relatively high since

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1989. The distribution of recruits is different, with highest abundances in general occurring in Sub-division 26 independent of the spawning stock size in that area, and always relatively low quantities in Sub-division 25.

1.2 Evaluate the temporal and spatial development of gonadal maturation in relation to sex, length/age and condition

Introduction

The main objective of this sub-task was to estimate sex specific maturity ogives, sex ratios and mean weights at age required for estimation of female spawning stock biomass. A database on Baltic cod maturity and sex ratio at age based on survey data was established during the CORE project. Analyses showed that males generally matured at a younger age than females, that the age at which sexual maturation occurred increased with distance from Kattegat and the Danish Straits and that female longevity exceeded male in all areas. Within the present project, the national survey data were recompiled and evaluated to ascertain the quality of the age-based data sets and time series of weight at age data were established. For Baltic sprat, the spawning stock biomass used in standard assessment is based on a constant, sex and area unspecific maturity ogive. A sampling scheme was developed and implemented to obtain sex specific maturity ogives and sex ratios. Data were compiled in collaboration with the ICES Study Group on Baltic Herring and Sprat Maturation. The variability in the proportions mature at age and sex ratios in sprat in relation to size/age was investigated using project derived and other available data. Similarly, data on weight at age in the stock have been collected on project related and regular research surveys to investigate variation in weight at age.

The macroscopic index scale applied to visually grade the reproductive status of female cod in the field during the CORE and STORE projects has been histologically validated to document the scale and ascertain the quality of data. Selected cod ovary samples representing different size groups, areas and times of the year were sampled and analysed. The presence/absence of various cytological features assigning oocyte and ovarian development was used to identify stage specific histological criteria and revise the macroscopic scale. In addition to the original work program, a histological validation of the applied sprat maturity scale was carried out during the project. This staging of sprat is more complicated than for cod scale, because sprat is an indeterminate spawner with asynchronous gametogenesis and repeated ripening periods within a spawning season. A detailed stage separation of spawning stages is needed for determination of the spawning frequency, which in combination with information on batch fecundity and number of batches is needed to estimate population fecundity.

To study the timing of cod spawning, a database on the gonadal maturation of cod containing information from project specific (CORE and STORE) and regular trawl surveys conducted since 1995 in the Bornholm Basin has been established. The compiled data consists of visually staged gonadal maturation according to sex and size. Analysis of the timing of maturation and spawning of different stock components has been carried out. Little information on Baltic sprat spawning time exists and this study is basically a pilot study.

Data were collected during the 1999 field phase of the project and used to determine seasonal and spatial

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variability in reproductive pattern, while an analysis of inter-annual variations in the timing of spawning covered April 1995-1999 in Sub-division 25.

The database on sex specific maturity ogives and sex ratios of cod includes all available survey information from Danish, German, Latvian, Polish, Russian and Swedish surveys from 1980-1999 for the months January-April. This period corresponds to the time when cod are in the late ripening phase, but is before the initiation of the spawning season. This makes the timing of the surveys suitable for estimation of maturity ogives. The revised time series of maturity ogives confirmed that males generally mature at a younger age than females independent of area and that the age at which sexual maturation occurs, increases with distance from Kattegat for both sexes. The most pronounced difference observed is for females with females in the eastern Baltic stock maturing on average a year later than in the Kattegat stock. The spatial differences also existed within national surveys covering most Sub-divisions and discrepancies in otolith age determination between ‘eastern’ and ‘western’ age reading laboratories were considered not to influence data significantly. The sex specific difference in the timing of sexual maturation was a consistent pattern. The earlier sexual maturation of males causes a skewed sex ratio in the spawning stock with the youngest age groups being heavily dominated by males. Furthermore, male longevity was shorter than female, which causes female dominance of the older age groups in the stock. This pattern was similar for all subdivisions.

The spatial and temporal variability as well as sex specific differences therefore were considered important and time series of sex specific maturity ogives and sex ratios were established separately for Sub-divisions to estimate are specific female SSB. Analysis of cod weight at age data from surveys showed little difference in mean weight at age between areas, but a tendency towards lower mean weight of males than of females.

This is likely to reflect the earlier sexual maturation and reproductive investment of males. The weight at age time series indicated substantially lower mean weights in the 1980s than in the 1990s. This pattern was identical for females and males and could reflect the changes in stock size relative to clupeid prey availability. However, survey weight at age data does not exist for the time period prior to 1989 and an alternative time series of area specific weight at age based on commercial landings was used to test the weight at age in different time periods. The average weight at age was found to be significantly lower in all subdivisions and quarters during 1974-1989 than in recent years, 1990-1999. The consistent differences between periods indicate that the main cause relates to changes in stock size and environmental condition inclusive relative food availability. Though the mean weights derived from the fisheries are not sex-specific, this time series was considered superior to the survey database, because the survey data would not properly consider the temporal changes. A too high mean weight at age for the early period would overestimate the female SSB and thereby the egg production when using relative fecundity.

The analysis of maturity ogives for sprat showed the sexual maturation occurred earlier in life for males than in females in all investigated Sub-divisions and years as for cod, and similarly the proportion of females increased with age, especially from age-group 5. However, these age groups contribute only little to the stock biomass applying sex ratios had only a limited impact on the estimated female biomass. The high proportion of young fish dominating the stock suggests that fluctuations in maturity ogives in these age

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groups may be considerably more important. From the available annual maturity data of sprat in Sub-division 25, a high variability in the proportion sexually mature is obvious for sprat up to 11 cm length. Utilising the yearly maturity ogives to calculate the spawning stock results in pronounced deviations compared to estimates based on the constant average maturity ogive used in standard assessment. These results indicate that the standard assessment procedure may introduce considerable error in the estimation of the spawning stock biomass, though the impact might be less severe than in the case of cod where more than one age group show high variability in proportion sexually mature. Significant changes in weight at age have been observed for Baltic sprat from early 1990’s to 1998 with indications of a reverse trend in most recent years. Shortage of food appears to be responsible for the reduction in weight at age with the availability of the calanoid copepod species that are the major prey organisms of sprat having declined since late 1980s.

The maturity determination of cod applied the 8-level maturity scale defined by Maier in 1908, but modified to include another two stages i.e. resting condition and specimens with reproductive malfunction. The histological characteristics used to judge ovarian development allowed establishment of unique criteria separating the 10 stages except the late immature and the resting stage. These stages were also combined in the original scale of Maier, but were kept separate in the revised scale to allow recording of spawning omission, which is distinguished from the time of occurrence. The interpretation of the female stage II in spring changes from being ripening to late immature (preparation) and V changed from being ripening to a spawning stage. The stages were aggregation into 6 phases based on the relationships between histological characteristics and the reproductive cycle i.e. juvenile, preparation, ripening, spawning, reconstitution and degeneration. The description of macroscopic characteristics of stages was revised by selecting photographs of histologically stage specific females and stage characteristic males, the latter supplemented by fertilisations experiments showing that the male stage V, similarly to the female was a spawning stage.

The photographs and revised stage descriptions were used to elaborate an illustrated manual. The macroscopic maturity scale for sprat established by Alekseev and Alekseeva in 1996 also was histologically evaluated and the periodicity of oogenesis and spermatogenesis investigated. As sprat is an indeterminate spawner, the maturation and spawning is cyclic during the spawning time, which is reflected in the differentiation of stages. The stages were characterised according to histological characteristics of gametogenesis and developmental phases reflecting the consecutive changes of sex cells and the morpho- functional state of the gonads. The revised scale for determination of sprat gonadal maturity is based on macroscopic criteria and focus on clear diagnostic features that can be distinguished by the naked eye.

The results from the analysis of timing of spawning of cod in the Bornholm Basin showed that both the duration of ripening process and the timing of spawning differ between females and males. The onset of the spawning period was substantially later for females than for males, which was caused by longer duration of the early female ripening stage. The time of spawning cessation was similar for sexes, but the average duration of the male spawning phase longer (about 4 months for males compared to 2.5 months for females) due the earlier ripening. The changed interpretation of the female stage V to be a spawning stage improved the agreement with ichthyoplankton data. The fertilisation experiments that ensured that male cod in stage V were in spawning condition confirmed this result and the long male spawning period. The proportion of spawning specimens generally peaked in July. The timing of spawning of different size groups of males and females showed that large specimens start spawning earlier in both sexes, but that the time of peak

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spawning was similar for the size groups within sexes. The time of peak spawning was used to adjust the spawning stock size for fishing mortality during the early part of the year. For sprat, spawning tended to start in January/February in Sub-div. 25 in 1999 but with the proportions of mature females in spawning condition being relatively low until end of April. May, June and early July showed the highest spawning intensity, while spawning ceased from July to August. The peak spawning time was estimated to be in early June which corresponded to the peak abundance of the youngest egg stage of sprat from concurrent ichthyoplankton surveys. The spatial variation in the timing of spawning showed that spawning time tends to be slightly later in Sub-div. 27 and 28 compared with 25 and 26. Both interannual fluctuations observed in Sub-div. 25 and spatial variability tended to be influenced by the ambient water temperature.

1.3 Examine the viability of sex products spawned in the various spawning areas in relation to parental age/size, condition and contamination by toxic substances

Introduction

Viability of offspring or egg quality aspects has been in focus of research lately in attempts to explain variability in stock-recruitment relationships. A number of investigation on different species suggest that maternal effects as female age, size and condition may influence viability of the eggs produced, e.g. that opportunities for survival of eggs and larvae are positively related to the size of the egg.

Potential paternal effects for the spawning success have been less studied. However, low saline water has been shown to influence the fertilisation success indicating that spawning success may differ between spawning areas and years due to different hydrographic conditions. To obtain information about paternal effects on viable egg production i) fertilisation capacity at different ambient salinities, ii) potential differences in fertilisation capacity during the spawning period and iii) variations in fertilisation capacity among males were examined.

Further, for fishes with pelagic eggs, as cod and sprat, egg buoyancy, determining vertical egg distribution and thus environmental conditions for egg development, has been identified as a major impact factor influencing egg survival. Consequently, studies on relationships between female size/age and condition, egg size and egg specific gravity, were conducted for both cod and sprat.

Besides stock related maternal effects on offspring viability, the contamination of spawning products by toxic substances may affect viability of eggs and larvae. With the aim to examine potential connections between the viability of eggs and larvae from individual running ripe-female cod and the concentration of organo- chlorines, stripped and artificially inseminated eggs were incubated up to day 10 after hatch. The evaluation included viable hatch, larval survival, larval growth, enzymatic activity (hepatic EROD and muscular AChE) of the mother fish and ovarian organo-chlorine concentration. In 1999, challenge experiments were additionally conducted on subsets of the larvae obtained from the incubations. In addition to organo-chlorine concentrations, the enzyme activities of EROD and AChE were also measured in mature male cod in order to estimate the influence of sex on these two biomarkers.

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In accordance with earlier investigations the present study shows that cod egg size is positively related to female size, both expressed as length and weight. However, our present study suggests no relationship between egg size and female condition (Fulton´s condition factor and hepatosomatic index ). Size of a fish, or rather growth is density dependent with higher growth rates at low stock densities as shown for Baltic cod within the present study. Higher growth rates and thus larger fish at low stock densities imply higher egg quality, i.e. may act as a compensatory mechanism with a higher relative reproductive success at low stocks.

In agreement with previous results, the investigation showed that the fertilisation capacity/success vary with salinity, i.e. that the fertilized egg production might vary with hydrographic conditions, e.g. between spawning areas. Further, studying sperm viability (assessed as duration of mobility) resulted in a positive relationship between sperm quality and fertilisation rate. Using repeated measurements of sperm mobility during the spawning period of individual males suggested that fertilisation capacity is lower early and late in the spawning period and that the fertilisation capacity is related to male length and weight but not to condition.

This suggests, consistent with egg quality and female characteristics, that size/age of the fish affects viability of male sex products and thus the spawning success, in particular at poor hydrographic conditions, e.g.

during stagnation periods.

With respect to salinity conditions, marine fish species in the Baltic are at the border of their distribution, inhabiting an environment with fluctuating salinity influencing spermatozoa activity and fertilisation success.

For Baltic cod, a minimum salinity of >11 psu is required for activation of spermatozoa. Differences in the percentage of fertilised eggs between salinities implies that the viable egg production may vary according to hydrographic conditions, i.e. differ between spawning areas and during periods with saline water inflows and stagnant conditions respectively. As salinity in the Bornholm Basin is higher, in general reaching 13-18 psu, compared to in the Gdansk Deep and Gotland Basin (10 to 13 psu), fertilisation success can be expected to be higher in the Bornholm Basin.

For sprat eggs no difference was found in specific gravity or in diameter of eggs from different spawning areas during peak spawning in May-June. Pooled data yielded significant relationships between female size and both egg diameter and egg specific gravity. Significant relationships between female parameters and egg characteristics suggest that stock related maternal effects involve selective egg survival affecting the viable egg production, e.g. that the vertical egg distribution is influenced by female size and/or condition and accordingly that eggs are subjected to different environmental conditions. It can be concluded, that along with the increase in stock abundance during the 1990s growth conditions have decreased substantially influencing egg parameters like egg diameter and egg specific gravity and thereby potentially the reproductive success. Despite the described relation between egg size and vertical distribution, the present study identified a significant increase in egg buoyancy without an increase in egg size for fish spawning during spring and summer respectively, suggesting that egg size cannot explain the change in egg buoyancy and vertical egg distribution during spring and summer that have been observed from ichthyplankton data.

The cod fertilisation/incubation experiments carried out in 1999 and 2000 did not reveal any significant correlations between the ovary burden of PCBs, hepatic EROD and muscular AChE activity, respectively, and the effect parameters hatching rate (%), survival after hatch (%), viable hatch (%) and malformations of

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The range of lethal body burdens (LBBs) found in the challenge experiments was between 55.6 - 185 mmol pyrene /kg lipid, indicating a difference in the intrinsic sensitivity between the different larvae batches.

However, there was no correlation between the contaminants measured in the parental fish and the LBBs measured in 5-6 days post hatch larvae. When EROD activity was particularly high in the females, the LBB in the larvae was always low. For AChE such an indication was not found.

A strong correlation between total amount of PCB and pesticides was found in both female and male cod, which was similar to our previous findings in the CORE project. All these correlations were statistically significant. Positive correlations between the ovary burdens of sum PCBs or DDTs, respectively, and length, total and gonad-free weight, gonad weight and gonadosomatic index of female cod were detected. The liver/testis burden of PCBs correlated positively with size and maturity of male cod.

Surprisingly, the mean concentrations of PCBs, DDTs and dieldrin in ovaries of Baltic cod appear to have increased steadily between 1996 and 2000. This is quite in contrast to the general trend of declining DDT and PCB concentrations in biota in the Baltic, and it is unclear why this general trend does not apply to cod.

Since p,p’-DDE concentrations increased between 1996 and 1999, but were little lower in 2000 compared to the preceding year, it can be assumed that there was either a fresh input of DDT or a remobilisation of DDT from contaminated sediments. The organo-chlorine concentrations in the gonads and livers of Baltic cod show a pronounced length dependence, which can be attributed either to the effect of variations in the lipid content or to the fact that the older fish have been exposed longer to the organo-chlorine concentrations in the environment. Positive relations were found between PCBs, DDTs and HCB with GSI of female and male cod suggesting that either gonad growth is enhanced by these compounds or larger gonads accumulate higher contaminant concentrations. The close correlation between PCBs and organo-chlorine pesticides indicates the principal existence of more or less similar correlations with many other contaminants of comparable physicochemical properties which could not be measured in the frame of this project, such as polychlorinated dibenzodioxins and dibenzofurans, polybrominated diphenylethers, several PAHs, synthetic steroid hormones or musk oil fragrances.

Activity of EROD was visible in most individuals, clearly lower in the females compared to the males. In the males, EROD activities also differed with season and between years. EROD activity correlated positively with maturity of female cod and condition index (CI) of male cod, but negatively with hepatosomatic index of male cod. The PCB congeners measured correlated with EROD activity neither in female cod nor in males. All individuals measured had AChE activities ranging between 10 to 80 nmol/min/mg protein. Activity of this enzyme was significantly lower in the females than in the males. Furthermore, AChE activities in the females tend to be lower in animals with higher loads of pesticides, particularly dieldrin. This negative relation is not significant, but in spite of the wide spread of data points, high AChE activities were not found at high dieldrin concentrations.

A comparison of EROD activities found in female and male cod from the Bornholm Basin with those of Atlantic cod from experimental and field studies indicate a significant induction of this biomarker to contaminant exposure in the Central Baltic. This finding is well in line with a similarly alarming report on recently increasing EROD induction combined with decreasing gonad sizes in perch from the Baltic Sea.

Size of the fish did not correlate with EROD activity, which is surprising since the older fish were mostly more contaminated with PCBs and DDTs, which are known to induce EROD activity. This and lack of positive

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correlations between the organo-chlorines measured and EROD activity indicate that other chemicals or factors are responsible for the higher EROD activity found in male cod.

As a further indicator of biological contaminant effects on Baltic cod, the activity of muscular AChE was measured (to our knowledge the first study on this biomarker in Baltic cod). The relatively low and variable activities of AChE, when compared to other fish species, suggest the presence of AChE-inhibiting substances, e.g. carbamates and organo-phosphates or specific trace metals. A negative correlation found between AChE and length, which means lower activities in older specimens, corresponds very well with the fact that older cod specimens show higher contaminant levels. This might be another indicator for AChE inhibition due to contamination. However, in order to better understand to what extent AChE is inhibited in Baltic cod, further investigations are needed.

The still high concentrations of the “traditional” contaminants measured (plus largely unknown concentrations of novel contaminants), the low condition of the few older cod left in the Baltic, the obviously induced EROD and probably inhibited AChE activities clearly demand further comprehensive investigations into the impact of xenobiotics on Baltic fish stocks.

1.4 Develop models of population egg production and characteristics (e.g.

viability, size, buoyancy) and evaluate ways to incorporate variability in adult growth and condition

Introduction

Egg production models for cod in the Baltic are presently not used in stock assessment, partly because the necessary input data for key parameters (e.g., individual fecundity, maturity ogives, sex ratios) were previously not available either for a sufficient number of years, or were not available on an annual basis.

Nevertheless evidence from other cod and haddock stocks shows that variations in maturity and fecundity are related to variations in food availability, condition or growth and might be responsible for significant amounts of variation in spawner-biomass recruitment relationships. It was therefore the aim of this contribution to establish time series of background information, e.g. fecundity, condition and growth, and based on this to derive time series of egg production or proxies that reasonably capture fluctuations in egg production to improve stock recruitment relationships for Baltic cod and sprat.

Two key variables influencing population egg production are individual potential and realized egg production.

Experience with cod and clupeids in other regions indicate that fecundity and atresia fluctuate strongly from year to year and that it may be necessary to include these variables directly into models of population egg production rather than assuming constant or randomly varying values. Specifically, this task has therefore investigated variations in individual fecundity and atresia, and the factors responsible for these variations.

Given the observations elsewhere, it is hypothesized that interannual differences in growth and condition may influence both variables. The main activities within this section were: i) establishment of otolith-based relationships between cod growth and individual fecundity; ii) establishment of time series of different indices of condition (i.e. Fulton´s condition factor and hepato-somatic index) and statistical analyses of variations in