4.1 Development, application and evaluation of Individual-Based Models (IBM) of drift and feeding of larval fish
Introduction
Recruitment of Baltic cod critically depends on egg survival, with oxygen concentration in dwelling depths and to a lesser extend predation by clupeids being important processes affecting egg mortality rates.
Surviving egg production and larval abundance are hardly correlated, however, larval abundance is significantly related to year class strength. This indicates either hatching and/or the early larval stage to be a critical period for cod recruitment. In sprat, egg production is significantly correlated to larval abundance, but larval abundance is not related to recruitment suggesting that in contrast to cod the later larval or early juvenile stage is critical for recruitment success. Based on these findings, he objectives of this task were to:
a) Employ 3-D, eddy-resolving circulation models to describe fluctuations of hydrographic parameters (e.g.
temperature, salinity, oxygen concentrations) in spawning areas.
b) Develop a coupled Individual-Based Model (IBM) of larval fish feeding and drift to evaluate how hydrographic processes may influence growth and survival of larval fish.
The approach used in constructing an individual-based model of the feeding and growth of Baltic cod was to couple a trophodynamic model with an existing multi-level ocean model. This coupled model allows to examine the feeding, growth and starvation mortality of larval cod in the Baltic Sea in the context of their transports by utilizing trophodynamic relationships along their potential drift routes. Prey fields were developed on basis of length specific diets of larval cod. Calanoid copepods Pseudocalanus elongatus,
Acartia spp., Temora longicornis and Centropages hamatus have been identified as the predominant prey components (Subtask 3.1).
For larval sprat, a preliminary IBM on temperature dependent early life stage development and drift has been established. In contrast to cod, this modelling approach was not able to resolve growth and survival of larval sprat in dependency on food availability and physical parameters (e.g. ambient temperature and transport).
Instead it allows the back- and forward tracking of individual larvae to their initial spawning ground as well as to their potential nursery areas.
Results
The transport of eastern Baltic cod larvae spawned within the presently only important spawning ground, the Bornholm Basin, was investigated by detailed drift model simulations for the years 1986 to 1999. In order to analyse in which habitats larvae and juvenile cod potentially dwell and where larvae and juvenile might change from pelagic to demersal habitat, a coefficient of overlap was utilized instead of simply counting the number of larval destinations within the different subareas of the Bornholm Basin. The results of these exercises on particles initially released within this spawning ground yielded a clear dependency on wind-induced drift of larval cod, which is mainly controlled by the local atmospheric conditions over the Baltic Sea.
Averaged seasonal simulated distributions were compared with field observations. The results suggested that juveniles caught during autumn trawl surveys in different areas of the Bornholm Basin can be assigned to different times of the spawning season. These observed patterns are very similar compared to our simulations, suggesting that numerical experiments on cod larval drift can explain broad trends in 0-group distributions of cod in and around the Bornholm Basin. Because of seasonal differences in the circulations patterns, the southern coastal environments are on average most important for early and late spawners, whereas larvae hatched in mid-summer were transported towards the north or to a higher degree remained in the spawning area. Observed lesser abundance of settled juveniles within the deep basin area might be explained by non-optimal feeding conditions. Thus, juveniles might only occur in the pelagic zone where they could not be sampled by bottom trawls.
The suitability of our hydrodynamic model of the Baltic Sea for predicting the circulation and correspondingly the transport of larval cod in the Bornholm Basin is clearly identified in this coupled field and modeling exercise. Occurrence of larvae within the different subareas of the Central Baltic can be explained by strong coupling with local atmospheric forcing conditions. When subdivided into more specific regions, these relationships partly became relatively weak, especially if considering the more western subareas of the Bornholm Basin. Here, larval transport might be influenced significantly by other processes (e.g. inflows from the west, complex bottom topography). However, the predictive power of larval drift patterns can be regarded as relatively high, because areas of the Bornholm Basin for which intense spawning activity has been observed are highly correlated with atmospheric forcing.
Field studies on foraging behaviour of larval gadoids indicated the dependence of feeding success on the availability of nauplii of specific calanoid copepod species. Thus, spatial and temporal variability in availability of this prey in combination with variation in ambient physical conditions, e.g. temperature and turbulence, might be responsible for fluctuations in larval growth and survival. Integrated measures of primary and secondary production in spawning and nursery areas are poor measures of the availability and are unlikely
counterintuitive early life stage growth and survival pattern, the development and utilisation of coupled bio-physical models became important for analysing these combined effects on fish recruitment. These models considered the spatial heterogeneity in environmental conditions by integrating larval trophodynamic IBM’s into 3-D hydrodynamic models.
In general, the hydrodynamic models using either Lagrangian particle tracking or advection-diffusion equation models resolve the small-scale vertical and meso-scale horizontal hydrodynamics quite well.
Because of the ephemeral nature of the atmospheric conditions over the Baltic Sea, we, in contrast to the majority of the previous studies, considered it necessary to apply realistic atmospheric forcing conditions in our model runs. The resulting hydrodynamic features in the Central Baltic are highly dynamic within the prolonged spawning period of cod.
The biological model components caused more problems, with larval growth, starvation and related mortality being difficult to model, especially as prey fields are normally not available in sufficient temporal and spatial resolution. This general problem lead in previous modelling approaches to the following assumptions: i) prey concentration do not limit larval growth, making inclusion of prey dependant feeding success and growth rates redundant, ii) mean prey densities are uniform in space and time, iii) variable over season, but invariate over years, partly considering feedback by predation on prey density, iv) varying horizontally as output of a quasi-static copepod population dynamic model or v) being variable in space and time obtained from a 3-D nutrient-phytoplankton-zooplankton (NPZ) model. While we believe that the latter approaches are of great potential in future modelling activities, we considered it pre-mature to couple copepod population models and more complex NPZ-models for the Baltic Sea into our hydrodynamic model. Instead, our exercise is based on horizontally and seasonally variable prey densities in the depths of highest larval abundance obtained from extensive vertically and horizontally resolving zooplankton monitoring, applying overall yearly weighting factors to account for interannual variability in integrated prey abundance. We consider this parametrization as the best available description of the prey field for cod larvae in the Central Baltic and as one of the first applications of a realistic prey field in a coupled tropho/hydrodynamic model of larval survival.
The approach allowed to re-address the question of food limitation in larval Baltic cod, considering explicitly temporal and spatial variability in prey species/stage distribution and physical environmental conditions. For modelling the trophodynamics, we used only sub-models developed and/or parameters determined for cod larvae, although mainly stemming from other geographical locations. The established coupled bio/physical model is able to assess the overall influence of small-scale turbulence on encounter rates, but does not consider that turbulence can have an overall detrimental effect on larval fish ingestion rate if exceeding a certain level depending also on larval behaviour. This is justified by the fact that wind-induced turbulence in the Central Baltic during cod spawning time in depths of larval occurrence does not reach intensities, which are expected to result in a decline in capture success.
For larval sprat, the preliminary IBM on temperature dependent early life stage development and drift identified spawning grounds being in relatively good agreement with observations. Predicted and observed distribution of juveniles showed higher deviations, though the trend of higher concentrations of 0-group sprat in eastern Central Baltic areas was similar.
Discussion
The primary aim of this task was to examine the influence of abiotic and biotic environmental variability on the potential larval survival success of Baltic cod. Generally larval survival is strongly influenced by transport into favourable feeding environments and thus depends on hydrographic and meteorological forcing conditions. Although, transport patterns of intermediate water layers where post yolk-sac cod larvae mainly occur are relatively well known, validation of the results of our coupled physical/biological modelling approaches is difficult. Several processes and factors are only partly resolved and thus parameterized in the model calculations. First, transport patterns of larvae are influenced by the initial spawning location assumed to be an even horizontal distribution, their initial vertical position in the water column and their behaviour.
Secondly, trophodynamics are the most difficult processes to implement in models of larval growth and survival, because of the difficulties in validating such models experimentally or by data obtained during field sampling. Besides the strong non-linearity of trophodynamic relationships utilized, these relationships are often simplified to be computationally feasible. Finally, tests and validation of the IBM model results can only be performed for specific time periods and areas, for which sufficient information on spatially resolved egg and/or larval abundance, its prey and physical forcing data are available; an exercise presently conducted for the spawning season 1999.
The present IBM approach has to cope with a relatively sparse temporal and spatial resolution of prey fields utilizing zooplankton data from the entire Central Baltic. However, a sensitivity study demonstrated that the coupled biophysical model is sensitive enough to show the effects of parameter uncertainty and to demonstrate the impact of natural perturbation especially in encounter and starvation processes. Other studies have suggested that biophysical models of larval growth and survival should additionaly include light attenuation as well as irradiance. Baltic cod larvae concentrate in the water column just below the thermocline, where prey availability and light conditions during day-time are optimal for foraging. Cod larvae do not migrate vertically into surface water layers during night, although this would enhance light conditions for feeding, most likely to avoid prevailing considerably higher temperatures. In consequence feeding activity ceases during night-time. While the latter process is included in our model formulation, we did not include variation in light intensity within a day or season assuming cod larvae to adapt to changes in light intensity by optimising their vertical position in the water column.
We computed idealized prey fields by using a multiple non-linear regression technique on existing zooplankton abundance data. This procedure yielded seasonal trends in abundance of copepod nauplii which resemble the life-cycle patterns of the different species. The univoltine P .elongatus has an abundance maximum in April and May. Contrary T. longicornis, Acartia spp. and C. hamatus having multiple generations accumulate in June to produce next summer generations. P. elongatus nauplii in the central Baltic are associated with the vertical distribution of adults, preferring higher salinities encountered in deeper parts of the water column. This explains higher nauplii concentrations in the centre of the basin. T. longicornis and Acartia spp., not confined to high salinities but to warmer waters, were generally found in the upper 50 m of the water column and are distributed in more shallow regions. Nauplii of C. hamatus were observed mainly in the centre of the basin which suggests a spawning in deeper layers similar to P. elongatus. However, the overall abundance of C. hamatus is low compared to other species. Although the constructed prey fields are considered to be realistic, simulating the decline in P. elongatus by excluding the abundance of this copepod
in the low food environment scenario, underestimated the food abundance and thus probably also larval survival success.
Another parameter not well represented in our model approach is the size specific selection of prey. Length specific larval gut content data have been used to specify preferred prey organism according to species groups and stages for feeding larvae of different size-classes. First feeding larvae rely nearly exclusively on copepod nauplii, which we treated as one prey group, as size differences between species and stages are limited. Thus, our main results referring to first feeding larvae are most likely not affected by a potential prey size selection process not considered in the model.
Our modeling results indicate central Baltic cod larvae > 6 mm, feeding on all juvenile stages of copepods and later on adults and cladocerans to be not food limited. In contrast, first feeding larvae (4.5 - 6 mm length) have changed from a non food-limited to a food limited state, thus representing a critical early life history stage. This food limitation was caused by the decline in abundance of the copepod P. elongatus within the last two decades. Zooplankton distributions suggest prey concentrations to strongly vary in time and space independent whether P. elongatus is considered or not. Including the abundance of this copepod, highest survival rates occurred during spring and early summer, whereas when neglecting P. elongatus, only late hatched larvae, although less in magnitude, had higher chances to survive due to increasing abundances of other copepod nauplii. The shift in peak spawning time of Baltic cod obviously accounts for the decline in P.
elongatus. Larvae are now born later in the season, thus profiting from the increasing abundances of juvenile stages of the remaining copepod species T. longicornis and Acartia spp. accumulating at this time of the year. It is not clear yet, whether spawning in general is delayed or late spawners are the only surviving part of the eastern Baltic cod stock. The spatial analysis of the model output revealed that larvae in recent years had only a high survival probability when advected relatively fast to the margins of the Basin. This is due to the concentration of these copepods in more shallow areas.
Our results show the dependence of Baltic cod larvae to climatic forcing conditions both directly in terms of transport and temperature and indirectly due to the impact on prey population development. In a recent study on the link between the North Atlantic Oscillation (NAO) and the Arctic sea ice export, it was found that the NAO underwent a secular change in the longitudinal position of the two pressure centers during the last two decades. The eastward movement of the centers of inter-annual NAO variability resulted in an increasing influence of the NAO on the Baltic Sea area, which was accompanied by a reduced flux of saline water masses into the deep basins of the Baltic Sea and an increased river runoff. A result of the decreasing salinity is the decay of the calanoid copepod P. elongatus during the last two decades. Our results showed that sufficient food to ensure high survival of Baltic cod larvae is strongly depended on the occurrence of P.
elongatus in the prey field. Thus, from the results of this task it can be concluded that variations of prey availability on a climatic time scale might be considered as input parameters for recruitment predictions of fish stocks.
However, our modelling approach has not yet reached the state where the effects of zooplankton variability on early life stage survival can be assessed in a quantitative way, though it is to our knowledge one of the very few attempts to include realistic prey fields with spatial and temporal resolution into a bio-physical model on larval survival. The modelling exercise has clearly demonstrated that traditional sampling methodology is unable to resolve food limitation of larvae without considering flow dynamics and the impact of physical conditions. It has demonstrated further that retention and dispersal of early life history stages can have
detrimental as well as beneficial effects on off-spring survival within a fish stock depending on seasonal specific prey availability.
For sprat, the constructed coupled IBM/hydrodynamic model has to be taken as preliminary. It revealed a substantially higher potential for transport into eastern Baltic areas than for cod confirming information from autumn hydroacoustic surveys that nursery areas are located along the eastern Baltic coast line. However, in contrast to the hydroacoustic surveys, the predicted distribution of juvenile sprat was more dispers, which may indicate either spatial differences in survival success or problems in modelling drift patterns, e.g.
associated to assumptions about the vertical distribution of larvae and juveniles. On the other hand hindcasted distributions of hatch locations match observed distributions of advanced egg stages very well without an indication that survival to the larval stage depends on the spawning location.