DISCUSSION

length, we always returned to the same conclusion: the salinity layer is the outstanding indicator in our analysis. As expected, recruitment success (RS) increased with increasing reproductive volumes and decreased, the deeper the halocline was observed in all periods tested.

In our example, depth at 11 psu is equivalent to the housing prices in the PTH calculation as it is a fundamental indicator that influences the living conditions significantly and therefore represents a different, not less important issue that we need to think about as it is of great ecological and economic importance for the Baltic ecosystem and community.

Studies (e.g. Plikshs et al., 1999) have shown that this hydrographic variable, which serves as an all-encompassing inflow indicator, can be related to cod’s life history and represents one of the most essential abiotic factors influencing the BE cod stock since a relative shallow halocline is needed for the effective development of cod eggs as it provides buoyancy and therefore avoids down-welling towards lethal oxygen conditions in deeper waters (Wieland et al., 1994; Nissling & Westin, 1997; Nissling

& Vallin, 1996). The results of the predicted scenario show that, regarding the 11 psu halocline, the EB cod stock underwent a favorable time period between 2003 and 2009. Findings of Eero et al. (2015) also indicate that the cod biomass increased during that time, providing possibilities for the cod stocks to reach sustainable levels again.

Another important abiotic factor influencing BE cod is the reproductive volume of the different basins, which is the water volume that contains at least 11 psu salinity and 2 mL·L-1 oxygen, both of which represent limiting conditions for cod egg survival (Plikshs et al., 1999). For its importance to BE cod recruitment, it seems obvious that the reproductive volume became evident as an indicator in this study. Nevertheless, we have to keep in mind that this indicator is based on a calculation of different abiotic components and therefore can obtain various error sources. A simple measurement such as oxygen or salinity alone might give more precise results in this approach.

By defining the recruitment threshold as the mean of the recruitment residuals, we can calculate the possible impact on cod recruitment success in an easy and understandable way if data is available. Further investigations might show the need

approach the mean of the residuals serves as a good proxy for a possible environmental threshold and can be used widely. The overestimation of high years in 1975 – 1985, which result in high residuals, need to be dealt with whenever high values such as these occur, as these high values drive the whole model and therefore dictate the results to a certain degree. We accounted for autocorrelation within the indicator selection process in order to limit the magnitude of this influence. Also, the choice of time frame within the analysis needs to be chosen with care as the regime shift periods R1 and R2 show.

Using RecRes as the response variable for defining environmental indicators seemed to be a relatively good choice depending on the indicator tested. The most prominent indicator (depth of 11 psu isohaline, Gotland Basin) proved to be a very strong driver explaining recruitment variability, as only three years differed in outcome over a time period of 38 years, indicating that correlation between recruitment and depth of saline waters are significant. Also, the second-best indicator, the reproductive volume in the Central Baltic, showed differences in only six years of the time series tested.

Both indicators seem robust measures and could be used together in a set of indicators to approximate recruitment success of EB cod. For recruitment predictions, all indicators showed 50 % or better accordance to respective thresholds, and can therefore serve as a guide along with other environmental information or model results to predict trends in recruitment if environmental drivers are changing.

Difficulties lie in the choice and availability of appropriate indicators that represent the ecosystem in hand. Indicators are widely used in management contexts as tools for evaluating the status of marine environments (CEC, 2008; USCOP, 2004) and are sometimes not easy to assess. Rice & Rochet (2005) provided a framework on how to select and derive appropriate indicators, which is a helpful tool but cannot be applied in every study or with every question at hand since sometimes there are only a few variables that can be used as available indicators such as in the present case. An appropriate indicator selection can only take place if applicable and fundamental research has been conducted regarding the history and biology of the studied species.

External circumstances like size-selectivity in commercial fisheries or high predator populations (Eero et al., 2015) may also cause misleading assumptions regarding the

indicator selection for recruitment success as mechanisms concluded from recruitment correlations, based on error prone data can be fatal (Kraak et al., 2010).

2.4.1 Conclusions

The present study provides a simple approach on how to define favorable and non-favorable recruitment environments for fish stocks by applying a hands-on statistical approach (Diagram 1). Here, we used the BE cod as an example to derive environmental indicators and calculate associated thresholds that can be used to understand stocks’ dynamics and assess possible future scenarios for the recruitment success. The study shows how important different environmental indicators become, depending on the regime that is evident in a system at the time: If data is dominated by high variance, abiotic indicators seem to be the driving factors that might overlap the biotic variables. With this method we can push EAF towards desirable goals and help understand the uniqueness of every ecosystem and its inhabitants.

Acknowledgements

We gratefully acknowledge the people who contributed to and provided the data from the ICES database, which are included in this study. Moreover, we would like to thank the colleagues from the Institute of Marine Ecology and Fishery Science who contributed with their feedback to this manuscript, particularly U. Jacob, A. Temming, R. Lauerburg and Maria Ley. First results from this study were presented at the ICES WGIAB meeting in Chioggia, Italy in April 2013. The feedback from several scientists at the meeting helped to develop the ideas of this manuscript. Furthermore, this study was presented in an oral presentation (Session B) at the annual ICES conference in Reykjavik in 2013. Title of contribution: Setting thresholds for management Advice – Environmental indicators as a tool for an integrated assessment of Eastern Baltic cod (Gadus morhua). This study was funded by MYFISH—Maximising yield of fisheries while balancing ecosystem, economic and social concerns Call (part) identifier FP7-KBBE-2011-5.