Knowledge about ecological interactions between species is of paramount importance in ecology and ecosystem-based fisheries management. To understand species interactions, studies of feeding habits are required. In the Baltic Sea, there is good knowledge of the diet of cod, but little is known about the diet of flounder, the second most abundant demersal fish in the region. In this study, we investigated the diets of cod and flounder for the first time using stomach content data collected simultaneously in 2015-2017 over a large offshore area of the southern Baltic Sea. The diet of flounder was relatively constant between sizes and seasons and was dominated by benthos, with a high proportion in weight of the benthic isopod Saduria entomon. The diet of cod differed between seasons and showed an ontogenetic shift with a relative decrease of benthic prey and an increase of fish prey with size. Historic diet data of cod were used to explore cod diet changes over time, revealing a shift from a specialized to generalist feeding mode paralleled by a large relative decline in benthic prey, especially S. entomon. Flounder populations have increased in the past 2 decades in the study area, and therefore we hypothesized that flounder have deprived cod of important benthic resources through competition. This competition could be exacerbated by the low benthic prey productivity due to increased hypoxia, which could contribute to explaining the current poor status of the Eastern Baltic cod. The results of this study point to the importance of including flounder in multispecies end ecosystem models.

Recreational fishing is increasingly included in fish stock assessments globally. Nonetheless, the extent and precision of recreational fisheries data are often limited. Common methods to collect those data are on-site and off-site surveys, both of which are prone to specific biases, whose magnitude is widely unknown. The present study compares data from an on-site access point intercept and a simultaneously conducted off-site diary survey, separately targeting charter boat, boat, and shore anglers in the German Baltic Sea recreational cod fishery to identify specific biases in the data sets. The analyses revealed indications of avidity and recall bias that might influence catch and release estimates. Catch rates and release rates differed substantially between the angling platforms. The catch and release rates of diarists and on-site anglers were in a similar range for every platform, and reasonably precise with standard errors between 0.1 and 0.4 fish per angler and day. Recreational fishing monitoring approaches should include complementary on-site surveys from time to time to control for biases whereby the angling platform should be taken into account. The low explanatory power of commonly collected socio-demographic variables indicated that variables capturing the heterogeneity of angler behaviour should additionally be included in future surveys.

Globally, marine recreational fishing is a popular activity that contributes substantially to fishing mortality for some stocks and therefore should be considered in stock assessments and fisheries management. Using the example of the German western Baltic Sea recreational cod fishery, this study examines the effects of the first-time introduction of a bag limit on a previously largely unregulated marine recreational fishery. Furthermore, the study simulates and compares effects of different bag limits, seasonal closures, minimum length, and harvest slot limits to inform scientists, stakeholders, and managers about alternative management strategies and their potential effects on the fishery. After the first-time introduction of the bag limit, recreational removals decreased more than expected and the fishing participation slightly declined. The simulations showed that management measures adapted to the fishing methods reduced recreational removals but with different effects on cod length distributions and angler welfare. A combination of a high bag limit, seasonal closure and size/slot limits were most suitable for limiting cod removals with minimal impacts on angler welfare. This study demonstrates that recreational fisheries management measures need to be evaluated considering fishing methods and angler preferences before their implementation to avoid unexpected biological, social, and economic consequences.

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