Abstract
Understanding how combinations of fishing effort and selectivity affect productivity is central to fisheries research. We investigate the roles of fishing regulation in comparison with ecosystem status for Baltic Sea cod stock productivity, growth performance, and population stability. This case study is interesting because three cod populations with different exploitation patterns and stock status are located in three adjacent but partially, ecologically different areas. In assessing stock status, growth, and productivity, we use survey information and rather basic stock parameters without relying on age readings. Because there is an urgent interest of better understanding of the current development of the Eastern Baltic cod stock, we argue that our approach represents partly a novel way of interpreting monitoring information together with catch data in a simplified yet more informative way. Our study reports how the Eastern and Western Baltic cod have gone toward more truncated size structures between 1991 and 2016, in particular for the Eastern Baltic cod, whereas the Öresund cod show no trend. We suggest that selective fishing may disrupt fish population dynamic stability and that lower natural productivity might amplify the effects of selective fishing. In support of earlier findings on a density‐dependent growth of Eastern Baltic cod, management is advised to acknowledge that sustainable exploitation levels for Eastern Baltic cod are much more limited than perceived in regular assessments. Of more general importance, our results emphasize the need to embrace a more realistic view on what ecosystems can produce regarding tractable fish biomass to facilitate a more ecosystem‐based fisheries management.
Highlights
As the TAC (Total Allowable Catch) has seldom been restrictive for Eastern Baltic cod (ICES, 2015), total catches (TC), which includes estimates of discards, is in this study considered as a valid estimate of P in the stocks
This study reports marked differences in how the relative size structure of Baltic cod has developed from the 1990s to present; the Eastern and Western Baltic cod have gone toward more truncated size structures, most pronounced for the Eastern Baltic cod
Lower ecosystem productivity may amplify the effects of selective fishing on cod growth in the Baltic Sea
Summary
Understanding how combinations of fishing effort and selectivity affect productivity is central to fisheries research (Worm et al, 2009). The stock is presently restrained in an unproductive state, due to a noticeable decline in individual growth rates (ICES, 2015), or at least in condition factor (Casini et al, 2016) This poor nutritional status of the Eastern Baltic cod stock has been correlated to density- dependent limitations of food (Eero et al, 2012). The Öresund cod is renowned for a relatively high abundance of, in particular, larger individuals and moderate or at least sustainable exploitation rates (Lindegren et al, 2013; Sundelöf, Wennhage, & Svedäng, 2013; Svedäng, Stål, Sterner, & Cardinale, 2010) This situation is partly due to the resident behavior of this stock in the central Öresund (Svedäng, André et al, 2010), and, partly as a result of a ban on towed fishing gears, that is, gillnetters is dominating the cod fishing in most of the Öresund. Due to increasing difficulties in aging (Eero et al, 2015), the Eastern Baltic cod stock is regarded as data poor since 2014 (ICES, 2014) and the status is uncertain
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