Recent research on the Baltic Sea salmon demonstrates perfectly why we need to consider entire ecosystems when we develop fisheries management plans, like the coming multiannual plan for Baltic cod, sprat and herring. This study shows how the survival of salmon might be affected by the poor status of the cod stock in the Baltic Sea. Salmon is an important predatory species in the Baltic, and it is also a valuable fish for both commercial and recreational fisheries.
Cod has been in a dire state for a long time in the Baltic Sea. First the stock was fished down by overfishing. Then, after stricter catch limits were reinforced, the promising recovery stopped. Currently the stock, though high in biomass, consists of mainly small and skinny individuals. The exact reasons for this are not known, but food scarcity, environmental conditions, diseases and fisheries effects have been mentioned. Notwithstanding the reason, all this has an interesting connection to salmon reproduction. With fewer cod feeding on sprat, salmon feeding behaviors have changed and their diet now consists of more sprat than ever before. Apparently the fat composition of sprat is not optimal for salmon and causes shortage of vitamin B, known as thiamin, making the salmon yolk sac fry more susceptible to M74 disease, which results in death.
As this case demonstrates, the consequences of the changes in one stock can be more far-reaching than just the species in question, ranging all the way from causing threat to another species to our dinner tables, by causing losses to commercial fisheries. Fisheries have much wider interactions with the ecosystem than just the target species. Therefore, the scope of fisheries management plans should also go beyond the effects on just the species in question, and take the whole ecosystem into consideration.