The eastern Baltic cod stock has recently started to recover, after two decades of severe depletion, however with unexpected side-effects.
A recent study by an international group of authors led by DTU Aqua shows that despite the still relatively low total cod abundance in the eastern Baltic Sea compared to historical levels, the cod abundance in the Bornholm Basin in the southern Baltic is currently the highest recorded in this area since the 1970s. This is because most of the stock is concentrated in this limited area, which is currently the only area where hydrographic conditions support the survival of cod eggs.
Yet, while the numbers of cod are increasing, the biomass of sprat and herring, a major prey for adult cod, is at a historic low in the main distribution area of cod. Consequently, the cod are having a hard time finding enough to eat.
"Adult cod prey largely on sprat and herring and the majority of this prey is currently found in northern areas of the Baltic Sea, out of reach for cod. In line with the low availability of prey, the body weight and nutritional condition of cod in Bornholm Basin has drastically declined," explains Margit Eero, researcher at DTU Aqua and lead author of the study "Spatial management of marine resources can enhance the recovery of predators and avoid local depletion of forage fish" published in Conservation Letters.
The combination of an increasing cod stock and a lack of prey has resulted in locally high predation mortality of forage fish and in cannibalism of cod, show the international group of researchers behind the new study.
Spatially explicit management
The eastern Baltic cod is one of the first examples of partial recovery of a heavily exploited demersal fish stock in North Atlantic waters in recent decades. The unexpected density dependent effects which have become apparent in the core distribution area of the stock illustrate that novel management measures, taking into account spatial distribution of the stocks, are increasingly needed to support the recovery of predator species in ecosystems, which in fact may not be any longer structured to sustain large cod stocks, conclude the authors.
"The Baltic Sea is currently chosen as a pilot case by EU for taking into account biological interactions in the new fisheries management plans under development in the European Commission. We hope that our study can contribute to this process and that the experiences from the Baltic Sea can serve as a basis for similar developments elsewhere," says Margit Eero, DTU Aqua.
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