Foraging performance of juvenile Atlantic cod Gadus morhua and profitability of coastal habitats

Overfishing and eutrophication affects coastal communities worldwide, leading to dwindling fish stocks and deteriorated habitats. Hence, attempts to rebuild overfished stocks to past fish productivities need to account for functional relations between habitat types and fish performance. Here we quantified resource availability, foraging performance, and anti-predator behavior of juvenile Atlantic cod Gadus morhua (hereafter 'cod') to assess the costs and benefits associated with different coastal habitats. In the laboratory, cod foraged more efficiently in sand habitats compared to the structurally more complex habitats of eelgrass Zostera marina and the canopy forming bladderwrack Fucus vesiculosus. Presence of chemical cues from a cannibal reduced cod consumption rates in all habitats, but most strongly in the sand habitat. Field observations in the 3 habitats showed highest resource density in the bladderwrack habitat and lowest in the sand habitat, irrespective of season. Habitat profitability, calculated by combining data from field estimates of prey density and experimental quantifications of foraging performance, revealed the bladderwrack habitat to be the most profitable, independent of season. The difference in profitability between the complex habitats was relatively small, suggesting that cod in the field contributed to drive habitat profitability towards equalization. The results strengthen the view that the ongoing loss of seagrass and macroalgae habitats may have significant ramifications for juvenile cod performance, which ultimately may lower the productivity of entire stocks. Consequently, future and ongoing rebuilding of commercial fish stocks should revise the expectations of stock productivity (and hence harvesting intensity) accordingly.