Habitat-mediated survivorship of juvenile (0-year) Atlantic cod Gadus morhua

Abstract

Fishing activity can impact fish populations in 2 ways. The first is the immediate effect on population demographics by the removal of fish. Second is the impact of fishing (e.g. bottom trawls and dredges) on the seafloor which can reduce habitat structure and thus increase the vulnerability of juvenile fish to predation by older conspecifics and other predators. We conducted laboratory experiments to investigate the role of variability in seafloor habitat structure on the survivorship of post-settlement juvenile (0-yr) Atlantic cod Gadus morhua. Groups of 0-yr cod were exposed to a foraging predator (3+ cod) over 5 seafloor habitats of varying complexity (sand, cobble, minimum density short sponge, maximum density short sponge, and tall sponge). These habitats were selected to mimic the range of impacts of mobile fishing gear given a gradient in fishing effort. Emergent epifauna resulted in a significant decrease in 0-yr mortality when compared to flat sand, the least complex habitat. Epifaunal density was shown to be more significant than epifaunal height in reducing 0-yr mortality. Predator reaction distance decreased with increasing habitat complexity, presumably due to the obstruction of visual cues by complex relief. Latency to first and second capture did not differ statistically between habitats. Alteration of seafloor habitat by fishing activity in the northwest Atlantic could magnify the effects of overfishing by limiting juvenile survivorship.