Habitat choice of juvenile coastal cod : the role of macroalgae habitats for juvenile coastal cod (Gadus morhua L.) in Porsangerfjorden and Ullsfjorden in Northern Norway

Abstract

Habitat choice of small juvenile coastal cod (Gadus morhua L. 1758) was investigated in a field study in Porsangerfjorden and Ullsfjorden in Northern Norway. A towed video rig was used to record video along depth transects in the subtidal zone. Depth, algae cover and occurrence of 0- group, 1-group and 2+ group cod was registered. Beach seine and angling was used to estimate relative abundances of cod and sample fish for length measurement and diet analysis. The occurrence of juvenile cod was significantly higher in areas with algae cover of 20% or more. In addition there seemed to be a depth preference for depths between 10-20 m. There was a difference in the degree of association to macroalgae between the various age groups of cod. The diet analysis showed variations in between the various age groups of cod. My findings suggest that macroalgae habitats play a vital role as nursing grounds for juvenile coastal cod by providing refuge from predators and lucrative feeding opportunities. There was observed an ontogenic shift in diet composition among juvenile coastal cod, which suggests that the role of macroalgae habitats is different for various age groups of juvnile cod. 0-group seemed more closely associated with the macroalgae than older individuals. Overgrazing of macroalgae by sea urchins may affect the survival of juvenile coastal cod by limiting the extent of nursing grounds. The recruiment of norwegian coastal cod north of 62°N is probably affected by the condition and extent of macroalgae habitats along the coast of Norway.