MEPS 265:227-234 (2003)  -  doi:10.3354/meps265227

ABSTRACT: A location-finding acoustic telemetry system (resolution ±1 m) was used to map ocean substrates and to continuously monitor the habitat associations (substrate, water temperature, and depth) of 58 Age 2 to 3 juvenile Atlantic cod Gadus morhua from 11 August to 20 December 1999. Substrate use was studied in 2 stages to determine if it differed from the pattern expected given an absence of selectivity (substrate use proportional to substrate availability). The first level compared the proportions of substrate in the study-area home ranges of Atlantic cod to those of the entire study site. The second assessed if the time spent over specific substrates was proportional to the substrate availability within the study-area home ranges. Boulder substrate was incorporated into the home ranges of juvenile Atlantic cod more than would be expected given its availability within the study area. Kelp substrates were included in these home ranges less than expected while sand and gravel were used in proportion to availability. Time spent over substrates within study-area home ranges varied over the diel and seasonal periods. Within study-area home ranges, juvenile cod occurred in boulder substrates at levels greater than or equal to availability. Conversely, the proportion of time spent over open substrates (e.g. sand and gravel) was either less than or not different from those of substrate availability. Kelp substrates were used less than expected in August and December and more than expected in September and October. Over the diel period, it was found that nocturnally, associations with structure diminished except during September and December when boulder habitats were still used to a greater degree. Unlike younger conspecifics, Age 2 to 3 cod in Buckley Cove were not strictly associated with structurally complex substrates, which suggests that these fish experience less vunerability to predation. Also, unlike younger conspecifics, the distribution of Age 2 to 3 cod did not shift inshore nocturnally and therefore they did not benefit metabolically by moving through the thermocline that existed from August to mid-November. This study indicates that substrate use by juvenile cod is dynamic and the strength of associations with structural complexity varies over diel and seasonal temporal scales.

KEY WORDS: Atlantic cod · juvenile · habitat use · telemetry · substrate · Newfoundland