Microhabitat Use of Juvenile Atlantic Cod in a Coastal Area of Bonavista Bay, Newfoundland

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Scuba surveys were used to examine the seasonal abundance and habitat use (substrate, temperature, and depth) of two species of juvenile cod Gadus spp. over the diel cycle in a coastal area of Bonavista Bay, Newfoundland. The abundance of age-0 cod increased throughout the study period (from early August until mid-November), while that of age-1 and age-2 Atlantic cod Gadus morhua peaked in mid-October. The diel abundance of age-1 Atlantic cod increased with the onset of dusk, indicating that these individuals moved from other habitats into the study area at this time. Although all age-classes used areas with kelp significantly more than other areas during the day, the differences between age-classes became apparent at dusk. Age-1 cod showed no significant difference in habitat density at dusk, while age-2 cod had higher concentrations over complex substrates (kelp and boulders); habitat use at dusk by age-0 cod was inconclusive. No significant differences in nocturnal habitat use were found for age-0 and age-1 fish; the results for age-2 fish were inconclusive. Throughout the study period, the 0 and 1 age-classes were found in shallower water at night. Previous studies suggest that diel inshore movements provide metabolic benefits to cod moving through a temperature gradient associated with a distinct thermocline. Although cod occupied colder water during the day in the summer and early fall, the continuation of this diel movement pattern after the disappearance of the thermocline suggests that there are other mechanisms influencing this pattern.