Environmental regulation of individual depth on a cod spawning ground

Fine-scale characteristics of spawning grounds may provide the opportunity for cod Gadus morhua L. to regulate their external environment by adjusting their depth. Ultrasonic telemetry was used to study how the environment influenced the depth-related behaviour of individual wild and farmed cod on a spawning ground. Of the environmental predictors examined, temperature explained most variation in depth (best predictor in 16 of 21 fish). A decrease in surface-water temperature was accompanied by a pronounced reduction in daily maximum depth of nearly half the fish examined and movement to a shallower area of the spawning ground. Wind and light (after controlling for diel periodicity) had a comparably limited and variable influence on fish depth. Some of the vertical movements seemed to be associated with feeding rather than spawning activity; wild females with diel vertical migrations (DVMs) had poorer condition than wild females with no DVMs, and DVMs were absent in farmed females in better condition. Generally, there were no clear differences in responses to the environment between wild and farmed cod. The ability of cod to regulate their external environment by changing their depth was likely to have been limited by the location of the spawning shoal at the seafloor.