Vertical distribution of larval cod (Gadus morhua) in experimental temperature gradients

Behavioural responses to gradients of temperature and light in the pelagic can potentially regulate the distribution and survival of early life stages of fish. Vertical temperature gradients (strong, mild and no thermocline, range 4–8 °C) were established in transparent experimental plastic bags (15 cm diameter and 1 m depth) to investigate changes in vertical distribution of larval cod in response to temperature and light conditions. The vertical position of the larvae (10 cm intervals) was recorded at three different developmental stages: after yolk absorption (10 days post hatch, dph), at established feeding (26 dph) and metamorphosing larvae (47 dph). Observations were first made after 2 h in light, and then after 2 h in darkness the upper and lower part of the bag was sampled by pursing the plastic bag at the level of the thermocline. Additional experiments with reverse light settings were done on consecutive days. At 10 dph larval cod were all found in the upper few cm of the water column regardless of temperature or light settings. At 26 dph larvae were generally found deeper in the water column, but with a larger variation in response to light exposure. In darkness fewer larvae was found in the colder water, depending on the strength of the thermocline. At 47 dph the cod actively avoided the coldest water in the light. However, these patterns disappeared in the dark. Larvae distribution did not change with reversed light settings at any of the developmental stages. Larvae in the upper part of the column were significantly heavier throughout the experiment. Our results indicate an ontogenetic change in the response to a thermal gradient through the first two months after hatching, as well as a complex response to different light settings at later stages.