Please use this identifier to cite or link to this item: http://hdl.handle.net/1946/6895
The effects of temperature and hatchery-rearing conditions on juvenile Atlantic cod (Gadus morhua) behavior
Atlantic cod (Gadus morhua) is a demersal gadoid that has been one of the most important species in the North Atlantic fisheries for more than 500 years, with major economic, ecological and cultural significance. However, in the past few decades many of the cod stocks have declined dramatically, because of overfishing and changes in the physical environment with major reorganizations of cod ecosystems.
Early in their first year of life Atlantic cod juveniles undergo a transition from pelagic to bottom habitats and become demersal. Depending on habitat type, shoaling and hiding are two of the main strategies used by small fish to reduce predation risk. In the first manuscript shoaling behavior of age 0+ wild and farmed Atlantic cod juveniles was examined in an experimental setting. Specifically, we tested if the process of rearing under aquaculture conditions affects the ability of juveniles to adjust shoal behavior according to the spatial complexity of the experimental environment. Trials were performed in the presence of a predatory cod, in the presence of an age 1+ juvenile and without older fish (control). Our results indicated that wild juveniles were more able to change their shoal behavior according to habitat, aggregating more closely in a simple environment but spreading out in the complex environment with shelters. In contrast, farmed juveniles did not vary their group behavior according to the spatial complexity of the environment. These findings suggest that hatchery-rearing does affect the development of shoaling behavior in age 0+ Atlantic cod juveniles and that hatchery-reared cod may be more vulnerable to predation than wild cod.
In a changing environment, behavior allows an organism to adjust to either an internal or external stimuli and is the result of adaptations to environmental variables. Thus, behavior is constantly adapting through direct interaction with physical (e.g. temperature), chemical (e.g. oxygen) and social aspects of the environment to ensure maximum fitness and survival of an individual. Thus, Atlantic cod juveniles when shifting from pelagic to benthic life stage and while interacting with several biotic and abiotic (e.g. temperature) features of the environment in order to survive must be able to adapt to multiple different conditions using their behavioral flexibility and overcome possible fitness trade-offs.
Juvenile cod tolerate a wide range of temperatures compared to the other life history stages, with temperatures varying between populations. Aquatic ecosystems are composed of several abiotic factors (e.g. light, salinity, current) with temperature being an all-pervasive attribute that limits the distribution, activity and survival of aquatic animals. Temperature affects fish physiology and thus several aspects of fish behavior. However knowledge on the effects of temperature on Atlantic cod juvenile behavior is limited. In the second manuscript age 0+ Atlantic cod juveniles (Gadus morhua) were exposed to two different temperatures treatments within their thermal limit (3ºC-13ºC) and their behavioral responses (e.g. swimming activity, aggressiveness, aggregation) were observed. We also examined the effects of temperature in feeding motivation and time needed to recover from a simulated predator attack. As anticipated juveniles held at the temperature of 13ºC were more active with higher feeding motivation than juveniles held at 3ºC. Atlantic cod juveniles held at the temperature of 13ºC were more aggressive than juveniles held at 3ºC when food was placed in the centre of the experimental aquarium and needed less time to recover from a simulated predator attack and start their feeding attempts again. Juvenile cod held at 3ºC aggregated more and used sheltering in the feeding and added risk treatments. These results show that temperature has a significant effect on activity and several aspects of juvenile Atlantic cod behavior. In general, the juveniles responded less to simulated risk at 13°C. This suggests that at the upper end of their thermal tolerance juvenile Atlantic cod have diminished abilities to alter their behavior in response to environmental change, either because of consequential increase in activity or higher metabolic demands. Thus, our results suggest that with the expected increase in northern sea temperature, several aspects of juvenile cod behavior, related to feeding and predation vulnerability, are likely to be disrupted.