Density-dependent habitat selection in marine flatfish: the dynamic role of ontogeny and temperature

Changes in habitat use with increasing conspecific density are well-documented, but such patterns are likely to be dynamic over the lifespan of the organism and responsive to changes in the environment. In the laboratory, we examined how habitat selection was mediated by ontogeny (6, 8 and 12 mo) and temperature (4 and 9 degrees C) in 2 juvenile, marine flatfish species: Pacific halibut Hippoglossus stenolepis and northern rock sole Lepidopsetta polyxystra. In a set of initial trials at 9 degrees C, groups, of same-aged juvenile flatfish (6, 8 or 12 mo) of either halibut or rock sole were given the choice of 2 habitats-fine sand (preferred) and coarse gravel (unpreferred) -at 1 of 6 densities (0.4 to 12.2 fish m(-2)). A second set of trials was conducted at 4 degrees C using 8 mo juvenile fish of both species over the same range of densities. At 9 degrees C, density-dependent habitat selection was observed among all treatment groups. As juveniles increased in age in the 9 degrees C treatments, both species began occupying the less-preferred gravel habitat at lower densities. However, at 4 degrees C, density-dependent habitat selection varied between species. Sand habitat supported higher densities of juvenile Pacific halibut at 4 degrees C whereas no change was observed in northern rock sole. Juvenile Pacific halibut activity was also lower than rock sole at 4 degrees C, suggesting that competitive interactions (e.g. interference, territoriality etc.) and/or physiological demands of halibut is sufficiently reduced at this temperature to increase the carrying capacity of the preferred habitat. Together, these results indicate that temperature, ontogeny and density interact to yield unique habitat selection. patterns in fish, mechanisms that may be important in area-abundance relationships.