Density-Dependent Antipredator Tactics and Habitat Selection in Juvenile Pollock

Juvenile pollock (Pollachius virens) school in open water at low tide and disperse in beds of intertidal algae at high tide. The goal of the present study was to determine whether both aggregation and use of algal habitat represent alternative antipredator tactics for pollock, and whether their occurrence depended on the number of fish present and the amount of algal habitat available. We systematically varied fish density and algal availability in large (8-m2) arenas and examined distributions before and after exposure to an avian predator model, a stuffed cormorant (Phalacrocorax auritus). Pollock preferred the algal habitat, and the proportion of fish in the algae increased as the amount of algal habitat increased. When density of fish in the arena increased, the number of fish in the algal habitat increased, but the proportion decreased. Following exposure to the predator model, the proportion of fish in the algae increased. Fish in the open were aggregated most of the time, but there was a trend toward increased aggregation of those fish remaining in the open following exposure to the predator. Thus, the use of two alternative antipredator tactics can produce very dynamic spatial distributions. Because the benefits of aggregations are positively density dependent while those of refuges are likely negatively density dependent, species that use both tactics are likely to show dramatic shifts in habitat distribution with changes in population size and refuge availability.