Aggregation in juvenile pike (Esox lucius): interactions between habitat and density in early winter

1. Juvenile pike (Esox lucius), a cannibalistic fish species, aggregates within habitat patches. The advantages to cannibals of aggregating in the absence of other predators and food constraints are not immediately obvious. In this study we explore the basis for this grouping by observing how spatial distributions of juvenile pike are mediated by the presence of conspecifics.2. Solitary pike preferred shallow-water (0.17 m depth) habitats. When fish density was increased, the average time spent in alternative deep-water habitat (0.33 m) increased, consistent with a despotic type of distribution and suggesting that interference was occurring.3. In pairs of fish, one pike, nominally the dominant individual, showed a habitat use similar to that of single fish. The second individual mostly occupied deep water, again consistent with a despotic distribution and apparently mediated by intimidation interference. However, dominant pike did on occasion enter deep water, at which times the subordinate pike remained with the dominant fish, appearing to aggregate in the pool.4. We propose that habitat-specific risk could explain aggregations of pike in deep water. Although remaining in close proximity to dominant individuals in deep water would seemingly put subordinate fish at great risk, the alternative of moving to shallow water may increase risk still further by reducing the capacity to perceive and/or evade attacks.