Coexistence of behavioural types in an aquatic top predator: a response to resource limitation?

Intra-population variation in behaviour unrelated to sex, size or age exists in a variety of species. The mechanisms behind behavioural diversification have only been partly understood, but density-dependent resource availability may play a crucial role. To explore the potential coexistence of different behavioural types within a natural fish population, we conducted a radio telemetry study, measuring habitat use and swimming activity patterns of pike (Esox lucius), a sit-and-wait predatory fish. Three behavioural types co-occurred in the study lake. While two types of fish only selected vegetated littoral habitats, the third type opportunistically used all habitats and increased its pelagic occurrence in response to decreasing resource biomasses. There were no differences in size, age or lifetime growth between the three behavioural types. However, habitat-opportunistic pike were substantially more active than the other two behavioural types, which is energetically costly. The identical growth rates exhibited by all behavioural types indicate that these higher activity costs of opportunistic behaviour were compensated for by increased prey consumption in the less favourable pelagic habitat resulting in approximately equal fitness of all pike groups. We conclude that behavioural diversification in habitat use and activity reduces intraspecific competition in preferred littoral habitats. This may facilitate the emergence of an ideal free distribution of pike along resource gradients.