Habitat use and feeding behaviour in two closely related fish species, the three-spined and nine-spined stickleback: an experimental analysis

1. In this paper I analyse experimentally the way in which nine-spined sticklebacks (Pungitius pungitius) and three-spined sticklebacks (Gasterosteus aculeatus) use weeded areas and open water and how this use is influenced by food availability. 2. Experiments were designed to answer two questions: what preference do the two stickleback species have for weeded areas and the open water and then how is this preference changed by the presence or absence of food? Further, differences in behaviour between the two species were analysed. 3. Results showed differential habitat use by the two species: nine-spined occupied the weeded area more often than three-spined and were less likely to utilize the bottom 10 cm of the water column. 4. While foraging, nine-spined swam and pointed more often than did three-spined and modified some of these behaviours in response to the distribution of food. 5. The nine-spined stickleback has a slimmer, more streamlined body than does the three-spined and the differences in habitat use and behaviour reflected this. 6. The results are discussed in relation to interspecific competition and speciation through the ecological selection of different trophic morphologies.