Resource partitioning and asymmetric competition between sympatric freshwater and Omono types of ninespine stickleback, Pungitius pungitius

The freshwater and Omono types of ninespine stickleback, Pungitius pungitius (L., 1758), coexist in several ponds and a stream in the Omono River system, Japan. We tested the hypothesis that coexistence of the two types is accomplished by resource partitioning through interspecific competition. First, the stomach contents of the two types were compared between sympatric and allopatric populations: the stomach contents of the sympatric freshwater type consisted of almost all Copepoda, whereas those of the sympatric Omono type consisted of not only Copepoda, but also many large benthic invertebrates; the stomach contents of both allopatric types were similar, consisting of Copepoda and large benthic invertebrates. Second, behaviour related to resource use in a sympatric pond was observed in the nonbreeding season: the freshwater type showed little aggressive behaviour, but the Omono type had a high frequency of aggressive behaviour. These results indicate that the sympatric freshwater type does not hold a feeding territory and its food resource is almost all Copepoda, whereas the Omono type has a feeding territory and its food resources are various. This suggests that asymmetric interference competition causes a diet shift of the sympatric freshwater type, allowing the two types to coexist by their resource partitioning.