Current status and distribution patterns of non-native freshwater fish on Okinawa-jima Island

Abstract

On Okinawa-jima Island, we collected 30 species and one hybrid of non-native freshwater fish during 1996-2011. Of these, 22 species native to various temperate and tropical regions were reproducing in the wild. The number of reproducing non-native fish was more than three times that of native fish (seven species). The main introduction pathway of non-native fish to Okinawa-jima Island was the release of ornamental pet fish. The distribution patterns of non-native fishes were classified into four categories: very wide (Oreochromis mossambicus and Poecilia reticulata), wide (four species, e.g., Gambusia affinis), intermediate (five species, e.g., Pterygoplichthys disjunctivus), and rare (20 species, e.g., Clarias batrachus). Positive relationships between the occurrence of non-native fish and human population density suggest that human activities and associated artificial environmental changes facilitate the invasion of non-native fish. The rate of range expansion varied across species. In particular, the range expansion of three species (e.g., Micropterus salmoides) that are classified as highly invasive in other regions was relatively slow, suggesting that the invasiveness of non-native fish depends on local environmental conditions. Establishing stricter rules regarding the release of ornamental fish is crucial for preventing new invasions on Okinawa-jima Island.