Print PageThe extreme isolation of the Hawaiian archipelago has resulted in low diversity, high endemism and strong representation by amphidromous taxa among the freshwater fauna. Due to the obligatory migration of the Islands’ freshwater fish species, modeling impacts of land use, in-stream habitat and exotic species presents additional and unique challenges not encountered in continental systems. Our research explores the most effective watershed land use model for predicting densities of native and exotic species across the Hawaiian archipelago. We also investigate the negative interactions between exotic and native fishes, in effort to isolate the impacts of exotic species from land use. Snorkel surveys were conducted at 75 sites across 30 watersheds on five of the Hawaiian Islands between May and November, 2009. Mark-Recapture experiments were then conducted from May 2010 to March 2011 at three sites in each of three watersheds on the Big Island and one watershed on Oahu. The four watersheds were chosen to represent a gradient of land use and stewardship, as well as presence or absence of introduced poeciliid fishes. Finally, larval production was measured by drift nets simultaneously placed at low and high sites within watersheds.
Results/Conclusions
Analysis of snorkel survey data indicates that population densities of native fishes, as a group, are negatively correlated with percent of agriculture and urban land use in watersheds, whereas introduced poeciliid species are positively correlated. Responses vary among species and between islands, with Oahu being particularly depauperate of native species even in relatively pristine watersheds. Upstream and downstream watershed land use varies in magnitude of influence on species densities. Densities of native and introduced poeciliid species are negatively correlated. However, it is unclear, from this data, what additional impact introduced fishes are having beyond land use. Mark-recapture experiments on two native species, Sicyopterus stimpsoni and Awaous guamensis provided data on demographic condition. Over 1800 individual fish were marked. We found that recruitment of new fish to the adult population was highest at sites devoid of introduced poeciliid species, while survival was highest at a site with poeciliids present. Additionally, demographic condition was not predicted by population density. Drift nets showed that downstream reaches are net producers of larvae, and larval production is greater in the absence of poeciliids. The correlation of production and recruitment is consistent with local retention of post-larvae. Future work will evaluate the potential benefit of poeciliid removal.
