Exploiting Macrofauna Diadromy for Assessing Anthropogenic Impact in American Samoa Streams

Stream biomonitoring is increasingly used to identify and monitor changes in water quality, stream habitat, and even the surrounding watershed. An effective biomonitoring protocol must comprise attributes able to discriminate human-caused changes from natural variation. We attempted to identify such attributes for streams of American Samoa, which, in turn, might also have widespread applicability to other oceanic islands. Owing to the diadromous nature of the macrofauna, we assessed species richness, diversity, composition, dominance, and biomass of freshwater fishes, crustaceans, and mollusks in 50 m sections in midreaches of five streams with and five streams without anthropogenic influences at the estuarine reach. We electrofished for fishes and crustaceans, and we picked mollusks from stream substrates. We discovered that two species of neritid snails of the pan-Pacific genus Clithon were significantly more abundant in the midreach of streams undisturbed by human impacts at the estuarine reach, making them potentially useful bioindicators throughout the South Pacific.