Abstract:
Freshwater mussels of the order Unionida have declined significantly worldwide, mainly due to habitat destruction, altered flow regimes, water pollution, and impacts from other invasive bivalves. New Zealand's three extant species are following a similar trend with declining populations and the lack of juvenile recruitment. Invasive freshwater fish can have a negative impact on other species, and their surrounding environment, but have not been considered in whether they exert impacts either directly (predation, competition) or indirectly (modifying ecosystem) on freshwater mussels. In this study both invasive freshwater fish; Mosquitofish (Gambusia affinis) and endemic freshwater mussel; kākahi (Echyridella menziesii) were manipulated in a 2 × 2 factorial design mesocosm experiment. The main aims of this experiment were to first assess if there were any direct impacts from mosquitofish on kākahi and second to determine if mosquitofish could alter the ecosystem, which could have potential indirect impacts on kākahi. The main findings were that there were no obvious direct impacts of predation or competition, but there was potential for mosquitofish to alter the ecosystem, which could have indirect impacts on the feeding ability of kākahi. Mosquitofish alone increased phytoplankton abundance and increased nutrients, but the combination of both mosquitofish and kākahi showed a reduction in phytoplankton over time. Stocking rates of both mosquitofish and kākahi were keep at low densities and this may have reflected the lack of direct interaction and competition. New Zealand has a high abundance of invasive freshwater fish and it is worth exploring other species interactions with kākahi in future experiments.
