Reduced Flows Impact Populations of a Specialist Fish Species Restricted to Spring-Fed Streams

Fish species restricted to streams with stable flow regimes may be especially susceptible to changes in flow variability. Bignose galaxias (Galaxias macronasus) are a New Zealand galaxiid species occurring in hydrologically stable spring-fed streams. They are recently described and little is known of their habitat or flow requirements, highlighting conservation concerns and contributing to their threatened status. We investigated how their populations changed over time in response to variation in flows. Ten reaches were established across a flow variability gradient associated with drying and flooding. Populations were surveyed on 4-5 occasions over 14 months. Bignose galaxias abundance decreased three-fold with increasing flow variability. Declines were largely driven by flow reductions, whereas in reaches with stable flows abundances remained high. These results suggest that specialised fish like bignose galaxias associated with stable spring habitats are highly susceptible to reductions in flow. Their distribution is restricted to a single large catchment with widespread flow modifications for agriculture and hydroelectricity generation. Future reductions in flow are a likely product of land-use intensification in the region. Furthermore, spring-fed systems are closely linked with groundwater levels, meaning changes to regional hydrological cycles will affect the rare spring-fed habitats needed to conserve fish like bignose galaxias.