Predicting biomass of resident kÅkopu

With the global decline of freshwater fishes, quantifying the body size-specific habitat use of vulnerable species is crucial for accurately evaluating population health, identifying the effects of anthropogenic stressors, and directing effective habitat restoration. Populations of New Zealand's endemic kÅkopu species (Galaxias fasciatus, G. argenteus, and G. postvectis) have declined substantially over the last century in response to anthropogenic stressors, including habitat loss, migratory barriers, and invasive species. Despite well-understood habitat associations, key within-habitat features underpinning the reach-scale biomass of small and large kÅkopu remain unclear. Here, we investigated whether the total biomass of large (> 90 mm) size classes of each kÅkopu species and the composite biomass of all small ([less than or equal to] 90 mm) kÅkopu were associated with components of the physical environment that provided refuge and prey resources across fifty-seven 50-m stream reaches. Because kÅkopu are nocturnal, populations were sampled by removal at night using headlamps and hand-nets until reaches were visually depleted. Based on Akaike's information criterion, greater large banded kÅkopu biomass was most parsimoniously explained by greater pool volume and forest cover, greater large giant kÅkopu biomass by greater bank cover and pool volume, and greater large shortjaw kÅkopu biomass by greater substrate size and pool volume. In contrast, greater composite small kÅkopu biomass was best explained by smaller substrate size, reduced bank cover, and greater pool volume. Local habitat associations therefore varied among kÅkopu species and size classes. Our study demonstrates the importance of considering the ontogenetic shift in species' habitat use and provides an effective modelling approach for quantifying size-specific local habitat use of stream-dwelling fish.