Predicting biomass of resident kokopu (Galaxias) populations using local habitat composition

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 Zealands endemic k[o]kopu species (Galaxias fasciatus, G. argenteus, and G. postvectis) have declined substantially over the last century in response to anthropogenic stressors, including habitat loss and fragmentation, invasive species, and over-exploitation. Despite well-understood habitat associations, key within-habitat features driving the reach-scale biomass of small and large k[o]kopu remain unclear. Here, we investigated whether the total biomass of small ([≤] 90 mm) and large (> 90 mm) k[o]kopu was associated with total pool area, average pool depth, total bank cover, average substrate size, and average forest canopy cover across fifty-seven 50 m reaches. These features were selected because generally pool habitats are productive feeding areas, bank cover and substrate interstices are important refuges, and forest cover provides greater food availability. Because k[o]kopu are nocturnal, populations were sampled with removal at night using headlamps and hand-nets until reaches were visually depleted. Using Akaikes information criterion, it was found that increases in large k[o]kopu biomass were most parsimoniously explained by greater pool area and bank cover, whereas increases in small k[o]kopu biomass were best explained by low bank cover and greater average forest cover. This study demonstrated the importance of considering the ontogenetic shift in species habitat use and provided an effective modelling approach for quantifying the size-specific habitat use of these stream-dwelling fish.