Abstract
Although some migratory fish species are broadly distributed across large-spatial scales, they might only attain high abundance in certain areas. This can be attributed to local habitat conditions and variation in recruitment strength, which can be related to distance from spawning or larval-rearing habitats. The presence of pelagic compartments in riverine systems can enhance the abundance of some migratory fish species by providing abundant food and habitat for larval development. In this study, we examined the composition and fine-scale abundance of migratory Galaxias brevipinnis Gunther, 1866 (Teleostei: Galaxiidae) and non-migratory Galaxias spp. in relation to the presence and absence of downstream lakes and upstream distance of sites from lakes. Fish abundance and microhabitat use were estimated from randomly selected point samples (about 1 m2) from twenty-two streams in South Island, New Zealand. The total length of G. brevipinnis individuals was also measured to examine possible changes in their size-frequency distribution with increasing distance from lakes. Galaxias brevipinnis was found in almost all of the stream reaches surveyed upstream of lakes, while none were recorded in no-lake stream sites. The mean abundance of G. brevipinnis was considerably higher than non-migratory Galaxias species at sites upstream of lakes. Lotic habitats closer to the lakes also supported a higher abundance of G. brevipinnis than habitats sampled further away from the lakes, suggesting that recruitment limitation with distance from the larval supply is more important than the effect of microhabitats for this species. Furthermore, G. brevipinnis size-distribution patterns differed greatly with distance upstream, with a higher proportion of smaller (younger) individuals found mainly within 2 km upstream of lakes, suggesting strong recruitment of this species close to lake recruitment sources. Although non-migratory species made up all of the Galaxias species caught in non-lake streams, they also occurred in streams upstream of lakes, but only at low abundance in stream sites close to the lakes. However, their abundance tended to be higher at sites further away from lakes, where G. brevipinnis was less abundant. Declining recruitment of G. brevipinnis with distance inland from lake, and competitive interactions with non-migratory species, are likely the main determinants of the adult abundances and composition of these stream fish at large spatial scales, and these effects may outweigh the role of micro -habitat characteristics and preferences in such river basins.