The role of landscape in determining freshwater fish distribution and abundance

Explaining patterns of species distributions and abundances in space is often challenging, especially in complex landscapes and for species that consistently need to use different habitats to complete their full life cycles. High abundance and range expansion of such species occur when habitats that are crucial during a particular life-history stage are available and accessible within the landscape, and also support species life-history traits. Although, many studies have shown the importance of adult habitat characteristics when predicting and interpreting changes in species distribution and abundance, relatively little research has evaluated how differences in larval habitat requirements may have a more important influence on the way different species respond to a given environment. This thesis examines the role of lakes, as a potential pelagic larval rearing environment for a migratory fish, and the influence of those lakes on the distribution, abundance, and population structuring of adult migratory and non-migratory Galaxias fish species. The opportunities for rearing larvae for high fecund migratory fish potentially leads to the competitive interactions with non-migratory fish, and contrasting patterns of these two groups of stream fish upstream of inland lakes. The analysis of fish presence-absence and habitat data showed that lake residency during the larval period of migratory, Galaxias brevipinnis, life history was the most important factor predicting large-scale distribution patterns of not only this species but also non-migratory Galaxias spp. in inland streams and rivers above lakes. Incorporating the spatial position of streams in relation to lakes (distance to the nearest downstream lake) into species (migratory and non-migratory) distribution models significantly improved distribution predictions. This pronounced effect of the lake was particularly apparent when meso- and micro-habitat-only models did not explain much of the variability observed in distributions of these fish species. While the highest predicted occurrence of G. brevipinnis was found in stream reaches immediately upstream of lakes, and generally 20 km, or upstream zone of highly-occurred G. brevipinnis. This suggests that non-migratory species are mostly excluded by G. brevipinnis from stream reaches close to lakes. Fine-scale quantitative fish surveys showed that migratory and non-migratory species didn’t coexist at high abundances upstream of lakes, suggesting that G. brevipinnis might outcompete non-migratory species from stream habitats closer to lakes where the abundance of G. brevipinnis was higher compared with sites sampled further upstream. A decline in abundance accompanied by the lower proportion of smaller individuals observed with increasing distance, suggests that recruitment limitation with distance from the source of pelagic larvae (lake) can regulate upstream populations of G. brevipinnis. The abundance of non-migratory species, on the other hand, increased with distance upstream of lakes, and was high in streams with no lake downstream where G. brevipinnis was in low abundance or absent. Microhabitat structure of sampling sites and species-specific microhabitat use didn’t appear to have a major influence, rather landscape features (supply of G. brevipinnis recruits to adult habitats) play a more important role in controlling the adult population structure of Galaxias species upstream of lakes. Analysis of the larval elemental composition of otoliths of adult migratory species from the tributaries of lakes revealed that their populations had originated from a common rearing environment in each respective system, most likely the nearest downstream lake. On a smaller scale, evidence of population structuring (catchment level) was also found within the lakes, suggesting that most larvae retained close to their natal streams. There was significant variation between larval and adult elemental concentrations in otoliths of G. brevipinnis, suggesting the spatial segregation between their adult and larval habitats, and that larval migration to the lake, here, is an obligate mechanism due to their pelagic larval phase. In contrast, the relatively consistent patterns of elemental concentrations found across the otoliths of non-migratory galaxiids suggest that they had completed their larval life-history stages at the same stream, or at least close to where the adults were collected. This thesis demonstrates that access to lakes for a migratory fish with a pelagic larval phase has major consequences for the distribution and abundance of both migratory and non-migratory stream fish in inland streams and rivers. It shows downstream lakes may primarily determine the distribution range of both migratory and non-migratory species due to interactions between life-history (high fecundity and pelagic larval stage) with the landscape. The result of this study can be relevant where ever lakes are an important component of riverine systems