Abstract
This thesis integrated habitat- and connectivity-based inquiries to better understand the population dynamics of Galaxias anomalus in the upper Taieri, Central Otago. Our overall aim was to evaluate the ecological and environmental processes that shape this threatened fish’s metapopulation, with particular focus on functional connectivity across fragmented sub-populations and the ecological conditions that enable co-occurrence with brown trout.
Habitat assessments, reach-scale disturbance indices, and electrofishing across 37 reaches revealed that trout abundance was negatively associated with Galaxias presence and density. In contrast, river disturbance and the availability of riffle and cobble habitats emerged as strong positive predictors of Galaxias presence within trout-occupied waters, while greater in-stream habitat diversity was a significant predictor of Galaxias abundance where the species was present. A negative relationship also emerged between disturbance and trout abundance, suggesting that floods reduce trout dominance and create opportunities for Galaxias to persist in sympatry. These findings suggest that disturbance-mediated refugia and local habitat complexity act as dual filters shaping G. anomalus persistence in invaded streams.
In freshwater systems, structural connectivity reflects the physical arrangement of habitats and hydrological linkages, while functional connectivity incorporates the biological capacity for individuals to move among patches. The chemical composition of otoliths provides a unique means to study the origin and movement of fish. We analysed otolith microchemistry (Sr:Ca, Ba:Ca, Li:Ca, Rb:Ca) from 357 individuals across 20 sites to infer natal origins and dispersal pathways, and test source-sink linkage hypotheses in G. anomalus. Larval signatures showed strong and consistent structuring at broad spatial scales, while patterns at finer within-tributary scales were more variable. Evidence for both local in situ recruitment and downstream larval export suggested a mosaic of demographic sources, sinks, and cryptic sources. Importantly, otoliths from G. anomalus in some trout-invaded reaches demonstrated in situ recruitment, suggesting that local reproduction can sustain populations under invasion pressure, alongside subsidies from upstream refugia.
Together, our findings indicate that under multiple pressures, population resilience in G. anomalus can emerge from the interplay of its fast life-history, habitat heterogeneity and a backdrop of intermediate disturbance. Local persistence in invaded reaches was supported by structural refugia and flow regimes that moderated trout impacts, while otolith chemistry revealed evidence for both source–sink subsidies and in situ recruitment. Viewed through the lens of a fast life-history strategist, G. anomalus is able to exploit shifting habitat mosaics and take advantage of transient recruitment opportunities. Conservation success will therefore depend on safeguarding both trout-free strongholds and the dynamic processes that sustain supportive habitats and connectivity across the riverscape.