Abstract
Complex life histories and a need to spawn in running water make salmonids susceptible to human activities such as dam construction, flow regulation, and water abstraction. Several aspects of salmonid migration and population dynamics are poorly understood including the dispersal dynamics, reproductive success of dispersers, and their combined influence on metapopulation dynamics and population resilience. The objective of this thesis is to improve our understanding of salmonid population dynamics by examining migration and recruitment patterns in a large regulated New Zealand river system.
The research presented here examines the utility of natural trace element (e.g. Strontium, Barium, Magnesium, Manganese and Calcium) and isotopic markers (the radiogenic strontium isotope ratio; 87Sr/86Sr) for fishery monitoring, using experiments and field studies on freshwater and sea migratory (anadromous) salmonid populations. Experiments expanded the utility of natural chemical markers in eggs and otoliths by testing (1) the temporal stability of chemical markers in salmonid eggs during incubation, and (2) the influence of diet on otolith composition, in order to provide realistic boundaries for applications to wild populations. To improve our understanding of salmonid population dynamics, field surveys investigated the natal origin and movement patterns of lake migratory brown trout (Salmo trutta) and anadromous chinook salmon (Oncorhynchus tshawytscha) in the Clutha River / Mata-Au catchment. Variations in otolith 87Sr/86Sr ratios reconstruct key life history events such as exit from natal habitats and ocean entry. Linear discrimination analyses, calibrated on samples from known sites, were used to select predictive models for assigning adult fish of unknown origin to likely natal sources.
The first experimental study (Chapter 2) confirmed, for the first time, that distinctive chemical markers in incubating trout eggs (high Sr, low Ba concentrations) remain unchanged for seven weeks after fertilisation, demonstrating that the chemical composition of eggs from spawning redds provide a reliable method for surveying migratory salmonid spawning distribution. The second experiment (Chapter 3) established that consumption of food sources that differ chemically from local environments (e.g. marine-derived forage fish) can have measurable effects on otolith chemistry within three weeks. Field surveys investigating population dynamics identified a significant lack of breeding success and recruitment contribution from a nursery stream where chemical analysis (87Sr/86Sr) confirmed approximately 30% of a lake migratory trout population spawn annually (Chapter 4). These surveys also classified 61% of all sea-run chinook salmon harvested below a hydropower dam blocking passage as progeny from self-sustaining landlocked lake populations above dams, and revealed that, on average, 82% of sea-run adults had adopted a stream-type life history (Chapter 5).
Field surveys integrating traditional and contemporary monitoring techniques suggest that water abstraction practices negatively impact a lake migratory trout population by causing a nursery stream to act as a recruitment sink. The surveys also demonstrated that metapopulation dynamics connect landlocked and anadromous chinook salmon populations. Combined, these outcomes illustrate (1) that habitat connectivity is essential to avoid ecological traps; and (2) that dispersal of migrants can be an important recruitment source that increases population resilience. This thesis highlights the need for water resource managers to consider the effects of anthropogenic water use on population dynamics in order to implement conservation actions for rebuilding supressed populations of migratory fishes. While anthropogenic use of water poses many challenges to migratory fishes, dispersal of progeny from landlocked populations above dams represents an important source of recruits that can help maintain anadromous populations in regulated river systems. Collectively, this body of research contributes to scientific knowledge by expanding the utility of natural chemical markers for fishery monitoring and advancing our understanding of salmonid metapopulation dynamics.