Abstract
The introduction of sports fish around the globe has significantly impacted recreational and commercial fisheries, contributing to local economies and cultural traditions. In Aotearoa New Zealand, species such as brown trout (Salmo trutta) and rainbow trout (Oncorhynchus mykiss) are highly valued sports fish. Sports fishing in Aotearoa New Zealand provides a range of benefits, including recreational, economical and positive effects on mental and physical health. However, due to ongoing anthropogenic pressures, our ecosystems are constantly changing, posing threats to the species that inhabit them, including brown and rainbow trout. Therefore, it is crucial to understand the ecosystem interactions in freshwater environments to help determine how species will respond in the face of change, as such changes may affect recreation, the economy and overall well-being.
Specifically, the ecosystem interactions of sockeye salmon (Oncorhynchus nerka) in Aotearoa New Zealand remain poorly understood. Introduced to Aotearoa New Zealand in 1902 in the Waitaki Catchment, sockeye salmon have since established populations in various hydro-lakes throughout the Central South Island. Recently, sockeye salmon population estimates have shown a trend of increasing and fluctuating numbers within Lake Benmore, which now hosts the largest sockeye population in the country. Lake Benmore is also home to both brown and rainbow trout, making it one of the most fished lakes in Aotearoa New Zealand. The increasing and fluctuating sockeye salmon population has raised concerns about how the trout population will respond in the event of a collapse. Sockeye smolt are thought to be a key food source for both brown and rainbow trout in the lake, though the extent of their reliance on this resource remains largely unknown.
Therefore, the aim of my study was to investigate the feeding ecology of trout inside and outside of the sockeye smolt season with a focus on their potential dependence on sockeye smolt as a key dietary component in Lake Benmore. Using a combination of research methods, such as stable isotope analysis, gut content analysis, and environmental DNA (eDNA) analysis.
My research found that snails were the main contributor to the diet of both species of trout outside the sockeye smolt season, followed by blowflies. During the sockeye smolt season, sockeye smolt was the most important food source for brown trout, whilst snails remained the most important food source for rainbow trout. Furthermore, brown trout tended to feed at a higher trophic level (higher δ15N values) and have more depleted δ13C values compared to rainbow trout. Findings also suggest that different habitats within the lake influences both species feeding habits, with tributary inflow and the subsequent nutrient input playing a significant role in shaping their diets.
My research aimed to provide guidance for management decisions of sockeye salmon and brown and rainbow trout in Lake Benmore. This study highlights that due to trout’s opportunistic feeding behaviour and the abundant availability of snails, sockeye smolt are not essential for sustaining healthy trout populations. However, they do provide a nutritional boost to their diet during the smolt season.