The interactions of sockeye and kokanee salmon (Oncorhynchus nerka), mysids (Mysis relicta), and macrozooplankton in Skaha and Osoyoos lakes, British Columbia

Skaha and Osoyoos lakes are within in the British Columbia portion of the Okanagan Basin. Each lake has an indigenous population of Oncorhynchus nerka and the lakes have been invaded by the opossum shrimp Mysis relicta through downstream migration from Okanagan L. where they were introduced in 1966. O. nerka and M. relicta compete for zooplankton food resources. Generally, in lakes where M. relicta have been introduced, the O. nerka population declines. Impacts of M. relicta include changes in seasonal abundance and species composition of zooplankton, leading to effects on phytoplankton, fish, trophic levels, lake nutrient processes, pollution, and parasitism. However, M. relicta is also a potential food source for O. nerka and may be a benefit. The interactions between these two species and zooplankton are variable and may be lake specific. Historical and current information related to O. nerka, and M. relicta and zooplankton population dynamics was collected for Skaha and Osoyoos lakes. This information was compared to the mysidinduced effects observed in other studies. In addition, the diel vertical migration of O. nerka and M. relicta was monitored and their diets analysed to evaluate the interactions between these species, including the potential of M. relicta as a food source. Within a decade of the introduction of M. relicta into Okanagan Lake, the'kokanee population in Skaha Lake underwent a five-fold decline; however, M. relicta is not directly responsible for this decline. The numbers of sockeye in Osoyoos Lake are highly variable, with many factors affecting their survival and M. relicta may become a future contributing factor. Since M. relicta entered Skaha and Osoyoos lakes there has been no significant change in the abundance of cladocerans, species diversity of cladocerans has declined, and the annual pulse in cladoceran densities has been delayed in Skaha L. M. relicta are a food source to O. nerka as early as their year of emergence. The interactions between M. relicta and O. nerka in each lake vary seasonally, and are influenced by zooplankton production and abiotic factors. Bioenergetic investigations are recommended to better quantify the competitive interactions between O. nerka, M. relicta and zooplankton in these lakes.Science, Faculty ofResources, Environment and Sustainability (IRES), Institute forGraduat