A spatio-temporal investigation of sockeye-kokanee hybridization (Oncorhynchus nerka) in the Okanagan Basin, British Columbia, Canada

Hybridization has complex fitness outcomes that require ongoing monitoring and assessment during active fisheries management programs. Sockeye salmon (Oncorhynchus nerka) is a species of cultural and economic importance that exhibits distinct life history (migratory) forms: anadromous (sćwin, “Sockeye”) and resident (kəkni , “kokanee”). In 2004, a restocking program for hatchery-origin Sockeye fry began in t̕uʔcin (Skaha Lake, B.C), successfully reestablishing Sockeye within the lake. Genetic monitoring over the past decade has revealed an increase in Sockeye-kokanee hybrids in this population, with adult hybrid spawners expressing a primarily (92%) resident life history. While previous work has effectively reconstructed these patterns, the fitness outcomes of this hybridization remained unknown. A similar restocking program was initiated in the much larger kɬusxnitkᵂ (Okanagan Lake) in 2017, which possesses both stream- and shore-spawning reproductive ecotypes of kokanee. Here, we used a 342 single nucleotide polymorphism (SNP) Genotyping-in-Thousands by sequencing (GT-seq) panel (OncoHyb; Chang et al., 2022) on tissue samples (n=1392), paired with otolith microchemistry (n=210), to investigate Sockeye-kokanee hybrid behavior and fitness within the Skaha Lake system. Additionally, we reconstructed migratory (Sockeye, kokanee, F1 hybrid) and reproductive (stream- and shore-spawning) ecotype proportions within Okanagan Lake using GT-seq on tissue samples (n=738). Within Skaha Lake, we detected very few hybrids smolts (~2%), and the majority of adult spawners exhibited resident life history, in line with prior findings. Together, these data suggest that hybrids primarily exhibit a resident life history, rather than anadromous hybrids exhibiting reduced fitness and survival. Stock proportions fluctuated between years, highlighting the need for continued monitoring as the system progresses toward equilibrium. Within Okanagan Lake, no hybrids were detected during the period of our study. Given that this program was only recently initiated, this is not unexpected and emphasizes the need for ongoing monitoring as Sockeye begin to re-establish. Overall, the findings of this research add to existing literature on hybridization frequency, fitness, and behaviour associated with a reintroduction program, which can help inform future management decisions within the Okanagan Basin system and beyond.