Estimating fishery harvest and spawning escapement (spawning stock size) are critical components of fisheries management; however, they can be particularly challenging to measure in systems where visually indistinguishable, but reproductively isolated populations mix within a single fishing area. Genetic stock identification is a common tool used in such mixed‐stock fisheries to improve estimates of spawning escapement and productivity; however, there are few references for management applications, particularly for inland recreational fisheries. The kokanee Oncorhynchus nerka population in Wood Lake, British Columbia, is a highly productive and valuable mixed‐stock fishery that includes two reproductively distinct ecotypes: shore‐ and stream‐spawning. Enumeration of shore‐spawning kokanee is logistically challenging, as the spawning population is not confined to a defined area or depth like stream‐spawners. Here, we combined in‐lake sampling (angler harvest and age‐0 trawl samples) over a 9‐year period (2008–2016) with genetic stock identification and Bayesian statistics to develop a new method for enumerating shore‐spawning kokanee. Our results suggest that angler‐harvested kokanee are representative of the spawner age structure and stock proportions. Therefore, we used the angler harvest sample combined with known stream‐spawner escapement to reconstruct the shore‐spawner escapement time series. Shore‐spawner abundance varied between 2,040 spawners and 13,460 spawners across years, which is over four times that previously predicted using the peak estimate of visual survey counts. Our results demonstrate the recovery of both the shore‐ and stream‐spawning kokanee in Wood Lake following a well‐documented crash in 2011 and suggest that a larger harvestable surplus is available for this high‐value kokanee fishery.
The ability to distinguish between different migratory behaviours (e.g., anadromy and potamodromy) in fish can provide important insights into the ecology, evolution, and conservation of many aquatic species. We present a simple stable carbon isotope (δ 13 C) approach for distinguishing between sockeye (anadromous ocean migrants) and kokanee (potamodromous freshwater residents), two migratory ecotypes of Oncorhynchus nerka (Salmonidae) that is applicable throughout most of their range across coastal regions of the North Pacific Ocean. Analyses of kokanee (n = 239) and sockeye (n = 417) from 87 sites spanning the North Pacific (Russia to California) show that anadromous and potamodromous ecotypes are broadly distinguishable on the basis of the δ 13 C values of their scale and bone collagen. We present three case studies demonstrating how this approach can address questions in archaeology, archival, and conservation research. Relative to conventional methods for determining migratory status, which typically apply chemical analyses to otoliths or involve genetic analyses of tissues, the δ 13 C approach outlined here has the benefit of being non-lethal (when applied to scales), cost-effective, widely available commercially, and should be much more broadly accessible for addressing archaeological questions since the recovery of otoliths at archaeological sites is rare.
Kootenay Lake is a large, oligotrophic waterbody in southern British Columbia renowned for recreational fisheries for piscivorous rainbow trout (Oncorhynchus mykiss) and bull trout (Salvelinus confluentus). Long-term datasets showed a build-up of large-bodied (>2 kg) piscivore abundance followed by a collapse of the kokanee (Oncorhynchus nerka) prey population in 2013 and subsequent decline of large-bodied piscivores. An unprecedented post-collapse state formed in 2015-2018, characterized by low kokanee spawner abundance and biomass and high catch rates for small-bodied (<2 kg), slow growing piscivores. Bioenergetics model estimates of average historic (1961-2008) piscivore consumption was 29.3% of the average historic (1993-2008) kokanee prey supply (biomass and production), but increased to 78.7% in 2011, immediately preceding kokanee collapse. From 2015-2018, kokanee did not recover due to persistently poor juvenile survival; estimated piscivore consumption relative to prey supply remained high (73.0%), suggesting that kokanee were trapped in a predator pit. Although the ultimate and interacting causes of the initial predator build-up remain uncertain, overcoming current depensatory dynamics may be aided by kokanee stocking or increasing harvest on still-abundant, unsatiated piscivores.
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