EVALUATING SEASONAL FOOD WEB RELATIONSHIPS TO OPTIMIZE KOKANEE ONCORHYNCHUS NERKA MANAGEMENT IN BUFFALO LAKE, WA

Brian Patrick Lanouette

Buffalo Lake, Washington is currently managed by the Confederated Tribes of the Colville Indian Reservation for the sustainability of a kokanee fishery, with secondary objectives of providing hatchery Rainbow Trout and quality Largemouth Bass fisheries. From 2014–2017, during variable environmental conditions, seasonal food web interactions were assessed with stomach content and sable isotope analysis, consumption quantified with bioenergetic modeling, and consumption rates compared against resource availability to determine if kokanee sustainability was impacted by competition from excess trout and predation from bass. In evaluating food web relationships a separate study was conducted to determine the impact of prey type on Largemouth Bass gastric lavage efficiency for pulsed syringe and mechanized pump techniques. Prey types with the most complex body structures, e.g. unidentified fish parts, were the most difficult to lavage, but overall lavage efficiency was high, 81–88%. There was no difference noted between the two methods for any prey types. Managers are therefore recommended to utilize the method most conducive to the study circumstances. Stomach content and stable isotope analysis revealed fish exhibited shifts in seasonal feeding behaviors driven by environmental conditions, resulting in a coupling/decoupling of pelagic-littoral food webs. Coupling of the pelagic-littoral food webs occurred in high water when Rainbow Trout capitalized on littoral prey. During warm, low water conditions, littoral access was limited, and Rainbow Trout switched to pelagic prey. This decoupled the littoral-pelagic food webs resulted in diet overlap between Rainbow Trout and kokanee. Largemouth Bass predation occurred during low water years and represented an additional pelagic-littoral food web coupling. Bioenergetic modeling of consumption indicated zooplankton exploitation was less than 50%, expect for summer when it exceeded 80%. Interspecific competition drove this demand, but zooplankton were able to recover as consumption demand decreased in the fall. Predation and competition did not impact kokanee productivity. Rainbow Trout should be stocked at current levels to prevent competition. The dynamic nature of seasonal food web relationships based on environmental conditions could allow managers to develop effective environmentally based adaptive management strategies on Buffalo Lake where more Rainbow Trout can be stocked during high water years.

EVALUATING SEASONAL FOOD WEB RELATIONSHIPS TO OPTIMIZE KOKANEE ONCORHYNCHUS NERKA MANAGEMENT IN BUFFALO LAKE, WA

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