Abstract
Predator-prey dynamics and their ecological drivers have absorbed the interest of population ecologists since models were first developed describing the cyclical nature of predator-prey populations in a static framework. Empirical data has demonstrated that species life histories evolve over ecological timescales in response to changes in broad ecological processes or specific changes in population densities and/or spatial distributions. Anthropomorphic environmental impacts have influenced ecological processes on a broad scale and in many cases impacted predator-prey relationships. Southern Resident killer whale, Orcinus orca, and its preferred prey, Chinook salmon, Oncorhynchus tshawystcha, provide an opportunity to evaluate human impacts from Chinook salmon ocean harvest and hatchery production to the predator-prey dynamic between these co-evolved species. This is particularly important when considering the management of ocean fisheries and hatcheries that support those fisheries relative to the recovery of the Southern Resident killer whale population, a federally-endangered marine mammal species. The purpose of this study was to: 1) investigate the impacts of modifications in Chinook salmon ocean harvest and hatchery production inputs on their abundance as a prey species for Southern Resident killer whale; and 2) evaluate how the Southern Resident killer whale population responds to the subsequent prey availability. Time-series (1984-2011) Chinook salmon terminal run (freshwater harvest plus spawning population) and ocean fishery harvest data from British Columbia, Washington, Oregon and California, were analyzed under twelve scenarios involving different combinations of ocean fishery harvest and hatchery production. Chinook salmon terminal run results were used as input to the Southern Resident killer whale population dynamics model to evaluate the whale population response to varying levels of prey availability. Two covariates were chosen for the Chinook salmon time-series analysis: reduction in ocean fishery harvest and reduction in hatchery production. The primary response variable chosen for this analysis was Chinook salmon terminal run, although ocean fishery harvest and Chinook salmon total abundance were also evaluated. Linear regression was used to evaluate the influence that reductions in ocean harvest and hatchery production have on the relative absolute change in Chinook salmon terminal run, fishery harvest, and total abundance between the status quo and each scenario. A Mann-Whitney Test was used to determine trends between early and late periods in the Chinook salmon time-series. Results of this study indicate significant interactions between covariate and response variables, revealing that reduction in ocean fishery harvest explains the amount of Chinook salmon in the terminal run to a weak degree, while the reduction in hatchery production explains the amount of Chinook salmon in the terminal run to a moderate degree. Although percent reduction calculations showed decreasing ocean harvest as hatchery production decreased, the linear regression showed that percent reduction in hatchery production does not explain the amount of Chinook salmon ocean fishery harvest. The ocean fishery harvest reduction percentage weakly explains the Chinook salmon fishery harvest. In addition, reduction in hatchery production explains the total abundance of Chinook salmon in the ocean to a weak degree, while ocean harvest reduction explains the total abundance of Chinook salmon in the ocean to a moderate degree. Results also indicate a significant decrease in fishery impacts from an early time period (1984-1997) to a late time period (1998-2011), while no significant difference was demonstrated between the two timeframes for terminal run and total abundance. Southern Resident killer whale modeling results revealed that all of the Chinook salmon scenarios (terminal run data) resulted in a positive Southern Resident killer whale population growth response. However, statistical analysis between scenarios indicated no significant difference in the Southern Resident killer whale population growth over a 10-year projection. These results have value to fishery managers with regard to potential modifications to Chinook salmon ocean fishing harvest and hatchery production that may result in a greater prey base for the Southern resident killer whale population. Identification of the Chinook salmon populations most important as prey to Southern resident killer whale could focus natural/wild Chinook salmon population restoration and hatchery reform efforts with the target of increasing the numbers of Chinook salmon produced in those important watersheds. Chinook salmon management decisions directed at changing the focus of hatchery operations to support naturally spawning populations and improving freshwater habitats to support natural/wild spawning Chinook salmon populations may provide more prey over the long-term for Southern resident killer whale than management actions directed at further restrictions on the commercial ocean Chinook salmon fishery. However, based on prior research (Ward et al. 2013) and the results of this study, Chinook salmon management practice involving a complete closure of the ocean fishery while maintaining current levels of hatchery production may produce a higher likelihood that the Southern Resident killer whale population would reach their recovery target.