Mixed-maturation fisheries compromise productivity and resilience of Chinook salmon

Most ocean-type Chinook salmon (Oncorhynchus tshawytscha) in the northeast Pacific Ocean are harvested in mixed-stock marine fisheries. Here, multiple populations with varying abundance and productivities are encountered. In addition to being mixed-stock, many of these fisheries encounter both mature and immature Chinook salmon. Hence, these fisheries are better described as mixed-stock and “mixed-maturation” fisheries. Harvest of immature fish can skew the age composition of Chinook salmon populations towards younger, and hence smaller, individuals. This shift reduces the proportion of older and larger Chinook salmon that contribute disproportionately to the productivity of their populations. We developed a deterministic individual-based demographic-genetic model of fall Chinook salmon to evaluate the effects of fisheries that harvest mature and immature Chinook salmon. We then compared these effects on age, size and weight to terminal fisheries that harvest only mature fish, evaluating the benefits to both landed catch and Chinook salmon rebuilding. In our modeled scenarios of mixed-maturation fisheries, we found that immature Chinook salmon can comprise up to 59% of the total numbers of fish caught, and 47% of the total weight of the catch. If instead, these Chinook salmon were not harvested until they reached terminal fisheries as mature adults, they would contribute greater biomass to landed catches. Unharvested and terminally harvested populations retain a more archetypal age and sex structure than contemporary fall Chinook salmon subject to marine mixed-maturation fisheries. Lastly, when harvested at rates that achieve maximum sustainable yield, terminal fisheries allow a greater proportion of larger, older Chinook salmon reach their spawning grounds, thereby increasing the productivity of their populations. The benefits of terminal fisheries would accrue to river-based fishers and communities, and other near terminal locations (such as inlets or estuaries). They would also support rebuilding of depleted Chinook salmon runs.