Productivity and resilience of Chinook salmon compromised by ‘Mixed-Maturation’ fisheries in marine waters

Abstract

Most 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, many of these fisheries generally encounter both mature and immature Chinook. Hence, these fisheries are better described as mixed-stock and mixed-maturation (MM) fisheries. Harvest of immature fish can skew the age composition of Chinook populations towards younger, and hence smaller, individuals. Older Chinook are generally larger and contribute disproportionately to the productivity of their populations. We developed an individual-based demographic-genetic model of ocean-type Chinook to evaluate the effects of fisheries that harvest immature Chinook. We then compared those effects to terminal fisheries that harvest only mature fish. Our model provides the ability to assess the benefits of terminal Chinook fisheries to both landed catch and Chinook rebuilding. Recovered populations show a more archetypal age- and sex-structure than contemporary ocean-type Chinook subject to marine mixed-maturation fisheries. In our modeled scenarios of mixed-maturation fisheries, we found that immature Chinook 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 were not harvested until they mature and reach terminal fisheries, they would contribute greater biomass to landed catches. These terminal fisheries allow a higher percentage of larger, older Chinook to escape, and would increase the fecundity and productivities of their populations. The benefits of terminal fisheries would accrue to fishers, sustainable wild harvesting, wildlife, and the rebuilding of depleted Chinook runs.