Incorporating harvest–population diversity trade-offs into harvest policy analyses of salmon management in large river basins

Abstract

Accounting for population diversity can be critical to the sustainable management of mixed-stock fisheries because harvest rates that can be sustained by productive populations may come at the cost of overfishing less productive ones. While these harvest–diversity trade-offs are well-recognized, their consequences for harvest policy performance are not often explicitly evaluated in contemporary fisheries management. We use closed-loop simulations to evaluate the ability of alternative harvest policies to meet population diversity and fishery objectives for one of the largest subsistence Chinook salmon (Oncorhynchus tshawytscha) fisheries in the world (Kuskokwim River Basin in western Alaska). We found clear evidence of population diversity that resulted in asymmetric trade-offs among fishery and conservation objectives whereby policies that forgo relatively small amounts of harvest result in relatively large increases in equitable access to Chinook and elimination of risk of weak stock extirpation. The performance of alternative harvest policies, and the magnitude of trade-offs, were sensitive to regime shifts and uncertainty in the drivers of recruitment variation. However, we found that harvest policies that prioritized meeting minimum subsistence needs were unlikely to jeopardize long-term sustainability.

Indigenous Systems of Management for Culturally and Ecologically Resilient Pacific Salmon (Oncorhynchusspp.) Fisheries

“…For many species, allocation decisions driven by the PST remain a hindrance to recovery and limit the potential for transformation toward more locally managed fisheries. Therefore, mixed-stock ocean fisheries will likely need to forgo some harvest opportunity if the biological and social benefits of terminal and selective fisheries are to be realized (Connors et al 2020 ). Including Indigenous people and local communities in future PST negotiations will therefore be essential, so that management of Pacific salmon fisheries can reflect their needs, aspirations, and understandings.…”

“…In the absence of transformation, salmon managers will continue to face a set of wicked tradeoffs posed by mixed-stock fisheries, where harvesting abundant salmon contributes to the long-term erosion of the biodiversity that underpins future fishing opportunity (Connors et al 2020 ). Fortunately, solutions to these problems are not out of reach, and there is growing momentum toward transformative changes in the way that salmon are caught and managed.…”

“…In general, mixed-stock fisheries tend to produce larger and more stable catches through time (Nesbitt and Moore 2016 , Freshwater et al 2020 ). Paradoxically, the stability conferred by harvesting multiple populations comes at the cost to small or depressed populations that comigrate with larger more abundant populations, threatening the very biodiversity that underpins stable catches (Connors et al 2020 ). In some notable instances, networks of locally interconnected terminal fisheries have alleviated conservation risks and produced stable harvest opportunities.…”

Potential for ecological nonlinearities and thresholds to inform Pacific salmon management

“…While nonlinearities underpin much of ecological theory and some research suggests that these relationships may enhance decisions involving salmon (Levi et al 2012, Martin et al 2017, Connors et al 2020), nonlinearities are not widely used to inform salmon management. This is unfortunate because appreciating nonlinearities may enhance harvest and recovery efforts, especially considering trade‐offs, competing human activities, costs, and resources that constrain management.…”

“…In addition, harvest may mediate nonlinear ecological trade‐offs and small adjustments to harvest may provoke substantially more desirable outcomes. For example, increasing escapement from low to medium levels may substantially increase grizzly bear density—a proxy for ecosystem function—while in some cases also increasing fisheries yields (Levi et al 2012) and policies resulting in small reductions (−20%) to mixed stock harvest may greatly increase equitable access to salmon resources (+84%) and virtually eliminate the risk of weak stock extirpation (Connors et al 2020).…”