1. Changes in biophysical conditions through time generate spatial and temporal variability in habitat quality across landscapes. For river ecosystems, researchers are increasingly able to characterize spatial and temporal patterns in habitat conditions, referred to as shifting habitat mosaics, yet rarely demonstrate how this translates into corresponding biological processes such as organism growth and production.

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.

Recent decades have seen an increased appreciation for the role infectious diseases can play in mass mortality events across a diversity of marine taxa. At the same time many Pacific salmon populations have declined in abundance as a result of reduced marine survival. However, few studies have explicitly considered the potential role pathogens could play in these declines. Using a multi-year dataset spanning 59 pathogen taxa in Chinook and Coho salmon sampled along the British Columbia coast, we carried out an exploratory analysis to quantify evidence for associations between pathogen prevalence and cohort survival and between pathogen load and body condition. While a variety of pathogens had moderate to strong negative correlations with body condition or survival for one host species in one season, we found that Tenacibaculum maritimum and Piscine orthoreovirus had consistently negative associations with body condition in both host species and seasons and were negatively associated with survival for Chinook salmon collected in the fall and winter. Our analyses, which offer the most comprehensive examination of associations between pathogen prevalence and Pacific salmon survival to date, suggest that pathogens in Pacific salmon warrant further attention, especially those whose distribution and abundance may be influenced by anthropogenic stressors.

No abstract

Variation among populations in life history and intrinsic population characteristics (i.e., population diversity) helps maintain resilience to environmental change and dampen interannual variability in ecosystem services. As a result, ecological variation, and the processes that generate it, is considered central to strategies for managing risks to ecosystems in an increasingly variable and uncertain world. However, characterizing population diversity is difficult, particularly in large and remote regions, which often prevents its formal consideration in management advice. We combined genetic stock identification of archived scale and tissue samples with state‐space run‐reconstruction models to estimate migration timing and annual return abundance for eight geographically and genetically distinct Chinook salmon populations within the Canadian portion of the Yukon River. We found that among‐population variation in migration timing and return abundances resulted in aggregate return migrations that were 2.1 times longer and 1.4 times more stable than if they had composed a single homogeneous population. We then fit state‐space spawner–recruitment models to the annual return abundances to characterize among‐population diversity in intrinsic productivity and population size and their consequences for the fisheries they support. Productivity and carrying capacity varied among populations by approximately 2.4‐fold (2.9 to 6.9 recruits per spawner) and three‐fold (8800 to 27,000 spawners), respectively. This diversity implies an equilibrium trade‐off between harvesting of the population aggregate and the conservation of individual populations whereby the harvest rate predicted to maximize aggregate harvests comes at the cost of overfishing ~40% of the populations but with a relatively low risk of extirpating the weakest ones. Our findings illustrate how population diversity in one of the largest salmon‐producing river basins in the world contributes to fishery stability and food security in a region where salmon have high cultural and subsistence value. More generally, our work demonstrates the utility of molecular analyses of archived biological material for characterizing diversity in biological systems and its benefits and consequences for trade‐offs in decision‐making.