Chinook salmon diversity contributes to fishery stability and trade-offs with mixed-stock harvest.

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.

Temperature, emergence phenology and consumption drive seasonal shifts in fish growth and production across riverscapes.

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. We assessed spatial patterns and processes determining seasonal changes in juvenile Chinook Salmon Oncorhynchus tshawytscha size, growth and production over 30-40 km in two NE Oregon subbasins. We quantified seasonal patterns of growth by combining estimated emergence dates and body size distributions in July and September. We then used analysis of bioenergetics, empirical fish diets and spatial models incorporating temperature, habitat and population density to evaluate mechanisms driving spatiotemporal patterns of growth. Lastly, we quantified seasonal contributions to individual fish growth and to total production as a function of position within the stream network. Spatial heterogeneity in incubation temperatures corresponded to later estimated emergence timing with distance upstream in both subbasins. During spring, estimated growth rates decreased with distance upstream, and coupled with emergence patterns, resulted in pronounced longitudinal gradients in body size by July. During summer, spatial patterns of growth reversed, with greater diet ration sizes and growth efficiencies upstream than downstream. These opposing spatiotemporal patterns of emergence timing and seasonal growth rates produced longitudinal gradients in the proportion of fish growth achieved in spring versus summer, with up to 80% of an individual's growth occurring in spring at downstream sites but as low as 10% at upstream sites. Coupling longitudinal patterns of fish density and growth revealed that in one subbasin the majority (65%) of total production occurred in spring, while in the other, in which fish were concentrated in headwaters, the majority (60%) of production occurred in summer. While recent work has emphasized inter-annual shifts in fish production across large spatial scales, this study demonstrates that longitudinal gradients of fish growth and production can reverse across seasons, and reveals important contributions of warmer, downstream habitats to overall production that occurred during cooler times of the year.

Thermal performance of the electron transport system Complex III in seven Alabama fishes.

Management of fish populations for conservation in thermally variable systems requires an understanding of the fish's underlying physiology and responses to thermal stress. Physiological research at the organismal level provides information on the overall effects of stressors such as extreme temperature fluctuations. While experiments with whole organisms provide information as to the overall effects of temperature fluctuations, biochemical assays of thermal stress provide direct results of exposure that are both sensitive and specific. Electron transport system (ETS; Complex III) assays quantify a rate-limiting step of respiratory enzymes. Parameters that can be estimated via this approach include optimum thermal temperature (Topt ) and optimal breadth of thermal performance (Tbreadth ), which can both be related to organismal-level temperature thresholds. We exposed enzymes of seven fish species (native fish chosen to represent a typical community in Alabama streams) to temperatures in the range 11-44°C. The resultant enzymatic thermal performance curves showed that Topt , the lower temperature for enzyme optimal thermal performance (Tlow ), the upper temperature for enzyme optimal thermal performance (Tup ), and Tbreadth differed among species. Relationships between enzymatic activity and temperature for all fish followed a pattern of steadily increasing enzyme activity to Topt before gradually decreasing with increasing temperature. A comparison of our enzyme optimum and upper-temperature limit results versus published critical thermal maxima values supports that ETS Complex III assays may be useful for assessing organismal-level thermal tolerance.