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Hatchery production of juvenile fish for release into the wild has been practiced for well over a century in an effort to increase the number of salmon available to harvest. In this study, we evaluate the net impact of the largest such program in North America, the hatchery program for pink salmon (Oncorhynchus gorbuscha) in Prince William Sound (PWS), Alaska. At the same time the hatchery program was increasing in output, there was a major change in productivity in the North Pacific so that throughout Alaska pink salmon increased dramatically in abundance between the 1970s and the 2000s. Using other regions of Alaska as reference sites, we estimate that the PWS hatchery program has increased the total catch by an average of 17 million fish, of which 8 million have been allocated to pay hatchery operating expenses. We estimate that the maximum sustainable yield (MSY) of wild spawning fish in PWS has increased slightly (28%), while in regions of Alaska without pink salmon hatchery programs the MSY has tripled. Our results support the use of a precautionary approach to future large-scale stock enhancement efforts.

Fishing can drive changes in important phenotypic traits through plastic and evolutionary pathways. Size-selective harvest is a primary driver of such trait change, has received much attention in the literature and is now commonly considered in fisheries management. The potential for selection on behavioural traits has received less study, but mounting evidence suggests that aggression, foraging behaviour and linked traits can also be affected by fishing. An important phenomenon that has received much less attention is selection on reproductive phenology (i.e., the timing of breeding). The potential for this type of "temporal selection" is widespread because there is often substantial variability in reproductive phenology within fish populations, and fisheries management strategies or fishermen's behaviours can cause fishing effort to vary greatly over time. For example, seasonal closures may expose only early or late breeding individuals to harvest as observed in a range of marine and freshwater fisheries.Such selection may induce evolutionary responses in phenological traits, but can also have demographic impacts such as shortened breeding seasons and reduced phenotypic diversity. These changes can in turn influence productivity, reduce the efficacy of management, exacerbate ongoing climate-driven changes in phenology and reduce resilience to environmental change. In this essay, we describe how fisheries management can cause temporal variability in harvest, and describe the types of selection on temporal traits that can result. We then summarize the likely biological consequences of temporally selective fishing on populations and population complexes and conclude by identifying areas for future research. K E Y W O R D Sfisheries induced evolution, fisheries management, migration, phenology, reproduction, selection

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The timing of breeding migration and reproduction links generations and substantially influences individual fitness. In salmonid fishes, such phenological events (seasonal return to freshwater and spawning) vary among populations but are consistent among years, indicating local adaptation in these traits to prevailing environmental conditions. Changing reproductive phenology has been observed in many populations of Atlantic and Pacific salmon and is sometimes attributed to adaptive responses to climate change. The sockeye salmon spawning in the Cedar River near Seattle, Washington, USA, have displayed dramatic changes in spawning timing over the past 50 years, trending later through the early 1990s, and becoming earlier since then. We explored the patterns and drivers of these changes using generalized linear models and mathematical simulations to identify possible environmental correlates of the changes, and test the alternative hypothesis that hatchery propagation caused inadvertent selection on timing. The trend toward later spawning prior to 1993 was partially explained by environmental changes, but the rapid advance in spawning since was not. Instead, since its initiation in 1991, the hatchery has, on average, selected for earlier spawning, and, depending on trait heritability, could have advanced spawning by 1–3 weeks over this period. We estimated heritability of spawning date to be high (h2 ~0.8; 95% CI: 0.5–1.1), so the upper end of this range is not improbable, though at lower heritabilities a smaller effect would be expected. The lower reproductive success of early spawners and relatively low survival of early emerging juveniles observed in recent years suggest that artificial and natural selection are acting in opposite directions. The fitness costs of early spawning may be exacerbated by future warming; thus, the artificially advanced phenology could reduce the population’s productivity. Such artificial selection is known in many salmon hatcheries, so there are broad consequences for the productivity of wild populations comingled with hatchery‐produced fish.