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Hatchery-Wild Salmon Stock Interactions:

Prince William Sound Pink Salmon

ABL Home

Marine Salmon Interactions (MSI)

Stock Enhancement

Hatchery-Wild Stock Interactions:

Southeast Alaska Chinook Salmon

Prince William Sound Pink Salmon

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Alex Wertheimer
Auke Bay Laboratories
Alaska Fisheries Science Center, NOAA Fisheries

Ted Stevens Marine Research Institute
17109 Pt Lena Loop Rd
Juneau AK 99801
(907) 789-6040
Alex.Wertheimer@noaa.gov

Hatcheries have been used to create or maintain fisheries by mitigating for habitat degradation and circumventing factors that limit production from a specific locale or region. Billions of Pacific salmon are now cultured and released into coastal ecosystems throughout the North Pacific Ocean. Some hatchery programs have been very successful in producing fish for harvest; for example, Japanese chum salmon hatcheries have produced annual returns of 40-87 million adults since 1990, exceeding historical production levels by more than an order of magnitude. As the scale of hatchery production has increased, however, concern for potential ecological effects on wild salmon stocks has also increased.

In Prince William Sound (PWS), Alaska, pink salmon have increased to historically high levels of abundance; total pink salmon returns to PWS averaged 31million fish annually from 1990 to 2000. Many of these fish have been produced from a system of four large hatcheries. The numbers of juveniles released by the hatcheries increased rapidly until the mid-1980s (Fig. 1); 500-600 million juvenile pink salmon have been released annually since then. Hatchery returns from these releases have averaged 25.3 million fish annually from 1990 to 2000.

Figure 1. -- Total run, wild run, and hatchery releases of pink salmon in Prince William Sound, Alaska, 1960-2001. Data from Alaska Department of Fish and Game.

Concurrent with increasing hatchery production, however, the number of wild pink salmon returning to PWS has declined from record high levels for brood years 1977-1983 (Fig. 1), and productivity (returns per spawner) of wild pink salmon has generally declined. The role of hatcheries in regard to the wild-stock decline is controversial. Some analysts have attributed the decline in wild-stock abundance to interactions with hatchery production, and have concluded that hatchery fish have produced little net benefit to the region’s fisheries.

Researchers in the MSI program, in collaboration with colleagues from the University of Alaska and the Alaska Department of Fish and Game, have addressed this issue of replacement versus enhancement of pink salmon in PWS from the hatchery program (Wertheimer et al. 2001). They compared historical catches and returns per spawner from different regions of Alaska, and concluded that wild pink salmon populations, in the absence of hatchery production, could not account for most of the record returns to PWS. They also used a generalized linear version of the Ricker spawner-recruit model to analyze the relationship of wild stock productivity with the number of hatchery juveniles released, and an array of other environmental variables (Wertheimer et al. 2004). Three time periods of wild stock spawner-recruit data were analyzed; the time periods were defined by the availability of the associated environmental data. For all time periods, indices of conditions in the marine environment best explained the variability in wild stock production in PWS. No significant effect of hatchery releases on productivity was observed for the 1980-1998 brood years (with the most comprehensive set of environmental variables) or for the 1960-1998 brood years (with the longest time series for spawner-recruit data and for some of the environmental variables). For the time period for the 1975-1998 brood, hatchery releases were identified as affecting wild stock productivity, but did not explain as much of the variability as did an index of density-independent marine survival conditions. Based on these results and a simulation model for the time period in which a detectable hatchery effect was identified, they estimated that for return years 1990-2000 the annual loss in wild production due to displacement by hatchery fish was 0-4.6 million pink salmon, and that the commensurate annual net gain in total returns was 20.6 million to 25.3 million pink salmon (Fig. 2).

Figure 2. -- Estimated annual loss in wild stock production, and annual net gain from hatchery production for return years 1990-2000, resulting from hatchery releases of pink salmon in Prince William Sound, Alaska, over three time periods. From Wertheimer et al. (2004).

These analyses did not take into account the potential for density-dependent effects of the hatchery program on body size of spawners in PWS. Abundance of Pacific salmon in the North Pacific Ocean has increased in recent decades, while body size of adult fish has generally decreased. Average body size of pink salmon in PWS has declined significantly since the inception of the hatchery program, from 1.9 kg for the period from 1965 to 1975, to 1.45 kg for 1990 to 2000. In a recent paper (Wertheimer et al. In Press), MSI researchers examined how body size at return of PWS pink salmon was related to 10 biophysical factors, including the scale of hatchery production, and the effect of body size at return on productivity of wild pink salmon in PWS. The success of large-scale enhancement in increasing the total pink salmon run in PWS may have contributed to the decline in body size because of density-dependent growth in the Gulf of Alaska. Variability in wild-stock productivity in PWS was again found to be primarily driven by density-independent factors in the marine environment, but body size of a year-class also significantly affected productivity of wild PWS pink salmon. The impact of hatchery-induced changes in adult body size on wild-stock production in PWS was estimated to be an annual wild-stock yield loss of 1.03 million pink salmon, less than 5% of the annual hatchery return of 24.2 million adult pink salmon for brood years 1990-1999 (Fig. 3). These results are very consistent with the estimates made by Wertheimer et al. (2004), and support the conclusion that sea ranching of pink salmon in PWS has provided large net benefits to the salmon fisheries of the region.

Figure 3. Observed and predicted body size at return (A) and wild-stock productivity (B) for 1975-1999 brood year pink salmon in Prince William Sound, Alaska. Predicted values are estimates from simulations of the effect of hatchery production of pink salmon. From Wertheimer et al. (In Press).

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