In a nutshell

• Enhancement of salmon stocks with hatchery-raised individuals is often used as a means of restoring depleted populations or supplementing healthy ones, but studies of its effectiveness have produced mixed results
• Although large-scale hatchery production is often cited as the reason for burgeoning North Pacific salmon abundance, there is evidence that salmon productivity would have increased in the absence of hatchery releases due to changing environmental conditions; hatchery fish compete for food in the ocean with wild salmon and thus might replace them rather than increasing total available salmon for fisheries
• Here, analysis of long-term pink salmon catch data from four regions of Alaska, two of which support large hatchery programs, suggests that hatchery-released pink salmon have replaced wild salmon in some regions instead of augmenting wild populations
• The authors caution that gains from hatchery programs for enhancing wild pink salmon populations may in some cases be significantly lower than expected, and that managers should adopt a more conservative approach toward the use of stock enhancement

These contrasting perspectives fall under the auspices of the “augmentation” and “replacement” hypotheses. According to the augmentation hypothesis, hatchery releases would generate additional productivity with no impact to existing wild stocks, whereas under the replacement hypothesis, hatchery fish would essentially simply replace wild fish in catches. “Imagine a stable system in which 1,000 wild fish are naturally produced each year and 500 of them are caught,” explains Michael Tillotson, a doctoral student at the University of Washington’s School of Fisheries and Aquatic Sciences and a coauthor of the paper. “A hatchery begins releasing juveniles to supplement the catch. If hatchery fish have zero negative impact on the productivity of wild fish, then each adult hatchery fish returning increases the population available for harvest by one fish, while productivity of wild fish remains unchanged. On the other hand, if each hatchery fish displaces one wild fish, for example by competition for a limited food resource, then the total fish available for harvest remains unchanged, but the productivity and maximum sustainable yield of the wild population decreases proportionally to the number of hatchery fish. This is a case of complete replacement and, assuming there is a cost to hatchery production, the net benefit of the hatchery is actually negative.” The hypotheses in fact represent the opposing ends of a spectrum, with most cases almost certainly falling somewhere in between the two extremes.

Average pink salmon catch sizes have increased over the past several decades in Alaska, and thus analysis of catches within enhancement sites over time, as well as between enhancement sites and regions in which no enhancement activities have taken place, provides an opportunity to examine the relative contribution of stock enhancement to the progressively higher catches of this species. Tillotson and his colleagues used wild escapement, total catch, hatchery fry release, wild catch, and cost-recovery data derived from the Alaska Department of Fish and Game’s Annual Management and Fisheries Enhancement reports over the period of 1960 to 2013 to examine what effect pink salmon enhancement has had on fisheries production in four regions of Alaska. Two of these regions, Prince William Sound (PWS) and Kodiak (KOD), support large-scale pink salmon hatchery programs, with production significantly higher in the former than in the latter, whereas little hatchery production occurs in the other two regions, that of southeastern Alaska (SEAK) and the southern Alaskan peninsula (SPEN). In addition, three distinct periods of productivity were identified, consisting of the “pre-regime-shift” period (1960–1976), defined as the period immediately prior to the onset of significant changes in North Pacific conditions and fisheries productivity; a transition period (1977–1987) characterized by increasing productivity of wild stocks in the absence of large-capacity hatchery releases; and the most recent period of 1988–2011, in which catch levels were high and total hatchery releases were both high and stable. Due to the 2-year life cycle of pink salmon, the researchers were also able to construct recruitment data for brood years over the 1960–2011 period, with salmon of the 1977 brood year assumed to be the first to experience the full effects of the changes in ocean conditions.

Although the maximum sustainable yield (MSY) of wild stocks in the SPEN and SEAK regions were found to have surged over the past half century, MSY in the PWS and KOD regions, while also larger, failed to display the same rate of increase despite the large-scale hatchery releases in these regions. The authors suggest that this may be due to differences in how the expanding carrying capacities driven by transforming ocean conditions were being filled in each of the regions: whereas increasing carrying capacities in the SPEN and SEAK were dominated by wild stock, the enlarged carrying capacities in the PWS and KOD regions were filled primarily by hatchery fish, and as such no apparent change in wild stocks were observed in these latter two areas. The growth in MSY in areas where hatcheries are absent, in combination with the much smaller response in areas where large-scale hatcheries are present, is interpreted as a strong indication that hatchery fish may be reducing the production of wild stocks.

“Because pink salmon have experienced high productivity throughout their North American range in recent decades, we consider it most likely that changing ocean conditions explain increased carrying capacity and MSY, and we would therefore expect all regions to benefit from this change approximately equally,” says Tillotson. “Thus, there is apparently some other process going on in KOD and PWS that has prevented the wild pink salmon populations from experiencing the same growth as in other regions. The large hatchery programs are a conspicuous difference between our study regions, with large increases in MSY and those with small increases. In regions without large hatcheries, we see much larger increases in wild production. Our interpretation is that in regions with large hatcheries, the hatchery fish are ‘filling up’ the excess capacity for pink salmon production that has been available under favorable ocean conditions in recent decades.”

Despite the increase in pink salmon catch size in PWS over recent decades, comparisons of this site to other Alaskan regions and over long periods of time suggest that the changes in ocean conditions favorable to pink salmon that began in the late 1970s would likely have resulted in greater pink salmon abundance regardless of enhancement programs. Thus, enhancement efforts have had only a relatively minor influence on pink salmon productivity in PWS. Amaroso et al. warn that gains from hatchery programs aimed at enhancing wild pink salmon populations in PWS – and potentially elsewhere as well – may be considerably lower than would be expected, and recommend that managers adopt a “precautionary” approach toward the use of stock enhancement. “Salmon enhancement can be a cost-effective approach to increasing fisheries yields,” suggests Tillotson, “but I think there is typically insufficient information to make a comprehensive assessment of the costs and benefits of hatchery operations. In many cases where the economic impact of hatcheries has been assessed, an assumption of complete augmentation is used; that is, each hatchery fish produced is considered one additional fish that would have otherwise not been available to catch. The same assumption also seems common in the planning and evaluation of hatcheries. So, what we mean by a precautionary approach is that, prior to the initiation or expansion of hatchery production, the true potential for enhancement should be evaluated – given local conditions, is it likely that a substantial enhancement effect can be sustained, or is it more likely that wild conspecifics or other species will be negatively impacted? In a similar vein, for existing programs, any evaluation of their cost effectiveness should also include these types of considerations.”

Although the authors were unable to identify the specific mechanisms driving these long-term patterns, the results of this research, along with those of similar studies, further serve to demonstrate that marine stock enhancement is a far more complex process than simply the addition of hatchery-raised individuals to existing wild stocks, and typically involves numerous and extensive trade-offs that may offset potential benefits.

Science Spotlight by Ken Ferguson

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