Impact of naturally spawning captive-bred Atlantic salmon on wild populations: depressed recruitment and increased risk of climate-mediated extinction

Abstract

The assessment report of the 4th International Panel on Climate Change confirms that global warming is strongly affecting biological systems and that 20–30% of species risk extinction from projected future increases in temperature. It is essential that any measures taken to conserve individual species and their constituent populations against climate-mediated declines are appropriate. The release of captive bred animals to augment wild populations is a widespread management strategy for many species but has proven controversial. Using a regression model based on a 37-year study of wild and sea ranched Atlantic salmon (Salmo salar) spawning together in the wild, we show that the escape of captive bred animals into the wild can substantially depress recruitment and more specifically disrupt the capacity of natural populations to adapt to higher winter water temperatures associated with climate variability. We speculate the mechanisms underlying this seasonal response and suggest that an explanation based on bio-energetic processes with physiological responses synchronized by photoperiod is plausible. Furthermore, we predict, by running the model forward using projected future climate scenarios, that these cultured fish substantially increase the risk of extinction for the studied population within 20 generations. In contrast, we show that positive outcomes to climate change are possible if captive bred animals are prevented from breeding in the wild. Rather than imposing an additional genetic load on wild populations by releasing maladapted captive bred animals, we propose that conservation efforts should focus on optimizing conditions for adaptation to occur by reducing exploitation and protecting critical habitats. Our findings are likely to hold true for most poikilothermic species where captive breeding programmes are used in population management.

Impact of climate change and human-mediated introgression on southern European Atlantic salmon populations

“…The level of introgression decreased dramatically after 1996 (Table 3), which may indicate dilution of foreign genes in the wild gene pool consistent with decreased fitness of alleles originating from farmed individuals (e.g. McGinnity et al, 1997McGinnity et al, , 2009Martinez et al, 2001).…”

“…In Atlantic salmon, hatchery releases may threaten genetic integrity of even geographically distant wild salmon populations (Vasemägi et al, 2005). The escape or deliberate introduction of captive bred animals into the wild can substantially depress natural recruitment and more specifically can disrupt the capacity of natural populations to adapt to higher winter water temperatures associated with climate variability (McGinnity et al, 2009). In addition, our results suggest that in the present conditions of accelerated climate warming the risk is even higher, because warm conditions will likely enhance mobility of hatchery individuals among rivers in a region.…”

“…Populations are currently managed by limiting catches per angler and conducting supportive breeding (Machado-Schiaffino et al, 2007;Horreo et al, 2008) and we expect that foreign genomes introduced in the past are being diluted in those populations because farmed and nonnative individuals released in the wild generally exhibit less fitness than native ones, e.g. Garcia-Vazquez et al, 1991;McGinnity et al, 1997McGinnity et al, , 2009Martinez et al, 2001;Finnegan & Stevens, 2008. The question addressed in the current study is, is it possible to disentangle the relative weights of climate and fisheries management in shaping contemporary regional population genetic patterns? Using microsatellite loci as genetic markers we determined gene flow and the level of population structuring in the region of Asturias in northern Spain, and estimated the relative effects of human-mediated introgression and climatedriven migration on the population pattern encountered in 4 different years over the last three decades.…”

“…Farmed salmonids have the potential for impacting on population productivity, disrupting local adaptations and reducing the genetic diversity of wild salmon populations (e.g. Fleming et al, 2000;McGinnity et al, 2009). It seems prudent to treat farmed fish as exotic species and to recognize the potentially negative consequences for wild populations, particularly when the latter are of conservation concern (Weir & Grant, 2005).…”

Compromising genetic diversity in the wild: unmonitored large-scale release of plants and animals

“…A 37-year study of Atlantic salmon in Ireland [57] found that naturally spawning farmed fish depress wild recruitment and disrupt the capacity of natural populations to adapt to higher water temperatures associated with climate change. Hansen et al [58] examined Danish populations of brown trout subject to hatchery supplementation for 60 years and found evidence for selection in the wild against alleles associated with non-native hatchery fish.…”