Long‐term changes and effects of significant fishery closures on marine survival and biological characteristics of wild and hatchery‐reared Atlantic salmon Salmo salar

Abstract

Long‐term data, over four decades, were analysed to examine temporal trends in survival indices and phenotypic characteristics of Atlantic salmon Salmo salar returning to the Burrishoole national salmonid monitored river in Ireland. Before 2007, the marine drift net fishery was the major capture method for salmon in Irish home waters, accounting for over 70% of the commercial catch and targeting mixed stocks from multiple rivers. The authors examined size differences in fish captured in marine and freshwater environments and the impact of closure of this fishery on long‐term survival indices and fish size. Return rates to Irish home waters for wild one sea‐winter (1SW) and a ranching strain of hatchery‐reared 1SW Atlantic salmon stocks showed a declining trend up to the time of closure of the fishery (1985–2006). In contrast, closure of the drift net fishery resulted in the anticipated increase in return rate to fresh water in the short term. Nonetheless, the short‐term upward trend was not sustained in the following years: the trend for return rate to fresh water (1985–2017) was found to be neither increasing nor decreasing. Mean return rates to fresh water 10 years pre‐ and post‐closure of the drift net fishery increased from 7.4% to 8.5% for wild 1SW and significantly from 2.4% to 3.7% for ranched 1SW suggesting some benefit had accrued as a consequence of drift net closure. For ranched 1SW salmon, entry into fresh water was found to be occurring earlier, which is likely a phenotypical response to changing climatic conditions. A declining trend in fish length was found in the pre‐closure period, followed by a more stable trend post‐closure. Similar patterns were observed for fish condition and weight parameters. Significantly, a step change in fish size occurred just before the closure of the Irish drift net fishery in both marine and freshwater habitats, when the average length decreased by 3.8 and 4.6 cm, respectively, between 2005 and 2006. This suggests an environmental effect on the population, rather than a fishery closure effect. Similar trends in fish length were observed in wild 1SW salmon kelts and ranched 2SW salmon in fresh water. The stable but not increasing trends post‐closure suggest that conditions at sea may not be improving. These findings show that a clear decline occurred in wild and ranched salmon populations' return rates and lengths, while the drift net fishery was still active. Closure of the fishery did not result in a rebound to pre‐exploitation levels of these indicators. Nonetheless, the trends went from declining to stable, suggesting the closure helped mitigate the impact of unfavourable environmental and rearing habitat conditions. These findings, based on four decades of data, highlight the urgency of strengthening monitoring of fisheries populations in face of climate change, so as to guide precautionary management measures that, as this study suggests, may be able to mitigate its impacts.

Body length changes for Atlantic salmon (Salmo salar) over five decades exhibit weak spatial synchrony over a broad latitudinal gradient

“…Marine exploitation rates are presently at historic lows and have been since the 1990s (Figure S7 ; ICES, 2023 ), and therefore, as in Cotter et al. ( 2022 ), these fisheries cannot explain the reductions in body size over the last two decades for salmon in some rivers. However, even though present harvest levels are low, the previously high exploitation rates may hinder the recovery of Atlantic salmon populations, possibly as a result of the loss of genetic diversity (Hauser et al., 2002 ; Law, 2000 ; Uusi‐Heikkilä et al., 2015 , but see Hutchings & Kuparinen, 2020 ) or changes in the frequency of alleles related to the age when salmon mature (Czorlich et al., 2022 ).…”

“…These fisheries use gillnets which can selectively harvest larger individuals (Chadwick & Claytor, 1990;Reddin, 1984). The inverse relationship between fisheries harvest and body size has been documented for Atlantic salmon in North America (Bielak & Power, 1986;Moore et al, 1995) and Europe (Cotter et al, 2022), as well as other marine species globally (Fenberg & Roy, 2008;Ricker, 1981;ter Hofstede & Rijnsdorp, 2011). The selective removal of larger fish may result in a positive feedback relationship that causes further reductions in body size and can impair sustainable harvests (Audzijonyte et al, 2013), slow recovery from overfishing (Birkeland & Dayton, 2005;Olsen et al, 2004) and affect the resilience of populations to other environmental perturbations such as climate change (Planque et al, 2010).…”

“…The selective removal of larger fish may result in a positive feedback relationship that causes further reductions in body size and can impair sustainable harvests (Audzijonyte et al, 2013), slow recovery from overfishing (Birkeland & Dayton, 2005;Olsen et al, 2004) and affect the resilience of populations to other environmental perturbations such as climate change (Planque et al, 2010). Marine exploitation rates are presently at historic lows and have been since the 1990s (Figure S7; ICES, 2023), and therefore, as in Cotter et al (2022), these fisheries cannot explain the reductions in body size over the last two decades for salmon in some rivers. However, even though present harvest levels are low, the previously high exploitation rates may hinder the recovery of Atlantic salmon populations, possibly as a result of the loss of genetic diversity (Hauser et al, 2002;Law, 2000;Uusi-Heikkilä et al, 2015, but see Hutchings & Kuparinen, 2020) or changes in the frequency of alleles related to the age when salmon mature (Czorlich et al, 2022).…”