The quest for successful Atlantic salmon restoration: perspectives, priorities, and maxims

Abstract

Atlantic salmon is often a focal species of restoration efforts throughout the north Atlantic and it is therefore an excellent case study for how best to design programmes to address and mitigate threats and correct population declines. This perspective is written to promote the work that has been accomplished towards restoration of Atlantic salmon populations and synthesize how we believe the lessons can be used effectively to support efforts by management agencies to restore populations. We reviewed where restoration is needed for Atlantic salmon, agreed on definitions for three levels of successful restoration, and then applied these criteria to 49 published papers focused on Atlantic salmon restoration. We identified 16 successful examples of restoration among 49 papers reviewed and discussed what interventions led to success versus failure. We then addressed key questions about when hatchery stocking should be used as part of a restoration measure and whether local restoration efforts are enough when these wide-ranging species encounter broad-scale changes in the north Atlantic, specifically related to issues of climate change and to marine survival. We advise to avoid restoration as much as possible by protecting and managing existing populations and when restoration is necessary, problems should be identified and addressed in partnership with river users. With appropriate resources and research to resolve ongoing mysteries, restoration of lost Atlantic salmon populations is absolutely feasible.

Atlantic salmon (Salmo salar) age at maturity is strongly affected by temperature, population and age-at-maturity genotype

“…Furthermore, some males never leave their home river and instead mature at a small size (down to 5 g) at the parr life stage, and so, mature individuals returning from the sea can be several thousand times larger (up to 25 kg and higher) than their mature river-bound counterparts. In recent decades, there have been conservation concerns for wild Atlantic salmon stocks due to population declines, with factors suggested to have contributed to these declines including climate change, aquaculture, illegal fishing, habitat degradation, hydropower dams and harvesting of prey species ( Einum et al , 2008 ; Chaput, 2012 ; ICES, 2019 ; Dadswell et al , 2021 ; Lennox et al , 2021 ; Czorlich et al , 2022 ; Harvey et al , 2022 ; Vollset et al , 2022 ). Some of these factors have also been associated with life history changes in the wild stocks, with some populations experiencing a decrease in the number or proportion of early-maturing individuals ( Vollset et al , 2022 ), while others are reporting a decrease in large, late-maturing individuals ( Czorlich et al , 2018 , 2022 ; Olmos et al , 2019 ).…”

Time series covering up to four decades reveals major changes and drivers of marine growth and proportion of repeat spawners in an Atlantic salmon population

“…Wild Atlantic salmon ( Salmo salar ) face a complex suite of environmental stressors throughout their lives. Some of these stressors are natural, while others are caused by constantly expanding anthropogenic activities in rivers and the coastal zone (Forseth et al, 2017 ; Lennox et al, 2021 ). With some exceptions in the northern areas (Niemelä et al, 2005 ), Atlantic salmon (hereon referred to as salmon) populations have declined throughout most of their distribution over the past several decades (Friedland et al, 2009 ; Jensen et al, 2011 ; Peyronnet et al, 2007 , 2008 ; Todd et al, 2008 ).…”

A Resist‐Accept‐Direct (RAD) future for Salmon in Maine and California: Salmon at the southern edge

“…Because the match of life‐history types, habitat, and climate is key to success now and in the future, loss of rare populations can limit future options. Conversely, others have suggested a range‐wide strategy should focus on saving the strongest populations in the healthiest habitats (Lennox et al, 2021; Walsh et al, 2020). While prioritizing resist efforts makes sense for strongholds, there is also value in conserving extant populations that have withstood transformational changes to this point, suggesting a mixed approach is prudent.…”