Science of salmon stocking: report

1. Introduction

Atlantic salmon (Salmo salar) are spawned in rivers and develop in freshwater for a number of years before undergoing a marine feeding migration of up to many 100s of km's with eventual return to natal spawning grounds after one or more years to reproduce. Some males spawn in freshwater without first migrating to sea, whereas in most populations all females are anadromous. Thus, populations of salmon are vulnerable to poor environmental conditions across a broad area of marine and freshwater environments. The Atlantic salmon is a cultural and conservation icon. It is also the subject of commercial net and recreational sport fisheries in many countries around the North Atlantic Ocean basin. For centuries, salmon populations have suffered pressures due to human use of river habitats, for example, suffering from pollution, abstraction of water and obstruction to migration. In recent decades, as many river conditions have been improved, the significant declines in populations seen throughout the range (Friedland et al., 2009; Chaput, 2012; ICES, 2019) appear to be associated with changes in marine conditions (Chaput, 2012; Olmos et al., 2019). For more than a century, stocking has been undertaken to try to increase production and offset such declines (Molony et al., 2003).

Stocking has been utilised in a wide variety of applications which are examined in detail below. However such interventions have been associated with significant controversies (Waples, 1999) and there has been much debate as to the effectiveness of stocking (Waples, 1999; Naish et al., 2007; Araki and Schmid, 2010) due to significant risks involved (Young, 2017). Under different jurisdictions and in differing circumstances, regulatory authorities have decided on different approaches, ranging from case-by-case justification to outright bans of stocking (reviewed in Chaput et al., 2017). Restrictions, and closures of stocking programmes (Harrison et al., 2019a), have been associated with conflict between stakeholder groups (Harrison et al., 2019b). Hence, it is fundamentally important that there is clarity regarding the reasoning behind decision making and the strength of available evidence.

The suitability of stocking as a management tool is governed by two cost-benefit considerations. Firstly, financial: does the cost of investing in stocked fish result in a nett benefit to income and/or capital value of a fishery? Secondly, environmental: does the financial or potential conservation benefit of stocking justify the associated biological risk? The first of these assessments is primarily a matter of business judgement that can be informed by scientific understanding of the biology of salmon. The second question is of more general interest because in this case the risks of stocking have potential to cause damage to populations that are a national resource of immense value. It is not a simple process to evaluate these risks and benefits, particularly because the scientific understanding is incomplete. In many countries Governmental regulation is applied to this risk assessment with the aim of safeguarding salmon as a national resource and in fulfilling legal conservation responsibilities. However, this is not always the case. For example, in Scotland, powers rest with devolved local fisheries management Boards comprised of owners of fisheries, where such Boards exist.

The first part of this review identifies various benefits of stocking and reviews evidence for associated risks that should be considered. Then, the review identifies decision hierarchies that can be followed by fisheries managers and regulatory bodies, considers relevant scientific understanding and defines uncertainties. It is intended that the overview will provide a useful decision-making tool for informed managers and policy developers. Although the focus is on Atlantic salmon, the review refers to salmonid fishes more widely and, as such, draws lessons from, and has relevance to, a wide range of stocking efforts underway or in development.