Science of salmon stocking: report

8. Conclusions

In consideration of this scientific consensus, regulatory bodies are re-examining their stocking policies, leading in some regions to complete prohibition (e.g. Wales: Natural Resources Wales, 2014; Uttley, 2014). In other jurisdictions, however, different conclusions have been reached and stocking is still widely undertaken (Aas et al., 2018).

There would seem to be a number of reasons for continuation of stocking in the face of the available scientific evidence. It may be, after careful consideration and investigation of all options, that stocking is determined to be a useful tool to be employed to fulfil a specific management objective. There are indeed a number of scenarios where stocking has been or is being rationally and successfully utilised in this way (ICES, 2017). In contrast, stocking is also being undertaken which is based not on scientifically rational management strategies but instead is driven by socio-political factors influenced to differing degrees by governmental agencies, local managers, commercial pressures, anglers, NGOs and other stakeholders (Young, 2017). Importantly among these drivers is the fact that, after considerable investment in hatchery infrastructure, it is difficult to reverse historic practices, especially when, in the absence of an understanding of the scientific consensus, it would seem, on the surface, that adding fish to a river is 'obviously' beneficial. Such well-intentioned interventions, carried out by people with a deep commitment to the well-being of their river systems, support an entire complex of management strategies and associated employment opportunities (Trushenski et al., 2018). As such, there is considerable sociopolitical inertia supporting the maintenance of these strategies despite the often lack of evidence of their potential benefit and lack of harm they bring to the system/s.

In these times of declining salmon numbers, there is often understandable and significant societal pressure to attempt to reverse such trends and the establishment of a hatchery and associated stocking can often be seen by stakeholders as a both visual and positive step. However, hatchery supplementation programmes are resource intensive, difficult to monitor, and carry well-established potential risks to both wild populations of the stocked species and to the wider ecosystem. Nevertheless, they can also, in certain circumstances, potentially provide a useful and, in some cases, the only tool to mitigate environmental disturbances and conserve or enhance natural populations. Each particular programme will have its own set of drivers and ecological constraints, so determination of where it is likely to fall along the continuum of potential positive and negative impacts must be evaluated on a case-by-case basis. Such evaluation must be science-based but also include wider socioeconomic impacts, together with local, national and international advisory and regulatory factors. These various factors should be considered and addressed during the development of a comprehensive project plan.

Decades of research supports a simple evidence-based scientific consensus (Young, 2017): if the integrity of wild salmon is a management priority, stocking hatchery fish should be avoided where possible (Hilborn, 1992; Blanchet et al., 2008; Araki and Schmid, 2010; Palmé et al., 2012). The genetic changes and loss of wild fitness, which has been well established in hatchery fish, place a significant risk to wild populations as such fish are released into systems. Competitive interaction with wild conspecifics at different life stages, together with impacts on the wider ecosystem may also negatively impact both the species of focus and the wider ecology of the river. Further, resources spent on potentially negative or ineffectual stocking might be better spent on other conservation strategies within the catchments. Such outcomes mean that, although as outlined above, in some situation's hatchery stocking may be an effective tool, depending on the objectives and circumstances (Arlinghaus, 2006; Lorenzen et al., 2013; Camp et al., 2014; Lorenzen, 2014; Arlinghaus et al., 2016; Amoroso et al., 2017; Johnston et al., 2018), in many cases extreme care should be taken in order to avoid negative outcomes.

Recent reviews of stocking programmes showed that successful programmes addressed all stressors acting on the population, in contrast to many unsuccessful ones where not all stressors were, or could be, addressed (Araki and Schmid, 2010; ICES, 2017). These outcomes are well illustrated by the differing outcomes of the live gene bank programmes in the Bay of Fundy and Norwegian rivers. The Inner Bay of Fundy programme has been operating for more than 15 years, yet there is little evidence of progress towards the stated goal of the re-establishment of self-sustaining wild populations in the face of continuing stressor pressures (Fisheries and Oceans Canada, 2018). In contrast, the Norwegian gene banking programme has successfully re-established self-sustaining wild populations in more than ten rivers following removal of stressors, which, in this case, were either controlling acidity (Hesthagen and Larsen, 2003) or the complete removal of the G. salaris parasite (Norwegian Envornment Agency, 2020). The removal of stressors is thus seen to be of paramount importance to the outcome of a restorative stocking programme. Indeed, in the absence of stressor removal, there is not only the danger of the programme failing in its aims, but it might actually result in negative outcomes (Araki and Schmid, 2010) and act as a further stressor on the already threatened population (ICES, 2017).

As is the case with all stocking programmes, together with removal of stressors, outcomes rely on following a set of well-defined design steps. Historically, however, such steps have not been followed and this has meant that a rigorous evaluation of the effectiveness of such schemes is often extremely difficult, if not impossible (ICES, 2017; Glover et al., 2018). Indeed, due to the fact that stocking has been such a widely used tool, it is impossible, in many cases, to single out the effect of the stocking versus the effects of other conservation measures and natural ecosystem changes which may be acting in parallel (e.g. Milner et al., 2004; Griffiths et al., 2011; ICES, 2017). Further, without such a rigorous evaluation, there is a risk of overestimating the stocking benefit whilst underestimating the role of alternative parallel restorative approaches, leading to an associated perpetuation of an inefficient restoration action which may inflict more harm than good (Carr et al., 2015).

Following a precautionary approach means that appropriate risk assessment methodology should be developed and applied during programme development and before stocking commences. This would include the provision of all information necessary to demonstrate that a proposed stocking activity will not have a significant adverse impact on wild salmon populations or have an unacceptable impact on the ecosystem (NASCO, 2007). This same precautionary approach can also be used at the regulatory policy development level, where the costs/benefits/risks of the different types of supplementation programmes can be evaluated at both the scientific and socioeconomic levels. This principle is especially relevant for rivers which have enhanced protective status due to their particular conservation importance. However, every situation is different, and so in some situations, where the threat of extinction can be identified and is imminent and extreme, hatchery supplementation may provide a vital tool in the right circumstances as long as careful consideration is given to the inherent risks of such an approach.

All stakeholders have the same basic goals for their rivers and fisheries: to develop the means to help species maintain, recover or enhance their populations. Hatchery supplementation can play a part in such endeavours but may be the right or wrong tool, depending on the situation. It is thus vital that stakeholders come together to jointly work toward defining and achieving their common goals.