Science of salmon stocking: report

2. Management objectives and potential benefits

In general, the intended benefit of a stocking programme is to increase numbers of salmon compared with taking no action. This simple objective covers situations ranging from replacing a population that has become extinct to increasing yield of an already productive fishery. In some cases, such as when salmon are reared to the smolt stage and then released (ranching), increasing numbers may be for the short-term benefit of a fishery harvesting the stocked fish irrespective of potential longer-term damage to the population. However, in most cases, fisheries managers are concerned with ensuring sustained increase in numbers of salmon within a population. The conservation potential, socioeconomic consequences and risks vary across the range of types of wild supplementation situations. A starting point for managers is to clarify intended benefits.

Figure 1 Steps in the development of a successful stocking programme (redrawn after Cowx, 1994b).

Define fishery management objectives by assessing stock and habitat status

Is production in danger of expiration?

If yes:

Consider stocking for conservation

If no:

Is production at carrying capacity?

If yes:

Consider alternative management options

If no:

Are constraints on production known?

If no:

Investigate cause

If yes, and after causes have been examined

Is stocking necessary or appropriate?

If yes:

Consider stocking for enhancement

If no

Can cause be ameliorated?

If no:

Consider stocking for mitigation

If yes:

Remove constraint

Consider stocking for restoration

Consideration of stocking as a management tool, and the eventual success of this approach, depends on an evaluation of the objectives for initiating the intervention following the process in Fig. 1. Such objectives fall into four broad categories: conservation, mitigation, restoration and enhancement (Cowx, 1994b).

2.1 Conservation

Conservation stocking is undertaken to maintain biodiversity in populations at high risk of decline and/or extinction (Cowx, 1994b). In such situations stocking is carried out with the aim of maintaining a population until pressures adversely affecting conservation status can be addressed or subside. In situations where wild populations are in critical decline, the window of opportunity for conservation of the remaining wild resource may be rapidly closing (O'Reilly and Doyle, 2007) and intervention by stocking may be the best and only option available. In such programmes, the hatchery can be considered to be a living gene bank (Gausen, 1993; O'Reilly and Doyle, 2007). Such conservation stocking is exemplified by application in a number of contexts in several countries. The approach has been adopted to seek preservation of the phenotypically and genetically distinct groups of Atlantic salmon residing in the Inner Bay of Fundy, Canada (Fisheries and Oceans Canada, 2018). It has also been applied in Norway towards conservation of populations threatened by acid precipitation and infection by the parasite Gyrodactylus salaris (Gausen, 1993; Norwegian Envornment Agency, 2020).

Implementation of a living gene bank requires considerable economic cost and robust science-based operating procedures that minimise the negative consequences a hatchery situation can create (O'Reilly and Doyle, 2007), as discussed in due course. Further, as with any supplementation initiative, unless and until the original pressure(s) has been addressed, the chance of success is questionable (ICES, 2017).

2.2 Mitigation

Mitigation stocking is defined as "Stocking conducted as a voluntary action or statutory requirement to mitigate lost production due to an activity that cannot be removed" (NASCO, 2007). This approach aims to offset losses of production in freshwater that are due to anthropogenic impacts and/or recent increases of salmon mortality in the marine environment (Chaput, 2012; Lehnert et al., 2019). A total of twelve anthropogenic stressors acting on salmon populations were identified by ICES (2017): 1) barriers, including but not limited to hydroelectricity production, 2) pollution, 3) water regulation, 4) exploitation, 5) aquaculture, 6) habitat degradation, 7) diseases and parasites, 8) climate change, 9) invasive species, 10) predators, and 11) other uncategorised stressors (e.g. light and noise pollution, shipping) and 12) stocking. Indeed, stocking was listed both as a potential action and a potential stressor, due to the negative impacts such a process may produce. There are a wide range of types of mitigation stocking associated with the diverse set of stressors acting and interacting in different populations. In many cases, due to the inability to remove stressors, such stocking can be significant and long lasting. For example, extensive stocking has been applied to attempt to mitigate for obstruction to free passage of migrating salmon associated with hydro-electricity dams in some cases for many decades (MacCrimmon and Gots, 1979; Parrish et al., 1998; Palmé et al., 2012; Lenders et al., 2016). Indeed, the inputs in some areas are so great that they now represent a larger proportion of the stocks than natural production (e.g. the Baltic Sea where annual releases of ~5 million juveniles represent c. 60% of the total production; ICES, 2018).

2.3 Restoration

The aim of stocking for restoration purposes is to promote the rapid recovery of natural populations that have been reduced in numbers after the cause of the decline has been identified and removed (Aprahamian et al., 2003). In their review of restorative stocking programmes, ICES (2017) found that such programmes can be effective in achieving these aims. Thus, notwithstanding the risks associated with stocking (reviewed below) and if used with due caution, correct planning and evaluation, restorative stocking has been shown to be a useful approach in the managers' toolkit. It should also be noted that, even if such a programme does not reach its ultimate goals, it can still provide a valuable positive impact by reducing short-term pressures while other human-led and/or natural restorative processors are underway, especially in the early stages of recovery (e.g. Milner et al., 2004).

2.4 Enhancement

Enhancement stocking is undertaken to augment the production of wild stocks through the release of hatchery-reared fish (NASCO, 2007), typically to increase recreational and/or commercial fishing opportunities (Utter and Epifanio, 2002). Such stocking is achieved through a number of routes. In some cases, hatchery produced fish are stocked into areas already containing a wild population, with the aim to boost the natural numbers (Bacon et al., 2015). Stocking of fish can also be carried out in areas of a watershed not usually accessible to the wild populations, due to natural barriers and so outside native range (Killinger, 1994), or lack of suitable habitat for certain life-history stages (Armstrong et al., 2003), again with the aim of increasing availability of juvenile habitat and associated carrying capacity of a watershed. Such stocking can also be carried out into rivers in geographic regions where the species is not found at all naturally (MacCrimmon and Marshall, 1968; MacCrimmon and Gots, 1979; Halverson, 2010; Gordeeva and Salmenkova, 2011). Finally, ocean ranching can be carried out using mass releases of juveniles directly into the marine environment, with the aim of completely avoiding the potential bottleneck of juvenile freshwater carrying capacity (Moberg and Salvanes, 2019).

Table 1

Risks mechanisms and impacts associated with hatchery supplementation. A full reference list for each issue can be found in Appendix 1.

Hatchery

Issue: Broodstock collection

• Mechanism/s: Broodstock mixing, restricted broodstock numbers, disruption of mate choice and reproductive timing, loss of natural production
• Impact: Loss of genetic diversity, homogenisation of populations, loss of local adaption

Issue: Use of non-native stocks

• Mechanism/s: Maladaptation to local environment
• Impact: Reduced fitness, increased straying, loss of population structure, introgression into wild stocks, decline in numbers following hatchery cessation

Issue: Domestication

• Mechanism/s: Adaptation to hatchery, gene expression changes
• Impact: Loss of fitness in wild, loss of reproductive capabilities, loss of population structure, phenotypic, physiological, behavioural and life history changes

Issue: Introduction of escaped farmed fish into broodstock

• Mechanism/s: Collection from wild without screening
• Impact: Loss of local adaption, reduced fitness

Issue: Hatchery adaptation

• Mechanism/s: Epigenetic changes
• Impact: Altered gene expression, physiological processes, migration, behaviour

Issue: Hatchery conditioning

• Mechanism/s: Hatchery rearing causing plastic phenotypic divergence
• Impact: Changes in growth rate, morphology, behaviour and life history traits

Issue: Loss of resilience

• Mechanism/s: Reduced genetic variability
• Impact: Long-lasting evolutionary impacts and loss of resilience to environmental changes

Environmental

Issue: Competition

• Mechanism/s: Physiological and behavioural interactions
• Impact: Compromised natural recolonization, reduced natural production, ecological disturbance

Issue: Displacement

• Mechanism/s: Displacement of wild gene frequencies/stocks
• Impact: Displacement of wild stocks and replacement with hatchery

Issue: Hybridisation (hatchery/wild)

• Mechanism/s: Hatchery fish interbreeding with wild fish
• Impact: Loss of fitness in wild-born hybrid offspring, loss of population structure, loss of genetically defined traits, changes in catchability

Issue: Hybridisation (inter-species)

• Mechanism/s: Increased hybridisation in areas of stocking
• Impact: Loss of wild reproduction, loss of fitness

Issue: Immunocompromisation

• Mechanism/s: Hatchery selection
• Impact: Reduction in disease resistance in wild population

Issue: Enhanced straying

• Mechanism/s: Maladaptation to local environment
• Impact: Enhanced straying of hatchery inputs

Issue: Enhanced predation

• Mechanism/s: Behavioural changes
• Impact: Attraction of predators to greater resource and/or to less risk adverse hatchery fish which also then impacts wild. Stocked fish may directly predate wild or same or other species.

Issue: Introduction of parasites/pathogens

• Mechanism/s: Infections from hatchery transferred to the wild
• Impact: Mortality / eradication of wild stocks

Anthropogenic

Issue: Overharvest

• Mechanism/s: Fishing mixed hatchery/wild stocks may impact weak/small wild populations
• Impact: Loss of population structure, decline/loss of wild populations

Issue: Sociological impacts

• Mechanism/s: Manipulated natural state
• Impact: Reduces sense of 'naturalness', false sense of security, undermining of incentives and divergence of resources from other management strategies