Wild Salmon Strategy Science and Evidence Board advice to the Delivery Group: recovery stocking of Atlantic salmon into weak populations

Appendix 1: Examples of risks of stocking methods

Intercept adults, artificially inseminate and stock offspring

• Artificial insemination by-passes natural mate selection and competition.
• Hatchery temperatures deviate substantially from those in the wild.
• Artificial insemination does not provide an appropriate contribution from mature male parr.
• Damage to adults during capture and handling.
• High mortality in artificial nests/egg boxes.
• Loss of eggs/fry in the hatchery.
• Insufficient information on the target system to verify that the approach is appropriate (e.g. stocking into inappropriate habitat).
• Handling pre-spawning adult salmon affects subsequent egg production and/or development of the offspring.
• Disruption of kin associations that benefit offspring.
• By-passing natural selection at early life stages.
• Mixing up of different genetic stocks in the hatchery.

Fry redistribution

• Damage to habitat and fish associated with electrofishing.
• Disruption of the chemical imprinting process occurring at the early fry life stage.
• Disruption of aggregations of kin-related fish and loss of associated benefits.
• Displaced fish may be highly vulnerable to predation due to loss of familiarity with local area and neighbouring conspecifics.
• Insufficient information on the target system to verify that the approach is appropriate (e.g. stocking into inappropriate habitat).
• Displaced fish may attempt to return home and are highly vulnerable in the process.
• Reduced local competition leads to reduced variation in life histories associated with variation in growth rates, such as multiple year classes of salmon parr and early male maturation frequencies.
• Spreading the fry at low densities when they are small and highly vulnerable means that they do not swamp key predators (should these be present) at the time. This may result in higher overall mortality than would have been the case at the original higher densities.

Smolt-adult supplementation

• No selection at sea in the fraction of the population treated by SAS, leading to proliferation of genes that are not fit in current and changing marine environments
• Damaging to an already weak local population by distorting the gene pool and phenotypes of spawned eggs and offspring if selected fish are not fully representative of the population.
• Damaging to the wider metapopulation if released fish stray and spawn successfully elsewhere.
• Reduction in adaptation of the population to changing marine conditions.
• Stocked salmon interfere with spawning by wild salmon.
• Progeny of stocked salmon compete with offspring of wild salmon for food, space and habitat, reducing numbers of true wild smolts.
• High densities of fry due to stocking reduce size and survival of wild and stocked salmon.
• Loss or damage of fish during the rearing phase.
• Transfer of disease between rearing and wild environments.
• Epigenetic consequences of captive rearing.
• Inadequate collection of smolt and pre-smolt migrants to sample the full gene pool.
• Spawning behaviour might be negatively affected by captive rearing, as has been recorded in salmon reared in aquaculture from egg to adult.
• Poor egg and offspring quality because of captive rearing of adults.
• Removes fish which otherwise may have spawned naturally.
• Hatchery-reared fish may come from a limited genetic pool depending on how many males are used to fertilise eggs.
• Domestication selection.
• Populations become dependent on SAS diverting resources away from addressing the root cause of decline, such as pollution, habitat degradation etc.

Kelt reconditioning

• Hatchery temperatures deviate substantially from those in the wild.
• Artificial insemination by-passes natural mate selection and competition.
• Artificial insemination does not provide an appropriate contribution from mature male parr.
• Damage to adults during capture and handling.
• High mortality in artificial nests/egg boxes.
• Loss of eggs/fry in the hatchery.
• Insufficient information on the target system to verify that the approach is appropriate.
• Handling pre-spawning adult salmon affects subsequent egg production and/or development of the offspring.
• Disruption of kin associations that benefit offspring.
• By-passing natural selection at early life stages.
• Reduction in egg number and increase likelihood in fungal infection due to the capture process.
• Loss of fish after spawning in captivity, where they can be more susceptible to disease.
• Transfer of stress effects on parents in captivity to quality and performance of offspring (maternal/paternal effects)
• Possible poor survival in the wild of offspring derived from reconditioned kelts.
• Fish not caught near their natural spawning location.