Research Document - 2016/030

Risks and benefits of mitigating low marine survival in wild salmon using smolt-to-adult captive-reared supplementation

By Dylan J. Fraser

Abstract

Most salmon supplementation programs capture and spawn wild adults and release large numbers of their captive-reared juveniles. A rarely adopted alternative is smolt-to-adult supplementation (SAS), wherein migrating smolts are captured, captive-reared until maturation and subsequently released in freshwater. Where marine return rates are low, SAS could potentially offer advantages over juvenile supplementation towards mitigating population declines by:

1. providing a predictable input to adult population size,
2. avoiding well-documented genetic risks to captive-rearing at early life stages experiencing high mortality, and
3. maintaining free mate choice in the wild.

Nevertheless, uncertainties exist concerning the potential genetic and ecological risks of SAS to wild populations, and whether the benefits of SAS are fully realized in the wild. In particular, the extent to which SAS reduces marine adaptation (or adaptation to freshwater-marine linkages) through unintentional or relaxed selection, or causes negative carry-over effects on fitness, is unknown. If such changes are as strong as with juvenile supplementation, SAS may not provide any additional demographic benefits to wild populations, according to life stage elasticities/sensitivities. There is an urgent need to quantify and compare the lifetime fitness of SAS progeny versus wild progeny, and their second generation progeny under natural conditions, but such controlled experiments require a decade to complete in salmon. To minimize risk in the interim, any potential adoption of SAS must ensure that:

1. deviations from wild phenotypic trait distributions are minimized as much as possible;
2. SAS is only used sparingly as a short-term approach to supplementing at-risk wild populations, and that
3. SAS comprises only a small-to-modest proportion of the total number of returning adults, unless a larger proportion is needed to avoid population extirpation.

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