Restoration and Enhancement of Salmonid Populations and Habitats with Special Reference to Atlantic Salmon
Bror Jonsson and Nina Jonsson
Abstract.—Populations of Atlantic salmon Salmo salar can be restored and enhanced through planting of green or eyed eggs (embryos) in rivers and by releasing fry, parr, smolts, or postsmolts. The success of the releases varies with time and site of release, broodstock origin, size and age of the fish, and rearing and release techniques applied. However, egg, fry or parr releases cannot be used for augmenting populations above the carrying capacity of the water course. To surpass the carrying capacity, the fish should be released as smolts or postsmolts. Smolts released in rivers during spring migrate to sea for feeding but return to the river of release for spawning. Atlantic salmon released at the postsmolt stage may return to the release site when adult, but thereafter, they may stray to any of a number of rivers for spawning. As a result of ecological interactions, released juvenile hatchery fish may partly displace, increase the mortality, and decrease the growth rate, adult size, reproductive output, biomass, and production of wild conspecifics through density-dependent mechanisms working in freshwater. Hatchery-reared Atlantic salmon are usually competitively inferior to wild conspecifics both during feeding and spawning in rivers, due to environmental impacts and genetic changes that occur during the juvenile rearing. Habitat restoration is preferred when restoring endangered, threatened, or weak populations. Degraded spawning habitats can be reconstructed, and poor freshwater quality can be mitigated. In regulated rivers, rapid fluctuations in water level should be avoided, and the migratory activity of the fish can be stimulated by increased water flow. Populations can also be enhanced by expanding the accessible nursery habitat by use of artificial fishways through human induced or natural migration hindrances. Adaptive management practice is useful when restoring and rehabilitating populations and habitats. More knowledge is needed about environmental and genetic influences on the phenotype of hatchery fish and how habitats constrain salmon production in rivers.