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Estimates of fish abundance from electrofishing surveys depend on accurate estimation of capture probability. We examine in this paper how estimates of capture probability and abundance of Atlantic salmon from multi-pass removal sampling can be improved by comparing the results of an experimental programme of closed electrofishing sites within selected rivers in west-central Norway, and those obtained from open electrofishing sites established for monitoring long-term juvenile Atlantic salmon population abundance within the Burrishoole catchment, western Ireland. We first establish that the Carle & Strub method provides a more robust estimate of population abundance than the Zippin and Seber methods. We then show how prior information on capture probability may be used to improve the accuracy of the abundance estimate in open sites. We also show that the use of prior information with single-pass electrofishing may improve the accuracy of the abundance estimate so that it is comparable with that of multi-pass electrofishing in terms of stock prediction while requiring less sampling effort.

A numerical method for computing the eigenvalue variance effective size of a subdivided population connected by any fixed pattern of migration is described. Using specific examples it is shown that total effective size of a subdivided population can become less than the sum of the subpopulation sizes as a result of directionalities in the pattern of migration. For an extension of the model with threshold harvesting and local deterministic logistic population dynamic we consider the problem of maximizing the total harvesting yield with constraints on the total effective size. For some simple source-sink systems and more complicated population structures where subpopulations differ in their degree of isolation, it is shown to be optimal, for a given total effective size, to raise the harvesting thresholds relatively more in small and in isolated populations. Finally, we show how the method applies to populations which are supplemented, either intentionally or unintentionally. It is shown that the total effective size can be reduced by several orders of magnitude if the captive component of a population is much smaller than the wild component, even with symmetric backward migration.

Most Norwegian salmon populations are characterized by small numbers of fish. The proportion of cultured salmon in these populations has increased together with the rapid growth of the Norwegian salmon farming industry. In several spawning populations, fish of cultured origin now exceed the number of wild fish. The cultured salmon occurring in Norwegian streams are largely dominated by escapees from fish farms, although some are released for stock enhancement and ocean ranching purposes. Life history characters in cultured salmon—for instance, age and size at spawning, time of spawning and migratory behavior—often differ from those of the local stock of wild salmon. Possible ecological effects of interactions in the freshwater stages are discussed. The most serious effects so far have been the introductions to wild populations of lethal parasites and diseases with cultured fish. In recent years, more than 30 populations of salmon have been completely wiped out by the monogean parasite Gyrodactylus salaris . High mortality of adult salmon has also been observed as a result of furunculosis in some streams. In biochemically detectable loci, small but statistically significant differences in allele frequencies exist between populations within and between rivers. Possible long-term effects of genetic interbreeding and erosion of local adaptations are discussed. Increased numbers of cultured salmon may also increase the fishing intensity on wild salmon. Suggestions to reduce negative effects of interactions from cultured fish are proposed.

This note summarizes a review by Hindar et al. (1991) outlining the genetic consequences of aquaculture on natural fish populations. Publication of the full paper (Hindar et al., 1991) has preceded that of the present volume; this note is included to permitt a more complete presentation of the papers read at the Loen symposium on interactions between cultured and wild Atlantic salmon ( Salmo salar ).