Stocks of Atlantic Salmon (Salmo salar) in Britain and Ireland: Discreteness, and Current Management
The null hypothesis that all Atlantic salmon (Salmo salar) in the North Atlantic belong to a common stock is shown to be untenable, on the grounds of differences between North American and European populations in transferrin gene frequencies, chromosome numbers, and numbers of circuli on scales. The further hierarchical breakdown of the European fraction into western, northern, and Baltic components is revealed through tagging data, which also shows distinctions within the western group, segregating into Icelandic, French, and British/Irish subpopulations. Within the latter, two groups are definable on the basis of differences in transferrin (Tf2) gene frequencies, the boreal and celtic races. Within the Celtic race, the populations of the neighboring rivers Bandon and Blackwater (Cork) have been shown to be distinct reproductively isolated stocks, on the basis of differences in gene frequencies in two enzymes (IDH-A, and AAT-A). Circumstantial evidence for the separate identity of 74 river stocks in Britain and Ireland, on the basis of differences in age structure at the smolt and adult stage, and size at return to the rivers, is supported by evidence from tagging experiments of a high incidence of homing to the native river, experimental evidence of the heritability of growth and developmental traits, and evidence from scale data of differences in specific growth rate of individual river populations during their 1st year at sea. Environmental pressures selecting for differences between river stocks are suggested by the significant positive correlation between river length and fish size at maturity. Differences in patterns of change in the commercial catch over the period 1952–76 indicate regional groups of river stocks within Scotland. The existence of such discrete stocks of Atlantic salmon implies the need for management on a stock-by-stock basis. Current national and international regulations do not achieve this goal.Key words: Atlantic salmon, transferrin gene frequencies, river stocks, management