Recruitment Mechanisms of Striped Bass and Atlantic Salmon: Comparative Liabilities of Alternative Life Histories

Naturally reproducing stocks of striped bass Marone saxatilis and Atlantic salmon Sa/mo salar have declined drastically in the last 20 years. These species face the common problems of anadromous fish, but have evolved very different solutions. Our purpose is to compare the relative sensitivities of these alternative solutions or life history strategies to decreases in conditional survival probabilities. Such decreases are relevant to the issues of increased fishing mortality or environmental contaminants. Eigenvalue analysis and Monte Carlo simulations of models by age (striped bass) or life stage (Atlantic salmon) suggested that striped bass are more sensitive than Atlantic salmon to fishing mortality. Striped bass recruitment is highly variable, and population persistence is contingent on the formation of strong year classes. The likelihood of formation of a strong year class diminishes rapidly as increases in fishing mortality decrease the average age of the population. Atlantic salmon have a more complex life history in which the durations of the freshwater and marine stages vary considerably. The persistence of Atlantic salmon populations hinges on demographic plasticity related to the chronological age at which maiden fish spawn. Repeated spawning is relatively unimportant. We use a model of alternative "lives" of Atlantic salmon to show that the selection for early age at smolting and decreased duration of the marine phase can be outcomes of increased fishing mortality alone. Annual variations in marine survival rates alter the relative fitness of various "lives" and act to maintain them in the population. Striped bass Marone saxatilis and Atlantic salmon Salmo salar are highly prized recreational and commercial species, but naturally reproduc­ ing stocks of both species are depressed. Com­ mercial landings of striped bass along the Atlantic seaboard of the United States declined from a peak of6.7 kilotonnes (kt) in 1973 to 0.8 kt in 1983 (Boreman and Austin 1985). Similarly, commer­ cial catches of Atlantic salmon in the north Atlan­ tic Ocean declined from a peak of 12.7 kt in 1973 to 6.7 kt in 1984 (Anonymous 1985). 1