Recruitment Mechanisms of Southern Gulf of St. Lawrence Atlantic Herring (Clupea harengus harengus)

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Abstract
Estimates of abundance of the southern Gulf of St. Lawrence herring complex by cohort analysis indicate that both biomass and population fecundity were reduced to low levels in the late 1950s following a widespread fungus disease in the mid-1950s. As a result of two strong year-classes in the late 1950s, however, abundance increased dramatically up to 1964 but has declined continuously since then due mainly to subsequent poor recruitment. Mackerel were also at low levels of abundance in the late 1950s and remained so until the mid-1960s when a series of strong year-classes produced a rapid increase in abundance to the extent that mackerel replaced herring as the dominant pelagic fish in the southern Gulf ecosystem. Changes in herring recruitment, growth, and maturation rates are investigated in relation to changes in herring biomass and total pelagic (herring + mackerel) biomass. Density-dependent changes in all three parameters have occurred in herring; mackerel also have interacted with the growth and recruitment of southern Gulf herring. This suggests that the carrying capacity of the southern Gulf for pelagic fish is limited and that competition and predation by mackerel intensifies the logistic response of herring. Thus, recruitment of southern Gulf herring in the period under consideration was largely controlled by the total pelagic biomass, acting mainly through herring up to the mid-1960s and through mackerel since then. The increase in mackerel abundance is attributed to a combination of favorable temperature regime and optimum spawning biomass. That being so, the probability of large year-classes of herring in the near future will depend heavily on the interaction between favorable survival conditions for mackerel and the effectiveness of ICNAF regulations to maintain mackerel biomass at maximum recruitment levels.

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