RECRUITMENT MECHANISMS OF SOUTHERN GULF OF ST-LAWRENCE ATLANTIC HERRING (CLUPEA-HARENGUS HARENGUS)

Estimates of abundance of the S Gulf of St. Lawrence herring complex by cohort analysis indicate that both biomass and population fecundity were reduced to low levels in the late 1950''s following a widespread fungus disease in the mid-1950''s. As a result of 2 strong year-classes in the late 1950''s, abundance increased dramatically up to 1964 but has declined continuously since then mainly due to subsequent poor recruitment. Mackerel were also at low levels of abundance in the late 1950''s and remained so until the mid-1960''s 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 S Gulf ecosystem. Changes in herring recruitment, growth and maturation rates were investigated in relation to changes in herring biomass and total pelagic (herring + mackerel) biomass. Density-dependent changes in all 3 parameters have occurred in herring; mackerel also have interacted with the growth and recruitment of S Gulf herring. The carrying capacity of the S Gulf for pelagic fish is apparently limited and competition and predation by mackerel may intensify the logistic response of herring. Recruitment of S Gulf herring in the period under consideration was largely controlled by the total pelagic biomass, acting mainly through herring up to the mid-1960''s and through mackerel since then. The increase in mackerel abundance is attributed to a combination of favorable temperature regime and optimum spawning biomass. The probability of large year-classes of herring in the near future may depend heavily on the interaction between favorable survival conditions for mackerel and the effectiveness of ICNAF [International Commission for the North Atlantic Fisheries] regulations to maintain mackerel biomass at maximum recruitment levels.