Dynamics of Rainbow Smelt during and after Exploitation in South Bay, Lake Huron

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Abstract
We examined population dynamics of rainbow smelt Osmerus mordax in South Bay of Lake Huron during intensive fishing of the spring spawning runs (1948-1957) and after exploitation was stopped (1958-1970). Modal ages of spawning rainbow smelt changed from 2 to 3 during the intensive fishing period to 3 and 4 after 1959. Percentage males in the spawning run samples from dip nets increased from 60% (1950-1957) to 78% (1958-1964). Percentage males in summer samples from gill nets increased from 39% to 50% between the same periods. Total annual mortality rate averaged 90% during exploitation and 67% after fishing stopped. Instantaneous natural mortality rate during 1948-1970 was estimated as 1.1. Relative stock sizes, estimated from cohort analysis, sharply increased after 1957; an extremely large 1960 year-class caused a peak in 1962. Growth of rainbow smelt to age 1 and to age 2 was decreasing while their own abudance and that of alewives Alosa pseudoharengus were increasing. Growth after age 2 was independent of abundance of 1-year-olds and total stock. Average fish growth for each cohort declined and abundance of 1-year-olds increased during and after exploitation, but growth slowed more rapidly after 1960 and greater numbers of 1-year-olds fluctuated more widely. Lengths at capture were much greater during 1948-1957 during 1966-1970. Total abundance of rainbow smelt in South Bay increased and growth declined during exploitation because the stock was expanding to a carrying capacity not yet reached after a severe die-off in winter 1942-1943. We suggest that immigration of spawners into South Bay from a discrete stock in Lake Huron, and relatively faster growth to maturity, allowed the population to withstand the fishing pressure.