The Impact of the Second Seasonal Spawn on the Nantucket Population of the Northern Bay Scallop

Nantucket, Massachusetts, has one of the last remaining commercial fisheries of the bay scallop Argopecten irradians , which is based largely on natural recruitment. Though previously thought to spawn only once in early summer at age 1, individuals of the northern subspecies often spawn again in late summer or fall, and recruits from this second spawning can survive to reproduce again in their second summer. We formulated an age‐based Leslie matrix model and estimated population growth rate with and without a second spawn based on data from 5 years of life history research. Elasticity analysis revealed that the population growth rate was most sensitive to juvenile survival, the major factor in recruitment rate, and year‐1 adult fertility was a close second. We varied those two rates randomly in a stochastic matrix model, which represented the effect of environmental fluctuations on population growth. A life history modeled with a second spawn had a negligible effect on the deterministic population growth rate under constant conditions, but under variable conditions the second spawn increased the mean of the stochastic growth rates up to 58.3% over that of a single early‐spawning life history. These results suggest that the second spawn is a successful bet‐hedging strategy. The northern bay scallop increases its chances for successful recruitment in a variable environment by spreading reproductive effort over more than one period in a season. This strategy appears to have sustained the Nantucket scallop population in spite of severe annual fluctuations and the eventual collapse seen in other locations. Received March 6, 2015; accepted June 19, 2015