Statistics of Reproduction and Early Life History Survival of the Georges Bank Sea Scallop (Placopecten magellanicus) Population

In a spatial analysis of the Georges Bank sea scallop (Placopecten magellanicus) population, variables of population reproduction and abundance are calculated from Canadian and USA research survey size-frequency data. Using commercial catch as a scaling factor to obtain absolute estimates, the time averages of harvestable-sized population numbers and meat-weight biomass, annual recruit numbers, and egg production were calculated from 1977 to 1988 for five principal subregions of Georges Bank. Average survival from egg to age 2 was obtained directly. Incorporating recent field sampling measurements of larval densities in the water column, survival and thus natural mortality was estimated for egg to larva and larva to age 2 juvenile. The size-specific fecundity vector was also employed in a lifetime egg productionper-recruit analysis. Detailed error analysis of the scaling coefficient and sensitivity analysis of potential error due to gear selectivity were combined into confidence intervals for the estimated population statistics. 1 Present address: Chesapeake Biological Laboratory, P. O.Box 38, Solomons, Maryland, USA 20688. Introduction The Georges Bank sea scallop (Placopecten magellanicus) population supports the third most valuable fishery in Atlantic Canada, worth approximately $100 million to Canada and the USA. As with all renewable resources, the processes of population reproduction are crucial to successful management. Detailed studies of sea scallop reproduction have included in particular: the annual reproduction cycle (Naidu, 1970; Robinson et al., 1981; Beninger, 1987; MacDonald and Thompson, 1986a; Shumway et al., 1988), fecundity (Langton et al., 1987), and feeding (Cranford and Grant, 1990; Shumway et al., 1987); the effect on growth (Wildish and Kristmanson, 1988; MacDonald and Thompson, 1985a, 1988) and reproductive output (MacDonald and Thompson, 1985a, 1985b; MacDonald et al., 1987; Barber et al., 1988) in response to changing environmental conditions; of spawning (Posgay and Norman, 1958) and larval development (Culliney, 1974); vertical distribution (Tremblay and Sinclair, 1990a) and vertical migration (Silva and O’Dor, 1988; Balch, MS 1990; Tremblay and Sinclair, 1990b) of larvae in the water column; of settlement (Merrill and Edwards, 1976; Dadswell and Sinclair, MS 1989) and longterm population trends (Dickie, 1955; Dow, 1977; Caddy, 1979). These are summarized in reviews by MacKenzie (MS 1979), Young-Lai and Aiken (1986) and Caddy (1989). In general it is known that Georges Bank scallops spawn in the early autumn, releasing large numbers of eggs into the water column, where the larvae, feeding on phytoplankton, grow through four stages in a period of roughly 4–6 weeks before settlement. Recruitment to five discrete subregions of the bank (Fig. 1) from sources of egg production is determined by the drift trajectories of pelagic J. Northw. Atl. Fish. Sci Sci., Vol. 13: 83–99 84 J. Northw. Atl. Fish. Sci. Vol. 13, 1992 Fig. 1. Georges Bank and its five subregions utilized in a spatial analysis of the sea scallop (Placopecten magellanicus) population. larvae and the survival rates after settlement, within and between subregions. The pattern of residual currents, characterized by a tidally driven clockwise gyre (Greenberg, 1983) which retains water most tightly above the bank during scallop spawning in early autumn (Butman et al., 1987), and direct measurements of daily tidal motion (Butman and Beardsley, 1987), and of the spatial distribution of adult beds (Robert and Black, 1990) and larvae (Tremblay, MS 1991; Tremblay and Sinclair, 1988) above the Canadian part of the bank, imply that two of the subregions, the Northern Edge and the Northeast Peak, are occupied by a single subpopulation. The three other subregions are occupied by spatially separated subpopulations (Fig. 1). A study of stock-recruitment relationships within and between the same subregions of Georges Bank studied here (McGarvey et al., 1992) suggests these subpopulations may be connected reproductively through larval transport. As noted by Sinclair et al. (1985), the persistence of aggregations year after year in the same general locations, even under intense harvesting, suggests that the spatial distribution of scallop beds is determined by the local persistence of hydrographic features, gyres, upwellings, mixing and other tidally driven current patterns which affect temperature, primary productivity, bottom flow rates and larval retention in these favourable habitats. The spatial processes of the Georges Bank scallop fishery were investigated by Caddy (1975) using a detailed model. Postulating stochastic annual recruitment with historical success probabilities in each 10' square area, Caddy confirmed that the intense levels of exploitation directed towards aggregations of high abundance, in particular on the Northern Edge, diminish the sustained yield, in part at least, by excessively rapid removal of younger scallops. Time-averaged spatial analysis is valuable for identifying distributions of population reproduction and abundance in relation to physical conditions especially geographically stable features of the marine environment. Yearly scallop population surveys have been carried out since 1977 on Georges Bank by the Canadian Department of Fisheries and Oceans in Halifax and the USA National Marine Fisheries Service in Woods Hole. Using data from these surveys for 1977–88, we generated time–averaged estimates of variables of population reproduction. These include recruitment, egg production, and abundance for the five subregions of Georges Bank (Fig. 1). The results yield a spatial breakdown of the demographics of the scallop populaton on the bank. In 40 70 69 68 67 66 41 42 71 Great South Channel (West) Great South Channel (East) Southeast Part Northeast Peak USA CANADA