Abstract
Through sperm limitation, population density can be a critical variable in fertilization success of marine broadcast spawners. The broadcast-spawning sea scallop Placopecten magellanicus is a commercially important species that has rebounded in population abundance within fishing closures imposed on Georges Bank (GB) and in the Mid-Atlantic (MA). Using video surveys, we tested whether closure increased population density, degree of aggregation and body size of sea scallops. Population densities and shell heights (SHs) were consistently higher in closed areas, significantly so in the MA. Shell height was significantly greater in all closed areas. Influence of area closure on degree of aggregation was less consistent. Using observed spatial patterns of adult scallops on GB and MA and classic steady-state sperm-plume models, we modeled expected fertilization success for two spawning populations with a 10-fold difference in adult density. To test model calculations, we measured fertilization success of manipulated scallop populations of these two densities over the 2012 spawning season in the Damariscotta River tidal estuary in mid-coast Maine. Contrary to predictions, our field experiments revealed no strong effects of population density on fertilization success. We did see significant spatial autocorrelation in adult scallop abundance, however, suggesting greater aggregation at low population densities within several days of high fertilization success on our recorded peak spawning date. Small-scale behavioral adaptation that reduces nearest-neighbor distances at low population sizes may ameliorate the effect of low mean density on fertilization success. Given the exponential relationship between SH and gonad mass, the greater average SH in fishing closures on GB and the MA imply that these regions may be particularly valuable by producing substantially more larvae per capita than areas open to fishing. Positive impact of high density on fertilization success predicted by fertilization models was not borne out, however, in our field trials over a 10-fold density difference. We observed higher-than-predicted fertilization success in our low-density treatment, which may be an encouraging sign that the reproductive performance of broadcast-spawning scallops at low mean densities could be greater than expected possibly due to aggregation behavior.