Large and small scale distributions of the giant scallop Placopecten magellanicus were determined in 2 scallop beds near Port Daniel in the Baie des Chaleurs, eastern Canada. Large scale (km) distribution was strongly associated with substrates of gravel or gravel mixed with sand. Within each bed, small scale (cm) distribution was contagious corresponding to a negative binomial distribution. Morisita's index confirmed the contagious spatial distribution and showed that clump size was approximately 1.13 m2 in one bed and 4.50 m2 in the other. The majority of scallops in both beds rvere ~vithin clumps. Small scale aggregation was not related to substrdte type. Mature and immature scallops were significantly associated In the less densely populated bed but not in the more densely populated bed. This suggests that immature scallops moved nearer to adult scallops when densities were low Both sexes were present in 79 %of clumps of adult scallops. In both beds the sex ratio ~7 a s 1 : l and scallops did not appear to seek out members of the opposite sex. In the 2 beds 91 and 75 " i of nearest neighbor scallop pairs were 100 cm or closer to each other. The short distance between scallops within clumps, the high proportion of clumps with both sexes present, and an average of 3 scallops per clump suggest high fertilization success within clumps. We suggest that the small scale aggregation of scallops is a n adaptation to increase fertilization success, and if this is true, disturbance of these aggregations may decrease reproductive success.

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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 spermplume 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.

We used a nested quadrat design to precisely and accurately estimate the absolute density a n d spatial distribution of juvenile spot Leiostomus xanthurus in the tidal fringe of a salt marsh. Fifty white quadrats (10 of each size: 32, 64. 128, 256, and 512 cmZ) were randomly placed within two 10 m2 plots adjacent to a pier. The number of fish over each quadrat was counted on 7 dates between l 7 March and 7 April 1995. Quadrats d~d not attract or repel fish and the observer's experimental bias was low. L. xanthurus had a contagious distribution, with aggregations covering approximately 128 cm2 L. xanthurus dens~ties ranged from 18.33 to 28.30 fish m-2 (3.0 to 5.9% level of precision), while densities estimated from shore seines ranged from 0 50 to 4.96 fish m-' (97.2 to 200.0% level of precision). Correction factors should be estimated before fish density estimates derived from net samples a r e used in ecosystem modeling, carrying capacity estimates or v~t a l statistics for this habitat.