The Distribution and Production of the Hard Clam, Mercenaria mercenaria, in Wassaw Sound, Georgia

Abstract

Listen

Translate article

Hard clams,Mercenaria mercenaria, occurred in four intertidal habitats in the outer, high salinity (18‰) region of Wassaw Sound, Georgia: small feeder creeks\((\bar x = 36 per m^2 )\); oyster shell bar deposits\((\bar x = 31 per m^2 )\); headwaters of shelly, sandy-mud, sand, and mud bottom creeks (\(\bar x\) = 26, 19, 13 and 3 per m2, respectively); and among live oysters (\(\bar x\) = 1 per m2). 0266 03 Clams from creek bottoms were larger (\(\bar x\) = 7.3 cm) than those from intertidal flats (\(\bar x\) = 4.7 cm) due to differences in predation or harvesting pressures, not to differeing rates of recruitment. Juveniles were absent from most areas, possibly because of increased juvenile mortality, natural sporadic settling, or restricted gonadal development due to abnormally low spring salinity from 1977 through 1979. Clams from Little Tybee Island, where sediments are sandy-mud, had faster growth rates than those from North Cabbage and Wassaw Islands, where clam beds were in shell deposits. Clams older than 7 years dominated at Wassaw and Little Tybee Islands (64 and 71%, respectively) and younger clams dominated (82%) at North Cabbage Island. These differences in age class structures are attributed to different harvesting or predation pressures. Clams from Little Tybee and Wassaw Islands occurred in creek bottoms, whereas clams from North Cabbage Island occurred on an intertidal flat in the open sound. Also, a greater variety of clam predators and greater densities ofUrosalpinx cinerea andBusycon spp. occur on intertidal flats of the open sound than on creek bottoms. Net reproduction ofMercenaria was 7.7 g ash-free dry weight (AFDW) per m2 per yr at Little Tybee Island, 6 g AFWD per m2 yr at Wassaw Island and 2.7 g AFDW per m2 per yr at North Cabbage Island. Differences are attributed to differences in standing stocks, age class structure, and growth rate. Standing stocks were low (11 g AFDW per m2) at North Cabbage, moderate at Little Tybee (50 g AFWD per m2) and high (120 g AFDW per m2) at Wassaw Island. The turnover ratios (P/B) of Wassaw, Little Tybee, and North Cabbage Island populations were low (0.05, 0.14, and 0.23, respectively) because a high percentage of older (>7 years) clams make up the populations.