Ultilization of a seagrass meadow and tidal marsh creek by blue crabs Callinectes sapidus. I. Seasonal and annual variations in abundance with emphasis on post-settlement juveniles

著者: Robert J. Orth, Jacques van Montfrans - Marine Ecology Progress Series 1987 被引用: 233

Blue crabs Callinectes sapidus Rathbun were sampled in a lower Chesapeake Bay seagrass bed and adjacent tidal marsh creek from October 1982 through December 1986, using a drop net and suction sampler. Suction sampling was 88 % efficient and provided more accurate estimates of juvenile blue crab abundance, that were at least an order of magnitude greater than those obtained by conventional trawl gear For data analysis, C. sapidus were separated into 4 size classes by carapace width--54.25 mm (recently settled 1st and 2nd juveniles); 4.25 to 11.00 mm (3rd through 7th juvenile crabs); > 11.00 to 525.00 mm (8th through 12th juvenile crabs); and >25.00 mm (older juveniles and adults). Seasonal and annual cycles of C. sapidus abundance were observed in both habitats with annual differences most pronounced in the grassbed. Densities of C. sapidus were significantly greater in the grassbed in all but 2 of the 48 sampling dates. Settlement of the new year class appeared to be by megalopae and occurred from August through December A pulse of small C. sapidus C4.25 mm was observed in the grassbed from early to mid-September, whereas in the marsh creek this pulse occurred 2 to 4 wk later and consisted of 3rd stage and generally larger crabs (>4.25 mm). The small numbers of 1st and 2nd stage juvenile crabs (<4.25 mm) in the marsh creek in contrast to their abundance in the grassbed may result from increased predation in the tidal marsh creek or selective settlement into the grassbed. C. sapidus were rare in the marsh creek in winter but densities of crabs 5 2 5 mm remained high in the grassbed from fall through early spring. Densities decreased gradually in both habitats to lowest levels by mid-August. Approximately 90 '10 or more of C. sapidus collected at both sites from late August to June were 5 2 5 mm in carapace width. Densities of individuals 11 to 25 mm and >25 mm throughout the study period did not differ significantly among years (1983 to 1986) even though there were significant differences in the abundance of smaller sized individuals between 1983-84 and 1985-86. This suggests mortality within or emigration from grassbeds around this size. A possible ontogenetic shift in habitat use by crabs 11 to 25 mm may reflect a refuge in size from predation.