Food availability and growth of the blue crab in seagrass and unvegetated nurseries of Chesapeake Bay

Variation in habitat quality and resource availability can affect the distribution and growth of animals. Thin-shelled clams dominate many benthic communities in Chesapeake Bay, both in numbers and in biomass, and they can comprise up to 50% of the blue crab (Callinectes sapidus) diet. Our objective was to determine which habitats were optimal for juvenile crab growth and how growth related to food availability. We experimentally examined benthic infaunal food availability (primarily bivalves) and concurrent growth of juvenile blue crabs at 30-40 sites along 50 km of the York River during fall 2000 and spring 2001. Each year, 4-10 replicate sites along the York River were established in each of five habitats: (1) Seagrass, (2) Mud at the river mouth, (3) Sand at the river mouth, (4) Mud upriver, and (5) Sand upriver. Food availability inside and outside of 0.43-m2 crab growth cages was examined, along with crab growth after 3-6 months inside cages. In both years, after 3-6 months, the Baltic clam, Macoma balthica, was abundant inside and outside the cages, whereas the soft-shell clam, Mya arenaria, was only abundant inside cages. Densities of Macoma were greatest in upriver mud and sand, while those of Mya were greatest in upriver sand. Crab growth was significantly greater in spring-summer than fall-winter and was significantly higher in upriver mud and sand, where clam densities were highest, than at the river mouth. The upriver region was near the turbidity maximum, which may enhance pelagic and benthic productivity and thereby provide more food for clams and therefore for blue crabs. Crab growth in seagrass was intermediate between that upriver and at the mouth, suggesting that upriver, unvegetated, subtidal habitats adjacent to salt marshes serve as valuable nursery habitats rivaling seagrass beds.