Influence of seagrass habitat patch size on growth and survival of juvenile bay scallops, Argopecten irradians concentricus (Say)

Patchy seagrass meadows are common features on high energy sand shoals in t emperate coastal waters. We tested the effect of seagrass patch size within patchy seagrass meadows on growth and survival of juvenile bay scallops, A rgopecten irradians concentricus (Say), during spring (two sites) and fall (one site) settlement periods in Back Sound, North Carolina, USA. Because p lant characteristics such as density and species composition (proportion of Zostera marina L. and Halodule wrightii Aschers) may influence growth and predation rates, we also quantified these variables during each time period . Weekly growth rates of scallops in the spring were more than twice that o f the fall (similar to 1.5 vs. 0.6 mm per week). Spring growth rates were s tatistically greater at Cape Lookout than at Oscar Shoal. The actual differ ence in growth, however, was < 1 mm (7.34+/-0.13 mm and 6.45+/-0.19 mm over 5 weeks, respectively), and most likely would not have produced significan t biological effects such as differential predation rates. Average growth o f juvenile scallops placed in the center (0.75 X 0.75 m area) of replicate small (similar to 1 m across) patches was faster than in large (> 8 m acros s) patches over 5-7 week periods during both seasons. The difference in gro wth, however, was small (similar to 1 mm) and would probably have little bi ological significance. In the spring, scallop survivorship after 2 weeks di d not differ between sites or among seagrass patches of varying size. After an additional 3 weeks however, scallop survivorship was approximately 30% greater at the site dominated by Zostera marina with lower shoot density, b ut there was no difference in survivorship among seagrass patch sizes. The same pattern was observed over the cumulative 5-week period. In the fall, p redation rates on juvenile scallops were significantly higher in small patc hes than in large over very short time intervals (e.g., 24 h). Over 3-, 4- and 7-week time intervals, however, there was no effect of patch size on sc allop survival, even though shoot densities were significantly greater in m edium- and large-sized patches than in small. Lack of a long-term patch siz e and/or seagrass density effect on scallop survivorship may be due to diff erential responses to patch size by the many potential predators that prey On juvenile scallops. (C) 1999 Elsevier Science B.V. All rights reserved.