Vol. 106: 85-91. 1994 MARINE ECOLOGY PROGRESS SERIES Mar. Ecol. Prog. Ser. l Published March 17 Separating effects of limited food and space on growth of the giant scallop Placopecten magellanicus in suspended culture Jean Ciite, John H. Himmelman, Michel R. Claereboudt Departement de biologie and GIROQ (Groupe interuniversitaire de recherches oceanographiques du Quebec), Universite Laval, Quebec, Canada G1K ?P4 ABSTRACT: Effects of limitations in space and food resources on the growth of the giant scallop Pla- copecten magellanicus (Gmelin, 1791) were examined by growing juveniles in pearl nets at different densities in 2 series, one in which density was increased by adding living scallops and a second in which density was increased by adding 'dummy' (non-living) scallops. The dummies occupied space but did not compete for food resources. For the first series, increasing the density from 25 to 250 ind. net-' caused a marked decrease in growth. In contrast, no significant decrease occurred in the series in which density was increased by adding dummies. This demonstrated that food depletion is the major factor causing decreased growth at high densities. The only evidence suggesting an effect of space was a decrease in the mass of the muscle and other soft tissues with increasing density in the series with dummies; however, this decrease was not significant. Density did not affect rates of mortality, either in the absence or presence of dummy scallops. KEY WORDS: Food/space competition . Density effect - Bivalve growth . Aquaculture . Placopecten magellanicus INTRODUCTION Competition for food and space are known as major factors determining growth and survival for suspen- sion feeders. Numerous field studies of bivalves demonstrate that growth decreases with increasing density (Okamura 1986, Peterson & Beal 1989, Vincent et al. 1989). This may be explained by the decrease in food resources per individual with increasing density, possibly because of depletion of food particles within the benthic boundary layer (Wildish & Kristmanson 1984, Frechette & Bourget 1985, Frechette et al. 1989). Competition for space may also cause decreased growth at higher densities. This is suggested because sessile suspension feeders in rocky habitats are often strongly limited by space (Connell 1961, Dayton 1971, Harger 1972).Peterson & Andre (1980) further suggest that competition for space largely determines the ver- tical zonation of infaunal bivalves in a coastal lagoon of California, USA, and Dijkema et al. (1987) report that in extremely dense populations, the cockle Cerasto- derma edule can even be pushed out of the sediment by conspecifics. Intraspecific competition may also limit growth of bivalves maintained in suspended culture by aquacul- ture industries, as studies demonstrate that growth decreases with increasing stocking density for scallops (Duggan 1973, Monical 1980, Parsons & Dadswell 1992, C6te et al. in press) and other bivalves (Jarayab- hand & Newkirk 1989, Eversole et al. 1990, Mallet & Carver 1991, Holliday et al. 1993). These studies emphasize food as a potential limiting factor. Competi- tion for space, involving physical interference among individuals, may also explain reduced growth at high densities. This is suggested by Duggan (1973), Monical (1980). Widman & Rhodes (1991) and C6te et al. (in press), although Parsons & Dadswell (1992) reject this hypothesis in a study of the giant scallop Placopecten magellanicus. Parsons & Dadswell did not observe signs of competition for space, such as decreased sur- vival or a higher proportion of deformed individuals at high densities. It is difficult to separate the importance O Inter-Research 1994