Effects of algal concentration and temperature on the feeding rates at three life stages (larva, spat, adult) of the purple clam, Saxidomus purpuratus

Abstract

The purple clam, Saxidomus purpuratus (Class Bivalvia, Family Veneridae) is a local species inhabiting relatively restricted areas around Korea, Japan, and China. This species is one of the most important shellfish resources for human consumption with high prices. Recently, the commercial yield from the traditional exploitation of natural fisheries by divers has been declining due to over-harvesting. The TAC (Total Accepted Catches) for S. purpuratus in 2002 was determined as 8,000 MT/year. Therefore much attention has been concentrated to the aquaculture and restocking of this species. In Korea, aquacultural studies on S. purpuratus were actively performed recently by South Sea Institute of KORDI in cooperation with Taean Marine Hatchery of NFRDI. Thanks to their efforts, several culture conditions for artificial seedling production were set up (Choi et al., 2003). However, rates of feeding and the optimal ration for inland culture were not understood yet. Thus the purpose of this study was established to know the feeding rates and to determine optimal ration level for each life stage of S. purpuratus. Experiments were carried out to know the clearance rate (CR) and ingestion rate (IR) as functions of algal concentration and temperature of each stage using the microflagellate Isochrysis galbana as food. Functional response of IR was expressed by the Michaelis-Menten equation. At all three stages, algal concentration strongly affected CR. With increasing algal concentration CR increased rapidly, but after a certain threshold level it decreased gradually. The maximum CR for larva, spat, and adult were 0.002, 19.9 and 238 ml/ind/hr, respectively. IR was also affected by algal concentration. In the experiments with larvae and spats, the changing pattern of IR could be divided into two phases: (1) as algal concentration increased with low level, IR increased rapidly, (2) but, as algal concentration increased further, IR did not increase any more. However, in the experim