THE EFFECTS OF SIZE GRADING AND STOCKING DENSITY ON GROWTH-PERFORMANCE OF JUVENILE ABALONE, HALIOTIS-TUBERCULATA LINNAEUS

The effects of size grading and stocking density on the growth of juve nile European abalone, Haliotis tuberculata L. (Gastropoda: Haliotidae ) were assessed in the hatchery. Animals were held in cages deployed i n a flowtray with recirculating seawater maintained at 18 +/- 2 degree s C, and fed ad libitum on the red alga Palmaria palmata (L.) Kuntze. In the grading experiment juvenile abalone were divided into four popu lations consisting of small (mean +/- s.e; initial shell length = 15.3 +/- 0.15 mm; n = 35), medium (initial shell length = 19.6+/-0.16 mm; n = 35), large (initial shell length = 23.8 +/- 0.17 mm; n = 35) and a n ungraded control group (16.8 +/- 0.41 mm; n = 35). Rearing was carri ed out over a period of 226 days, after which it could be concluded th at an advantage in overall growth performance had been gained by gradi ng the animals. There was evidence that growth of small abalone improv ed in the absence of larger conspecifics. In the stocking density expe riment with two size grades and three stocking densities, a decreasing exponential relationship was found between growth (body weight and sh ell length) and stocking density, with a substantial amount of growth occurring at all densities. Final total biomass for both grades of aba lone increased with density. For the smaller grade biomass gain increa sed as density increased. However, for the larger grade biomass gain d eclined at higher density. Overall mean survival (+/- s.e) was 98.3 +/ - 0.4% and was not influenced by density. It is suggested that the cho ice of stocking density is essentially a trade-off between maximum gro wth, optimal biomass gain, and economic considerations which may dicta te densities that would result in a net reduction in overall productio n costs.