Transplant experiments on growth and mortality of the fan mussel Pinna bicolor

Abstract

Growth and mortality for different size classes (30–49 mm, 50–79 mm and 80–100 mm shell width) of the fan mussel Pinna bicolor transplanted at open sea bottom, within enclosure cages, and on suspended rafts were studied and compared over an 18-month study period. Growth, in terms of shell width, dry tissue weight and condition index of the same size class, did not show significant differences ( P>0.05) between the transplanted populations at the open sea bottom and in enclosure cages. Fastest growth was noted for the small-size class during the initial 12-month period, with a net increase in maximum shell width of 50–60 mm (100–120 mm shell length). Thereafter, growth tended to slow down and all growth parameters became similar for all the three size classes. In comparison of the small-size fan mussels between the bottom (open and caged) transplants and that on the suspended rafts, a better growth result was registered in the suspended raft population possibly due to a faster induced flow and availability of food in the water column. Mortality of transplanted P. bicolor was the highest at open sea bottom, and predominantly caused by predation by fish and crabs. Highest mortality was also noted in the small-size class as the larger fan mussels were less vulnerable to predation than the small juvenile. The number of transplanted P. bicolor at open sea bottom fell to the same natural density within the first 12 months of the study and stabilized afterwards, suggesting that the natural population of P. bicolor was controlled by its predators. In rearing of fan mussels, both growth and mortality should be considered in concert, so as to select an optimal size class of transplants which would attain the maximum growth, with minimum mortality, over the culture period.