Evaluation of live microalgae and microalgal pastes as supplementary food for juvenile Pacific oysters ( Crassostrea gigas)

Abstract

Supplementary feeding was an effective method of enhancing growth of juvenile Pacific oysters ( Crassostrea gigas) at a nursery site with low to moderate levels of natural phytoplankton (chlorophyll a, 0.68±0.29 μg l −1). Over five experiments, oysters (500–700 μm) were fed supplementary rations of live microalgae or microalgal pastes. Supplementary diets were added to naturally occurring seston at a ration of 7 mg day −1 ml −1 oysters (initial oyster bed volume). These levels increased the total phytoplankton concentration by 50 to 207%. Variation in the environmental conditions between experiments influenced oyster growth rates. Increases in oyster growth rates were high in animals supplemented with Chaetoceros calcitrans (instantaneous growth rate, k=0.062 day −1), Dunaliella tertiolecta ( k=0.059 day −1), Isochrysis sp. (strain T. ISO) ( k=0.059 day −1), an Australian isolate of Rhodomonas salina ( k=0.074 day −1) and pastes of Skeletonema costatum ( k=0.064 day −1) and C. calcitrans ( k=0.059 day −1). These rates were significantly greater than in the oysters fed a reference supplementary diet, Pavlova pinguis (range for experiments; k=0.049–0.066 day −1) and in the control (i.e., non-supplementary fed) oysters (range for experiments, k=0.034–0.043 day −1). P. pinguis was a more effective diet when it was grown under 24:0 h L:D ( k=0.066 day −1) than when grown under a 12:12 h L:D regime ( k=0.061 day −1). C. calcitrans paste (4- to 14-days-old) ( k=0.059 day −1) gave similar growth to live C. calcitrans ( k=0.062 day −1), which indicated the promise of pastes as an off-the-shelf alternative to live diets. The effectiveness of D. tertiolecta—a diet lacking the polyunsaturated fatty acids 20:5 n−3 and 22:6 n−3—indicated that the oysters received sufficient quantities in background phytoplankton to sustain a high growth rate. The essential nature of these fatty acids was demonstrated when all oysters retained higher percentages of 20:5 n−3 and 22:6 n−3 than other fatty acids. Initial cost estimates for supplementary feeding show that it will add 2 to 3% to juvenile oyster production costs during the nursery phase when oysters are 0.5 to ∼3 mm in size.