Effects of stocking density, ration, and temperature on growth of early post-settled juveniles of the basket cockle, Clinocardium nuttallii

Abstract

The effects of stocking density, ration, and temperature on seed growth – from early post-settlement to a size suitable for out-planting ( i.e. ~ 4 mm shell length) – and the effect of stocking density on metamorphic rates were examined for the basket cockle, Clinocardium nuttallii, to determine optimal hatchery rearing conditions for this species. The effect of stocking density on seed growth depended on sampling time. A significant finding of the research was that optimal seed growth can be maintained when less than ~ 100% of the rearing unit's bottom area is covered, regardless of seed size. Metamorphic rates were high (88.2–91.0%) and not significantly affected by stocking density in the tested range (20–320 ind cm −2). Optimal rations were determined for five different size classes of seed fed a bi-algal diet of Chaetoceros muelleri and Isochrysis sp. (Tahitian strain, TISO) and these were, respectively, 3.2, 6.4, 6.4, 12.8, and 25.6 × 10 6 equivalent-TISO cells ind − 1 d − 1 for size classes 1 (initial mean shell length: 0.74 mm), 2 (1.59 mm), 3 (2.10 mm), 4 (2.60 mm), and 5 (3.00 mm). Whilst optimal rations generally increased with an increase in seed shell length, the opposite trend was observed for weight-specific rations. Faster-growing seed built up higher organic matter content in their body than slower-growing individuals, suggesting that satiation feeding for maximum growth should be preferred during hatchery rearing. Due to the rapid growth of seed over a short period of time, frequent adjustments of both stocking density and ration are necessary in order to best utilise available resources ( i.e. space and food) and to maintain maximum growth over time. Seed were grown at various temperatures (range of means: 5.9–26.4 °C) with optimal growth occurring at 18.3 °C.