Effects of stocking density and immersion time on the performance of oysters in intertidal off-bottom culture

Intertidal culture in meshed bags on trestles is worldwide the most established culture method for oysters. Culturists can affect oyster performance by adjusting stocking density and immersion time, and it can be expected that these factors are not independent. The combined effect of stocking density and immersion time on survival, growth and condition of oysters was investigated on a culture site, where oysters are usually stocked at 8 kg bag(-1). Half-grown oysters were stocked in three biomass densities: 4, 8 and 12 kg bag(-1), nested within three immersion times (87%, 76% and 73% of tidal cycle). Chlorophyll-a concentration peaked in summer (similar to 3-10 mu g l(-1)) and was low in autumn (similar to 1 mu g l(-1)), and high chlorophyll-a levels coincided with oyster spawning period. Survival was not affected by density or immersion time. Shell growth per oyster and biomass production per bag were density dependent-higher at 4 than at 12 kg bag(-1)-but neither were different from 8 kg bag(-1). Growth rates were negativity related with immersion over the spawning period but showed a (non-significant) positive trend over other periods. At harvest, condition per oyster decreased with stocking density and increased with immersion. There was no interaction between density and immersion. Treatments had a more pronounced effect on meat content than on biomass production. Hence, oysters might be kept at higher stocking densities to increase biomass production during most of the growth cycle but restocked in lower densities, with longer immersion times prior to harvest to maximize meat content at harvest.