Abstract

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 (~ 3–10 μg l−1) and was low in autumn (~ 1 μg 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.

Highlights

  • Extensive shellfish aquaculture is largely dependent on the environment for seed and feed

  • In period I, growth rates decreased with density (F(12,2) = 12,14, p = 0.001); oysters placed in low-density showed significantly higher growth rates than at the highest density, but there was no difference with the reference biomass of 8 kg bag−1

  • Growth rates decreased with stocking density and were significantly higher at the lowest density compared to the other densities (F(12,2) = 4.92, p = 0.03)

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Summary

Introduction

Extensive shellfish aquaculture is largely dependent on the environment for seed and feed. Optimizing culture practices or system design for culture is based on anticipating to this. Shellfish farmers anticipate food availability by trying to optimize shellfish density, mostly based on experience, by adjusting stocking density and thinning-out practices. Shellfish density affects growth and survival when individuals compete for food or for space (Marshall and Dunham 2013; Honkoop and Bayne 2002; Fréchette and Bacher 1998). Oysters are the foremost cultured species in volume in global aquaculture production, and much of the oyster culture takes place in intertidal areas (FAO 2018). Oysters in intertidal areas are subjected to different immersion times that at some point affect growth and survival (Vincent et al 1994). It might very well present an opportunity for the oyster growers to manipulate growth and survival

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