EFFECT OF INUNDATION TIME ON GROWTH, QUALITY AND SURVIVAL OF PACIFIC OYSTER Magallana gigas FOR SITE-SPECIFIC (OFF-BOTTOM) AQUACULTURE IN THE OOSTERSCHELDE, THE NETHERLANDS
In this study, a field experiment on the effect of inundation time on growth, quality and survival of Pacific Oysters (M. gigas), was carried out at an intertidal off-bottom cultivation site. The cultivation site was located in the central part of the Oosterschelde, the Netherlands. By using off-bottom cultivation techniques, farmers are able to control the inundation time of the oysters by adapting the height of baskets attached to the lines. Longer inundation has an effect on food condition and therefore on the growth of oysters but shorter inundation can increase the survival of the oysters.
Oysters (8-12 mm hatchery reared spat) were cultivated in hanging baskets in an off-bottom farm. For the experiment two lines, to which the baskets were attached, were adjusted to 0.61m and 1.27m below mean sea level, resulting in a inundation time for the oysters of 65% for the upper and 88% for the lower line. At the start of the experiment 6 baskets (three per inundation time, 6 mm mesh) were each stocked with 5000 live oysters (visually checked). Every six weeks the mortality in every basket was recorded and a subset of 50 oysters was measured for shell length (mm) and the wet weight (g) of the oyster spat. At the end of the experiment the condition index (CI) of the oysters was calculated as: [dry tissue weight (g) × 100] / dry shell weight (g).
The experiment was carried out from May 27th 2014 until April 19th 2016.
Results show that oyster growth rates, especially in spring, were higher at 88% inundation time (SGR - 8.5 mg/d), than at 65% inundation time (SGR - 7.5 mg/d (figure 1). This same pattern was also measured in the oyster wet weight. Overall there was an oyster survival of 17% for the 65% inundation treatment and 11% for the 88% inundation treatment (P=0.000).
The condition index differed between the treatments (P=0.000) and was 4.33 ±0.98 and 4.85 ±1.05 for an inundation treatment of 65% and 88% respectively for April 2016.
This study concludes that a longer inundation time results in higher GR and CI, probably caused by better food conditions. In contrast, shorter inundation times promotes oyster survival. This can be a result of a shorter exposure time to the oyster herpes virus.
It is recommended to increase the inundation time of the complete oyster culture cycle but decrease inundation time in periods with high mortalities (e.g. in spring due to herpes virus).