OPTIMIZING OFF-BOTTOM OYSTER CULTIVATION IN THE NETHERLANDS: IMPROVING YIELD WITH SITE-SPECIFIC OYSTER FARMING

Off-bottom oyster cultivation in the Oosterschelde , the Netherlands, started in 2010 . Off-bottom cultivation typically result in oysters (Crassostrea gigas) with faster growth and a  higher meat quality  in comparison with the  conventional bottom  oyster cultivation, due to a higher food quality and availability  higher in the water column. Using triploid oysters , which invest more energy in growth , ensures a shorter cultivation process . In off-bottom cultivation oyster spat is placed at a high density (app.  5000  individual pieces) in fine-meshed bags  or baskets and will be manually thinned in density during the growth process to consumption size. The starting material, oyster spat, can be of various origins and usually comes from hatcheries,  as the Netherlands, Ireland and France.

Currently two di fferent cultivation systems are  being used  by Dutch oyster farmers in the intertidal area in the Oosterschelde  The first system consists of firm plastic bags (available in 4 different mesh sizes) whic h are placed on iron frames. The second system is the so-called BST system which consists of hanging baskets (available in 3 different mesh sizes) attached on longlines or on iron tables . Although t hese systems are widely and successfully used around the world, there seems to be quite some room to optimize this way of farming. Main issue s are i) their labour intensiveness  in combination with  the  relatively high wages in the Netherlands and ii) high losses caused by the variety wave- and flow dynamics in the Oosterschelde . It is questioned if other off- bottom cultivation systems  that are also successfully deployed around the world would be a good addition and or alternative to improve efficiency and productivity of  Dutch oyster cultivation.

In 2020 the European Marine and Fisheries fund ed project “Oyster yield improvement through knowledge transfer and monitoring” was initiated . The goal of the project is to optimize  oyster farming and  it  should lead to efficiency improvements of sustainable oyster cultivation. In this project  a comparative study between current off-bottom oyster cultivation and alternative off-bottom farming systems will be conducted. The effect of the farming systems on the performance of oyster  growth, quality and shell shape

 will be examined for  four different systems in  three different  off-bottom  cultivation locations in the Oosterschelde. In a comparative study between the currently used systems and alternative off-bottom cultivation systems ,  additionally the following  two methods  are being tested:

SEAPA - hanging baskets

FlipFlop bags - horizontal hanging bags

 For this study T15 triploid oysters, o riginating from the hatchery Roem van Yerseke, the Netherlands., were placed in four bags or baskets at 3 different locations in the Oosterschelde (Prinseplaat-C, Yerseke bank-74/75 and Hooge Kraaijer-46) , see

. The number of oysters per system is calculated according to the total volume of each individual system. All the systems and all the locations  are sampled  every 10 weeks for the period March  2021 till December 2021 . During sampling, 1 bag or basket of all the systems (and of all three locations) will be collected . Per sample  randomly  60 individual oysters are analyzed on  - shell shape / growth: length, width, thickness, quality and wet fresh weight. The Condition Index (Rainer and Mann, 1992) is determined  for 30 individual oysters per sample.

M onitoring and experiments are ongoing till December 2021. During the presentation the results of the study will be  shown and discussed.