Abstract
At present, European flat oyster beds are rare or absent in most of their natural range and only a few individuals remain in the North Sea. The goal of is to establish a self-sustaining bed in terms of recruitment. As part of the Gemini oyster bed restoration project of De Rijke Noordzee (DRN) an initial population of flat oysters (Ostrea edulis) was introduced in Gemini ZeeEnergie in 2021 and 2022. Substrate, such as cleaned Pacific oyster shells, can be introduced to facilitate expansion of the bed. Timing of the introduction of shells is crucial. An earlier study showed that covering substrates with Reefpaste enhanced larval settlement. To study this at larger scale oyster shells were coated with Reefpaste. To test the timing deployment of settlement shells larvae were sampled and oyster shell substrate (with and without Reefpaste) was introduced during the larval period in 2023 (year 3) of the Gemini project. It was envisioned to install substrate in four research trips: during the peak in larvae, two weeks after the peak in larvae, four weeks after the peak in larvae and six weeks after the peak in larvae. Results of the study can be used to decide on the type of shells (with or without Reefpaste) and to time the introduction of the shells with the purpose of expanding off-shore oyster beds. Finding a suitable weather time-window proved to be a challenge. Sometimes the waves looked good three days ahead, but then at the planned date they were too high again, or the window was too short to do the work.
Finding a suitable and approved vessel was also challenging. Because of this, field work started later than anticipated. Only three trips to install the frames during the larval season were carried out. This resulted in placing two frames on the same date. Retrieval of the frames was planned for October 2023. However, after 13 unsuccessful attempts (where in two cases the park was actually visited) the frames are still in the park.
The effect of temperature on larval abundance could not be determined since there were no oyster larvae detected in the larvae (water filtrates) samples. Sampling may have been too late in the season. The effect of deployment week of the spat collectors on spat abundance and the effect of addition of Reefpaste to the shells on spat abundance could not be determined because the frames were not retrieved from the field.
The WINOR oyster spat collection experiment required frequent field trips to be able to follow larval abundance over time and introduce settlement substrates at different times during the larval season. Next to that, the frames should be retrieved before the winter. For this, the use of small vessels like CTV’s were chosen as they are flexible in time and cheaper than larger ships. Furthermore, frames to hold the bags with substrates were designed in such a way that they could be deployed from a small vessel. The use of small vessels required a calm sea with waves of 1 m or less. The year 2023 was characterised by long periods with a lot of wind and very few weather windows in which the wave height was less than 1m. Locating the experiment in a wind farm came with extra measures which sometimes delayed the process of accessing the site. In conclusion, under the present circumstances testing the timing of oyster spat collection is demonstrated to be too complicated in practice to be studied in a windfarm.