Native oysters discovered in the Port of Rotterdam

A surprising number of native oysters (Ostrea edulis) have been found in the western part of the Port of Rotterdam. This is the conclusion of a survey by Bureau Waardenburg, Wageningen Marine Research and Sas Consultancy, financially supported by the Port of Rotterdam Authority and the Dutch Ministry of Agriculture, Nature and Food Quality. The discovery is special, since native oysters are currently only present in limited areas along the Dutch coast, mainly in commercial cultivation.  

The researchers investigated 6 vertical transects, from water surface to harbour bottom, on a quay wall of ca. 1 km length. They found an average density of 8 oysters per m2. This agrees with the OSPAR definition of a reef (minimum density of 5 individuals per m2). The total estimated amount of oysters along the quay is ca. 100.000 individuals. In the recent summer period, native oyster larvae were detected in the port water too.

The native oysters population along the quay is mixed with Crassostrea gigas. At low water depth (above 10 m), Crassostrea dominates, but as depth increases, native oyster density increases. Below 15 m, the native oysters dominate the population in 4 out of the 6 investigated transects.

It is not known whether there are similar quantities of native oysters elsewhere in the port, nor the total size of the total population. A more extensive survey will be required to find this out. 

The population appears to be infected by the Bonamia ostreae parasite. Paradoxically, this is rather good news, since the population is apparently thriving. Hence, it may be assumed that it has developed tolerance against the disease, analogous to the population in the Dutch Delta (the area 30-80 km to the south of the port). 

The occurrence of native oysters in the port is probably promoted by a combination of factors, such as the great depth (up to 20 m), the limited tidal action and the presence of suitable substrate. The origin of the population is still unknown.

The Port of Rotterdam Authority will support future research into the native oyster population in the western port area. It is pleasantly surprised by the discovery of the native oyster. “Of course, there were already clear signs that the marine environment in the port is in good condition. Research into the presence of fish indicated this. This new find confirms that a port can very well go hand in hand with a rich biodiversity under water. We are doing our best to further improve this combination,” says Boudewijn Siemons, COO of the Port of Rotterdam Authority.

Figure 1. Diver in action (9 dec 2020).
Figure 2. Japanese oysters are present at the quay from the high water level.
Figure 3. Impressions of oyster densities in one of the transects, 2,5 meter
and 15 meter below average water depth (9 dec 2020).

(Photos from native and Japanese oyster survey in the harbour of Rotterdam, Bureau Waardenburg report 21-005)