The Rise And Fall Of An Oyster Bed In The German Bight (North Sea) Before The Dawn Of Industrial Fishery

The European flat oyster (Ostrea edulis, EFO) has been common in the North Sea at least throughout the late Holocene. Aside from its important ecological function as a reef builder the EFO has been of high economic value ever since man invented commercial fishery. During first half of the 20th century CE the EFO became functionally extinct in the German Bight. It is commonly agreed that industry-style overexploitation using dredges and trawls destroyed the habitats and eventually caused the extirpation of the EFO. Today, the EFO is a severely endangered and protected species.

Recently, an as yet uncharted fossil EFO bed was encountered close to the island Helgoland (German Bight, SE North Sea), that does not reveal any obvious signs of the physical disturbances typical for heavily harvested sites. This suggests possible environmental forcing behind the decline of this particular EFO bed, which could also have contributed to the fall of the entire oyster population of the German Bight.

The area was surveyed using drift videos along with grab samples to locate the EFO bed, measure its perimeter and evaluate characteristics, such as the density of shells. A total of 590 shells from 17 locations were measured (length, width) and weighed. A total of 19 shell samples from 17 locations were AMS radiocarbon dated to obtain absolute age control.

The EFO bed is located on a rocky slope to the west of Helgoland, its area is about 0.6 km² and the water depth ranges between 32 and 43 m. The shallower parts are characterized by rubble whereas the EFO bed disappears under muddy sediments in the deeper parts. A rough estimation reveals around 200,000,000 single oyster shells in this bed. The oldest shell dated to about 4000 years BP, the youngest age dates to the beginning of the 19th century CE. Most shell ages are between 2700 and 2000 years BP. A hiatus occurs between 1300 and 300 years BP, and only one measurement reveals an age younger than 1300 years.

This EFO bed had perished already 1300 years ago, hence intense fishery as a reason can be ruled out. Today, the deeper part of the EFO bed is affected by muddy sediment that temporally buries the seafloor and occasionally forms turbid clouds of suspended matter. The sediment likely originates from large rivers (e.g. Rhine, Weser, Elbe) and the mudflats of the Wadden Sea. Climatic/oceanographic fluctuations that had modified the discharge and transport of muddy sediments in combination with increased influx of sediment from fluvial sources as a result of land-use changes in early medieval central Europe may have been a major stressor for the oyster habitats, albeit biological stressors cannot be ruled out.