Limfjord oyster dredge fishery
Certified as sustainable in May 2012.
Species: European flat oyster (Ostrea edulis)
Location: Western Limfjord, Denmark
Fishing methods: Oyster Dredge
Number of fisheries: 1
More about European flat oyster
Native oysters can be found intertidally but generally occur offshore from about low water down to some 80 m on firm, comparatively immobile bottoms of mud, rocks, muddy sand, muddy gravel with shells, hard silt or old peat bottoms , but the main concentrations are usually in shallower water, down to some 20 m. They are sessile animals, cementing to hard objects on the seabed by the left (lower) shell, so the substrate must contain suitable, clean, hard surfaces forsettlement (known as cultch). In appearance, O. edulis is usually roughly circular in shape, but the shell is often distorted, conforming to the shape of the surface to which it is attached. This distortion can be excessive when more than one oyster is attached to the same object. The irregular shell has a distinct hooked beak patterned with delicate foliation. The two halves (valves) of the shell are of different shapes.
Both the planktonic larvae and adult oysters are filter feeders, extracting particles from the water by means of ciliary mechanism that sort particles and pass them to the mouth. The diet now includes organic detritus, bacteria, diatoms, dinoflagellates and a variety of protozoans, together with the smallest planktonic crustaceans and fragments of larger animals. Ostrea edulis is a protandric sequential hermaphrodite: the young oyster first becomes sexually mature as a male, then changes relatively slowly to a functional female, after which it very quickly becomes a male again and so on alternately throughout life. Spawning takes place in the summer months, when the temperature reaches about 15°C. The eggs and sperm are discharged from the gonad into the chamber above the gills. For male oysters the sperm is immediately pumped out in the exhalent water current. For the females, however, the eggs are forced against the water current through the fine channels in the gills into the inhalant chamber, where they are fertilized by sperm brought in with the inhalant water current.
Oysters have numerous invertebrate predators and are also eaten by some fish. In general, the thin shelled spat are the most vulnerable and predation declines as the oysters becomes larger and the shell thickens. Various crabs, but notably the common shore crab Carcinus maenas, and the common starfish Asterias rubens, are serious predators in many fisheries. Many animals, like ascidians and barnacles, compete with Ostrea edulis for space, or food, but perhaps the most important in many areas is the slipper limpet, Crepidula fornicata. This species was introduced with imported oysters from North America in about 1880 and has subsequently spread throughout Europe where it can occur in such high densities that it causes problems for oyster cultivation.
More about the fishing methods
There are 101 oyster dredgers operating in the Limfjord. Vessels are restricted to a maximum overall length of 12m (although there are 4 vessels larger than this size that have “grandfather” rights to fish because they were operating in the fishery prior to the introduction of this regulation). Around 50 of the vessels are 12m long, and the rest are smaller than 12m. Dredges used in the oyster fishery are restricted in their size and weight by fishing licence requirements. They must be no more than 1m in width and 20cm high, with their weight limited to 35kg. Vessels can use no more than two dredges at a time, and are prohibited from carrying mussel dredges aboard when they are fishing for oysters.
1,049 tonnes in 2010
Main markets have been identified as Europe.
Actual eligibility date
17 October 2011