European plaice (Pleuronectes platessa)
Also known as: flatfish, fluke, henfish, plaice, plaice-fluke
Morphology & Biology
The European Plaice (Pleuronectes platessa) is a diamond-shaped right-eyed flatfish. The upper side (or right side) is grey-brown and speckled with characteristic orange dots, the lower or left side is whitish. Plaice have smooth skin with small scales. Between the eyes they have a strip-like thickening of bony humps. Along their sides, the plaice has long, soft-rayed dorsal and anal fins. The average length for plaice is 25-40cm, with a weight of 2-3kg. However, they can reach a maximum length of 1m, and a weight of 7kg. Plaice can be long-lived, reaching a maximum age of around 45 years.
The females reach sexual maturity at 4-7 years. The size and age from which the plaice becomes sexually mature has changed since the beginning of the 20th century. The fish are now smaller and become sexually mature earlier. It is believed that this is an evolutionary adaptation to fishing. They lay 60,000-100,000 eggs per spawning season, which is from January to Mid-February. Spawning takes place in offshore waters. Larvae that are bilaterally symmetrical hatch from the pelagic eggs, with one eye on each side of the body. The larvae are too weak to propel themselves against water currents and so they drift with the currents. Organisms which drift in this way are called plankton (after the Greek word, meaning drifter or wanderer). They remain plankton for around 3-4 months, which is longer compared to other flatfish. After this, the larvae undergo a complete metamorphosis to a flattened, asymmetric body shape. The young animals settle into their nursery grounds in estuaries and flat beaches. Around 7 months later, they migrate into deeper waters. They use the current to move between spawning areas and hunting grounds. The young plaice are carried onto the sandy flats in the Wadden Sea by the high tide and return to the channels around them by the ebb tide.
The plaice is normally found at depths of 1m-100m and prefer sandy bottoms. Plaice can quickly bury themselves in sand using the ribbon of soft-rayed fins which run along their sides. They can also partially adjust the colour of their skin to camouflage into their environment. The plaice is a nocturnal animal that hunts in shallow waters for bristle worms, small crabs, thin-shelled mussels and snails. During the day they bury themselves in the sand for protection.
The European plaice is found along the coast of Europe all the way to Iceland and Greenland. They are particularly widespread in the Northern Sea. Coastal waters are well suited as nursery grounds, the Wadden Sea being particularly popular. However, spawning takes place further from the coast in deeper waters, particularly in the southern North Sea and the eastern coast of Britain. Many eggs are found in the eastern part of the Channel and the ‘southern bight’ (this is the pinched point at the right-hand end of the channel where the UK and France are closest).
Reports that European plaice are also found in the east Mediterranean Sea have been questioned. A confusion with the flounder (P. flesus) could be an explanation. In the past, plaice may have existed in the east Mediterranean Sea, but none are found there today.
The plaice is listed by IUCN as LC (least concern). This means that these fish are not considered at risk of extinction in Europe. In the 1970s and 1980s, the plaice population was heavily overfished, but since then they havebeen slowly recovering. According to the IUCN, there has been a slight increase in the annual amount of spawning. Nevertheless, the fact that plaice are often bycatch endangers the European plaice. Often the mesh size of the nets are too small, meaning that juvenile plaice are captured and die. This poses a problem for new generations of plaice and their likelihood of producing offspring. Additionally, oil and gas extraction in the plaice’s habitats endanger the population. The plaice is very particular about the composition of the soil in their habitat; especially for nursery areas. This makes the fish very vulnerable to any human activities.
Fisheries & Aquaculture
For the flatfish fishery, European plaice is the most important economically. Denmark and the Netherlands capture the largest number of plaice.
They are caught in demersal fishery vessels that use bottom trawls, mainly beam trawls. There are high fuel costs, due to the high towing resistance caused by dragging beam trawls across the sea floor.
This fishing method is particularly harmful as beam trawling destroys the seabed and has a high rate of bycatch for other bottom-dwelling species (fish and invertebrates). Not much is able to survive being a victim to trawling or becoming bycatch. Species composition, food web and biomass can be significantly altered by this fishing method. It is one of the legal fishing methods with the highest direct impact on the marine environment.
The European plaice is also called “Goldbutt” because of their golden dots on the upper side. Even though they are a flatfish, the plaice is not as flat as you think – as a larvae they look like a “normal” fish. Nevertheless, the larvae cannot swim against the current, so they swim up and down in the water and use the tides to move around.