Plaice: Successful transition to a sustainable fishery

PlaicePlaice is one of the commercially most important target species that are fished in the North Atlantic. What makes this flatfish equally significant for both fishermen and fish lovers is not only its excellent meat quality but also the available biomass. Following the low in the 1990s plaice stocks have in the meantime recovered significantly. Nearly 150,000 tonnes are currently caught per year under the ongoing management plan.


Flatfish like plaice are the most extreme example of lateralization in vertebrates. Organs, body functions and behaviours shift to one side of the body, and no other vertebrate is so asymmetrically built and flattened. And yet after they hatch, flatfish larvae look just the same as “normal” fish. They are symmetrical in build, and float upright in the water with their back facing upwards. With the onset of metamorphosis far-reaching changes begin, however, which characterize the shape of the future flatfish. The symmetry of the head, gills, mouth and nose region is increasingly lost. One eye moves over the edge of the forehead to the other side of the body. With just a few exceptions the left eye usually moves over to the right side and so plaice are assigned to the group of right eyed flatfish. The arrangement of both eyes on one side of the body and the 90° turn of the fish – flatfishes do not lie on their bellies but on one side of their bodies – necessitate radical amendments to the construction plan of “normal” fishes. Parts of the skull have to be redesigned, optic nerves shifted, and the fish’s sense of balance “reprogrammed”. At the same time the visual perception upon which the brain models a three-dimensional image of the environment changes. Apart from that, flatfishes swim lying on their sides which requires a special coordination of the fins. And even that works: plaice are relatively good swimmers. With undulating movements of their fin edges they move elegantly through the water. Linnaeus could hardly have chosen a more suitable name for the species: The translation of Pleuronectes is “swimming on the side” (Greek pleura – side, nēktos – swimming.)


Biologists were for a long time puzzled about why millions of years ago plaice gave up the body plan that proved useful and successful for other fishes. The evolutive development of flatfish was difficult to verify as long as no fossils were found that could confirm the development process. Flatfish evolution remained a mystery, particularly since Darwin’s principle of natural selection was a contradiction to every attempt at an explanation. For the principle of natural selection means that deviations from the species-typical norm that arise constantly through mutation are rigorously selected out when put to the test of nature if they do not offer any benefits. It is hardly conceivable that the selection process would not have identified and eradicated such severe abnormalities as the shirting of an eye. In the 1930s the evolutionary biologist Richard Goldschmidt tried to explain the evolution of flatfish through “macromutations”. His theory was that in the course of evolution dramatic genetic changes could sometimes suddenly occur that in some species led to development leaps and changed an organism’s build from scratch, making them “hopeful monsters” – like flatfish. Although evolutionary biologists were never really happy with Goldschmidt’s macromutations they couldn’t offer a more plausible explanation either.

It was not until Matt Friedman, a British palaeobiologist, scoured the fish fossil collections of several European museums a few years ago that more light was brought into the darkness. He for the first time found fossil remains of symmetrical fish during the transition to asymmetrical flatfishes. Scientists had been searching for these since Darwin. In computed tomography images it was recognizable that in these fossils one eye had shifted in the direction of the vertex (crown of the head). With that Friedman had provided definitive proof that the asymmetry of the flatfish is not caused by macromutation but developed in a gradual evolutionary process. An approximate impression of the course of this change in shape that took place during evolution over millions of years is today conveyed by the early development phase of flatfish in which this remarkable chapter of evolution is run quasi in fast motion. Depending on water temperature the spectacular reorganisation of the symmetrical yolk sac larvae to the asymmetrical flatfish takes barely two months.


Plaice (Pleuronectes platessa) is mainly found in the North Sea, Skagerrak and Kattegat and the Western Baltic.

Plaice stocks are susceptible to overfishing

Plaice prefer cool, saline waters. Their lower tolerance limit is around 5 parts per thousand. They are gregarious bottom dwellers and are relatively faithful to their habitat, which is preferably at water depths of between 10 and 50 metres on a seabed with sand or broken shells. During the day the fishes usually burrow into the sediment but at night they are active and swim up to 30 km a day in their search for food. Plaice use the water currents in order to save energy. At low tide they allow themselves to be carried by the current into deeper waters and then later on the tide carries them back to more shallow zones near the shore. This energy-saving technique is called selective tidal stream transport. The feed spectrum of plaice is broad. They eat anything they can overpower and that fits in their mouths: in particular polychaetes, brittle stars and shrimps but also small fishes such as gobies, smelts, young herring and cod. Their staple diet is molluscs, primarily thin shelled species such as cockles and clams, young scallops and razor shells, blue mussels, and soft clams. Plaice grind the shells down with their pharyngeal teeth and the powerful pharynx muscles.

Dietmar Lill
At a length of nearly 47 cm this plaice is of a size that fishermen rarely find in their nets today.

Spawning normally takes place at temperatures of 5 to 7°C. Plaice eggs need a salinity of at least 12 to 15 parts per thousand for the eggs to float pelagically in the water. In the relatively low-salt Baltic, plaice can thus only spawn in the western regions or at great water depths of around 60 m. In the North Sea the spawning grounds are usually at depths of between 20 and 40 m. The best known spawning grounds include the deep channel off the Dutch-Belgian coast, the eastern channel, parts of Heligoland Bight and the central southern North Sea, but also England’s east coast between Aberdeen and Montrose, in the Firth of Forth, off Flamborough Head and north of Fraserborough, or off the Norwegian coast and off southern Iceland. Plaice lay their eggs in small portions at intervals of 3 to 5 days for the duration of about a month (portion spawners). Depending on water temperature the yolk sac larvae hatch two to three weeks later. After completion of metamorphosis the young plaice measure about 12-14 mm in length and begin their demersal existence. In their second year of life the fishes are about 15 cm long and migrate to deeper waters. Lack of food, bad weather and the massive development of jellyfish can harm young plaice.

The North Sea is the main area of distribution and also the major fishing ground for the plaice fishery. The species is susceptible to overfishing and the stocks react sensitively to changes in environmental conditions or strong pressure from fishing. Special protective measures, fixing of minimum landing sizes and closed seasons were introduced in an attempt to maintain the stocks at a sustainable level. Such measures alone are rarely sufficient, however, to avert overfishing. As from the early 1970s the biomass of the plaice spawning stocks was greatly decimated by intensive fishing until in the 1990s an all-time low was reached. In 1989 a “plaice box” (a defined closed area to protect fish stocks) was set up in the North Sea. The 40,000 square kilometre region in the Wadden Sea off the Danish, German and Dutch coast was to offer more effective protection of undersize plaice juveniles. The region was closed for plaice fishing with beam trawlers over 300 horsepower (221 kW), at first only temporarily but then, as from 1995, all the year round.


Rigorous implementation of EU management plan a success

Opinions vary on whether the plaice box has fulfilled its objective. Fisheries scientists assure us that without it the plaice stock would today be smaller. In contrast to this view, a lot of fishermen see the box as an example of failed regional development policy at a high cost for minor benefits. Important goals, particularly the reduction of plaice juvenile discards and an increase of plaice revenue were not achieved at first. In spite of this, the EU Commission decided to hold on to the protection zone until scientific research could deliver a more exact assessment of the situation. And until then, the restrictions in this region will continue to apply. It is controversial what part the plaice box plays in the recovery process of the plaice stocks. In contrast, there are no doubts about the value of the EU long-term management plan. Part of this plan has meant that plaice and sole have since 2008 been managed on the basis of maximum sustainable yield (MSY) and that has paid off! After the low in the 1990s the number of spawners has risen continually. Plaice stocks in the North Sea and the Irish Sea which account for three quarters of the total European plaice catch are within safe biological limits. According to ICES, the number of plaice at reproductive age (spawning stock biomass) in the North Sea was 670,000 t in 2014, the highest value since 1957 and more than twice as high as the long-term average! It is expected that in 2015 spawning stock biomass will exceed 700,000 t.

Dietmar Lill
The plaice stocks in the North Sea and Irish Sea that account for three quarters of the total European catch of this flatfish species are at present within safe biological limits.

This positive development has also been helped by the fact that the responsible politicians have recently based their TAC decisions strictly on scientific recommendations. The plaice stock in the Central North Sea is currently at a record level, and stock biomass is likely to increase further in the medium term although the fishing quotas were raised several times by about 15 per cent. Plaice is the most important flatfish for European fisheries and is a major source of income for cutter fishermen in the North Sea and the Baltic. According to FAO statistics, total landed volume amounted to 108,774 t in 2013. Due to the positive stock development it was possible to raise the TAC for 2015 to 152,075 t. The Dutch fishing sector, in particular, benefits from this because it is geared very strongly to plaice and landed 33,748 t in 2013, followed by Denmark with 21,893 t and UK with 21,226 t.


New fishing gear supports sustainability and protects the environment

Numerous different kinds of fishing gear are used for plaice fishing, both traditional ones like gillnets and traps and new types, too, that enable a more gentle fishing process. In the North Sea, where plaice are mostly fished together with sole, beam trawls continue to play an important role. This fishing gear accounts for about 80% of Dutch flatfish landings. Environmentalists criticise beam trawling because the tickler chains beneath the beam that hold the net on the bottom and startle the flatfish and drive them out of the seabed are allegedly harmful to both the seabed and the animals that live there. The fishermen have reacted to this criticism and taken measures to better protect the seabed ecosystems and undersized juvenile plaice. A lot of fishermen now use nets with larger mesh sizes than required by law and more and more fishing gears are equipped with escape panels, sorting, selection or guide grids that enable small fishes to escape. In cooperation with fishing technologists some traditional fishing methods have been modified or completely new concepts developed and tried out that are gentler, more environmentally friendly, more energy-saving and more sustainable. For example, the Danish seine has been further developed to enable its use in deeper waters. In contrast to the shallow water method, fly shooting, also called Icelandic seining, uses a kind of harpoon to shoot the line away.

The new developments in fishing gear include the pulse trawl in which the heavy chains beneath the net are replaced by thin electrodes which emit slight electric shocks to disturb and stun the fish. Electro beams are lighter than traditional beam trawls, fuel consumption is reduced by 20-40 per cent and fishing causes less damage to the seabed. The catch ability of hydrorigs is based on hydrodynamic effects. The transverse beam at the upper net edge produces a strong pressure wave in front of the net opening during trawling in the water and this startles the fishes. Sumwings or jackwings do without runners completely and float like aeroplane wings above the seabed. Plaice fishing using a twinrig even fulfils all MSC requirements. Each of these technical alternatives to the beam trawl solves individual parts of the criticised problems but not all of them at once. Compromises in this area are inevitable

Development of plaice catches between 1950 and 2013
Source: FAO, Figis

Björn Marnau
Mesh size for catching flatfish is mostly about 100 mm. A lot of fishermen voluntarily use larger sizes so that smaller fishes can get out of the net.


Marketing initiatives to increase demand for plaice

Plaice is marketed both fresh and frozen. The centre of European flatfish processing is The Netherlands and about one quarter of the Dutch fish and seafood processing companies (over 400 in all) specialises mainly in flatfish. A share of the plaice goes to the retailer’s kitchen-ready and h & g (“head off, gutted” or simply “headed and gutted”). By far the larger share, however, is hand- or machine-processed to fillets. Filleting is relatively easy because the skeleton structure of plaice has just a few short ribs and pinbones which hardly disturb the required cuts. The fillets are removed completely from the backbone on both sides of the body in one piece, partly skinned and then traded fresh or frozen. Fresh plaice fillets are part of the traditional standard range of products at service counters in the fish trade in northern and central Europe. Since fresh fillets and also fillets packed in modified atmosphere (MAP) became available in supermarkets and discounters fresh sales have increased further.

Because plaice are relatively flat and the amount of meat on single fillets is accordingly low the fillets of the eye side and the blind side are sometimes placed on top of one another and frozen together. These products, mostly called “double deckers” or “married”, offer numerous possibilities for further processing and value adding. At the retailer’s plaice is often sold breaded or in a batter coating. Double deckers often have cream fillings or similar ingredients between the two fillets. Single fillets can be rolled and then filled, seasoned or combined with a suitable sauce. Such convenience products are suitable either as finger food or as the main components of complete fish dishes. Whether briefly fried or steamed, breaded, rolled and filled with a tasty filling, with or without sauce, plaice is suited to a variety of preparation methods. They can be grilled and even used for some Mediterranean meals. Young plaice are smoked in some regions, a processing method that is particularly popular around the Baltic. Due to the good stock situation and large catches some companies are even using plaice fillets for the production of canned fish products. During food controls plaice products occasionally fail due to high extraneous water content in the fillet which is often the result of an excessive use of polyphosphates. If the use of such additives is not correctly declared it is regarded as attempted fraud. The water absorption feigns a higher fillet weight and there is a real risk that water is then sold for the price of plaice.

The abundant plaice catches of recent years have led to an oversupply in the market. Particularly in 2013 and 2014 supply was sometimes in excess of demand and there were serious problems selling this high-quality table fish. A lot of fishermen complained about sales problems and falling prices. Whilst in 2008 they could still get an average of 2.15 EUR per kilogram for plaice the price in 2013 was just over one euro. Some landings could not be sold at the fish auctions even for very low prices of below 0.80 EUR/kg and had to be processed to fishmeal. It is thus hoped that special marketing campaigns will be able to improve the marketing opportunities for plaice products. Initiatives in this direction usually come from the Dutch fish industry which would like to establish the month of September as a second marketing peak (in addition to May). The first promotion test in 2012 under the slogan “Let plaice into your heart” was successful. The following campaign “Schol is smullen” roughly translated “Plaice is a feast” which ran in September 2014 in Germany, too, could offer more potential.

A typical and particularly striking feature of plaice (Pleuronectes platessa) are the round orange-red spots on the eye side. The skin is smooth.

Towards the end of 2015 the mood of many plaice fishermen brightened slightly after a long dry spell. For the first time in years producer prices were slightly better. The auction price for the smallest plaice grade was stable for weeks at 1.30 EUR/kg which industry experts put down to the continually good, sometimes even growing demand and declining inventories in the wholesale sector. Despite this, the peak prices of earlier years when plaice fishermen could get over 2 EUR/kg are still a long way off. But recent developments raise hopes that the efforts made for sustainable management of the plaice stocks in the North Sea will at last pay off for the fishermen, too.