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Plaice: Pleuronectes platessa (L.) Phylum: Chordata Class: Osteichthyes Order: Pleuronectiformes (flatfishes) Family: Pleuronectidae
Species description The plaice is a flatfish with eyes on the right side of the body and a pointed snout/mouth. The head is less than 0.25 of the total fish length [1] and has line of 4 to 7 bony warts between the eyes extending to the lateral line [3]. The upper side is dark brown with distinct orange blotches and the underside is pearly white [1]. To some degree plaice can adapt their colour to match their substrate [2], thus increasing camouflage. BIOLOGY and ECOLOGY
Reproductive behaviour Plaice spawn from December until March, peaking in January and February [5] in well defined spawning grounds that are 20-40m deep [5 & 7]. Spawning occurs above the seabed with the female across the upper side of the male [4]. At the time of spawning the eggs are approximately 2mm in size, are yellow and have no oil globule [4]. Hatching occurs at approximately 7mm; growth to this size takes between 12-21 days depending on the water temperature, 10 to 5°C respectively (i.e. growth and thus hatching is faster in warmer water) [4]. At approximately 40 days old the left eye shifts round to the right side of the body [4], metamorphosis is complete at 60-120 days depending on the water temperature [5] and the fish takes on the typical flatfish form. By this time the juvenile plaice will have drifted inshore to an appropriate shallow nursery ground, sunk to the bottom, adopted the benthic environment and be approximately 3cm in length [3 & 5]. Juvenile plaice spend up to 2 years on the nursery grounds before joining the adult stock [5]. Migratory behaviour A tagging study of adult plaice has indicated that there are three groups of plaice present in the English Channel; resident plaice in the west English Channel, resident plaice in the east English Channel and migratory North Sea plaice which enter the English Channel in the autumn, spawn and rapidly leave [5]. There is very little traversing between the English Channel/North Sea plaice and the Irish Sea plaice [5]. Interestingly however, hydrographic studies indicate that juvenile plaice do not recruit to nursery grounds near to the area they were spawned, instead showing an eastwards movement [5]. This can be explained as the eggs/larvae have a period of several weeks in the water column before they settle to the benthic environment during which they are subject to an easterly moving current system [5]. These hydrographic studies suggest that plaice spawned in the western English Channel recruit to nursery grounds in the eastern English Channel, whilst plaice spawned in the eastern English Channel, recruit to nursery grounds in the southern North Sea [5]. Tagging of juvenile plaice on these nursery grounds found that over one third of the adult plaice in the eastern English Channel came from nursery grounds in the North Sea, whilst the western English Channel received 34% of its recruits from the eastern English Channel and 53% from the North Sea [5]. Studies by CEFAS [15] have shown that plaice adopt ‘selective tidal stream transport’; this is where the fish leaves the bottom at slack water and swim down tide i.e. with the current, and return to the seabed when the tide turns. Maturing fish select the tidal stream flowing towards the spawning ground and spent fish use the opposing tidal stream to return to the feeding grounds. This behaviour is very energy efficient.
Habitat Adult plaice can be found on sandy or muddy substrates to over 100m deep [3], whilst the juveniles are found predominantly in inshore shallower waters [4]. Plaice often increase their camouflage on the seabed by flapping their fins to cover their body with a fine layer of bottom sediment, often only leaving the eyes protruding [3].
Predators and prey Plaice most commonly feed upon bivalves including; cockles, razor shells, and small scallops [3], they also predate upon some polychaetes and crustaceans and larger plaice may even take small fish. Plaice have ‘cutting’ teeth on the ‘under side’ of the jaw which they use to bite off soft protruding parts of the bivalve e.g. the siphons, whilst further back in the throat region they have crushing teeth used for crushing e.g. the shells [3]. Predators of the plaice include anglerfish, weever fish, gurnards, cod, conger eels, rays, seals and dolphins [8].
THE SUSSEX FISHERY Fishing methods Fishing for plaice within Sussex IFCA district predominantly occurs at a commercial level.The most common practices used are fixed gear, beam trawling and stern (otter)trawling. Small quantities are also taken by recreational fishers using fixedgear or rod and line.
Fishing activity There are no defined seasonal or daily movements of plaice which play a role in the fishery; however, it is common for flatfish to be more active at night. Plaice are predominantly taken as a valuable bycatch in the sole fishery thus the activities of the sole fishery play a large role in the activities of the plaice fishery. The trawlers targeting sole will often fish for 24+ hours; the sole are mostly caught during the hours of darkness but fishing continues during the day as plaice are still caught. Plaice appear to be in their best condition and have the greatest meat yield during autumn and winter. Fig 2. Plaice for sale © Sussex IFCA References [1] Hayward, P. J., and Ryland, J. S. 1998. Handbook of the Marine Fauna of North-West Europe. Oxford University Press, New York. [2] Irving, R. 1998. Sussex Seasearch: Sussex Marine Life: An identification guide for divers. East Sussex County Council Publication, Lewes, England. [3] Lythgoe. J., and Lythgoe, G. 1971. Fishes of the sea: The coastal waters of the British Isles, Northern Europe and the Mediterranean, A photographic guide in colour. Blandford Press, London, England. [4] Miller, P. J., and Loates, M. J. 1997. Collins Pocket Guide: Fish of Britain and Europe. Harper Collins Publishers, London, England. [5] Pawson, M., G. 1995. Biogeographical identification of English Channel fish and shellfish stocks. Fisheries Research Technical Report (number 99), MAFF Direct Fisheries Research Lowestoft, England. Available from: http://www.cefas.co.uk/Publications/techrep/tech99.pdf [6] Bagenal. T. B. 1973. Identification of British Fishes. Hulton Educational Publications Ltd, Buckinghamshire, England. [7] Dipper, F. 1987. British sea fishes. Underwater world publications limited, London, England. [8] FishBase. 2000. A global information system on fishes. Available from: http://www.fishbase.org/Summary/SpeciesSummary.php?id=1342 (accessed 08/04/09) [15] Movements of plaice and cod. Centre for Environment, Fisheries and Aquaculture Science (CEFAS), Lowestoft, England. Available from: http://www.cefas.co.uk/publications/files/fish_movements.pdf (accessed 08/04/09)
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