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plaice report

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    An investigation into the Fisheries Biology of Plaice ( Pleuronectesplatessa) Introduction: Plaice ( Pleuronectes platessa) is the sole member of the Order Pleuronectiformes, and is one of the several species within the Family Pleuronectidae occurring in the waters surrounding Ireland. Pleuronectids are flatfishes characterised by the eyes being on the right side of the body in an asymmetrical position. Living in the demersal environment, plaice are usually immediately distinguished by their striking colour with the eyed side being warm brown with red or orange spots. The blind, demersal side is a clear pearly white. The mouth is terminal and located slightly to the right of the eyes. A series of four to seven bony knobs run in a curved line from the eyes back to the lateral line which, like in most flatfish, is well-developed on both sides of the body. Body scales are cyloid with the body being smooth to touch. Plaice can grow up to 91cm in length and weigh up to 7kg in exceptional cases. Most fish however usually reach a maximum length of 50  –  60cm. The plaice is the most familiar flatfish in northern Atlantic waters largely due to its excellent edible quality and its relatively high abundance. It is a typical shelf species found in shallow water from the shoreline (juveniles & smaller specimens) to about 120m in depth. It is most common in sandy benthic habitats but is also found on mud and gravel. “Occasionally in certain areas, larger plaice may move up into intertidal areas and estuaries. However, plaice are not as tolerant of fresh wa ter as flounders and do not typically penetrate far up estuaries” (Wheeler, 1969).  Spawning takes place off the Irish coast in March  –  April. The spawning period differs in different regions and at different latitudes. There are a number of particularly important spawning grounds where mature fish congregate and to which they migrate over some distance. One of these grounds is in the southern North Sea (ICES Division 4c) and being of particular importance to many plaice fish stocks in North-West Europe, including Ireland. Other important spawning grounds exist in the Irish Sea. Plaice typically produce 10,000 to 600,000 eggs which drift as plankton for 2-3 weeks in the upper water column. Upon hatching, the larvae and post larvae are surface living feeding on diatoms and zooplanktonic copepods as they progressively grow and age. After around 6 weeks metamorphosis occurs and the left eye migrates to the right side. At this stage, the juvenile plaice become bottom living and spend their first year of life in shallow inshore water where feed on crustaceans and amphipods. As they grow larger they gradually move into deeper waters, changing their diets to principally feed on molluscs and other benthic organisms including polychaetes. The food an individual eats depends on the availability in the particular bottom sediment. Plaice feed mainly during daylight hours and feed less intensively in the winter months compared with other times of the year. Females grow faster and live longer than males, with females reaching sexual maturity between 3  –  7 years and males between 2  –   6 years. “The plaice is thus a long -lived flatfish,   Fig 1. External biology of plaice  growing rapidly early in life after which the growth rate slows” (Wheeler, 1969). The growth rate varies from area to area due to availability of food, temperature and the population density. The plaice is the most important flatfish to the fisheries of Europe. The total European catch amounted to roughly 7kt (7,000 tonnes) tonnes of landed fish in 2015. Ireland holds rights for 47.6% of the total allowable catch within its own waters within the E.U. Roughly 295 tonnes of plaice was caught annually in mixed demersal fisheries by Irish vessels between 2013 and 2015, 49% of which were caught in ICES Division 7.a (Irish Sea). The landings were worth approximately €708,454 for Irish vessels in 2015 alone. Ground fishing gear is primarily used because plaice is a demersal species. Several fishing types are used but mostly bean and otter trawls. ICES has set the minimum landing size for plaice at 27cm so these fishing gears have to specifically tailored to avoid catching smaller juvenile individuals which would damage the stock. Juveniles dwell in shallower water hence destructive trawling is not carried out at shallower depths in order to preserve the recruitment stock and juvenile habitat. ICES sets TACs for five stocks around Ireland including the Celtic Sea, Irish Sea, South-west of Ireland, West of Ireland and the West of Scotland & Rockhall. Over the years, the plaice has become over exploited with the average size of landed fish dropping as the effort to land a given unit has increased. Huge problems exist at this moment in time in relation to the plaice stock and fishery, especially in the Celtic Sea (ICES Division 7f-g). Here, plaice are taken as a minor by-catch in mixed fisheries and are landed with whiting and anglerfish. High discards of other non-target species occur Fig. 2. Showing the distribution of plaice (Pleuronectes  platessa) in North –   West Europe. Plaice may be found from the western Mediterranean Sea, along the coast of Europe as far north as the White Sea and Iceland. Occasionally they occur off Greenland. Plaice populations also show spatial distributions e.g. within the North Sea: juveniles are concentrated along the Danish and German coast but do not occur in the centre or north-western regions. Fig 3. Landings distribution maps for plaice in NW Europe and in Ireland.  leading to a total of 73% of the total catch being discarded. Before 2004, the reporting of discards was not mandatory so the magnitude of discards from the previous decades is not known. International landings of plaice in ICES Divisions 7a (Irish Sea) and 7b (West of Ireland) peaked in the 1980’s at more than 20kt. Landings have now declined to around 7kt per year. The plaice has been subject to many trans-plantation experiments, where numbers of small fish were caught and released in areas where growth and health was confidently assumed to be better. The method however has not proven to be a financial or economic success. Material and Methods: A population of plaice ( Pleuronectes platessa) was supplied by Galway Sea-foods and used for the examination of the health of the stock and to analyse the various parameters of the fisheries biology of plaice. The precise location or ICES division from which the population was taken was not known nor whether or not the fish were taken from the same stock. For the purpose of the investigation it was taken that the location from which the fish were taken was unknown but in Ireland, and the data obtained used for comparison between known stocks. A sample of 71 plaice were taken from the sample population and the various physical properties for each were measured and recorded in the lab, i.e. length (cm), weight (g) and meristics (dorsal fin ray count). An accuracy level of 0.1cm was established when measuring the length and 1.0cm when measuring the weight. The bodies of each fish were then cut open to expose the gonads which were hence used in sex determination. An accurate cut was then performed diagonally between the bony knobs on the lead in order to expose the two otholiths within the head. Both otholiths were carefully removed, cleaned and placed under a stereoscopic microscope in order to determine the age in years (which was then recorded). Once all of these vital properties (for fisheries biology) were measured and recorded, the data was processed and interpreted using MS Excel into appropriate graphs and tables from which the results were interpreted and compared to previous fishery studies for plaice.   Fig. 4(a) Showing Irish landings for gear type & (b) proportion of discards vs landings.   Results:   1).  Age Determination: Fig 5. Morphology of outer and inner surface of otholith. The outer surface exhibits both opaque and transparent growth rings. Opaque growth rings are much wider and represent summer growth, whilst transparent growth rings represent winter growth and are much narrower. These two type of growth rings combined are equal and deposited by one year of growth. By observing and counting the numerous growth rings carefully, the age in years can be accurately estimated. The older a fish becomes the more tightly packed the growth rings appear which makes it more difficult to estimate the age accurately. Variation in width of growth rings signifies differing environmental conditions from year to year.
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