Population dynamics of plaice Pleuronectes platessa L in the inshore waters of North West Wales and Eastern Anglesey
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- School of Ocean Sciences
This study examines the population dynamics of juvenile and adult plaice Pleuronectes platessa L. in the coastal inshore area of North West Wales and Eastern Anglesey. The population dynamics of the adult population were studied by conducting autumnal otter trawl surveys in 2004/05. The data obtained were compared with an earlier study conducted in the same area in 1974/77 and with current data for other plaice stocks in North West Europe. The size/age structure, length/weight relationship, growth rate, mortality rate and size/age at first maturity (L5o/A5o) were described for male and female plaice. The results of the survey showed that the size/age structures of male and female fish were different with the older, larger and heavier fish comprising predominately female fish The length/weight relationships were different for male and female fish with males exhibiting isometric growth (b=3.02) and females exhibiting positive allometric growth (b=3.20). Growth patterns, described using the von Bertalanffy growth model, were significantly different between male (K= 0.62 year 1; LW=26.4 cm SL) and female (K= 0.25 year 1; L. =43.8 cm SL) plaice. The instantaneous rate of total mortality was higher for male plaice than for females (Z: 3=1.05 year I cf. Y=0.64 year ). Male plaice matured at a significantly smaller size and younger age than female plaice (L50, d'=16 cm SL cf ? =20 cm SL; Aso, d=1.33 years cf. Y=2.11 years). A comparison of the results of the 2004/05 survey with the earlier 1974/77 survey showed differences in the size/age structure, growth rate and maturity ogives between the two surveys. A meta-analysis of the population dynamics of plaice stocks in North West Europe revealed consistent similarities in the size-structure and patterns of growth and maturation between the populations. A detailed analysis was conducted of the population dynamics of plaice in the coastal inshore area of North West Wales and Eastern Anglesey, based on the autumnal RV Prince Madog fisheries surveys, for the time period 1970-2006. A cyclical trend in abundance (number of plaice hour-1) was apparent in the catch data which was significantly correlated with the winter North Atlantic Oscillation Index (wNAOI). The results indicated no significant changes in the size composition of the plaice catch during the 4 decades (average sizes; (3=24.0 cm SL cf. ? =29.1 cm TL) but a decline in the proportion of older age classes (>_5 years old) in the catch (average ages; c3=2.6 years cf. Y=3.1 years). Analysis of the sex-ratio data between 1970 and 2006 indicated a decrease in the proportion of male plaice caught in the fisheries surveys over the last 4 decades. Male and female plaice both showed contrasting changes in growth patterns between 1970 and 2006. For female plaice, L. tended to increase over time with no change in K values. In contrast, male plaice showed no long term change in L. over time, but an increase in K-values post-1990. The Lso/Aso datasets were limited but analysis of the available data indicated a decrease in L5o/A5o values for male and female plaice over time. The population biology data for plaice (currently underexploited but historically overexploited in the 1970s and 1980s) for the time period 1970-2006 were compared with comparable data for dab Limanda limanda L. (an unexploited species in the Irish Sea) and whiting Merlangius merlangus L. (an overexploited species in the Irish Sea) obtained from the same fisheries surveys. Analysis of the time series data revealed changes in the size and growth patterns for the three species that were consistent with their level of exploitation (i. e. no changes for dab cf changes for plaice and whiting). A similar analysis of the Lso/Aso data was more limited but suggested decreases in the maturity ogive values of plaice and dab. The results were suggestive of fisheries-induced evolution in plaice and whiting in the South East Irish Sea. The population dynamics of plaice were examined in relation to summer/winter/annual sea surface temperatures (SST) and wNAOI to examine whether any changes were related to recent climate change. This analysis indicated a correlation between abundance and wNAOI with higher abundances (number plaice hour') recorded in the survey when the wNAOI was in negative phase. A simple model outlining the effects of increased/decreased SST on size and growth was proposed. For male plaice, the correlations between LC/K and SST/ wNAOI followed those predicted by the model, but the response was not observed for female plaice. A detailed three year survey of the growth, length/weight relationship and condition of juvenile plaice on two nursery grounds (Conwy and Red Wharf Bays) between April 2004 and March 2007 are reported. Modal progression analysis was used to follow the growth of the 2004,2005 and 2006 year classes with the average size increasing from 1.5-2.0 cm SL in April/May to ca. 8-10 cm SL the following February/March Seasonal variations in the length/weight relationships and in condition factors were observed on both nursery grounds. Condition was lowest in the winter/spring and increased in the summer/autumn as a result of increased food availability and growth opportunity. No differences were observed between the two nursery grounds.
|Award date||Jun 2009|