Population dynamics of plaice Pleuronectes platessa L in the
inshore waters of North West Wales and Eastern Anglesey
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Abstract
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