Maternal effects and fecundity of plaice (Pleuronectes platessa) in the Irish Sea
The fecundity of Irish Sea plaice caught from Liverpool Bay, the Cumbrian
coast and in the western Irish Sea was estimated in 200 1, 2003 and 2004 and was
compared with data previously collected for the years 1953, 1995 and 2000. The
fecundity was also estimated for plaice caught during September 2004 and 2005.
Temporal variation in fecundity was greatest in the western Irish Sea, followed by
the Cumbrian coast and there were no significant differences between years in
Liverpool Bay. Fecundity estimates from September did not differ between years
or between areas but was higher than fecundities estimated during the spawning
season. The maximum fecundity of an individual fish was determined by the
weight of the fish at the end of follicle proliferation. This was then down regulated
by atresia during the period between the cessation of follicle proliferation and
spawning. The differences in fecundity between years and areas are hypothesised
to be due to differences in the degree of down regulation.
To examine if the degree of down regulation was affected by the feeding
level during late vitellogenesis, plaice were housed in individual pens and fed on
either a high or low ration of food in late autumn. Biopsy samples were taken at the
beginning, middle and end of the experiment and follicle size was determined using
image analysis, the percentage of atretic follicles was noted and the change in ovary
weight was monitored. Follicle growth rate increased with food level and the level
of atresia was negatively correlated with change in condition factor. As food level
decreased there was an increased dependence on stored reserves for metabolism and
The Total Egg Production (TEP) for plaice for the whole Irish Sea was back
calculated using available fecundity estimates and the Virtual Population Analysis
(VPA) data from 1964 to 2004 and compared with indices of recruitment at age 1.
TEP was positively related to SSB. Recruitment at age 1 was not related to TEP or
SSB. Mortality between the egg stage and recruitment was positively related to
TEP which is believed to be due to density dependent processes occurring during
the nursery ground phase. The estimates of TEP were approximately one third of
the direct estimates of stage 1 egg production from plankton surveys in the eastern
Irish Sea. This is hypothesised to be due to inaccuracies in the VPA data.
The effects of maternal size on various egg and larval characteristics were
examined using plaice caught from coastal waters around the Isle of Man (Great
Britain) and Bergen (Norway). Egg batches were incubated at 7°C with larvae from
one batch being monitored at the individual level. Egg size increased with maternal
size, with larger eggs producing larger larvae with a greater yolk sac volume. Eggs .
from earlier batches had greater incubation times than eggs from later times. A
longer incubation time led to bigger larvae but with a smaller yolk sac volume.
Growth during the two weeks after hatching was related to size at hatching and yolk
sac volume, with smaller larvae with larger yolk sacs having the greatest growth.
Larger larvae had no survival advantage under the present experimental conditions,
which had a plentiful supply of food and no predators.