Production and mortality of early life stages of flatfishes.
Plankton sampling established the presence of a plaice spawning ground off
the west coast of the Isle of Man in the Irish Sea. The total production of stage I
plaice eggs from this spawning ground in 1993 was estimated at 7.6 x 101
Theoretically this spawning ground could supply the local nurseries with plaice
larvae. Spawning was most intense in March, before the peak in the plankton bloom
in the Irish Sea.
Predation on plaice eggs by clupeids was studied in March in an area of high
plaice egg density to the east of the Isle of Man. Fish eggs formed the bulk of the
stomach contents of sprat and herring due to the lack of alternative zooplankton
food at this time of year. The later developmental stages of plaice eggs were more
vulnerable to predation. This was thought to be due to the increase in pigmentation
with embryonic development increasing the contrast between the egg and the water
to predators that detect prey visually. Sprat and herring >80 mm showed a strong
selection for plaice eggs over smaller pelagic fish eggs. However, the large eggs of
plaice had a refuge in size from predation by sprat <80 mm. Smaller clupeids were
feeding more actively than larger clupeids, as indicated by stomach fullness and the
total number of fish eggs in stomachs. Stomach content data was combined with
published biomass estimates and the daily instantaneous mortality rate of plaice
eggs due to predation was estimated at 0.023 for sprat and 0.001 for herring. Sprat
were therefore an important predator of plaice eggs in the Irish sea, whereas herring
seemed of limited significance.
Predation by fishes on O-group flatfishes was studied on a Scottish nursery
ground by stomach content analysis. Fish predation was shown to be a significant
source of mortality and I-group grey gurnards and gadoids were the major
predators. Flounder suffered higher predation rates than plaice or dab due to the
smaller size at settlement. Predation on plaice and dab was size-selective,
concentrated on the smaller individuals. Smaller flatfishes were vulnerable to a
greater size range and greater taxonomic range of predators. Predation by O-group
cod on O-group dab was limited by the sizes of predator and prey. It was
hypothesised that the times of recruitment of flatfishes and their predators to
nurseries, and relative growth rates, would affect predation and hence mortality.
The handling time of O-group flatfishes by O-group cod in laboratory
experiments was positively related to flatfish size and negatively related to cod size.
The handling time of plaice was longer than for dab of a similar size due to the
difference in body shape. Profitability of flatfishes (wet weight gained per unit
handling time) decreased monotonically with flatfish size over the range of prey and
predator sizes used. It was therefore predicted that when O-group cod forage in the
field the smallest O-group flatfishes would be the most profitable and would be
behaviourally selected for. However, the selection of prey by cod was suggested to
be determined by both behavioural and physical processes. There was evidence
that behavioural selection of prey occurred at the ingestion stage of feeding.
The current evidence for density-dependent processes in the juvenile stage
of plaice was reviewed. Both the data in support of, and the processes thought to
cause, density-dependent mortality in the juvenile phase were equivocal. It was
hypothesised that density-dependent mortality occurs in the early egg stages of
plaice and evidence, and the rationale for a potential predatory process resulting in
such mortality, were presented.