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.
| Identifer | oai:union.ndltd.org:bl.uk/oai:ethos.bl.uk:386805 |
| Date | January 1994 |
| Creators | Ellis, Timothy Roberts |
| Publisher | University of Liverpool |
| Source Sets | Ethos UK |
| Detected Language | English |
| Type | Electronic Thesis or Dissertation |
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