No abstract

Experiments were conducted on the ability of TaqMan molecular probes to detect plaice Pleuronectes platessa DNA from eggs, and cod Gadus morhua DNA from eggs and larvae following ingestion by a teleost predator, whiting Merlangius merlangus. Estimated half-life detection rate (T50) for eggs was 31 h, and 26 h for larvae, with some positive detections occurring even after visual inspection indicated complete gut clearance. Because TaqMan probes are taxon specific, the results presented demonstrate that this technique can provide a means of rapid and unambiguous detection of predation by teleosts on fish eggs and larvae.

For species with extended spawning seasons, short periods of beneficial conditions often lead to disproportionate survival of sub-sets of the offspring. This has been demonstrated for freshwater fish and for marine pelagic species, but has been less frequently tested for other teleost groups. Using offshore plankton surveys, we constructed egg production curves for plaice Pleuronectes platessa L. in the eastern Irish Sea. Data from 5 yr showed that spawning began sometime before January and was completed by the end of April. Over these years the timing of peak spawning varied between mid-February and mid-March. In 2001, 2002 and 2003, we also sampled postsettlement plaice from inshore nursery grounds. Otolith micro-increment analyses and temperaturedependent egg development rates were used to back-calculate the birth dates of the settled fish. By June, immigration to the nursery grounds was largely completed and the shape of the reconstructed birth-date distributions corresponded closely to the relevant egg production curves. In contrast to many other studies, there was little evidence for significantly disproportionate survival of specific portions of the egg production curve between spawning and the recently post-settled stages. Such a survival pattern could be generated by a gradual switch from starvation to predation as the principal cause of larval mortality as the spawning season progressed.

Mortality during the egg and larval stages is thought to play a major role in de termining year-class strength of many marine fish. Predation of eggs and larvae is normally considered to be a major factor but the full suite of predators responsible has rarely been identified. Potential predators on a patch of plaice Pleuronectes platessa eggs located in the eastern Irish Sea were mapped using acoustics and sampled by trawl and a plankton multi-net. Gut contents of 3373 fish, crustacea and cephalopods sampled in the area were screened using a plaice-specific TaqMan DNA probe. Herring Clupea harengus and sprat Sprattus sprattus dominated trawl catches and showed high positive TaqMan responses (77 and 75% of individuals tested respectively). Locations of clupeid schools also broadly corresponded with the distribution of fish eggs in the plankton. Whiting Merlangius merlangus were also reasonably abundant in trawl hauls and 86% of their stomachs tested positive for plaice DNA. Species showing lower levels of positive TaqMan response included mackerel Scomber scombrus, poor cod Trisopterus minutus, squid Loligo spp., dogfish Scyliorhinus canicula and weever Trachinus vipera. Nonre activity of all negative controls precluded the occurrence of cross-contamination, and positive reactions from demersal species, such as dogfish and weever, may have resulted from secondary predation. No benthic macro-crustaceans tested positive. Samples of planktonic organisms yielded 13% positive TaqMan reactions, mainly from clupeoid or sandeel (Ammodytidae) larvae, but also included some Malacostraca and Amphipoda. Use of the molecular approach allowed rapid screening of a large number of potential predators of plaice eggs, and the results provide a more holistic description of the predator community than has previously been achieved.

Transport models for planktonic fish eggs and larvae often use temperature to drive growth because temperature data are readily available. This pragmatic approach can be criticised as too simplistic as it ignores additional factors, such as food availability and growth-rate-dependent mortality. We examined the extent to which temperature could explain the larval duration and growth variability in post-settlement plaice from the southern North Sea, Irish Sea and west of Ireland. Larval duration and individual growth were estimated from the otolith microstructure of juvenile plaice collected from nursery grounds in each region, and average sea surface temperatures were reconstructed for each region from satellite data. Hatch dates, larval duration and settlement dates all differed among the regions, reflecting the timing of spawning and the physical characteristics of each region. Models incorporating presumed temperature history and region explained 73% of variability in individual larval growth (as estimated from otolith microincrement widths), again demonstrating important differences among the regions. The results suggest that sea temperature can be used to drive growth models of plaice larvae, although allowance must then be made for interregional differences. However, larval duration can be estimated using a pan-regional model based on the average temperature experienced during the larval phase.

No abstract