Ray Guillery

Netherlands report in the current issue of the Proceedings of the Royal Society B (published online) the first ever survey of freshly spawned fish eggs to cover the whole of the North Sea. The team highlight the former significance and current concern about cod stocks. “Rebuilding these stocks remains a key policy aim in Europe, the USA and Canada,” they argue. “Historically, the North Sea contained one of the most productive cod stocks but the present abundance of mature fish is the lowest on record,” they write. And the population has been considered as a single entity, which may mask the presence of subgroups within the North Sea. “In these assessments, data are aggregated over large areas but this can mask local trends caused by differences in population dynamics, exploitation, and environment.” Key to the assessment of North Sea cod stocks is the determination of their breeding sites. But this presents many problems: freshly released and fertilised eggs need to be identified quickly to locate key spawning grounds but this has traditionally been hard because cod eggs are difficult to distinguish from the eggs of related species such as haddock and whiting in the earliest stages. But the team report that this problem has been overcome by the use of molecular techniques to distinguish the cod eggs. The researchers report the first survey of the whole of the North Sea to use genetic probes to identify unambiguously cod eggs within the plankton samples trawled from varying depths by vessels involved in the research project over the latewinter spawning season. Alongside the trawl for eggcontaining plankton, the researchers compared their results with trawls for mature fish, to help estimate egg production. In total, the researchers carried out 502 plankton trawls and nearly 9,000 cod-like eggs were identified using genetic probes. Analysis of hatchery-spawned cod and haddock eggs revealed that 95 per cent of the plankton eggs were correctly identified. The researchers studied cod eggs up to the end of stage 1 of development, which represents approximately three days’ growth. Identification of these eggs gives a good indication of where they were spawned. The survey revealed hot spots of cod spawning. Eggs were found around the southern and eastern edges of the Dogger bank in the central North Sea with another concentration in the German Bight. Significant numbers of eggs were also found off the Moray Firth in Scotland and to the east of the Shetland Isles. The researchers found that the spawning sites they discovered matched the presence of mature fish found in other trawling surveys. The one discrepancy was that of mature fish found off the UK’s Yorkshire coast at Flamborough Head, but no evidence of spawning in this area from the fish egg survey. “The application of modern molecular methods holds the promise of rapid, unambiguous identification of cryptic species or life stages. However, applications to large-scale distribution mapping of marine plankton have been hampered by the need to analyse very large numbers of specimens, by the relatively high per-sample cost of molecular identification and by the time and specialist skills needed,” the researchers write. “The application of modern molecular techniques combined with traditional plankton survey methods allowed us to map the occurrence of early-stage cod eggs across the whole of the North Sea for the first time.” The researchers believe their results show that the severely depleted North Sea cod are still spawning in most areas where mature fish were found. But the situation is difficult. Recent studies found slightly increased numbers of cod in the North Sea last year and the EU has increased the fishing quota for 2008 by 11 per cent. But last year’s catch of 20,000 tonnes is a shadow of the 170,000 tonnes caught 20 years ago. “Some localised populations may now have been reduced to levels where it is difficult to find cod eggs in the plankton. These populations could be at particular risk of severe depletion and may require targeted conservation measures,” the researchers write. Ray Guillery