ICES coordinated acoustic survey of ICES divisions IIIa, IVa, IVb and VIa (North) 2003 Results
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Six surveys were carried out during late June and July covering most of the continental shelf north of 52°N in the North Sea and to the west of Scotland to a northern limit of 62°N. The eastern edge of the survey area was bounded by the Norwegian and Danish, Swedish and German coasts, and to the west by the shelf edge between 200 and 400 m depth. The surveys are reported individually in the report of the planning group for herring surveys, and a combined report has been prepared from the data from all surveys. The combined survey results provide spatial distributions of herring abundance by number and biomass at age by statistical rectangle; and distributions of mean weight and fraction mature at age. The estimates of North Sea autumn spawning herring are consistent with previous years at 3.1 million tonnes and 18,400 million herring. The survey also shows two exceptional year classes of herring (the 1998 and 2000 year classes) in the North Sea, which is consistent with the observation of exceptionally large year classes observed in the MIK and IBTS surveys. The estimates of Western Baltic spring spawning herring SSB are 106,000 tonnes and 823,000 herring and show a substantial decrease compared with the previous year. The Western Baltic survey produces a rather noisy signal but the indications are of a stock that is slightly higher than during the period 1996 to 2000. The West of Scotland survey estimates of 739,000 tonnes and 4,000 million herring and shows the high 1995 year class again this year. The 1998 year class now (4 ring) is now confirmed also a large one. Indications are that the 2000 yearclass is also good. Total adult mortality shows low mortality again ( 0.1 ) but the mean mortality over the last 4 years has been around 0.3: this is consistent with the 2003 assessment that the stock is lightly exploited.