Report of the ICES Working Group on the Bløden Tagging Experiment, 1969/70
In 1966 the North-East Atlantic Fisheries Commission asked the Liaison Committee of ICES for guidance regarding the effects of measures proposed to conserve the Downs stock. In particular it enquired into the effect of regulation of the Bløden industrial herring fishery and the consequent effects on the Downs stock. From previous international tagging experiments it had been calculated that in 1957 and 1958 the juvenile fishery removed about 16-19 per cent of the stock per annum. Because of the limited extent of these experiments and the changed nature of the juvenile fishery, it was proposed that a new tagging experiment should be made, spreading the liberations of tagged herring over a longer period and at a higher rate of tagging.
The history of the project is outlined in Appendix I and the detailed plans are given in Appendix II. Because of delays in arranging the financing of the experiment, and changes in the North Sea fisheries since the inception of the idea in 1967, the object of the proposed experiment was not restricted to Downs herring alone but included the total juvenile stock wherever it occurred.
The Bløden Herring Tagging Experiment took place during the period July 1969 to March 1970. Tagged herring were expected to remain in the area of the juvenile herring fishery until early 1971 at least. The Working Group met in March 1971 ( Anon. 1971) to examine the material so far available and to consider useful methods of analysing the data. The development of a more extensive model of dispersion than that used for the 1957-58 experiments was considered essential. From these taggings, a higher percentage of tagged fish had then been recaptured, over a relatively long period, than from those of 1957 and 1958 (Aasen et al. 1961). It was decided to meet again at ICES headquarters prior to the Statutory Meeting in September 1971, to further consider the available data and to discuss the type of dispersion model required.
The Marine Laboratory, Aberdeen offered the assistance of a mathematician/programmer and the facilities of a computer of adequate capacity. A timetable was drawn up for transference of the data from computer files in Copenhagen to Aberdeen and for the development of the program, which was expected to be operational during autumn 1972. It was agreed that the Group should meet when output from the computer was available.
The development of the computer program took longer than anticipated, the first outputs not being available before June 1973, when the Working Group was convened. The preliminary results from the dispersion model were discussed and a number of modifications were proposed. A report was presented to the Pelagic Fish (Northern) Committee at the ICES 1973 Statutory Meeting, dealing basically with the returns from the juvenile fishery. Stock sizes and fishing mortality rates were derived.
As considerable numbers of tags had already been recovered from the adult fisheries in the northern North Sea and west of the Orkneys, and since some were still being reported from the 1973 season, the final meeting of the Working Group was delayed until May 1974, by which time these latest returns would be processed and the new outputs from the dispersion model would be available.
