The International Council for the Exploration of the Sea (ICES) Mackerel Working Group recognises two mackerel stocks in European waters, the North Sea stock and the Western stock. From March to July 1977 an Anglo-French plankton survey was carried out in the shelf waters of the Bay of Biscay, Celtic Sea and west of Ireland to estimate the size of the Western stock. Samples were collected with a 76 cm Lowestoft pattern high-speed plankton sampler fished to a maximum depth of 100 m. The number of eggs m−2at each of five development stages were raised to daily production estimates with development rate coefficients measured with live material collected in March. The mean number of stage I eggs m−2d−1was raised to a total daily production estimate for each month by the area within the minimum contour level, and a production curve was drawn for the spawning season. The total egg production was estimated at 1.98×1015eggs,+30,−20. With a mean fecundity of 361 000 and a sex ratio of 0.64 males per female this is equivalent to 8 995 mill
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