In the horse mackerel the gametes develop asynchronously and are released in batches. The maturity cycle is characterized by a rapid post‐spawning recovery period and a relatively long period of oocyte maturation. Mean gonad weight reached a peak in June and intraovarian hyaline oocytes occurred from May to August. Determination of fecundity was complicated by resorption of oocytes both before and after spawning, and by the fact that resting and developing oocytes were not clearly separated by size. An estimate of‘potential fecundity‘was obtained by a combination of histology and oocyte measurement. Plankton surveys showed that a major spawning area exists in the Southern Bight of the North Sea, with eggs occurring from May to late August. An estimate of annual egg production was combined with the fecundity data to give an estimate of stock size of about 500 000 tons.

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The distributions of four species of larval Ammodytidae in the Southern North Sea are described. Two separate broods of Ammodytes lancea larvae were found, in spring and autumn. The spring larvae occurred in the areas of the Thames and Wash, and the autumn type mainly off the French and Dutch coasts.A description of these larvae is given. Larvae of Gymnamtnodytes semisquamatus were found in the Southern Bight area and in several areas off the Humber. Few larvae of Ammodytes lanceolatus were taken: they occurred in the Southern Bight and Dowsing areas. A. marinus larvae were found in two areas off the Humber and in the Southern Bight. There is evidence for a spawning area some distance to the west of the Dover Strait.