ON THE ABUNDANCE OF MACKEREL (SCOMBER SCOMBRUS L.) IN THE NORTHERN AND NORTH-EASTERN NORTH SEA IN THE PERIOD 1959-1969 By

Although fishing intensity on North Sea mackerel during the years 1959-1966 was estimated to be very low as indicated by a low total mortality-rate (Z = 0-28), the abun-dance of the mackerel as measured by the catch-per-unit-effort in the Dutch trawl-fisheries declined due to a failing recruitment. Recruitment to the North Sea population was poor after the strong year-class 1958 appeared in the trawl-fisheries in 1962. The combined effect of a very moderate fishing intensity and poor recruitment, gave the mac-kerel population the appearance of an old accumulated stock by the middle sixties. Since the introduction of the purse-seine fishery in 1964 the total international catch of North Sea mackerel started to rise, reaching an unprecedented level of about 900000 tons in 1967. As a result of these substantial catches mortalities rose to an average of Z = 0-80 in the late sixties. In 1968 and 1969 mackerel abundance in the North Sea dropped to a very low level as a consequence of the high fishing intensity and the persisting failure of recruitment to the now decimated stock. Although the yield curve in mackerel does not suggest large gains from protection-measures such as minimum size or effort restriction, the danger of an unlimited fishery on a stock of small sized mackerel are discussed