The effect of recent changes in the North Sea mackerel fishery on stock and yield

The paper deals with biological aspects and exploitation of the mackerel (Scomber scombrus) stocks occurring in the North Sea. The state of stocks and exploitation in the North Sea area are investigated on the basis of annual releases of tagged fish and samples of commercial catches. Two populations occur, one spawning in the North Sea, the other in the Celtic Sea. The stocks are quite mixed, especially in the older age groups. Following the introduction of the purse-seine fishery, the North Sea stock (age three years and older) was fished down from a level of 2 5 million tonnes in the early 1960's to about 250 000 tonnes in 1971. Since 1970 the fishery has been regulated, resulting in some recovery of the stock . The recruitment of the 1962 to 1971 year classes has fluctuated within the range 12:1. Assuming a recruitment level equal to the average recruitment of these year classes, the maximum sustainable yield (MSY) of the North Sea stock is estimated at 250 000 tonnes per year. The distribution area of mackerel is found to be related to stock size. The decline in stock size resulted in the disappearance of mackerel from fishing grounds previously exploited by conventional gears, whereas the availability of mackerel to the purse seine in offshore waters is only slightly affected. Density-dependent growth is indicated by an increased growth rate of the 1969 year class compared with that of the 1962 year class