Otolith measurement as a method of identifying factors affecting first-year growth and stock separation of mackerel ( Scomber scombrus L.)

Abstract

The amount of growth in the first year can be measured from the otolith (L ₁ ). This difference in first-year growth has been associated with the difference in the peak spawning times between the North Sea and Western mackerel stocks. These differences in growth are analysed, and their suitability considered for stock separation when catches are taken in areas where the two stocks are known to be mixing. Firstyear growth has also been shown to vary between year classes. While the variation for both stocks follows the same trends, the difference in growth between Western and North Sea fish has not always been in the same direction. Prior to 1974. firstyear growth was significantly greater for the Western stock than for the North Sea stock. Since 1974 the amount of growth in the first year has been the same or larger for the North Sea stock. The changes in growth of the North Sea fish have been linked with density dependence.