Changes in age--size distribution and sex ratio during spawning and migration of Western mackerel (Scomber scombrus L.)

Abstract

The migrations of the Western stock of mackerel (S. scombrus L.) to the spawning, feeding, overwintering, and back to the spawning grounds are interpreted with reference to changes in mean length, mean weight, mean length-at-age, mean weight-at-age, and sex ratio. Migration to the spawning grounds occurs in an age—size succession. In fish of the same age and size, the males start spawning earlier than the females, while spawning ends at about the same time for both sexes. The return migration after spawning occurs only in an age–size succession. Nearly the whole spawning stock of Western mackerel is gathered in the spawning area by the end of April and the beginning of May, except for the two-year-olds. Larger fish of a given age group arrive earlier in any area and leave it earlier than the smaller fish, which results in a decrease in mean length-at-age and mean weightat-age in all areas where mackerel appear and disappear again. The “fast” and “slow” growing components are the result of the migrations of the age groups in a length succession; they are not based on two subpopulations or populations. The mean length of female mackerel in maturity stage 6 (hyaline eggs present in the ovary) during all months of the spawning season proved to be higher than the mean length of males in this stage: females in maturity stage 6 should be used forstock-size estimation by calculating the mean fecundity at the mid-points of the egg-survey cruises. During the spawning season different sex ratios are observed, which are necessary to calculate the number of males and females in the spawning area on the basis of the number of spawning females which are derived from the egg production. The male/female ratio for the whole Western mackerel stock was estimated as one male per female, but a slight deviation could be caused by the two-year-old males' maturing at a smaller length than the females; the size of this deviation will therefore depend on the contemporary abundance of the two-year-old fish. Throughout the whole spawning eason the male/female ratio of maturity stage 6 mackerel was 56/44, which indicates a longer duration in maturity stage 6 for males than females. The testes seemed to develop earlier than the ovaries, but more slowly.