Seasonal and ontogenetic changes in the vertical distribution of eggs and larvae of mackerel ( Scomber scombrus L.) and horse mackerel ( Trachurus trachurus L.)

Abstract

Results are summarised from vertical distribution sampling for eggs and larvae of mackerel ( Scombrus scombrus) and horse mackerel ( Trachurus trachurus) to the west of the British Isles and in the Bay of Biscay between 1974 and 1995. Eggs of both species were predominantly in the upper 50 m of the water column and generally in increasing numbers towards the surface. For both mackerel and horse mackerel, the eggs had a deeper distribution earlier in the spawning season when there was little thermal stratification and became progressively more restricted to the surface layers as stratification developed. Eggs at the latest stages of development tended to be deeper in the water column than earlier stages and exhibited a sub-surface peak of abundance. Larvae of both species were also concentrated in the upper 50 m of the water column above the seasonal thermocline and exhibited sub-surface peaks of abundance in the depth range of 10–30 m. There was little evidence of diel migration for larvae of either mackerel or horse mackerel. The optimum sampling depth for quantitative egg surveys is discussed.