The relationship between the location of western mackerel spawning, larval drift and recruit distributions: A modelling study
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Summary: Mackerel spawning distributions have changed over the last 20 years. From 1977-1995 there has been a progressive westerly shift in the distribution of spawning in May/June (peak spawning), with a resultant increase in the proportion of spawning west of the shelf break. Also during this time period there has been a marked increase in the proportion of spawning in the north of the spawning area. This shift occurred most obviously between the surveys carried out in 1986 and 1989. At the same time it is shown that there has been a northerly shift in the distribution of first winter juveniles. These observations are derived from data collected on the ICES coordinated winter bottom trawl surveys. This study used data on mackerel egg distributions during May, derived from the ICES triennial mackerel egg surveys as input to the NORWECOM transport model, using real weather fields. The outputs from the model indicated that most eggs and larvae could expect to be transported south after spawning and that there had been no significant change in this transport pattern during the period studied. The combination of more northerly spawning and the prevailing transport pattern may explain, in part, the increase in the recruit population in the northern nursery areas. It is concluded that passive transport can explain the juvenile distribution in some areas, but that active migration must also play a role in the area of the Celtic Sea. The interactions between the modelled transport patterns and the real egg distribution data are discussed with reference to these topics and to the potential survival of larvae in the first weeks after hatching.