Density dependent growth changes through juvenile and early adult life of North East Atlantic Mackerel (Scomber scombrus)

Density dependence of somatic growth has an important, but overlooked, impact on fisheries management advice. We therefore examined how growth relates to abundance in a case where growth has been observed to vary substantially in recent years, namely North East Atlantic mackerel, one of the most widespread and commercially important fish stocks in the North Atlantic. Growth of juvenile and early adult North East Atlantic mackerel was found to be decreasing since the late 1990s. Modelling showed that growth was related to density. Mean growth rate during the first year was tightly correlated with the density of juveniles and especially with juveniles from the previous cohort. This putative effect of juvenile density could be tracked in the length-at-age up to adult mackerel at commercially targeted sizes. However, as the mackerel grew towards adulthood, the effects of the same cohort became dominant. The effect of adult density was minor, but increasing with age. The ontogenetic progression in density dependant regulation of growth appeared to reflect the spatial dynamics (migration patterns) in the feeding season. This is the first time growth patterns quantified in the adult mackerel population have been linked to density related processes occurring within the nursery areas during the first year. These aspects of mackerel production have a substantial and direct impact on the management plan evaluations used when fisheries scientists provide advice for fisheries management.