Changes in the abundance and spatial distribution of the Atlantic chub mackerel (Scomber colias) in the pelagic ecosystem and fisheries off Portugal

This study describes changes in abundance and spatial distribution of Atlantic chub mackerel in the Portuguese waters and the Spanish waters of the Gulf of Cadiz using data from acoustic surveys from 1995 to 2010 and data from the fishery since 1958. The distribution and dynamics of chub mackerel and sardine (Sardina pilchardus) are compared and species interactions are discussed. From 2007 to 2009, chub mackerel biomass, as measured acoustically, ranged between 69000 and 238000 t concentrated off the western coast (~50%) and in the Gulf of Cadiz (~30%). Portuguese landings have varied with periods of high landings around 1970 and in recent years. Since 1986 landings have been between 4000 and 23000 t. Both survey catches and commercial landings are taken mainly off the southwestern coast and one- to two-year-old individuals (20-24 cm) predominate in both. Scarcity of larger individuals may be a consequence of their deeper distribution and avoidance of fishing gear, migration of older individuals or a combination of both. In years with high abundance, the fishery expands to the northwestern areas of the Iberian Peninsula. The expansion appears to be caused by improved recruitment although other factors might have contributed (such as targeting to compensate for low sardine catches and the appearance of new markets). The complementary spatial distribution of chub mackerel and sardine and the inverse correlations between their frequency of occurrence, landings and recruitment indices suggest some interaction between the dynamics of the two species, possibly associated with climatic variation.