Changes in Western mackerel (Scomber scombrus) spawning stock composition during the spawning season

Abstract

INTRODUCTIONTwo groups of mackerel (Scomber scombrus L.) are recognised by the International Council for the Exploration of the Sea (I.C.E.S.) for stock assessment purposes in the north-east Atlantic. The North Sea ‘stock’, which overwinters along the edge of the Norwegian Trench and spawns off the south coast of Norway, in the Skagerrak, Kattegat and the central North Sea (Hamre, 1980), and the Western ‘stock’, which overwinters and spawns along the edge of the continental shelf from the west of Ireland to the Bay of Biscay (Lockwood, Nichols & Dawson, 1981).

Growth variability of mackerel (Scomber scombrus) off north and northwest Spain and a comparative review of the growth patterns in the northeast Atlantic

“…The catch composition at age also varies in the different areas of the northeast Atlantic (ICES, 1996(ICES, -2001. As previously stated, Dawson (1986) and Eltink (1987) linked differences in growth to gradual spatio-temporal changes in length at age during the migration of mackerel through European waters. Another cause of variability in mackerel growth rate may be the effect of population density, as found by Mackay (1973), Overholtz (1989) and Neja (1995) in the northwest Atlantic, especially in the youngest ages.…”

“…Nevertheless, Eltink and Gerritsen (1982), Dawson (1986) and Eltink (1987) rule out the hypothesis that there are two subpopulations in the western area and postulate an alternative explanation, stating that the growth differences are due to gradual spatial and temporal changes in the length at age during migration. The largest fish of a certain age reach spawning areas earlier and also leave for feeding areas earlier than smaller ones, which would lead to successive changes in length and weight at age.…”

“…In addition to the age-length succession in the migratory routes of the mackerel, the interpretation of growth will be dependant upon the season when samples are obtained and seasonal differences in growth were reported (Castelló and Hamre, 1969). Population samples that are not representative will cause differences in growth (Dawson, 1986). In migratory species like the mackerel, input data in growth studies may often be biased (elements might be missing as the whole (Kästner, 1977), 3. western area (Division VIa) (Kästner, 1977), 4. western area (Divisions VIIb and VIIj) (Eltink and Gerritsen, 1982), 5. northern and western area (Skagen, 1989).…”

“…There have been descriptions of different growth groups in the northeast Atlantic mackerel, which in some cases point to the existence of different subpopulations to the west of the British Isles (Kästner, 1977;Corten and Van de Kamp, 1978) and others that have put forth the alternative hypothesis that the growth differences in the mackerel may be attributed to the changes in length at age observed after they have migrated (Eltink and Gerritsen, 1982;Dawson, 1986;Eltink, 1987). This paper examines the growth pattern of southern component mackerel and its interannual variability, taking into account a wider geographical area and a greater number of years than the studies previously cited for this component.…”

Analysis of the handline fishery targeting mackerel (Scomber scombrus, L.) in the North of Spain (ICES Division VIIIbc)

“…During spawning in the southern area, the length of fish reveals a gradual phenomenon in which the largest spawners appear in the catch in March and April, while a greater abundance of young spawners appears in May, when recruitment to the spawning stock is effectively taking place (Eltink et al, 2002). Dawson (1986) and Eltink (1987) also found that migration of western component mackerel to the spawning area occurs in an agelength sequence, from the oldest to the youngest. The exploitation pattern of the handline fishery was constant in the period studied except for 1992, when there was a small increase in exploitation of individuals of age 1, which were hardly exploited at all in other years.…”