Life history parameters as basis for the initial recognition of stock management units in horse mackerel (Trachurus trachurus)

Information on growth, reproductive biology and the distribution of commercial landings was considered in the EU funded project HOMSIR as basis for the initial recognition of management units for horse mackerel. Twenty sampling sites were considered, which covered the entire range of the species distribution. More than 4400 otoliths were aged and the length distributions at age of different sampling sites were compared. Horse mackerel showed great variability in growth although some patterns can be identified. The Mauritanian coast shows the highest median length at age. The increasing trend in median length at age according to the increment in latitude in two different areas (the Portuguese coast and along the west Ireland coast) suggests the possibility of a particular length-dependent migration pattern during the spawning season in these areas. In the Mediterranean, the horse mackerel in the Thyrrenian and the Ionean Seas showed the highest length at age values. The reproductive strategies of the horse mackerel were analysed using the maturity ogives and the batch fecundity. Maturity ogives were estimated in terms of length and age at first maturity. In the Atlantic, both the length and age at first maturity seem to decrease with decreasing latitude. In the Mediterranean, no similar tendency was evident between its western and eastern parts. Statistical differences between all areas were obtained in the analyses of both length (P < 0.0001) and age at first maturity (P < 0.05). Batch fecundity had a decreasing trend with latitude in the Atlantic. In the Mediterranean, data were obtained only from two areas: in Greek waters and off Southern Spain. The individuals from the latter had higher fecundity than those from Greek waters. Statistical differences were found between areas (F = 2.1377; P = 0.0346). The distribution of commercial landings in the northeast Atlantic showed that fish move from the south of Ireland to the Norwegian Sea and northern North Sea to feed in the autumn early winter.