Distribution and population structure of the rockfish Helicolenus dactylopterus (Pisces : Scorpaenidae) in the western Mediterranean

The distribution and spatio- temporal variation of Helicolenus dactylopteru s (Pisces: Scorpaenidae) population was studied from 816 bottom trawls deve loped along the Iberian coast. Multifactor analysis of variance Mras used t o test the differences in abundance and biomass and cluster analysis was ap plied to detect differences in population structure. The results showed a b athymetric and latitudinal gradient in abundance and population structure a long three geographic sectors in the surveyed area. In the southers area, t he species was more frequent and showed a wider bathymetric distribution ra nge than northwards. The depth range with the maximum frequency of occurren ce was also wider in the southern area than northwards. Similar trends were observed in terms of abundance and biomass, with the highest and lowest in dices in the southern and northern areas, respectively. The age composition of the catches showed that the population consisted mainly of young-of-the -year and juvenile fish. This fraction of the population is well represente d, at a mesoscale, along the whole surveyed area, hut adults are well repre sented only at a local scale, at the deepest strata surveyed in the souther n area. In this case, recruits of age 0 and juveniles up to 4-y old were re stricted to depths shallower than 500 m, while adult fish older than 6 y of age appeared below this depth. In the southern ar ea, seasonal changes in the population structure were also observed, with modes of small fish (3-6 cm) from March to June, as a consequence of the species recruitment to the bottom. Some direct and indirect factors of biogeographic, environmental an d anthropogenic origin affecting the observed gradient are discussed.