The objective of this study is to substantiate the pattern of geographical distribution of the three swordfish stocks in the Atlanto-Mediterranean region, the existence of which was evidenced by mtDNA analysis (Chow et al. 1997, Mar.Biol. 127,359-367). More specifically, it will be sought to estimate the extent of population mixing of these stocks in the feeding grounds (north and south Atlantic), and to estimate the levels of gene flow between them. Special attention will be paid on the Mediterranean stock, which as strong evidence suggests, should be considered as an endangered population. We propose to describe the migration and distribution patterns of the three stocks in the feeding grounds of the Atlantic, to check for a within Mediterranean sub-structure by genetic characterization of eggs and/or larvae, or juveniles, and to study variation in gene frequencies between different generations in the Mediterranean. As very little is known on the distribution of the Mediterranean stock in the feeding grounds of Atlantic, it is impossible to correlate a specific fishing pressure in this area with the observed response of the Mediterranean stock. The presently proposed project could give the information needed, by intensive sampling and assignments of specimens to differentiated groups on the basis of their combined genotypes on three to five microsatellite markers (already developed and proved efficient). The reference material on the basis of which fished animals all around the Atlantic will be classified, will be constituted by samples from the three main spawning grounds (Mediterranean, Gulf of Mexico and Tropical Atlantic).
This project can constitute the base for future monitoring of swordfish populations, by the creation of a databank with genetic and biological information on sampled fish in the region under study, as well as a tissue and/or DNA bank.
The swordfish, Xiphias gladius, is a commercially very important species with a worldwide distribution. Constrained by the need of spawning. in warm waters and high energy demanding metabolic rates, it undertakes long migrations between the warm spawning, grounds of tropical Atlantic and the Mediterranean and the rich feeding grounds at much colder areas of north and south Atlantic. Three main spawning grounds have been described for this species in the Atlanto-Mediterranean biogeographic province: tropical waters around the Gulf of Guinea, Gulf of Mexico and surrounding, waters, and Mediterranean.
Stuies of mtDNA polymorphism on sample from the spawning grounds have shown that three very differentiated stocks exists in the area under consideration. The discreteness of these stocks is most probably maintained by some kind of homing behaviour. Parallel studies of physical tagging suggested that an important mixing of stocks takes place in the feeding grounds of north Atlantic. Although mtDNA permits to discriminate well-defined samples of the spawning grounds in terms of allelic frequencies, this marker is not diagnostic at the individual level, and consequently does not allow unequivocally assign individuals to stocks. Genetic Stock Identification analysis on the other is- compromised by the fact that several days of sampling are needed in order to get samples of adequate size.
Funding SchemeCSC - Cost-sharing contracts
15080 La Coruna
61032 Fano (Ps)