In mainland Portugal, rays constitute an important
by-catch of multi-gear shelf fisheries. However and despite their economical
importance, landing information discriminated by species is scarce. The
relative stability in Portuguese annual landings of rays and skates can be
misinterpreted as no evidence for species declines. The present concern on
the overexploitation of individual species arises from both theoretical
considerations about biological and ecological traits and for historical
reasons.
This work constitutes an approach for a retrospective analysis of ray
landings in mainland Portugal. Data from research surveys carried along the
Portuguese coast was used to infer possible changes in species composition
on Portuguese landings. Clusters were determined based on data collected
during surveys from species composition for three time-periods, 1989-1991,
1995-1997 and 2001-2003. The relative importance of the associate species
within clusters was the input data to define discriminating rules
posteriorly used to assign clusters to landings from each commercial vessel.
The results show an increase of species relative importance and changes on
species abundance: Raja brachyura and Raja clavata decreased, whilst Leucoraja naevus, increased. Furthermore, it was
observed a decrease on the mean weight of the two first species and a
relative stability in the last one. Such results reflect the importance of species
life-history characteristics in particular, on the response to fishing impact.