Spatial segregation of two species of Mullidae (Mullus surmuletus and M-barbatus) in relation to habitat

The distributions of 2 congenerics, sympatric species of Mullidae (Mullus s urmuletus and M. barbatus) were studied by means of abundance and length fr equencies, obtained in experimental trawl surveys carried out in the spring time (1994 to 1998) throughout the Spanish Mediterranean area. From the dat a obtained we compared the spatial segregation between these species in rel ation to the bottom type and to the depth of their habitat. Although M. bar batus was the most abundant species throughout the sampling area (except at the shallowest rough-bottom sites), spatial segregation was observed betwe en the 2 species, with the ratio between the species varying according to t he bottom type on which they lived. M, barbatus showed maximum abundance an d frequency on muddy bottoms in waters between 50 and 200 m deep. For M. su rmuletus, maximum density and frequency were at a shallower depth (between 10 and 50 m) and largely on rough bottoms. At overlap sites, the relative a bundance of M, barbatus increased on muddy bottoms and decreased on rocky a nd sandy bottoms. The size frequencies encountered for both species were ma de up by preadults and adults. Significant size differences between species existed in all the bathymetric strata studied. In all cases, except the mo re coastal muddy bottom stratum, M, surmuletus had a range of sizes and a m ean size greater than M, barbatus. The structural heterogeneity of the Span ish continental shelf gave rise to a distribution characterised by patches. M. barbatus had a clear preference for the areas where the shelf became wi der, and M. surmuletus was more abundant and frequent in narrow shelf areas .