Is the African armoured searobin (Peristedion cataphractum, Peristediidae) stock off the south Sicily and North African coasts a "unit stock"?

Distribution, abundance, biological traits and growth of Peristedion cataphractum L., 1758 coming from two areas of the Strait of Sicily were analysed. The two investigated areas correspond to the north (A-area) and south (B-area) of the midline located between South Sicily and North Africa. Both published and new elaborated parameters were estimated from data gathered in autumnal experimental bottom trawl surveys; the surveys in the B-area, in particular, were carried out between 1997 and 2004. The abundance resulted to be ten fold higher in the B- than in the A-area. The biological parameters ranges resulted in 0.38-0.48 and 0.42-0.50 (sex ratio), 200-230 and 200-225 mm (median total length of mature stage), 200-232 and 200-222 mm (mean total length of mature stage) and 168-221 and 177-199 mm (size at 50% of sexual maturity) for stocks from A and B areas, respectively. The length frequency distributions were always significantly different. The von Bertalanffy growth function (derived after sex combined length frequency analysis) revealed a high closeness: 379-390 mm and 0.38-0.36 for L(infinity) and K(y)(-1), respectively. These results support the hypothesis of a common genetic-biological features of the population on one side, but also suggest maintaining them operationally distinct (i.e., as "unit stocks" sensu Gulland) since the likely low rate of spatial interchange (as a consequence of the low mobility of this species) and the reported differential in fishing exploitation.