Towards a better management of Nassarius mutabilis (Linnaeus, 1758): biometric and biological integrative study

Abstract

      Fishing of Nassarius mutabilis is by far the most important activity carried out by artisanal fisheries in the central and northern Adriatic Sea, accounting for more than 35% of total fishing effort and yielding from 2000 to 3000 t of landings each year. This gastropod is targeted from the beginning of autumn to the end of spring using basket traps. Despite its importance under a socioeconomic point of view, the scientific studies on the biology and ecology of this species are very scarce and the failure of the current management measures could also be attributed to the scarce knowledge on the biology of this species in the area. Taking this statement into consideration, the results of the present study contribute to fill a few existing gaps and may be useful to improve the management measures currently in force. Samples were collected from March 2006– March 2008 during monthly fishing surveys and carried out using basket traps in the central Adriatic Sea. A total of 383 males (size range 7–29 mm SH) and 504 females (size range 15-32 mm SH) were caught. Mean SH (± SD) of males was 15.84±3.57 mm and mean SH of females was 25.31±3.06 mm. A significant difference in the biometric characteristics between the two sexes was discovered. The condition index (CI) showed similar seasonal trends for both sexes, with a decrease from late winter to spring followed by an increase from summer to fall - early winter. The minimum values were recorded in summer and the maximum ones in winter, following an opposite trend in respect to water temperature. In the males this trend was observed starting from size classes larger than 11 mm SH, hence this size might be very close to the size at first sexual maturity. The size at first maturity hypothesized for females (16-20 mm SH) agrees with the minimum landing size (MLS) currently established for this species (20 mm SH). The absence of small females (SH < 15 mm) and large males (SH > 23 mm) leads to suppose a sex reversal from male to female at around 20 mm SH. On this basis, the management measure of MLS leads to the selective retention of females and, hence, to a sexual gap between the two sexes with possible consequences on the biology of the species and the resilience of the stock. The peaks of CI observed for males and females confirmed that the spawning season of this gastropod occurs in late winter – early spring, according to the abundant presence of egg capsules attached to submerged substrates in that period at sea. Consequently, from a management point of view, it would be advisable to shorten the fishing season that at present extends from fall to later spring.