REPRODUCTIVE BIOLOGY OF POLYBIUS HENSLOWII LEACH, 1820 (DECAPODA, POLYBIIDAE): A DISCARD SPECIES FROM THE NORTHWESTERN PORTUGUESE BEAM TRAWL FISHERY

Polybius henslowii Leach, 1820, a Portunid crab distributed from the British Isles to Morocco and the Mediterranean Sea is caught in high densities by many types of fishing gear and frequently thrown back to the sea. Along the northern coast of Portugal, P. henslowii is an important species discarded in beam trawl fishery. There are no quantitative estimations of discards and very little is known about its biology, information that is needed to value this species. The present work was conducted from March 2007 to December 2010 in order to improve the current knowledge of the reproductive biology of P henslowii. By-catches were registered aboard commercial vessels that volunteered to participate in the study. In that period P. henslowii represented 57.3% of the total catch and was present in all hauls analysed. The target species, Palaemon serratus (Pennant, 1777), represented 1.2% of the total catch, showing the low selectivity of the fishing gear and the need for suppression of this gear to catch this shrimp species. Differences in size between sexes were found, with males being larger and heavier than females. Mature females occurred mainly in autumn and the carapace width at which 50% of the females reach maturity was estimated to be 37.8 mm, which could be set as the minimum landing size of this potential exploitable resource. A macroscopic ovarian maturity stages classification was proposed, allowing a rapid field assessment of female sexual maturity. This study fills a gap in the knowledge of a very important species discarded in beam trawl fishery that may be basis of further work.