Catch composition and damage incurred to crabs discarded from the eastern Ligurian Sea “rapido” trawl fishery

Abstract

Catch composition and damage incurred to the populations of three crabs (Liocarcinus depurator, Medorippe lanata and Goneplax rhomboides), discarded by the “rapido” trawl fishery of Viareggio (eastern Ligurian Sea), was investigated. Data were collected on a monthly basis (January–December 2001) through embarks on board of a commercial fishing vessel, so as to reflect commercial fishing practice. The three species represented from 7 to 31% of the discarded biomass, varying with season. Density and biomass indices were characterised by a clear seasonality, each species showing a peak in a different period of the year. A six level scale, based on macroscopic external injuries was used to assess the damage sustained by each species after fishing and sorting operations. For the three species, very similar percentages (56–58%) of individuals were macroscopically intact, while inspection of damaged individuals revealed some differences in the distribution of injuries. G. rhomboides showed the highest proportion of serious damage, followed by L. depurator and M. lanata. The severity and frequency of damage was mainly correlated with interspecific and intraspecific (sexual dimorphism) morphological and behavioural characteristics. For example, specimens with longer and thin appendages, like males of G. rhomboides, seemed to be particularly vulnerable to the fishing and sorting operations; this could also explain the positive correlation between damage level and size found in this species. In M. lanata, small individuals proved to be more vulnerable, while for L. depurator no significant correlation was found between damage and size. Although these estimations did not take other injury typologies into account (e.g. internal and physiological damage), these results can be seen as a first indirect estimation of discard mortality for these species and can represent an important term of reference both for similar studies performed in other areas and for the monitoring of this fishery.