Selectivity parameters and size at first maturity in deep-water shrimps, Aristaeomorpha foliacea (Risso, 1827) and Aristeus antennatus (Risso, 1816), from the North-Western Ionian Sea (Mediterranean Sea)

Selectivity experiments were carried out during trawling targeting deep-water shrimps Aristaeomorpha foliacea (Risso, 1827) and Aristeus antennatus (Risso, 1816) (Crustacea, Decapoda, Aristeidae) in the North-Western Ionian Sea (Eastern-Central Mediterranean). Different criteria were employed to analyse maturity, however the proportion at 50% of retained, mated and mature specimens was always used to indicate the size, expressed as Carapace Length (CL, mm), at first capture (CLc), mating (CLsp) and at first maturity (CLm), respectively. In order to estimate the size at 50% maturity (CLm) for females of both species three criteria were adopted. In particular, such a size was computed for the mature females not considering the presence of spermatophores, for the mature females with spermatophores and for the mature females intersected by the decreasing proportion with size of females without spermatophores. Three diamond stretched mesh codends of 40, 50 and 60 mm were tested using a cover of 20 mm. The 40 mm stretched mesh size (European Union legal size in the Mediterranean) was not selective for the sampled population of each species. The size at first capture (CLc), calculated in both species for sex combined in term of carapace length, increased significantly with mesh size. Even for the mesh size of 60 mm, the size at first capture was still smaller than the sizes at 50% maturity, whatever the criterion adopted. Since the differences between the size at first maturity and the sizes at first capture are greater in A. foliacea than A. antennatus, the former species appears in this respect to be more vulnerable to trawling than the latter.