Preprint volume - SIBM

Pre-print Volume –Oral presentations
Topic 4: THE ELASMOBRANCHS
C. PINTO, A. MANNINI, G. RELINI
Centro Biolmar
DIP.TE.RIS., Università degli Studi di Genova, V.le Benedetto XV, 3 – 16132 Genova, Italia.
pinto.cecilia85@gmail.com
REMARKS ON GALEUS MELASTOMUS
IN THE NORTHERN LIGURIAN SEA
OSSERVAZIONI SU GALEUS MELASTOMUS NEL NORD DEL MAR LIGURE
Abstract – Galeus melastomus (Chondrichthyes, Scyliorhinidae) is a deep sea elasmobranch that is fished
as a by-catch species of the bottom trawl fishery targeted to norway lobster and red shrimps. The mapping
of its distribution shows that adults and juveniles occupy bottoms at different depths. Analyses of
abundance doesn’t show significant trends in MEDITS while a decrease is detected in GRUND surveys.
Key-words: Galeus melastomus, Selachians, trawl surveys, GIS, Ligurian Sea.
Introduction – The study of by-catch species is considered significant in those kinds
of fisheries that capture a wide range of species, as bottom trawling fishery does.
Elasmobranchs are often part of by catch species and are reckoned as more vulnerable
than other species, due to their biological and reproductive characteristics (Walker,
1998). This group is characterised by a slow growth, late attainment of sexual maturity
and long life spans (Stevens et al., 2000), moreover they are often top predators in the
trophic web, that is why they are taken in account as good indicator, both, of the “topdown”
effect of the trophic chain, and of populations variations due to overfishing
(Carbonell et al., 2003; Serena et al., 2002; Baum et al., 2003). The main objective of
this work is the identification of areas of distribution of a deep sea elasmobranch,
Galeus melastomus, in the northern Ligurian Sea, dividing the population into adults
and juveniles to verify if the last ones have preferential habitats. Having a 15 years
time series of data derived from seasonal bottom trawl surveys, we got the chance to
observe the trend of abundance, biomass, mean weight and lengths distribution, to
determine the population status of adults and juveniles. This species has been studied
in the past, in the same area, concerning reproduction and feeding patterns (Orsi Relini
and Wurtz, 1975).
Materials and methods - The study area is the Northern Ligurian Sea, from the
French borderline to the North Gorgona Isle (belonging to Tuscany Archipelago). The
dataset used was made taking data from GRUND (autumn season) and MEDITS
(spring season) scientific trawling campaigns based on a random stratified sampling
scheme, using a dataset of 15 years, from 1994 to 2008. A total number of 248 and 213
hauls were carried out on bathyal grounds respectively in GRUND and MEDITS
surveys. To realize the cartography representing the species distribution, we used mean
points of trawling transects, representing them through ArcGIS 9.2. To have a more
robust representation of the distributions using abundances of species, we put together
abundances of all the 15 years for each fishing point and then realized an interpolation
through the IDW technique building different maps for adults and juveniles (when data
were available). Our interpolation has been limited to fishing points comprised
between 200 m and 80 0m, since this species has never been found in shallower waters.
We built trends to observe fluctuations of the population trying to assess the population
41 st S.I.B.M. CONGRESS Rapallo (GE), 7-11 June 2010
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