Preprint volume - SIBM

Pre-print Volume –Posters
NECTON AND FISHERY COMMITEE
M. STAGIONI 1,2
1 Department of Evolutionary Experimental Biology, University of Bologna, Italy.
2 Laboratory of Marine Biology and Fisheries of Fano, University of Bologna, Italy.
marco.stagioni3@unibo.it
CONSIDERATIONS ON MINIMUM COMMERCIAL SIZE
OF CHAMELEA GALLINA (L.)
CONSIDERAZIONI SULLA TAGLIA MINIMA COMMERCIALE
DI CHAMELEA GALLINA (L.)
Abstract – Sorting sieve selectivity experiments were carried out on a hydraulic dredge fishery exploiting
the clam, Chamelea gallina (Mollusca: Veneroida, Linneus, 1758). Six sieves (from 20 to 23 mm) were
tested. The material collected shows that all sieves capture undersized clams. As it is impossible, with
current sorting techniques, to avoid presence of undersized clams, the alternative approach “number per
kilogram” has been proposed. The result of the study shows in 220 the allowed number of clams per
kilogram.
Key-words: Chamelea gallina, clam fisheries, size-limit regulations, imaging techniques.
Introduction – The clams fishermen complain about the practical impossibility to
observe the minimum landing size in force for Chamelea gallina, 25 mm of maximum
diameter (Annex IV art. 15 EC Reg. 1967/06). Therefore the Laboratory of Marine
Biology and Fisheries of Fano carried out experimental surveys to evaluate selectivity
of sieves with different diameter used by the local hydraulic dredge fishery.
Materials and methods – C. gallina samples were collected on February 4 th and 9 th
2010 onboard of a commercial hydraulic dredger, in the maritime compartment of
Pesaro, Adriatic sea. During the experimental survey three vibrating and sorting sieves
per day were tested, on three different hauls: 20.5, 21.5, 23 mm the first day; 20, 21, 22
mm the second day. Subsamples of clams collected from each sieve (about 10 kg) were
taken to laboratory and immediately weighed (1 g), sorted to remove bycatch, broken
shells and onshell epifauna, photographed by backlight illumination, counted and
measured with magnification of 10 pixel/mm (0.1 mm) by proprietary macro image
analysis program developed on ImageJ (Rasband 2010). A further set of tests
considered the weight loss of 5 kg subsample for three sieves (20, 21, 22 mm), since
arriving in the laboratory after fishing for three days, storing at 5 °C and re-weighting
every day.
Results – Results are summarized in Tab. 1. The selection curves (not showed) show
that there is always a capture range of clams less than 25 mm, albeit with low
percentage. The weight loss after three days storage reaches a maximum of 2% and
ANOVA analysis on transformed data (arcsine of root percentage) do not shown
significant differences between sieves (F2,6=0.22 p=0.8088), but shows significant
differences between hauls (F2,6=8.9913, p=0.01566).
Conclusions – In all tests (at the ideal slope of the vibrating sieve and with calm sea),
the regular dredges carry on a large number of undersized clams. Using the dredge
prescribed by the current law, high rates of undersized clams are caught, according to
41 st S.I.B.M. CONGRESS Rapallo (GE), 7-11 June 2010
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