Preprint volume - SIBM

Pre-print Volume –Oral presentations
Topic 4: THE ELASMOBRANCHS
kilograms live weight) and divided into three categories: “Asià” (all sharks but
Scyliorhinus spp.); “Gattucci” (Scyliorhinus spp.); “Razze” (all skates). Landings of
1945-2008 were compared with annual mean sea water temperature at 2 m depth in
Trieste; annual mean Po river discharge (m 3 s -1 ) at Pontelagoscuro (Ferrara); the annual
mean North Atlantic Oscillation index (NAO), positively correlated to surface
temperature in the NA; the December to February Western Mediterranean Oscillation
index (WeMO), related to temperature and salinity in the NA.Time series of landings
and environmental variables were tested for temporal trends with the non-parametric
Mann-Kendall test (Z), and slopes computed with Sen’s robust estimator. Inter-annual
landing periodicities were identified through spectral analysis based on Fast Fourier
transform. Correlations between environmental variables (lagged and non-lagged) and
the three landing categories were tested with Spearman’s rank correlation coefficient rs
after applying Bonferroni correction. At the fish market a total of 79 samplings were
performed once or twice a week from October 2006 to July 2007. Catch composition
(species) was estimated; sex was assessed (based on the presence of claspers); total
length was measured to the nearest mm; the percentage of sexually mature males was
estimated (those presenting claspers rigid, due to calcification, and at least 1.5 cm
longer than the pelvic fins). This method, the only feasible given that fish market
elasmobranchs are eviscerated, can overestimate the number of mature males (Capapé
et al., 2008; Saïdi et al., 2008). To simulate present trends of populations, an
Individual-Based Model (IBM) was set up to simulate present and future trends of M.
mustelus and S. canicula, using a stochastic approach (Rose et al. 1999). For each
female specimen, at each time step (month), the following processes have been
simulated: growth, with the VBGF (parameters from Froese and Pauly, 2010);
reproduction (data from Capape’ et al., 2008; Saidi et al., 2008); natural mortality (data
from Frisk et al., 2001); fishing mortality (data inferred by best fitting between
observed trends in the last 10 years, when Mustelus species are categorized in a
separate group than other sharks, and simulated population trends). Population
dynamics has been inferred by simulating every single specimen and relative offspring
for 50 years. Uncertainty has been estimated averaging 20 runs of the model. Two
management scenarios have been considered: a “minimum catch size” scenario, where
it was assumed that any individual caught before maturity is discarded and survives;
and a day-off scenario, where the fishing effort was recalculated assuming that
fishermen work just for 4 days at week instead of 5.
Results - Elasmobranch landing declined significantly from 1945 to 2008 (Z = -6.90, p
< 0.0001, Sen’s slope = -1 755.9 kg y -1 , Fig. 1), with a reduction of 85.6% in the last
five years compared to the ‘40s. Skates showed the most dramatic reduction, 97.9% (Z
= -8.44, p < 0.0001, Sen’s slope = -1 584.6 kg y -1 ), while Scyliorhinus spp. showed a
reduction of 93.8% compared to the ’40s (Z = -6.50, p < 0.0001, Sen’s slope = -268.4
kg y -1 ). The category “Asià”, including all the other sharks, showed a weak increase (Z
= 3.68, p < 0.001, Sen’s slope = 303.2 kg y -1 ), but a strong decline after 1993 (Z = -
3.51, p < 0.001, Sen’s slope = -1 947.9 kg y -1 ). All the three categories exhibited
periodic peaks in landings during 1945-2008. Spectral analyses highlighted a principal
period of 7 years for skates, 13.5 for Scyliorhinus spp. and 9 for “Asià”. For skates and
Scyliorhinus spp., oscillations disappeared when landing decreased consistently.
Landing peaks of skates correlated with WeMO (rs = 0.38, p = 0.0019) and NAO (rs =
41 st S.I.B.M. CONGRESS Rapallo (GE), 7-11 June 2010
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