Martinho et al.: Comparative feeding ecology of two elasmobranch species, Squalus blamville and Scyliorhinus canicula 
75 
Number of stomachs with contents 
Figure 3 
Randomized cumulative trophic diversity curves (Shannon-Wiener index) for Scyliorhi- 
nus canicula and Squalus blainville. The number of stomachs refers to those in which 
at least one prey item was identified, with the exception of unidentified materials. 
er evenness (J’= 0.73), compared to that of S. canicula 
(J'= 0.63). Schooner’s diet overlap index indicated high 
overlap ( / s . =0.81) between S. canicula and S. blainville. 
The relative importance of each prey group was 
evaluated by using the dietary coefficient (QI) and 
the index of relative importance (IRI) (Table 3). For 
S. canicula, Crustacea were considered dominant and 
preferential by both QI and IRI, respectively, and Te- 
leostei were only considered preferential according to 
the QI. All other prey items were considered accessory 
by both classification systems, with the exception of 
Annelida and unidentified materials, which were con- 
sidered secondary by the QI. For S. blainville, Crus- 
tacea and Teleostei were also classified as dominant 
prey items according to the QI, whereas Mollusca and 
unidentified materials were considered secondary prey 
items. According to the IRI, Crustacea were classified 
as preferential items, Teleostei as secondary, and all 
other prey groups as accessory prey items (Table 3). 
The %IRI values were in accordance with these pat- 
terns (Table 3). 
Seasonal and maturity-stage-related variability 
in the diet of S. canicula 
No differences in vacuity levels of S. canicula were 
observed between adults and juveniles for either sex 
(F=2.622; P>0.05). Nevertheless, maximum values, 
which indicate a lower feeding activity, reached -25% 
in adult females and -20% in adult males — all in the 
autumn (Fig. 5). For juveniles, maximum vacuity was 
observed in the autumn for females (-17%), and in the 
summer for males (-16%) (Fig. 5). The higher vacuity 
values (i.e., lower feeding activity) were observed in 
adult females. 
In S. canicula males, Crustacea were the dominant 
prey items in both adults and juveniles, with %IRI val- 
ues between 44.3% and 69.7% in adults, and between 
56.4% and 92.4% in juveniles (Fig. 6). Teleostei were, 
in general, the second most important prey group for 
both adults and juveniles according to the %IRI, but 
higher in the spring and summer. In the autumn, both 
adults and juveniles relied more on crustaceans, with 
also a marked increase in the presence of sipunculid 
worms. This finding, however, was more evident in ju- 
veniles. During the winter, the importance of Annelida 
(namely Polychaeta) for the diet of adults was noticeable 
(%IRI=17.4), but the reliance on this group was reduced 
in the subsequent seasons. 
For S. canicula females, the seasonal dietary patterns 
were similar to those of males (Fig. 7). Likewise, crus- 
taceans composed the majority of the diet in juveniles 
(%IRI between 43.6% and 91.5%) and adults (%IRI 
between 32.3% and 57.4%) across seasons. For adult 
females, Teleostei were also an important part of the 
diet in spring and summer (%IRI = 39.0% and 64.8%, 
respectively). In juveniles, the increase in importance 
of Teleostei during the same period was even more 
marked (%JRI=5.2% and 42.7%, respectively) (Fig. 7). 
As with adult males, S. canicula adult females had a 
more equally distributed diet between the main prey 
items in the winter, in contrast with the rest of the 
year. However, no significant differences were found 
between maturity stages for each sex and season after 
taking into account the diet composition evaluated by 
the %IRI (ANOVA; P>0.05). 
Diet overlap was measured with Schoener’s index (I s ). 
With a comparison of the different maturity stages, 
sexes, and seasons, a significant overlap was observed 
for all possible combinations (Table 4). The exception. 
