72 
Fishery Bulletin 110(1) 
10°0'0"W 0°0'0"E 8°54'00"W 
Figure 1 
Location of the sampling area (shaded) adjacent to Figueira da Foz harbor off the coast of 
Portugal, where specimens of Scyliorhinus canicula and Squalus blainville were collected 
by commercial fisheries for stomach content analysis in 2006. Gray isolines represent 
bathymetry (m). 
and to its congener S. acanthias (Avsar, 2001; Domi 
et al., 2005). 
Studies of stomach contents and feeding ecology have 
been widely used, mainly for determining the possible 
competition for food resources (Braccini et ah, 2005; 
Domi et al., 2005), for describing life history patterns 
(Demirhan and Seyhan, 2007), for determining mean 
trophic levels of fish species (e.g., Cortes, 1999), and 
for establishing ecosystem-based models (e.g., Chris- 
tensen and Pauly, 1992). In this context, changes in the 
functioning of marine ecosystems, as a consequence of 
overfishing of elasmobranchs, will, according to Stevens 
et al. (2000), involve changes in trophic relationships, 
particularly through selective removal of predators or 
prey groups, as well as through species replacement 
and enhancement of food supply by fishery discards. In 
this study, we tested the hypothesis that S. canicula 
and S. blainville have distinct diets, and that possible 
intraspecific differences in <S. canicula may be attrib- 
uted to sexual maturity and seasonal variability in 
prey resources and habitat use. Therefore, our main 
objectives were to assess differences in diet composition 
between S. canicula and S. blainville captured off the 
Portuguese Atlantic coast, as well as the consistency 
between seasons and maturity stages in the feeding 
ecology of S. canicula. 
Materials and methods 
Acquisition of data 
The shark specimens for this study were captured off 
Figueira da Foz, in the central Atlantic coast of Portu- 
gal (ICES Division IXa), in depths up to 70 m (40°10'N, 
9°9'W; Fig. 1). Both species were captured between 
January and December 2006 by local trawl and mul- 
tigear fishing fleets (mainly trammel and purse-seine 
nets), and were acquired at the Figueira da Foz Regional 
Office of Docapesca-Portos e Lotas, Sociedade Anonima, 
the company in charge of first fish sales along the Por- 
tuguese mainland. Sharks were frozen at sea, and the 
whole fish were transported in ice boxes to the labora- 
tory, to minimize further digestion of stomach contents. 
In all fish, sex was determined and total length (TL) was 
measured to the nearest mm. 
The diets of S. canicula and S. blainville were studied 
by examining stomach contents, which were removed 
and preserved in a 4% buffered formalin solution, for 
later identification to the lowest possible taxonomi- 
cal level. Stomach contents were identified by using a 
set of references for several taxonomic groups: Fauvel 
(1923; 1927), Crothers (1983), Whitehead et al. (1986), 
and Hayward and Ryland (1995). All prey items were 
counted and weighed (wet weight, 0.001 g precision). For 
statistical comparisons among groups, prey items were 
grouped into the following major taxonomical groups: 
Sipuncula, Annelida, Crustacea, Mollusca, Echinoder- 
mata, Chondrichthyes, Teleostei, Urochordata, as well 
as unidentified material. 
For S. canicula, an elasmobranch species with little 
sexual dimorphism in total length (Filiz and Taskavak, 
2006), individuals of both sexes with TL above 500 mm 
were considered adults, whereas for S. blainville, males 
above 510 mm and females above 600 mm were consid- 
ered sexually mature (according to Sion et ah, 2003). 
Analysis of diet composition 
The diet breadth of both species, determined to the 
lowest taxonomical level possible for each prey item, 
was compared with two indices: 1) the Shannon-Wiener 
index (H') as a measure of diversity, which increases 
with increasing species diversify: 
s 
7 = 1 
(1) 
