62 
Fishery Bulletin 119(1) 
Spain. In these latter 2 papers, a higher dependency on 
crustaceans, particularly euphausiids, was reported. 
As far as differences in diet compositions between the 
European hake and blue whiting in our study are con- 
cerned, the European hake was found to prey mainly on 
pelagic fish species (i.e., sardines and European anchovy), 
as well as on benthopelagic (e.g., blue whiting) and demer- 
sal (e.g., poor cod) fish species, and the blue whiting was 
found to feed mainly on benthopelagic fish species (e.g., 
the horned lanternfish and jewel lanternfish). This ability 
of European hake to prey on different groups resulted in 
the European hake occupying the broadest trophic niche 
of all the species studied. Hence, in relation to ontogenetic 
shifts in diet, both species are characterized by a decreas- 
ing consumption of euphausiids and an increasing propor- 
tion of fish species with size. All of these results agree with 
those reported by Bozzano et al. (1997) for work in the 
Gulf of Lion and by Cartes et al. (2009) for a study in the 
Balearic Islands, a small archipelago off the northeastern 
coast of Spain. 
Results for examination of niche breadth indicate 
a pattern that relates size class to niche breadth. The 
small size classes had a broader trophic niche than that 
of the large size classes, for all species studied except the 
Mediterranean ling, with ontogenetic growth leading to 
greater specialization in these species. In contrast, the 
Mediterranean ling is a specialist piscivorous species, 
the larger size of the specimens of which, in comparison 
to the size of individuals of other species studied, allowed 
it to have access to a greater number of potential prey in its 
diet. Therefore, its trophic niche increased with the onto- 
genetic development of specimens. Our findings for the 
Mediterranean ling indicate a relationship between size 
class and trophic level, with the trophic level being lower 
in the small size classes and increasing after the ontoge- 
netic shift. This difference in the trophic level between size 
classes was greater for those species located higher in the 
trophic web (e.g., the European hake, blue whiting, and 
Mediterranean ling). 
We found that the studied species of Gadiformes exploit 
food resources at different trophic levels of the food web, 
indicating ontogenetic shifts in diet and resource parti- 
tioning. The silvery pout, for example, plays a role closer to 
that of a primary consumer, with the specimens that com- 
posed the small size class being those with the lowest tro- 
phic level. Fish species that play a similar role within the 
food web include species of Gobius, Mullus, Symphodus, 
and Diplodus, according to Karachle and Stergiou (2017). 
An intermediate trophic position is occupied by the poor 
cod, greater forkbeard, Mediterranean bigeye rockling, 
and the small size classes of blue whiting and European 
hake, as well as of the blackmouth catshark (Galeus melas- 
tomus) and species of Serranus, Pagellus, and Chelidon- 
ichthys (Karachle and Stergiou, 2017). Large specimens of 
European hake and blue whiting have a position in the 
upper part of the food web, along with species of Raja 
and Scorpaena (Karachle and Stergiou, 2017). Finally, 
the Mediterranean ling, primarily a piscivorous species, 
is in the highest trophic level, similar to other piscivorous 
species such as the common dentex (Dentex dentex), 
greater amberjack (Seriola dumerili), bluefin tuna (Thun- 
nus thynnus), and John dory (Zeus faber) (Karachle and 
Stergiou, 2017). 
If an ecosystem is structured in accordance with the 
hypothesis of niche complementarity (Ebeling and Hixon, 
1991), on the basis of the distribution and diet of the spe- 
cies involved, the structure should have consistently low 
niche overlap such that the coexistence of similar species 
occurs because of differences in resource use (Pianka, 
1973; Schoener, 1974). We investigated the dietary over- 
lap of coexisting species for which a partitioning of trophic 
resources was found among species and size classes. This 
finding indicates that the 7 species studied occupy differ- 
ent trophic positions, ranging from a generalized feeding 
behavior to specialized piscivorous habits. These results 
might complement those reported by Macpherson (1978a) 
and Morte et al. (2001, 2002), who investigated changes 
with season and depth, respectively, in the diets of blue 
whiting, silvery pout, poor cod, and greater forkbeard. 
They related availability of prey to diet depending on the 
season. Moreover, differences in the patterns of species 
distribution reduce niche overlap even further. 
Only 3 pairs of species had a co-occurrence greater 
than 50%, with the pairs of blue whiting and greater 
forkbeard and of European hake and poor cod having by 
far the highest values of spatial overlap but correspond- 
ingly low values of trophic overlap. This distributional 
and trophic strategy minimizes interspecific compe- 
tition. Cases in point are the Mediterranean ling and 
blue whiting, which may avoid intraspecific competition 
through this mechanism. Only the species pair of silvery 
pout and blue whiting had high values for both indices, 
indicating strong competition for resources between 
these 2 species. 
For 5 of the 7 species, excluding the Mediterranean 
bigeye rockling and poor cod, ontogenetic shifts in diet 
were found, shifts that allowed them to occupy several 
niches or subniches simultaneously (Deselle et al., 1978; 
Keast, 1978). These ontogenetic shifts were associated 
with an increase in inferred trophic levels for all the 
species, contributing to increasing segregation between 
size classes (Pauly et al., 2001). Although the size classes 
described in this study are related to shifts in trophic 
strategies, intraspecific trophic overlap was observed for 
those 5 species that had ontogenetic shifts in their diets, 
and the intraspecific overlap was generally higher than 
the interspecific overlap. This result is in line with that 
of Bergstad (1991), who found that the effects of com- 
petitive interactions are more likely to occur between 
size classes of the same species than between different 
species. 
Of those 5 species, the greater forkbeard, European 
hake, and silvery pout had high co-occurrence that 
involved strong competition for food resources, resulting in 
high intraspecific trophic overlap. An important aspect of 
this kind of competition is cannibalism, which was found 
in European hake, and this result is in agreement with 
findings reported by Macpherson (1979), Bozzano (1997), 
