Garcia-Rodriguez et al.: Ontogenetic shifts and feeding strategies of Gadiformes in the Mediterranean Sea 61 
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Figure 4 
Level of trophic overlap and percentage of co-occurrence for pairs of 12 catego- 
ries of 7 gadiform species for which stomach contents from specimens caught 
in the western Mediterranean Sea during 2011-2017 were examined. The 12 
categories combine species and size class (small [S] and large [L]; length ranges 
for size classes are provided in Table 2). Diamonds and circles indicate inter- 
specific and intraspecific interactions, respectively. Pairs in the dark gray area 
had low niche overlap, and pairs in the light gray area had high niche overlap. 
@ 
GAD.-GAD, 
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80 90 
such as euphausiids and shrimp species, 
confirming the feeding habit observa- 
tions previously reported by Macpherson 
(1978a) for silvery pout in the western 
Mediterranean. Therefore, these results 
indicate that the silvery pout consumes 
a broad range of prey items, a diet char- 
acteristic of a generalist feeder. We also 
found ontogenetic variation for this spe- 
cies, with it becoming a more specialized 
predator as its trophic level increased. 
This result contrasts with the findings 
reported by Macpherson (1978a), who 
observed no clear link between diet and 
size for the silvery pout. 
The last of the 4 feeding guilds we 
identified is represented by both size 
classes of European hake and blue 
whiting, which occupy a position high 
in the food web. As has been observed 
for Mediterranean ling, both the Euro- 
pean hake and blue whiting seem to be 
primarily piscivorous, although their 
diets in addition include shrimp spe- 
cies (Pasiphaea spp.) and large plank- 
The species studied include the silvery pout (Gadiculus argenteus) (GAD), Med- ton, particularly euphausiids (e.g., 
iterranean bigeye rockling (Gaidropsarus biscayensis) (GAI), European hake 
(Merluccius merluccius) (MER), blue whiting (Micromesistius poutassou) (MIC), 
Mediterrenean ling (Molva macrophtalma) (MOL), greater forkbeard (Phycis 
blennoides) (PHY), and poor cod (Trisopterus minutus) (TRI). 
changes; we restricted our study to the spring, when the 
MEDITS is carried out. This high degree of predation on 
decapod crustaceans by greater forkbeard and poor cod is 
indicative of a narrow trophic niche breadth compared 
with that of the Mediterranean bigeye rockling, which 
was found to have more generalist feeding habits. More- 
over, and in agreement with results reported by Morte 
et al. (2002), an ontogenetic shift occurred in the diet of 
greater forkbeard in our study, but our findings differ 
from those of Morte et al. (2001) for poor cod in that no 
ontogenetic dietary shift was found. 
The second group consists of both size classes of Med- 
iterranean ling, and this species had the highest trophic 
levels among the 7 species studied. The diet of this spe- 
cies is primarily composed of benthopelagic and demersal 
fishes, such as the silvery pout and poor cod. This evidence 
indicates that the Mediterranean ling occupies a narrow 
trophic niche, a finding that is in agreement with the 
results previously reported by Macpherson (1981) for the 
same study area. 
A third trophically distinct group is composed of both size 
classes of silvery pout, which is the only species primarily 
exploiting resources at the base of the food web, playing 
an important role in the energy flow from the lowest lev- 
els to the top of the food web. The majority of the diet of 
the silvery pout consists of a variety of small crustaceans, 
northern krill). For European hake, 
our findings are consistent with those 
of other studies from the western Med- 
iterranean Sea (Bozzano et al., 1997; 
Cartes et al., 2009). However, slight 
differences were found when compar- 
ing the diets of fish sampled in waters 
of the North Atlantic Ocean and the eastern Mediterra- 
nean Sea, where crustaceans were relatively less impor- 
tant and mackerels play the role reserved for sardines in 
our study area (Guichet, 1995; Velasco and Olaso, 1998; 
Philips, 2012). Regarding blue whiting, our results agree 
with those reported by Olaso and Rodriguez-Marin‘*, Vel- 
asco et al.’, and Gutiérrez-Zabala et al.® for this species 
in the Cantabrian Sea off the northern coast of Spain, but 
they contrast with the findings of Preciado et al. (2002) 
for blue whiting in the south of Galicia in the northwest 
of Spain and of Torres et al. (2013) for this species in the 
Gulf of Cadiz off the southern coasts of Portugal and 
6 Olaso, I., and E. Rodriguez-Marin. 1995. Alimentacion de veinte 
especies de peces demersales pertenecientes a la division VIIIc 
del ICES. Otofo 1991. Inst. Esp. Oceanogr., Inf. Téc. 157, 56 p. 
[In Spanish.] [Available from Inst. Esp. Oceanogr., Calle Cora- 
zon Maria 8, 28002 Madrid, Spain.] 
’ Velasco, F.,, I. Olaso, and F. de la Gandara. 1996. Alimentacién de 
veintidés especies de peces demersales de la divisién VIIIc del 
ICES. Otonos de 1992 y 1993. Inst. Esp. Oceanogr., Inf. Téc. 164, 
62 p. [In Spanish.] [Available from Inst. Esp. Oceanogr., Calle 
Corazon Maria 8, 28002 Madrid, Spain.] 
8 Gutiérrez-Zabala, J. L., F. Velasco, and I. Olaso. 2001. Alimentacién 
de veintiuna especies de peces demersales de la division VIIIc del 
CIEM. Otonos de 1994 y 1995. Inst. Esp. Oceanogr., Datos Resim. 
16, 61 p. [In Spanish.] [Available from Inst. Esp. Oceanogr., Calle 
Corazon Maria 8, 28002 Madrid, Spain.] 
