Figure 2 shows the evolution of the digestive vacuity of A. anguilla from Mellah lagoon and Wadi El Kebir, according to sampling seasons and silvering stage. The seasonal maximum and minimum values of the coefficient of digestive vacuity are higher in Mellah lagoon (72.2% and 31.3%) than in Wadi El Kebir (56.1% and 25.8%), with respective annual averages of 53.1% and 42.9%. Statistical analysis showed the existence of seasonal changes in the digestive vacuity in both sites (χ2 ML = 71.014; χ2 WEK = 37.803; P = 0.000). The digestive vacuity in yellow and intermediary eels are less than 50% while those of silver eels is almost 90% in Mellah lagoon and null in Wadi El-Kebir. In these two sites, digestive vacuity is significantly different between the three silvering stages (χ2 ML = 33.309, P = 0.000; χ2 WEK = 7.907, P = 0.019).
Tables I and II summarize the composition of the diet of A. anguilla and the classification of taxa ingested in Mellah lagoon and Wadi El Kebir, respectively. Qualitative analysis of digestive contents shows poor diet diversity in both sites.
In Mellah lagoon, A. anguilla feeds mainly on arthropods, bony fish, molluscs and macrophytes (Tab. I). According to IRI, arthropods are preferred foods (IRI = 89.98%), particularly decapod macrurans (IRI = 58.78%). Bony fish are secondary favourite preys (IRI = 9.59%) while shellfish and macrophytes were ingested accidentally (IRI <0.5%). Preys identified belong to 11 species attached to the following phylums: Arthropoda (Palaemon adspersus, Penaeus kerathurus, Carcinus aestuarii, Microdeutopus gryllotalpa), Chordata (Aphanius fasciatus, Atherina lagunae, Gobius niger), Mollusca (Abra ovata, Cardium glaucum, Mytilus galloprovincialis) and Macrophyta (Ruppia sp.). We recognized a total of 462 preys for a total weight of 633 g, which represents approximately 1.6 preys for a weight of 2.16 g per digestive tract.
In Wadi El Kebir, molluscs and nematods und. are only accidental preys in the eels feeding (Tab. II). Decapods crustaceans are the basis of their diet (IRI = 91.37%) with a preference for small shrimp Atyaephyra desmarestii (IRI = 67.02%). Bony fish, especially small Mugilidae, are ingested in second position (IRI = 3.21%). The Atherinidae and Gobiidae fish are consumed in negligible quantities. The majority Osteichthyes could not be identified due to their advanced state of digestion. Qualitatively, 9 items are part of its basic food, including 3 crustaceans (P. adspersus, C. aestuarii, Sphaeroma sp.), 1 insect (Libellula quadrimaculata), 4 Osteichthyes (Mugil cephalus, Chelon saliens, C. ramada, C. labrosus) and a snail (Hydrobia sp.). Each sampled eel ingested approximately of 2.2 preys for a total weight of 4.4 g per digestive tract.
Figure 3 compares the IRI of the 4 main prey categories eaten by A. anguilla from the two study sites. Arthropoda are always main preys (IRIML = 89.98%; IRIWEK = 96.15%). Chordata (Osteichthyes fish), Mollusca and Macrophyta are rarely consumed in both sites (IRI <10%).