Diet of Chelidonichthys obscurus and Chelidonichthys lastoviza (Pisces: Triglidae) from the Gulf of Gabes (Tunisia)

Abstract

The most common gurnard species in the Gulf of Gabes (Tunisia) are Chelidonichthys lastoviza and C. obscurus. These two species were collected by trawl nets between February 2000 and July 2002 at depths ranging from 80 to 150 meters. In the laboratory the stomach contents were examined for diet assessment and to determine diet variation in relation to fish size, season and sex. The importance of different prey items was analysed utilising several feeding indexes, while diet overlap was tested using Schoener's index and the null model test. The two species showed similar diets, with the same qualitative and quantitative composition. Basic food consisted of crustaceans, mostly mysids and decapods. For Chelidonichthys lastoviza:Gastrosaccus sp. (F = 48.57%), Anchialina agilis (F = 30.57%) and Sicyonia carinata (F = 21.71%); for Chelidonichthys obscurus:Gastrosaccus sp. (F = 80.95%), A. agilis (F = 55.41%) and Pontocaris lacazei (F = 15.15%). A change in food composition based on fish size was observed in both species: mysids more frequently in the diet of small specimens, and decapods in the largest fishes. Seasonal variation in their diet was also recorded. The two fishes showed a significantly higher number of empty stomachs in late autumn and winter. Regardless of species and size class, a biologically significant diet overlap was observed between C. lastoviza and C. obscurus. This result may indicate a lack of competition among triglid species size-class combinations or strong competition that has not yet led to resource portioning.