Dietary choices of siganid fish at Shiqmona reef, Israel

The diet of two Erythrean algivorous siganid fish, Siganus rivulatus and S. luridus, was studied off the Shiqmona vermetid reef, south of Haifa Bay, Israel, in conjunction with the spatial and seasonal variability in composition of the local algal community. The algae identified from the gut contents of reef-dwelling siganid populations indicate that both species feed on the vermetid reef platform, especially in spring. In June, green algae made up 66% of the gut contents of S. rivulatus, whereas brown algae made up 69% of the gut contents of S. luridus. In October, brown algae constituted 90% and 95% of the diet of S. rivulatus and S. luridus, respectively. This is the first record of S. rivulatus feeding on and selecting large amounts of this brown, coarse, leaf-like algae along the Mediterranean coast of Israel. During spring the food niche overlap (Renkonen's measure) was ca. 36%, whereas during autumn it was ca. 80%. The results suggest that in reef-dwelling siganids, weakening of dietary niche-partitioning occurs when the preferred algae are scarce.