Siganus rivulatus, a successful herbivorous immigrant fish from the Red Sea, consumes along the Israeli east Mediterranean coast a wide variety of algae. Only four of them composed about a half of the bulk of its food, and only twelve, about 80% of it. Seasonal changes in quantitative representation of different algae in the food followed several patterns. Most algae were taken throughout the year. Proportions in the food of some algae fluctuated only slightly during the year, proportions of others were very great during short periods and much smaller during the rest of the year. Polysiphonia is an example for the former, Ulva and Spatoglossum, for the latter. Among the latter, Ulva dominates the food contents during the end of winter and spring; Spatoglossum, during the late summer. As shown previously, S. rivulatus prefers certain algae as food. Nevertheless, food composition in fish captured at inshore sites differed from that of fish captured offshore. This reflects, to a certain extent, the difference in vegetation between these sites, and the fact that the fish tend to graze in a rather limited area at a time. Small and large fish take different algal food species. Unexpectedly, small fish clearly preferred some large and robust algae, consuming them in much greater proportions than bigger fish. Also unexpectedly, medium size fish showed preferences and avoidances of their own in regard to the consumption of certain algae, and not merely occupied an intermediate position between small and large fish.
