DEFENDED TERRITORIES OF AN AGGRESSIVE DAMSELFISH, POMACENTRUS TRILINEATUS, IN THE NORTHERN RED SEA, HURGHADA, EGYPT

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The territorial behavior of the pomacentrid damselfish Pomacentrus trilineatus was studied on three reefs in the northern Red Sea, Hurghada, Egypt in order to investigate difference between defended and undefended areas. Seven families of reef fishes (Pomacentridae, Chaetodontidae, Acanthuridae, Labridae, Scaridae, Holocentridae and Siganidae) were dominant among the 21 recorded families at study sites. The territories of P. trilineatus were very small in area, ranging from 0.7 to 0.9 m2. Remarkable variation was observed for substrate composition between both inside (defended) and outside (undefended) P. trilineatus territories. Eight genera of hard corals were recorded inside P. trilineatus territory of them four genera were dominant and widespread outside the territory. Two soft coral genera were recorded inside and outside territory of P. trilineatus. Fleshy algae were dominant inside P. trilineatus territory. Three groups of associated fauna (Molluscs, Echinoderms and sponges) were recorded inside territory of P. trilineatus while two groups (Molluscs and Echinoderms) were recorded outside territory. Dead components of substrate showed more dominance outside the territory than inside. The percentage cover of algae had the highest values inside territory especially fleshy algae while that of hard corals had the highest values outside territory. Nine species belonging to four families were resident inside P. trilineatus territory. Of the 60 species recorded, 42 species were intruders. Siganids, the fish P. trilineatus(which is not the territory holder), Scarus ghobban and the juvenile fish Chlorurus sordidus had the lowest trespass times recorded. P. trilineatus showed high number of agonistic acts against conspecifics, Siganus luridus, S. rivulatus and Rhinicanthus assai with strong level intensity (A). P. trilineatus did not show any agonistic acts against resident species. The highest values of agonism elicited were recorded against nonspecific, S. luridus, S. rivulatus, R. assai, C. sordidus (juv.) and S. ghobban. The fish existed in high numbers at study sites and found to cultivate its territory with high density of macro-algae and the algae were observed growing at faster rates inside the territories of the fish than outside. The fish found to defend aggressively herbivorous fishes and excluded them from its territory especially browser fishes. Such behavior is considered to have detrimental effects on benthic structure through allowing fleshy macro-algae to grow faster and out-compete coral recruitments for space.