Territorial behaviour is a conspicuous determinant of social organisation in many reef fishes including parrotfishes. Most parrotfish studies in the Caribbean have focused on the species Scarus iserti and Sparisoma viride over limited ranges of reef habitat. By contrast, our study has included all common parrotfishes in Belize (Sc. iserti, Sp. viride, Sparisoma aurofrenatum, Sparisoma chrysopterum, and Sparisoma rubripinne) at three sites with different physical and biotic conditions and a wide range of fish densities. Density in Sc. iserti was strongly positively correlated to substrate rugosity. In contrast, densities of Sp. chrysopterum and Sp. rubripinne were unrelated to rugosity and territories were large. Territory size was smallest in Sc. iserti (mean areas at the three sites ranged from 41 to 120m2) and largest in Sp. rubripinne (ranged from 168 to 1400m2). All species except Sp. chrysopterum exhibited significantly larger territories where density was low as suggested by territory theory. Territory size decreased rapidly with increasing density of competitors. Patterns of harem size differed between two groups of parrotfishes. (1) Sc. iserti, Sp. viride, and Sp. aurofrenatum exhibited an expected positive correlation with territory size. (2) Harem size was smaller in Sp. rubripinne and Sp. chrysopterum, and showed no spatial pattern. Aggression in Sp. viride and Sc. iserti was directed entirely towards intraspecifics and positively density dependent. Interspecific interactions accounted for only 10% of observations and were recorded exclusively whilst following Sp. chrysopterum, Sp. rubripinne, and, to a lesser extent, Sp. aurofrenatum. A meta analysis of species interactions suggested that intraspecific interactions were most common where overall fish density was greatest and conversely, interspecific interactions occurred more often at lower densities. This may suggest that the economic defensibility of territories is largely confined to intraspecifics where density is greatest. Most (62%) of the interspecific interactions comprised Sp. rubripinne chasing the smaller species Sp. chrysopterum, suggesting that territorial behaviour has at least some non-reproductive origin and may therefore be associated with either food or shelter. It is feasible that at such low population densities, it is economically feasible for Sp. rubripinne to defend against intraspecifics and Sp. chrysopterum. Social behaviour in Sp. chrysopterum and Sp. rubripinne, and to a lesser extent Sp. aurofrenatum, differs to that of Sc. iserti and Sp. viride which conform to existing theories of social behaviour in reef fish.
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