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Composition Changes and Movements in Mixed-Species Groups of Algae Grazing Fish in Jamaica and Grand Cayman Island. Part II

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Document pages: 14 pages

Abstract: Although Caribbean mixed-speciesherbivorous fish groups are an important component to the reef community byhelping to crop algae that often overgrow and kill corals, little is known ofhow they organize their foraging groups. In spite of a highly flexiblemembership, the basic structure of these groups consists of a “core species,”that leads the group and often is either the striped parrotfish (Scarus iserti) or the ocean surgeon (Acanthurus tractus). These species lead their groups toopen areas where they feed largely on low profile turf algae. Other membersprefer macro algae and are termed “associate species,” of which the two commonspecies we studied were the stoplight parrotfish (Sparisoma viride) and theredband parrotfish (Sparisoma aurofrenatum). In spite of the large differencein group sizes between Jamaica and Grand Cayman Islands, the relationshipsbetween movement patterns and compositional changes were largely consistent.There was no support for the hypothesis that these dramatic and continuousgroup changes were related to foraging success. Instead, we speculated thatthese group changes perhaps were designed to maintain cohesion among amembership that was spread over a wide area. We also examined if associatesspecies may be more than just passive followers of core species but ratherinstigated the attracting or the building of core groups. Both associatespecies do attract striped parrotfish in open areas and thus appear active ininitiating mixed-species groups. Finally, given that associate species seem toderive little foraging benefit from following core species, we tested thehypothesis that associate species joined core groups to gain protection againstpredators. Associate species do not selectively join the larger groups ofstriped parrotfish but appear to join core species randomly and the groupsthey joined resembled the wide assortment of core groups available in thearea. Thus, while associates may be joining core groups for protection, thisprotection was not based on sizes of core groups.

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