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

Although Caribbean mixed-species\r\nherbivorous fish groups are an important component to the reef community by\r\nhelping to crop algae that often overgrow and kill corals, little is known of\r\nhow they organize their foraging groups. In spite of a highly flexible\r\nmembership, the basic structure of these groups consists of a “core species,”\r\nthat leads the group and often is either the striped parrotfish (Scarus iserti) or the ocean surgeon (Acanthurus tractus). These species lead their groups to\r\nopen areas where they feed largely on low profile turf algae. Other members\r\nprefer macro algae and are termed “associate species,” of which the two common\r\nspecies we studied were the stoplight parrotfish (Sparisoma viride) and the\r\nredband parrotfish (Sparisoma aurofrenatum). In spite of the large difference\r\nin group sizes between Jamaica and Grand Cayman Islands, the relationships\r\nbetween movement patterns and compositional changes were largely consistent.\r\nThere was no support for the hypothesis that these dramatic and continuous\r\ngroup changes were related to foraging success. Instead, we speculated that\r\nthese group changes perhaps were designed to maintain cohesion among a\r\nmembership that was spread over a wide area. We also examined if associates\r\nspecies may be more than just passive followers of core species but rather\r\ninstigated the attracting or the building of core groups. Both associate\r\nspecies do attract striped parrotfish in open areas and thus appear active in\r\ninitiating mixed-species groups. Finally, given that associate species seem to\r\nderive little foraging benefit from following core species, we tested the\r\nhypothesis that associate species joined core groups to gain protection against\r\npredators. Associate species do not selectively join the larger groups of\r\nstriped parrotfish but appear to join core species randomly and the groups\r\nthey joined resembled the wide assortment of core groups available in the\r\narea. Thus, while associates may be joining core groups for protection, this\r\nprotection was not based on sizes of core groups.