An ecological comparison of Cephalopholis argus between native and introduced populations

Abstract

The blue-spotted grouper, Cephalopholis argus, was introduced to the Hawaiian islands from Moorea, French Polynesia in the late 1950s in an attempt to establish a grouper fishery in Hawaii. C. argus was introduced from a region of high grouper diversity into an environment with little competition from other large sedentary piscivorous species, and has flourished over the last 50 years. Total length, weight, and body condition were each significantly greater in introduced populations of C. argus in Hawaii than in native populations in Moorea. Both regions showed significant positive relationships between C. argus total length and: (1) prey total length, and (2) prey body depth. There were significant regional differences in diet; in Moorea C. argus consumed significantly deeper-bodied prey than their counterparts in Hawaii. These differences are consistent with competitive release experienced by C. argus in Hawaii. Active and passive tracking confirmed that C. argus are diurnal and highly site attached. In Moorea, home range size ranged from 230 to 1389 m2, mean = 700 m2 in, and 425 to 2300 m2, mean = 1236 m2in, in Hawaii. There was a significant positive correlation between the size of C. argus and home range size, and C. argus in Hawaii held significantly larger home ranges than C. argus in Moorea. C. argus in both regions spent the majority of their time in core use areas which corresponded with areas of high rugosity and reef complexity. Differences in home range area and habitat utilization between regions may be due to the lack of competing species, especially other groupers, in Hawaii. C. argus is known in Hawaii for causing ciguatera fish poisoning (CFP). CFP is caused by Gambierdiscus toxicus, which produces ciguatoxins (CTX) that are incorporated into the fish tissues. Standard length of C. argus and CTX score were positively correlated, however, small fish could be strongly positive, and cause CFP. At all sites, the majority of C. argus tested contained CTX levels at or above those considered unsafe for consumption. C. argus of a given length in Hawaii had lower CTX scores than fish of the same size in Moorea.