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DOLPHINS AND THE TUNA INDUSTRY
for example, a speedboat, moving by means of its noisy engine, approaches the corkline near such a herd, the entire dolphin herd has been observed to move slowly away (Norris et al., 1978). Speedboats also are able to move the herd, whereas sometimes swimmers and rubber rafts deployed inside the net cannot. The trapped dolphins tolerate swimmers moving near them, without evident flight responses. Such behavior would be unthinkable in an unrestrained herd. Encircled dolphin herds probably can be maneuvered to specific locations in the net circle to allow release efforts.
Norris et al. (1978) observed that aggression is a notable feature of active non-rafting dolphins. Aggression was also typical of patterns described in another study by Norris et al. (1985). In undisturbed spinner dolphin herds studied elsewhere in the Pacific Ocean, aggressive patterns were observed much less frequently (Norris et al., 1985).
Both Norris et al. (1985) and Pryor and Kang (1980) reported the prevalence of male coalitions in trapped dolphin herds, which has also been reported for undisturbed Hawaiian spinner herds. These coalitions consist of small, closely coordinated groups of dolphins that move as clearly distinguishable subgroups through the trapped herds. Their sex is obvious for spinner dolphins but much more cryptic for spotted dolphins (Perrin, 1972).
A protective role for these coalitions has been suggested because they frequently interposed themselves between observers and the rest of the herd, even in undisturbed herds (Norris et al., 1985). Such coalitions are active in tuna nets before backdown but have not been reported to remain during backdown. Similar male coalitions of bottlenose dolphins have been observed to cut out and sequester sexually ready female bottlenose dolphins (Connor, 1987). Although bottlenose dolphins are seldom associated with tuna or involved in the ETP tuna fishery, this observation strengthens the probability that the male coalitions have a protective role as part of their purpose.
These features indicate an organization of dolphin herds that persists early in a set and that might be utilized to assist release. If given an opportunity to escape while this protective structure is still in place, dolphins may be able to help themselves escape. In the crowding of the backdown channel, however, such social arrangements may be crowded out of existence.