Dolphins and the Tuna Industry (1992) / Chapter Skim
Currently Skimming:
5 The Behavior of Dolphins and Tuna in the ETP
5 The Behavior of Dolphins and Tuna in the ETP
Pages 42-51
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From page 42... ...
In addition, sporadic direct studies of dolphin and tuna behavior have been done during the course of purse-seining operations. The major impediment to doing comprehensive behavioral studies has been that such studies require work aboard seiners.
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From page 43... ...
TABLE 5-1 Species of Dolphin Found in Association with Tuna as Determined by Frequency of Sets on Different Speciesa Dolphin Percent Sets in 1988b Northern Offshore Spotted Eastern Spinner Northern Whitebelly Spinner Central Common Southern Offshore Spotted Northern Common Southern Common Southern Whitebelly Spinner aData from IAITC. 81.6 23.7 8.6 4.7 2.1 2.0 1.2 0.8 bThe sum of the percentages exceeds 100 because many sets are made on mixtures of species or stocks; thus many are counted twice (e.g., almost all sets on eastern spinner dolphins also include spotted dolphins)
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From page 44... ...
. ~.._ —,~ _...._..._ ~ , 1 1 _ 120° 1 10° 100° 90° 80° W FIGURE 5-1 Known distribution of the spotted dolphin in the eastern Pacific, showing the 1979 species range and stock boundaries presented at the Status of Porpoise Stocks (SOPS)
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From page 45... ...
If, for example, the dolphins are central in the association and the tuna are followers, an operation that attempts to release dolphins through the net perimeter is likely to result in tuna following. Some modestly convincing evidence indicates that the dolphins, especially the spotted dolphin, are the central species in the association and not the tuna.
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From page 46... ...
· No evidence suggests that dolphins protect tuna, rather the association appears passive on the part of the dolphin. It seems reasonably clear that even though fishermen repeatedly remove tuna from dolphin herds, the two species continue to reaggregate daily and thus to sustain the fishery.
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From page 47... ...
, although spotted dolphins may also feed at night to some extent (Leatherwood and Ljungblad, 1979; Scott and Wussow, 1983~. The spinner dolphins that often are seined with the spotted dolphin and tuna are a diving species that feeds primarily at night on small mesopelagic prey that typically concentrate in daytime below the thermocline.
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From page 48... ...
A possible exception is the so-called "untouchables," an apparently regional collection of dolphin herds that may have learned to escape seines before pursing takes place. Some dolphin species, such as the bottlenose dolphin, routinely dive to safety through still open seines.
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From page 49... ...
Once a dolphin herd has been located, the seiner launches its speedboats, which race into position outside the fleeing animals and then move ahead and turn them while the seiner follows. The entire entourage changes the dolphin's swimming path into an inward bending arc.
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From page 50... ...
has dubbed a "teacup formation," in which passive dolphins are to some extent left above and within the excursions of these more active animals, and in a fluid way the active animals roughly define the boundaries of a "cup." Entire herds of dolphins, including rafters and more active animals, can be moved within the net circle by bringing strongly aversive stimuli near them.
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From page 51... ...
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.
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