Figure l . — Photograph taken during backdown of a tuna purse seiner showing three passive dolphins in the foreground. Note that the 

 middle animal is resting on its dorsal fin. The four animals that are hanging flukes downward in the water are displaying the sinking 

 behavior associated with rafting. (Photo courtesy of the Cooperative Dedicated Vessel Research Program, 1978.) 



were making no apparent attempt to surface, were 

 at depths that varied from 2 or 3 m to as much as 12 

 to 15 m. At first glance, the animals appeared to be 

 struggling feebly, perhaps against the current 

 being produced as the seine was pulled through 

 the water during backdown. Their movements 

 were weak and lacked the grace that long observa- 

 tion leads one to expect in dolphins. The majority 

 of animals lying on the net in this manner were 

 oriented with their heads toward the release area 

 of the net (i.e., in the direction of the current), 

 whereas the rafting and active animals were nor- 

 mally oriented away from the release area (i.e., 

 heading into the current). After about 3-5 min, the 

 passive dolphins began to rise singly or in two's 

 and three's to breathe and were either backed out 

 of the net or hand-released if the backdown proce- 

 dure had already been terminated. No animals 

 were seen returning to rest on the bottom of the 

 net after surfacing. 



Prior to promulgation and adoption of a federal 

 regulation requiring use of a rescuer in a raft 

 using a mask and snorkel, which resulted from the 

 chartered cruise of the Bold Contender, this pas- 

 sive behavior probably was an important con- 

 tributor to dolphin mortality in purse seines. 



When viewed from the deck of a speedboat tending 

 the corkline in the dolphin-release area during 

 backdown, these animals appeared to be dead if 

 they were noticed at all, and as a result the release 

 efforts were often prematurely terminated. Dol- 

 phins not released during the backdown have a 

 high probability of being killed ( Coe and DeBeer^). 



The reasons for passive behavior in purse 

 seine-caught spotted dolphins are not understood. 

 The behavior has only rarely been observed in 

 spinner dolphins in the tuna fishery. A similar 

 behavior pattern has been described for two newly 

 captured Hawaiian spinner dolphins during es- 

 cape behavior experiments designed to delineate 

 the dimensions of a dolphin release gate for 

 possible use in purse seine nets (Perrin and 

 Hunter 1972). 



Animal trainers and biologists have noted what 

 appears as similar behavior in individual captive 

 dolphins of several species. Caldwell et al. (1966) 

 reported prolonged inverted "resting or sleeping 



2Coe,J.M.,andJ.DeBeer 1977. Results of the 1976 twenty 

 vessel test of two fine mesh systems to reduce incidental porpoise 

 mortality in tuna purse seining. Unpubl. manuscr., 75 p. 

 Southwest Fisheries Center, PO. Box 271, La Jolla, CA 92038. 



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