PERRIN and HUNTER: ESCAPE BEHAVIOR OF PORPOISE 



MONOFILAMENT AND POLYVINYL 

 PANELS 



When the monofilament panel was inserted in- 

 to the 1.1-m-deep openino- at the end of a series 

 of depth trials, Westward (IV) "s'ot u]i a full 

 head of steam and plowed into the monofilament" 

 (extracted from field notes of W. Wasden) and 

 became entangled. Insertion of polyvinyl panels 

 produced similar results in multiple trials; West- 

 ward (VI) in each trial hit the panel and slid 

 over it and out of the chamber. There was 

 nothing in the behavior of the porpoise to indi- 

 cate that they recognized the presence of the 

 panels. 



DISCUSSION AND CONCLUSIONS 



The swimming behavior of the naive porpoise 

 Westward and, to a lesser extent, of Moana, the 

 first few days after capture was very similar to 

 that of porpoise {Stenella spp.) in tuna purse 

 seines as ol)served by one of us (Perrin) off Cen- 

 tral America. A typical "failure to escape" epi- 

 sode is illustrated for Moana in Figure 7. Im- 

 mediately after a purse-seine net has been set, 

 when the diameter of the encircled area is great- 

 est (approximately 250 m), the porpoise swim 

 about quite rapidly in small tight groups of a 

 dozen or so individuals, the mem])ers of a group 

 diving and surfacing together (Figure 8). As 

 the net is hauled and the area enclosed becomes 

 smaller, especially after the backing down oper- 

 ation (see Perrin, 1969) , the porpoise congregate 

 and raft near the center of the enclosure and mill 

 very slowly, holding their bodies in a semiup- 

 right position with blowhole exposed and ros- 

 trum at or slightly below the surface (Figure 9) . 

 At this point, individual animals can be seen to 

 leave the grou]) and dive. When the net has been 

 completely hauled, animals are often found with 

 their snouts entangled in the webbing several 

 meters below the corkline. 



Although the head bobbing exhibited by West- 

 ward was not observed in the purse-seine situ- 

 ation, the similarities in Ijehavior between 

 freshly captured animals and those captured in 

 a purse seine were striking. In both cases the 

 animals did not display normal motor patterns; 



they rested or swam at abnormally slow speeds, 

 and this behavior was often ended l^y a rapid 

 dive beneath the surface with no noticeable 

 change in behavior preceding the act. The prin- 

 cipal characteristics of this behavior, the inhi- 

 bition of activity in a fear-inducing environ- 

 ment, resembled fear responses described for 

 many other vertebrates and frequently classified 

 as an immobility or freezing response (Ratner 

 and Thompson, 1960; Hinde, 1970). Hogan 

 (1965, 1966) suggested that withdrawal and im- 

 mobility are separate, mutually inhibitory sys- 

 tems. If this view is correct, then driving por- 

 poise through an escape route in the luirse seine 

 would not be successful once the animals began 

 to show the immobility response, because with- 

 drawal would be inhibited. Under these circum- 

 stances the additional fear stimulus associated 

 with driving might be the catalyst for the rapid 

 dive to escape, which results in entanglement. 

 Driving may have to be carried out before im- 

 mobility begins. Once the animals became im- 

 mobile the only strategy may be to ])ull the net 

 out from beneath them as is currently done dur- 

 ing the "backing down operation" (Perrin, 

 1969). 



That the behavior of Westward and Moana 

 evolved into more typical behavior during the 

 course of a single ex])eriment also supports the 

 notion that their unusual behavior was caused 

 by the circumstance of captivity rather than ill 

 health. 



Our conclusions with respect to projected de- 

 sign of a rescue gate for removing porpoise from 

 a purse seine during fishing operations were: 



1. The gate should be sufficiently wide so that 

 when the perimeter of the net circle buckles 

 after pursing, the width does not become less 

 than 1.5 m. Considering the equivocal results 

 of the ex])eriments for o])enings wider than 

 1.5 m, the opening should be as wide as prac- 

 tically possible. 



2. Depth of the opening should be not less than 

 1 m and as deep as it is possible to make it with- 

 out causing loss of the fish in the net. 



3. There should be no line, corkline, or other 

 barrier across the oi)ening at the surface. 



4. A self-actuating release port that will open 

 when struck by a porpoise swimming into it 



57 



