35 



observed to be somewhat behind the deeper than the porpoise school, while 

 lesser amounts of tuna were common at the sides of the porpoise school. 

 Occasionally tuna could even be seen ahead of the porpoise school and :il-o 

 under the porpoise school. When, as sometimes occurred, the porpoise school 

 fragmented during the chase, the tuna school also would usually fragment. In 

 these situations, the helicopter was a valuable tool, which allowed the captain 

 to evaluate which segment of the porpoise school was "carrying" the most tuna. 

 A major tool in herding porpoise was the wake of the tuna seiner. Porpoise 

 almost never penetrated the seiner wake. The wake of the Elisabeth C.J., when 

 operating at near maximum speed, was distinctly visible for a considerable 

 distance, probably well over 2 km. That long wake was used as a fence to 

 contain the porpoise and thereby stop them from bypassing the tuna-seiner. 

 As the chase progressed, the speedboats would be used to reinforce the wake 

 of the seiner, in areas where the wake had begun to deteriorate. Thus as the 

 seiner spiraled inward toward a circle small enough to set the net, the wake 

 would become more and more effective and the speedboats could be concentrated 

 more in the weakest portion, e.g., directly in front of the bow of the seiner. 



DURING SET BEHAVIOR 



When the net was let go, porpoise generally stopped running and would be 

 observed milling about in approximately the middle of the area to be encircled 

 by the net. Throughout the rest of the set, one could seldom see more than 10 

 to 25 percent of the animals at the surface at any one time. Spotter dolphins 

 and spinner dolphins showed different behavior and will therefore be discussed 

 separately, in both their surface and underwater behavior. 



Spinners on the Surface. — Spinners spent most of their time on the fringes 

 of the aggregation of spotters. Spinners were present in 50 percent of the sets 

 made during the behavioral portion of the cruise. They were seldom observed 

 to stop moving around the net. In several sets, good data on the diving 

 frequency and duration of spinners was obtained since they were present in 

 small numbers and stayed in one group. Small groups of spinners generally 

 made synchronous dives. They remained submerged for more than 4 minutes 

 occasionally. The group would surface at approximately the same time, take 

 a breath, and dive again in approximately the same order that they had 

 surfaced. Most dives were deep enough to take them out of sight of the 

 observers on the mast of the seiner. Occasionally, they would swim just barely 

 under the water and resurface at intervals so that they were moving in an 

 approximate sine wave pattern. The higher level of activity and their location 

 in the school may well explain why spinners have a higher mortality rate 

 than do spotters. 



Spotters on the Surface. — Spotters were in the majority in all but three sets. 

 Their presence in normally high numbers made it difficult to make observations 

 on dive rhythms except in those sets where marked animals were present. 

 Spotters exhibited several types of behavior. The first was "milling", a 

 situation in which the animals are all swimming but the visible portion of the 

 school does not go anywhere. Apparently, when animals surface to breathe, 

 they swim near or on the surface for a short distance before diving again. 

 When resurfacing from the dive, they tend to do so in mid-school. Infrequently, 

 this behavior appeared to be oriented so that all of the surface animals would 

 swim in the same direction while at the surface, giving one the feeling of a 

 water wheel with only a small portion of the whole wheel being visible. 



In contrast to spinners, spotted dolphins exhibited several types of aerial 

 behavior which occurred throughout the period in the net prior to backdown. 

 Aerial behavior usually involved only one animal at a time, and the animal was 

 most often a juvenile (two-tone coloration). The most spectacular form of 

 aerial display was a leap out of the water followed by a large splash on 

 reentry either as a result of a belly flop or landing on one side. The animal 

 that made one leap would most often continue leaping at short intervals, 

 occasionally up to 9 times. 



Other types of aerial behavior included slapping the water with the flukes, 

 bringing the anterior portion of the body up out of the water to various 

 degrees. The most common behavior of this type was called "nose out" behavior 

 in which the porpoise is quiescent in the water but holds the beak, eyes, and 

 blowhole above the water. 



