37 



the majority of animals headed in the same direction. Usually animals in the 

 smaller group would ruin, after traveling less than 1 km, and rejoin the 

 majority. Released porpoise were followed by helicopter on several occasions. 

 In each case, large splashes were visible for about 4 l<m. after which the 

 animals would begin to slow. They apparently essentially stopped at a distance 

 of approximately 7 km, and were very difficult to observe. At this point, 

 behavior approximated that observed during the pre-chase situation. 



GENERAL COMMENTS 



During the course of this cruise, several items became apparent. Probably 

 the most encouraging one is that a properly equipped vessel with a well-trained 

 and concerned crew can fish for tuna by setting on porpoise with a very low 

 mortality rate. The vessel must be equipped with the super apron system that 

 was designed to take advantage of the fact that fish and porpoise maintain 

 different positions in the net — the tuna remaining deeper and more toward the 

 boat and the porpoise remaining generally shallower and more toward the 

 center or far side of the net from the boat. The porpoise must surface to 

 breathe, and. during backdown, surfacing will result in the net being pulled 

 under the porpoise each time it surfaces, until the animal finally ends up over 

 the super apron in shallow water. Tuna, on the other hand, usually stay deep 

 and are thus contained in the deep portion of the net, boatward of that section 

 where the apron begins. 



This net configuration should be considered in contrast to either the old 

 style nets or the newer idea of the double depth safety panel, where there is 

 no ramp created to force the porpoise into shallow water. Indeed there can be 

 a large canopy formed in either of these designs which can trap and drown 

 porpoise by cutting them off from the surface. In the super apron design the 

 netting is cut so as to virtually eliminate canopies and reduce the volume of 

 the backdown area. 



The second item of interest was "sleeping behavior". This behavior, coupled 

 with the non-super apron types of nets, results in substantial numbers of 

 animals being left in the net after backdown. After backdown, the net reopens 

 and the porpoise become difficult to catch until they are sacked up with the 

 fish. Such a situation results in high mortality on the porpoise, due to being 

 confined, in the extreme, with several tons of fish. 



A third item related to steps to be taken in tuna-porpoise mortality reduction 

 research is the following. Given that all boats are properly equipped and have 

 well-trained crews aboard, the mortality rate should decline to the point where 

 all populations will be able to return to approximately a pre-fishing level. 

 Assuming that this is so, the next step should be to develop a fishing method 

 that does not rely on encircling porpoise with a purse seine. 



Another area requiring research is how unexploited porpoise respond to 

 purse-seines. Both the stocks outside the CYRA and those south of the equator 

 have higher mortality rates than those in the traditional fishing areas. The 

 behavioral basis for the difference in mortality should be investigated with an 

 eye towai'd possible modifications that would solve the problem. 



A final item meriting further research is to determine how one might remove 

 porpoise from the net prior to backdown. It seems reasonable to expect that 

 the longer a porpoise is confined in the net the larger its chances are of being 

 killed. Thus if simple, practical, and reliable methods of prebackdown removal 

 can be developed they would be valuable, at least in situations where an 

 equipment breakdown has occurred which may occasionally take several hours 

 to repair. 



Avoidance of objects in the water is a very frequently observed behavior of 

 porpoise which makes them herdable. Their behavior in the net and particularly 

 their location in the net may result from a process in which they attempt to 

 minimize incoming signals. Thus, they end up in center net or slightly more 

 toward the far edge of the net as the boat is presumably a better sonar target 

 and also generates much more noise than the net. With this hypothesis as the 

 basis for design of a system, it may be possible to design a herding and release 

 method for the porpoise that would not affect the tuna. 



