Discussion Leader - John J. Magnuson 

 Rapporteur - Everet C. Jones 



Reference: Papers No. 



IV - 1. Strasburg, D. W. - An aerating device for salt well water 



2. Nakamura, E. L. - The establishment and behavior of skipjack tuna (Katsuwonus 



pelamis) in captivity 



3. Yuen, H. S. H. - Experiments on the feeding behavior of skipjack at sea 



Behavior has been defined as the total 

 movements of an intact animal. More recently 

 the field has been broadened to include the study 

 of these movements relative to physiology, ecol- 

 ogy, and phylogeny. From a practical view- 

 point, behavior studies may provide information 

 useful in developing fishing gear, in predicting 

 a tuna's location in time and space, and in under- 

 standing behavioral mechanisms which have con- 

 sequences in population dynamics. 



Observations of skipjack both in captivity 

 and in the field indicate that certain color changes 

 and fin movements are associated with feeding. 

 Transient vertical bars observed on skipjack 

 appear to result from the contraction of melano- 

 phores in vertically oriented zones on the sides 

 of the fish, producing a pattern of alternate light 

 and dark bars. The speed of these changes indi- 

 cates nervous control rather than endocrinalcon- 

 trol. Such transient vertical bars have been 

 observed on other large pelagic fishes in the 

 living state, but fade on dead or dying fishes . 

 Vertical bars are a common non-transient pat- 

 tern on juveniles of many fish species. 



Various people have observed reddish 

 colored skipjack; perhaps this is a spawning 

 coloration. A more commonly observed pattern 

 is one of bright blue horizontal streaks on the 

 dorso-lateral surface. It was suggested that 

 these blue streaks may be fluorescent or lumi- 

 nescent. Since this color is not observed on dead 

 fish the possibility of iridescence was questioned. 

 Transient color patterns, together with erection 

 of the first dorsal fin (resulting in the prominent 

 display of the white leading edge) and opening of 

 the mouth (resulting in the display of the silvery 

 tongue), may be social releasers . The possibil- 

 ity that these changes are involved in communi- 

 cation among fish opens up new fields of research. 

 Avenues of investigation included the use of arti- 

 ficial skipjack painted in appropriate colors to 

 test the reactions of both captive and wild fish. 



The feeding response of skipjack at sea 

 appears to be influenced by the quality of the 



stimulus, i.e., the characteristics of the bait 

 species used. It is possible that a sufficiently 

 attractive bait fish might actually attract the 

 tuna away from the hooks and result in a lower 

 biting rate. Experiments have been conducted 

 in Hawaii using dead bait fish because of the 

 scarcity of live bait. Both the catch rate and 

 feeding response of skipjack were less with dead 

 bait than with live bait, butyellowfin tuna in the 

 eastern Pacific react to such inanimate material 

 as macaroni, rice, or nails. The Galapagos 

 fishery was started on dead bait. 



Large schools of many species of pelagic 

 fishes, including the tunas, have often been ob- 

 served to be associated with floating logs, boxes, 

 ordriftwood. In the Atlantic , blackfin tuna have 

 been observed associated with whale sharks, 

 and various tunas in the Pacific have been ob- 

 served associated with porpoises. It is a com- 

 mon practice of fishermen to investigate floating 

 material to locate schools of tuna. In Japan, 

 skipjack did not aggregate under driftwood which 

 was planted at sea for that purpose. Ahypothe- 

 sis putforwardto explain the association between 

 fish schools and logs or driftwood was that the 

 floating object attracted small fish which were 

 preyed upon by the large fishofthe school. Ob- 

 jections to this idea centered about the obser- 

 vation that the schools of predaceous fish under 

 floating objects were often too large t o be 

 attracted by a few small fish. A counter sug- 

 gestion was made that we do not know the rate at 

 which small fish accumulate and that it may be 

 greater than presently expected. 



Skipjack schools in the Japanese fishery 

 have been observed to extend from 100 to as 

 much as 1,000 meters from the driftwood and 

 to move inandout from it. In Hawaii, 3 months 

 of investigation of floating wood resulted in no 

 observed association with skipjack. Dolphin 

 ( Coryphaena) were often seen in such association, 

 but examination of stomach contents indicated 

 that they were not feeding on Kyphosus , the small 

 fish most often found under driftwood, but were 

 feeding on trunkfish and flying fish. 



