134 



FISHERY BULLETIN OF THE FISH AND WILDLIFE SERVICE 



strains of skipjack was raised. The conclusion 

 was in favor of age groups. 



There has been httle study of age and growth 

 determination of Hawaiian tunas. Some measure- 

 ments of the skipjack {Katsuwonus pelamis) were 

 collected by Bonham (1946) in 1944 and 1945, 

 and length frequencies were plotted from these 

 data. Bonham suggested the possibility that 

 two successive year classes were present, but 

 recognized the limitations of his data and did not 

 attempt to assign ages. Brock (ms.) made a 

 rather detailed study of size frequencies of skip- 

 jack. He was able to identify modal groups in 

 the catches of successive years and to demon- 

 strate progression of the modes during the year, 

 whence he concludes they are year classes. No 

 previous studies have been made of the Hawaiian 

 yellowfin tuna. 



SOURCES OF DATA AND METHODS OF 

 COLLECTION 



The data for this study were obtained from two 

 different types of fishery, the long-line or flag-line 

 fishery, ami the pole or live-bait fishery. The 

 long-line fishery in Hawaii is carried on through- 

 out the year in most of the waters around the 

 main Hawaiian Islands. The catch from this 

 fishery is sold primarily to the fresh-fish markets 

 by auction. The live-bait fishei-y, on the other 

 hand, is more seasonal and the catch is primarily 

 for the cannery, although some of the fish are 

 sold on the fresh-fish market, especially when 

 the cannery is not operating during the slack 

 season. 



The long-line fishery is conducted by means of 

 setlines made up of units of gear known as baskets. 

 The term "basket" is derived from woven bamboo 

 baskets in which the units of gear are stowed. A 

 vessel fishes a long-line composed of about 30 

 baskets, each of which is from 140 to 203 fathoms 

 in length and has 4 to 6 brancii lines with hooks. 

 When the baskets are fastened together and the 

 long-line is set, the hooks fish from 30 to 50 

 fathoms in depth (June 1950). 



Long-lines are set in the early morning and are 

 fished only during the daylight hours. LTsually 

 a few tunas are taken each day, and the catches 

 may also contain several marlin, swordfish, and 

 sharks. Fish taken by this method are generally 

 large in comparison with those taken in the live- 



bait fishery. Yellowfin tuna caught by this 

 method average about 140 pounds in weight, and 

 the big-eyed tuna {Parathunnus sibi) are heavier. 

 The total landings, by months, vary considerably 

 in both the numbers and the species of fish caught. 

 The yellowfin is the most abundant species taken 

 during the summer months, and the big-eyed tuna 

 dominates the winter catch. Albacore also are 

 caught on the long-lines during the winter months, 

 but the numbers are small in comparison with 

 either of the other two species. Although the 

 tunas are definitely seasonal in availability, some 

 fish of all three species usually are taken during 

 the entire year. 



The second source of data was the live-bait 

 fishery. This fishery is seasonal; most of the 

 catch is taken during the summer months. The 

 fisheiy is dependent on small live fishes which are 

 used as chum to lure the tunas within reach of the 

 feather lures or live bait on hooks on the poles of 

 the fishermen. The fish caught by this method are 

 much smaller than those taken by the long-line 

 method; the largest weigh near 25 pounds. The 

 fact that no large fish are caught on the surface by 

 the pole or live-bait fishery and no small fish are 

 caught at depths fished by the long-line gear indi- 

 cates that there may be a possible vertical migra- 

 tion downward of yellowfin tuna during the early 

 years of life. 



Although this fishery is primarily for the skip- 

 jack, mixed schools of skipjack and yellowfin or 

 skipjack and big-eyed tuna are sometimes found. 

 Approximately 12 to 15 catches from mixed schools 

 are landed at the cannery each season. It was 

 from schools such as these that tlie data on small 

 yellowfin were collected for this study. Schools 

 of tuna, whether a pure school of skipjack or 

 mixed with either yellowfin or big-eyed tuna, 

 tend to contain fishes with little range in size. 

 Brock (ms.) says of skipjack schools, "no indivi- 

 dual school of fish sampled contained fish differing 

 by more than 20 centimeters in length and usually 

 much less." Differences in sizes of fish from dif- 

 ferent schools, however, were as much as 50 

 centimeters. 



Weight and length frequencies of the long-line 

 catch were taken from fish sold at auction by the 

 Kyodo Fishing Co., Ltd., Honolulu. The officials 

 of this company were kind enough to allow measur- 

 ing of the fish on the auction floor before the 



