JUVENILE OCEANIC SKIPJACK FROM THE PHOENIX ISLANDS 



By Bell M. Shimada, Fishery Research Biologist 



Studies by various investigators have added 

 substantially to oiu- hitherto hinited knowledge of 

 the spawTiiag of oceanic skipjack {Katsuwonus 

 pelamis Linnaeus 1758) ui the Pacific Ocean. 

 From evidence based on the examination of 

 gonads or the capture of juveniles (see table), 

 spawning grounds have been indicated in waters 

 ofi' Central America (Schaefer and Marr 1948), 

 the Hawaiian Islands (Eckles 1949), the northern 

 Marshall Islands (Marr 1948), the Ti'uk Islands 

 (Inanami 1942), the Phihppine Islands (Wade 

 1950), and the northern Ryukyu Islands (Kish- 

 inouye 1923). The existence of additional spawn- 

 ing grounds near the PhoenLx Islands in the south 

 central Pacific is demonstrated by the captm-e of 

 juveniles incidental to biological, oceanographical, 

 and exploratory-fishing studies conducted in this 

 locality during the summer of 1950 by the Pacific 

 Oceanic Fishery Investigations of the U. S. Fish 

 and Wildlife Service, Honolulu, Hawaii. 



During a regular hydrographic cruise of the 

 Pacific Oceanic Fishery Investigations research 

 vessel Hugh M. Smith, between Hawaii and the 

 Phoenix Islands, two juvenile scombroids were 

 collected on July 18, 1950, at 3°50.5' S. and 

 171°48.5' W. by collaborating scientist V. E. 

 Brock,' and subsequently identified as oceanic 

 skipjack, Katsuwonus pelamis. These young fish, 

 measiu-mg 35 mm. and 48 mm. m total length,- 

 were captui-ed by dipnet under a night light while 

 the vessel was adrift. 



On August 5, 1950, a sister ship, Hem-y O'Mal- 

 ley, visited the Phoenix Islands for exploratory 

 fishing. Wliile night-light collectuig from this 

 vessel at a position approximately 400 yards ofi' 

 the west end of Hull Island (4°30' S., 172°11' W.), 

 K. Yee,' caught thi-ee additional specimens of 

 juvenile K. pelamis. Total lengths of these fish 

 were 20 mm., 22 mm., and 36 mm. 



 Director, Division of Fish and Game, Territory of Hawaii. 



3 Defined as the distance from the tip of the snout to the tip of the shortest 

 median caudal ray. 



s Fishery Methods and Eriuipment Specialist, Pacific Oceanic Fishery 

 Investigations, U. S. Fish and Wildlife Service. 



953184—51 



All five specimens exhibit body contours typical 

 of juvenile oceanic skipjack and possess a very 

 slightly pigmented first dorsal fin and a colorless 

 second dorsal fin, which are characteristic of young 

 fish of this species (Schaefer and Marr 1948, Wade 

 1950). The 48-mm. juvenile of the Smith col- 

 lection was stained with alizarin red S and found 

 to have a "trellis" and a total of 41 vertebrae, 

 m'ostyle included. The 20-nun. specimen of the 

 O'Malley collection was stained and cleared after 

 HoUister's (1934) method and was found to have 

 a vertebral count of 20-1-21. These characteristics 

 are definitive of Katsuwonus pelamis as shown by 

 Kishmouye (1923), Frade and de Buen (1932), 

 and Godsil and Byers (1944). 



The 35-mm. specimen is colored with light- 

 brown pigmentation except for the belly, which is 

 colorless, and the head. Pigmentation is more 

 concentrated dorsally and along the sides of the 

 body where it outlines a narrow band along the 

 midline. Scattered melanophores on the peri- 

 toneum are visible tlnough the thin body wall and 

 extend caudally to the anus. The top of the 

 head forward of the nape is brown in color with 

 subcutaneous melanophores on the underlying 

 brain covering. The upper portion of the oper- 

 culum, the posterior and inferior orbit, as well 

 as the sides of the upper and lower jaw, are lightly 

 pigmented with brown. The membrane between 

 the first and second dorsal spme is irregularly 

 marked witli black spots from the Inise to tlie 

 distal ends of the spines; the membrane comiecting 

 the remaining dorsal spines is similarly marked 

 but only near the tips of the spines, the basal 

 half beuig colorless. The second dorsal is with- 

 out color. Black pigment spots are present along 

 the upper pectoral raj's and along the upper base 

 of the fin. Similar spots are present along the 

 insertion of the median fins and finlots. 



The first dorsal fin is composed of 16 spines of 

 which the second is the longest. Fouileen rays 

 are present in the second dorsal fin. There are 

 8 dorsal fuilets and 7 anal finlets. An interradial 



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