122 



FISHERY BULLETIN OF THE FISH AND WILDLIFE SERVICE 



OBSERVATIONS ON YOUNG 



Since the tunas are subject to intensive fishing 

 in many parts of the world their biology has long 

 been under investigation. Kishinouye (1919) 

 outlines the early work done on the larval stages 

 of these fishes. As he points out in another 

 paper (1926), the work of Ehrenbaum (1924) is 

 probably the most important single work on the 

 young stages of these fishes. The fishes described 

 by Ehrenbaum (1924), Kishinouye (1926), Liitken 

 (1880) and other early workers were generally less 

 than 15 millimeters in length and were taken with 

 plankton nets. For the larger sizes the investi- 

 gator is dependent primarily on collections made 

 under lights or on specimens found in the stomachs 

 of adult fish. Such specimens as these have been 

 described by the more recent workers, Schaefer 

 and Marr (1948a, 1948b), Eckles (1949), Wade 

 (1949), and others. In this paper I shall describe 

 specimens principally between 10 mm. and 18 mm. 

 in length, larger than those taken in planlcton 

 hauls, and note characters I have fomid useful in 

 their identification. 



As is generally the practice, the Alphecca often 

 drifted at night while on the fishing grounds, 

 offering an excellent opportunity for night col- 

 lecting. The collections were made imder a drop- 

 light suspended immediately above the water. 

 Fourteen such collections yielded, among others, 

 juveniles of the following three scombrid fishes: 

 Neothunnus macropterus, the yellou^fin tuna; 

 Euthynnus lineatus, the black skipjack; and Auxis 

 thazard, the frigate mackerel. Early stages of 



Table 1. — Data on postlarval Auxis thazard, Neothunnus 

 macropterus, and Euthynnus lineatus taken from the 

 Pacific Ocean off Central America, May 1949 



all these species have been described by Schaefer 

 and Marr (1948a, 1948b) from specimens taken 

 in the spring of 1947 off Central America. The 

 identification of their specimens made known 

 spawning grounds for the yellowfin tuna, oceanic 

 skipjack, black skipjack, and frigate mackerel off 

 Costa Rica and Panama. The present collec- 

 tions extend the known limits of these spawning 

 regions for three of these species 350 miles north- 

 west up the Central American coast. Dates, 

 positions, and other data for the collections are 

 reported in table 1. 



KEY TO THE POSTLARVAE OF FIVE 

 SPECIES OF CENTRAL AMERICAN 

 SCOMBRIDS 



A workable key for the identification of the 

 postlarvae of scombrids known to occiu- off 

 Central America is dependent on a few discrete 

 external characters. The teeth and body shapes 

 are similar in all species. Pigmentation, gill 

 rakers, preopercular spines, viscera, and, to some 

 extent, fin rays are in the process of development 

 and show variation within each species at a given 

 length. The characters used in the key presented 

 here were taken from specimens of Euthynnus 

 lineatus fi'om 7.5 mm. to 32.5 mm., Neothunnus 

 macropterus from 10.5 mm. to 26.5 mm., Scom- 

 beromorus sierra from 21 mm. to 71 mm., and 

 Auxis thazard from 10 mm. to 48.5 mm. in length. 

 The characters used separate species within these 

 ranges but may not hold true for larger or smaller 

 specimens. No specimens of Katsuwonus pelamis 

 were examined but the description of Schaefer 

 and Marr (1948b) based on two individuals, 21 

 mm. and 44 mm. in length, has been referred to 

 in preparation of the key. Thei'e is no spot on 

 the isthmus of the smaller of these two specimens. 

 The larger fish was cleared and stained for bone 

 study, thus destroying all pigmentation. 



la. More than 17 spines in the first dorsal. Total number 

 of vertebrae more than 46, usually 47 or 48. First 

 dorsal pigmented distally. Pigment spot on point 



of isthmus Scomberomoriis sierra. 



lb. Less than 17 spines in first dorsal. Less than 46 

 vertebrae. 

 2a. First dorsal separated from the second by a distance 

 equal to or greater than half the length of the first 

 dorsal; usually 11 spines in first dorsal. Spot on 

 isthmus. Vertebral count usually 20+19=39 

 . x'iuxis thazard. 



