12 



Fishery Bulletin 105(1) 



ment may have been precluded by the patchiness in 

 their distribution and the difficulties in species iden- 

 tification. The only other growth study done on yel- 

 lowfin tuna larvae was conducted in the Gulf of Mexico 

 (Lang et al., 1994), but the species identifications were 

 based on morphology and meristics, and the larvae 

 were younger than those in our study. Results from 

 our mtDNA analysis enabled us to examine species- 

 specific growth rates of older larval stages of yellowfin 

 tuna and associated factors affecting their growth and 

 distribution. 



Distribution 



Yellowfin tuna larvae have consistently appeared in 

 the night-light collections near the Frailes Islands 

 during the reduced upwelling season, but not during 

 the season when strong upwelling occurs and other spe- 

 cies of scombrid larvae and plankton levels are more 

 abundant (Smayda, 1966; Forsbergh, 1969; Lauth and 

 Olson, 1996). The absence of yellowfin tuna larvae from 

 our sampling area during the upwelling season may be 

 associated with a cessation of spawning by yellowfin tuna 

 during this period (Schaefer 1998, 2001; Margulies et 

 al., in press) and with the temperature threshold of their 

 larvae. Lower mean water temperatures typically occur 



during the upwelling season (Lauth and Olson, 1996) 

 and have ranged from 17.3° to 25.8°C within the upper 

 50 m (Owen'^). In the laboratory, survival of first-feeding 

 yellowfin tuna larvae is poor at ambient water tempera- 

 tures of <21°C and at dissolved oxygen levels <2.2 mg/L 

 (<33.0 % of oxygen saturation) (Margulies et al."^). These 

 temperature and dissolved oxygen requirements probably 

 determine and limit the distribution of yellowfin tuna 

 larvae within the mixed layer and determine whether 

 or not they can survive during the upwelling season 

 when water temperatures are lower. The distribution of 

 yellowfin tuna larvae during the upwelling season may 

 also be strongly infiuenced by the occurrence of strong 

 westerly directed currents and northerly winds resulting 

 in larval transport away from the coastal areas of the 

 Panama Bight during this season. 



The area of larval distribution since hatching may ac- 

 tually be smaller or larger than what we have estimat- 

 ed, depending on the amount of passive transport and 



' Margulies, D., V. P. Scholey, J. B. Wexler, R. J. Olson, J. M. 

 Suter, and S. Hunt. In press. A review of lATTC research 

 on the early life history and reproductive biology of scom- 

 brids conducted at the Achotines Laboratory from 1985 to 

 2005. Inter-American Tropical Tuna Commission, Special 

 Report 16. lATTC, 8604 La Jolla Shores Drive, La Jolla, 

 CA 92037. 



