FISHERY BULLETIN: VOL. 74, NO. 1 



spatially but not temporally (there are two co- 

 horts of 3.33 X 10'' postlarvae spread approxi- 

 mately evenly over the year), approximately 9.13 

 X 10"* postlarvae enter the system daily. (This 

 is the equivalent of nearly 1% reproductive suc- 

 cess of either one 155-cm female or five 87- 

 cm females.) 



The relative fecundity of yellowfin tuna is 

 given by Joseph (1963) to the following: 



Number of eggs = 8.955 x lO'^ Z2.791 

 where / is the fork length of the fish in mm. 



If we assume the average spawning female to 

 weigh 25 kg and we estimate the presence of 

 175,000 metric tons of females of reproductive 

 age in our unexploited population, then the equiv- 

 alent number of reproductive females is ap- 

 proximately equal to 7 x 10^. These females 

 would be an average of 107 cm in length and 

 therefore: 



(8.955 X IQrH (l,0702-''9i) (7 x 10« females) 

 = 1.79 X 10^^ eggs produced. 



So if 6.67 X 10"'' postlarvae start the process we 

 need invoke only 3.72 postlarval survivors per 

 million eggs spawned. This estimate is conserva- 

 tive due to the assumption that females only 

 spawTi once per year, whereas they could spawn 

 more often. (No evidence for or against multiple 

 spawnings is in existence for yellowfin tuna.) 

 It does, however, seem likely that spawning suc- 

 cess (survival to postlarvae) is greater than 3.72 

 individuals per million eggs produced (Sette 

 1943; Farris 1961). It is also important to mention 

 that all attempts at relating spawning biomass to 

 recruitment estimates for yellowfin tuna in the 

 CYRA have been futile. This could be due to error 

 in either, or both, estimates of spawning biomass 

 and recruitment and/or the possibility that 

 environmental conditions indeed override any 

 obvious relationships. 



These comments are presented to point up the 

 likelihood that the density dependent factors for 

 limiting yellowfin tuna abundance are probably 

 more effective on the egg to larvae to juvenile 

 stages than at 40 cm or more. The larvae to 40-cm 

 fish are likely very narrowly distributed in the 

 water column (approximating a two-dimensional 

 distribution) due to thermal and energetic re- 

 quirements. The recruitment at 40 cm in the 



highly productive regions such as the periphery 

 of the Costa Rica Dome and the Panama Bight- 

 Ecuador coastal regions can perhaps be best 

 explained by the high productivity levels in these 

 regions which ranges from 500 to 700 mg carbon 

 m'2 day"^ as compared to the 205 mg carbon m"^ 

 day^ average CYRA carbon fixation rate, in con- 

 junction with the relatively shallow oxygen mini- 

 mum and thermal optima which probably act to 

 compress the available habitat toward the sur- 

 face. If one could invoke the ability of yellowfin 

 tuna to climb a food gradient, a simple volume 

 change in the preferred thermal-oxygen regime 

 combined with a negatively correlated food 

 gradient could result in the observed coastal 

 "emergence" of recruits, which "grow out" of 

 their previous thermal-oxygen limitations as they 

 develop, and exploit a significantly wider niche 

 than they could as relatively poikilothermal enti- 

 ties at sizes below 40 cm. 



To summarize, larval tunas are relatively im- 

 mobile and for survival are probably dependent 

 on aggregations of food resources. The ability 

 of tunas, particularly postlarval sizes, to detect 

 food gradients is unknown, but may indeed ac- 

 count for the easterly trend in abundance of 

 recruits. The wider distributions of larger fish 

 (postrecruits) probably is a response to competi- 

 tive feeding problems and changing physiologi- 

 cal capabilities. These larger fish are increasing 

 their daily demands but are gaining in adaptive 

 physiological and morphological characteristics 

 which widen their niche as compared to smaller 

 sizes. Their mass and mobility insure their ability 

 to move rapidly from low to high availabilities 

 of food resources, in response to seasonal and 

 areal fluctuations in productivity, perhaps ac- 

 counting for the cyclic migratory behavior ob- 

 served in their first few years in the fishery. The 

 relative offshore surface distribution of the larger 

 fish (>40 cm) may be roughly correlated vdth the 

 depth distribution of the 22°-23°C isotherms, a 

 relationship which we are now starting to study. 

 As the larger fish grow in mass, they can afford 

 deeper and longer forays into colder than optimal 

 zones with low O2 availability to obtain larger and 

 more calorific food sources; and by thus increas- 

 ing the maximum excursion depth, competition 

 is likely to be less severe. The disaggregation of 

 larger sized fish into smaller schools (number 

 of individuals) may be accounted for by these 

 effects. The large yellowfin tuna in the offshore 

 areas are certainly concentrated at the surface 



48 



