MURPHY and SHOMURA: PRE-EXPLOITATION ABUNDANCE OF TUNAS 



continuity and sinking at the discontinuity, a 

 mechanism that should concentrate certain types 

 of tuna forage or food for tuna forage. 



We are still confronted with the basic differ- 

 ence in the distribution of surface and deep- 

 swimming tunas (Figure 38), though we have 

 logically advanced two hypotheses that seem ade- 

 quate to explain each distribution pattern when 

 the problems are considered independently, i.e., 

 basic enrichment and northward drift of up- 

 welled water in the instance of the deep-swim- 

 ming tunas, and "fronts" in the instance of 

 surface schools. An entering wedge is offered 

 by the difference in social behavior between the 

 deep-swimming and surface schools. 



Sette (1950) , in discussing mackerel, proposed 

 that schooling was advantageous to any predator 

 feeding on prey that was aggregated. In fact, 

 the very existence of schools of carnivores would 

 seem to require schools or aggregations of prey, 

 for if prey were distributed at random, it could 

 be most effectively harvested by pursuit and cap- 

 ture of individuals by individual predators. In 

 other words, where we find scattered prey we 

 should expect scattered predators. In the pre- 

 sent instance, where we have a mechanism 

 ("fronts") for concentrating prey, we find sur- 

 face schools, and where the mechanism is not 

 present we do not find the schools. The deep- 

 swimming tunas, on the other hand, are not ag- 

 gregated into large, compact schools (Murphy 

 and Elliott, 1954) and so might be expected to 

 use more or less scattered forage organisms more 

 efficiently, and the distribution of such tunas 

 might be expected to conform more closely to 

 the mean distribution of forage, rather than the 

 distribution of concentrating mechanisms. 



SUMMARY 



1. Surveys of tuna populations and environ- 

 mental conditions were made in the equatorial 

 central Pacific in 1950-53. 



2. Tunas were sampled by longline, trolling, 

 live-bait fishing, and sighting from the bridge 

 of a research vessel. 



3. The longline was used more than the other 



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5 10 15 20 



WIND SPEED (KNOTS) 



25 



Figure 39. — Daily frequency of fronts as a function of 

 daily noon wind speed (lat5°S-10°N, long 148°-170°W). 



FISH SCHOOLS PER DAY 



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• FRONTS PER 1.5 HOURS 



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0*-5°N 

 LATITUDE 



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Figure 40. — Rate of crossing temperature discontinuities 

 and rate of sighting fish schools near the equator (vessel 

 speed about 8 knots, vessel course always north or south). 

 Longitudinal limits 140° to 170°W. 



methods of fishing, because it was the most con- 

 sistently productive sampling method, particu- 

 larly on the high seas. 



4. Tunas, especially yellowfin tuna, may be 

 divided into two groups, small surface-swimming 

 tunas and large deep-swimming tunas. 



5. On the basis of longline catches, deep-swim- 

 ming yellowfin in the central Pacific were most 

 abundant near the equator. 



6. Deep-swimming bigeye tuna were nowhere 

 as abundant as yellowfin tuna. Two tongues of 



907 



