but some were recorded there during the second 

 period. 



On the other hand, tuna aggregations upon 

 banks which are not affected by upwelled water 

 might be primarily attributable to a bank effect. 

 Further work is needed to establish the reality of 

 the bank effect and to explain it if it does exist. 

 Obviously, this research should be attenii^ted in 

 a situation where upwelling is not likely to inter- 

 fere with the bank environment, such as one of 

 the offshore southern banks toward the end of the 

 tuna season. In the meantime, it is reasonable to 

 assume that some feature of banks makes them 

 slightly more attractive to tunas than other areas 

 when temperatures are suitable, and that this fea- 

 ture affects tuna distributions in situations where 

 upwelled water does not interfere. Off Baja Cali- 

 fornia these situations are probably most common 

 in the southern j^art of the area, where upwelled 

 water does not seem to reach far offshore; at the 

 begimiing of a tuna season when tuna are unable 

 to enter upwelled water; and after all upwelling 

 has ceased. 



Fishermen often make an additional observation 

 that tunas aggregate near boundaries between blue 

 and green water. Some of the results of this study, 

 especially for cruise TO-64-2, are consistent with 

 that opinion. Blue water would frequently have 

 suitable temperature but not much food; green 

 water may contain suitable food but be too cold. 

 Tuna would be expected to aggregate at the bound- 

 arj- under those circumstances. 



The relation of tuna to the Cape San Lucas front 

 was discussed under cruise T0-6ni— 1. Evidence is 

 lacking that this particular front has any effect 

 upon tunas independent of the tuna-limiting tem- 

 peratures that may occur in it. Low temperature 

 tends to be limiting, whether located in the front 

 or not, and no other feature of the front seems 

 to have any effect upon the tunas. 



If the area west of Baja California could be 

 thoroughly and frequently monitored for surface 

 temperature and surface chlorophyll a during a 

 tuna season, it would be possible to specify areas in 

 which aggregations of yellowfin and skipjack tunas 

 would be expected — including offshore areas which 

 fishermen might not otherwise visit — and those in 

 which tuna would not be expected. This work could 

 perhaps be done from ships, which already yield 

 much data on surface temperature and could be 



equipped to yield data on surface clilorophyll a 

 (Lorenzen, 1966). The chlorophyll equipment 

 would be costly, however (about $2,000 per ship), 

 and require careful maintenance aboard ship. 

 Overflying aircraft or satellites offer another possi- 

 bility. They would probably yield much more use- 

 ful data than ships except in cloudy situations. 

 Methods of measuring surface temperature from 

 sensors above the ocean already exist, and measure- 

 ment of surface chlorophyll is said to be feasible 

 (Duntley, 1965). Obviously, the chlorophyll in- 

 formation could assist in mapping distributions 

 of other useful organisms besides tiuia — for exam- 

 ple, the red crab itself, a possible human resource 

 (Longhurst, 1968b), and other herbivores. Tem- 

 perature data alone would be insufficient to specify 

 distributions of tuna or red crab. 



ACKNOWLEDGMENTS 



Many people assisted in the work reported in this 

 paper. The Inter- American Tropical Tuna Com- 

 mission supplied data on tuna catches, and the fol- 

 lowing persons commented on the first draft : G. 

 Flittner, J. Joseph, "W. Klawe, A. Longhurst, C. 

 Onuige, E. Owen, "W. Thomas, F. Williams, and B. 

 Zeitzschel. 



LITERATURE CITED 



Ahlstrom, Elbert H. 



1967. Co-occurrences of sardine and anchovy larvae 

 in the California Current region off California and 

 Baja California. Calif. Coop. Oceanic Fish. Invest. 

 Rei>. 11 : 117-135. 

 AxvEKSON, Franklin G. 



1963. The food of yellowfin and skipjack tunas in 

 the eastern tropical Pacific Ocean. Inter-Amer. 

 Trop. Tuna Gomm., Bull. 7 : 293-396. [English and 

 Spanish.] 

 Anonymous. 



1963. CalCOFI Atlas No. 1: CalCOFI atlas of IO- 

 meter temi)eratures and salinities, 1949 through 

 1959. State of California. Marine Research Com- 

 mittee, iv -\- 1S)7 pp. 

 B.\xTER. John L. 



1967. Summary of biological information on the 

 northern auchovy EngrauHs mordai Girard. Calif. 

 Coop. Oceanic Fish. Invest. Rep. 11 : 110-116. 

 Bennett, Edward B., and Milnejs B. Schaefer. 



1960. Studies of physical, chemical, and biological 

 oceanography in the vicinity of the Revilla Gigedo 

 Islands during the "Island Current Survey" of 

 1957. Inter-Amer. Trop. Tuna C-omm., Bull. 4 : 217- 

 317. [English and Spanish.] 



174 



U.S. FISH AND WILDLIFE SERVICE 



