less important role, in the region between 

 Baja California and the Equator and offshore 

 to the limits of the fishery. The statistical 

 evidence for this is not strong, but it does 

 exist for the two periods for which most data 

 are available for yellowfin and food chain 

 animals in the same localities. The rela- 

 tionship to environnnental factors of abundance 

 of yellowfin in other regions (including Baja 

 California itself), and abundance of skipjack 

 in any region, is still obscure. 



7. Observations made in summer between 

 Baja California and the Equator were con- 

 sistent with an approximately steady state in 

 the biotic part of the tuna food chain, with 

 abundance of phytoplankton positively corre- 

 lated with abundance of zooplankton and a 

 similar relationship between zooplankton and 

 micronekton, from place to place; abundance 

 (standing crop) of phytoplankton was also 

 positively correlated with rate of primary 

 production. It would therefore be possible 

 to estimate abundance of tuna prey from 

 contemporaneous data on phytoplankton and 

 this might lead to estimates of abundance 

 of yellowfin (6 above). 



8. Abundance of zooplankton can also be 

 estiniated within reasonable confidence limits 

 fronn contennporaneous measurements of cer- 

 tain physico-chennical variables (e.g., meam 

 light attentuation coefficient and mean dis- 

 solved oxygen concentration in the mixed 

 layer, and the depth of the mixed layer itself), 

 both in summer and winter, in the region 

 between Baja California and the Equator. 

 This offers greater practical possibilities for 

 estimating abundance of tuna food and tuna 

 than does the method of measuring phyto- 

 plankton (7 above), because the physico-chenn- 

 ical ocean properties would probably be more 

 easily nneasured on unmanned moored stations 

 ( 16 below). 



9. Four cruises were made to the Gulf of 

 Tehuantepec, an innportant fishing area for 

 yellowfin, to learn by repetitive observation 

 the particular form of the assumed connection 

 between yellowfin, its food, physico-chennical 

 properties of the water including temperature, 

 and local wind (cf. 3, 4, 6, 7, and 8 above). It 

 was found that cooling, chemical enrichment, 

 and biological production in near-surface 

 waters result from the seasonal effect of 

 local wind in (a) setting up a circulation which 

 results in a domed or ridged discontinuity 

 layer, and (b) stirring the upper part of the 

 layer in the region where it is closest to the 

 sea surface. This process is different from 

 upwelling (4 above). The wind-induced circu- 

 lation also appears to act on the biota pro- 

 duced, distributing them in a way that is 

 spatially and temporally consistent with the 



major features of yellowfin distribution if the 

 latter is considered to be related to the dis- 

 tribution of tuna food. 



The following additional scientific accom- 

 plishments of the program (10-15) are less 

 directly related to the study of tropical tuna 

 ecology at present, either because the investi- 

 gations are incomplete or because there has 

 been insufficient opportunity to integrate them 

 with other work, but they are all considered 

 to be pertinent to the inain investigation. 



10. Unialgal cultures of several tropical 

 oceanic species of phytoplankton were suc- 

 cessfully maintained in the laboratory and 

 used there to study algal growth in response 

 to various conditions of light, temperature, 

 and chemical nutrients; some of the artificial 

 environments simulated those of the ocean. 

 Similar work was done at sea on cultures of 

 mixed natural populations. This work is re- 

 quired to improve the understanding (and 

 therefore the practical efficiency) of some 

 of the more empirical parts of the investiga- 

 tion, such as the attempts to substitute physico- 

 chemical variables for phytoplankton in sta- 

 tistical relationships predictive for tuna food 

 (7 and 8 above). For example, some experi- 

 ments showed that nitrate is limiting for the 

 growth of Gymnodinium, which suggested that this 

 variable should receive attention in further 

 work; efforts were then made to make meas- 

 urement of nitrate concentration a part of 

 the shipboard oceanographic routine in the 

 eastern tropical Pacific. 



11. An experimental evaluation of the C 

 method of measuring productivity, or plant 

 production rate, was made with laboratory 

 cultures of Dunaliella. It was concluded that 

 the method gives adequate estimates of syn- 

 thesis of plant material except under ex- 

 tremely nutrient-deficient or light-deficient 

 conditions. 



12. Studies were made of the distribution 

 of physical properties and ocean structure 

 and circulation in the Pananna Bight and 

 Gulf of California, which are tuna fishing 

 areas. In Panama Bight there is a doming 

 of the discontinuity layer, like that in the 

 Gulf of Tehuantepec (9 above), which is prob- 

 ably connected with the abundance of tuna in 

 the area. The mouth of the Gulf of California 

 is a region where sharp horizontal gradients 

 of temperature (fronts) are frequent; since it 

 is widely supposed that these fronts play an 

 important role in the seasonal aggregations 

 and migrations of tuna, perhaps by causing 

 concentrations of animals on which tuna feed, 

 a program of oceanographic observations in 

 this area was begun. 



43 



