fully analyzed for cruises after TO-59-1; for 

 other cruises (e.g., Tuna Longline Expedition 

 1953, Expedition SCOPE 1956) the numbers 

 of 1 -degree rectangles with both zooplankton 

 and tuna infornnation were much lower than 

 those above. 



For the above-mentioned four cruises only 

 one zooplankton-tuna correlation (TO-59-1, 

 zooplankton-yellowfin) was significant at P = 

 0.05, positive. 



The next step was to introduce surface 

 temperature data into the study. At present 

 four regressions of tuna abundance on surface 

 temperature and zooplankton, in the same 

 1 -degree rectangles and months, are avail- 

 able: EASTROPIC, both tunas, n = 14; SCOT, 

 yellowfin only (not enough skipjack data), 

 n = 33; TO-59-1, yellowfin only (not enough 

 skipjack data), n = 24. None is significant, 

 hence for TO-5 9-1 the combination, tennpera- 

 ture-zooplankton, is less closely related to 

 yellowfin than zooplankton alone. 



For SCOT (n =20) and TO-59-1 (n = 14) 

 total nnicronekton and its four connponents 

 were tested for correlation with yellowfin. 

 No correlation coefficient was significant. 

 The largest of them was obtained with the 

 large Crustacea component for Expedition 

 SCOT, and this was significant and positive 

 when used with abundance of total tuna (i.e., 

 including skipjack in the few 1-degree rec- 

 tangles where this species was taken). 



The regression of yellowfin on temperature 

 and large Crustacea was investigated for 

 SCOT and TO-59-1 but in neither case was 

 it significzmt. The regression of total tuna 

 on the same two variables was not significant 

 either for SCOT stations (for TO-59-1, total 

 tuna and yellowfin were identical); that is to 

 say, the relationship between total tuna and 

 large Crustacea lost its statistical significance 

 when temperature was introduced. 



These results are scanty, as are the data, 

 but more consistent than those in table 4. 

 There are indications of a positive relation- 

 ship between standing crop of yellowfin and 

 that of animals in its food chain, as would 

 occur if yellowfin aggregated upon supplies 

 of food. The independent effect of surface 

 temperature on yellowfin aggregation is com- 

 paratively slight. For skipjack, in view of the 

 situation shown in figure 5, it could be ex- 

 pected that temperature would sometinnes 

 have a significant direct effect. 



Observations from a few Baja California 

 stations, those at which zooplankton and mi- 

 cronekton measurements were available and 

 comparable with those from Middle America 

 stations, were included in the material dis- 

 cussed above. 



Gulf of Tehuantepec 



Inspection of txona abundance data by 1-degree 

 rectangles and months showed that yellowfin 

 are most abundant in the winter and spring 

 and least abundant in summer, and skipjack 

 generally scarce in all seasons. The seasonal 

 cycle of abundance of yellowfin agrees broadly 

 with that of zooplankton as inferred from 

 STOR cruise data (higher standing crops in 

 November and January- February than in May- 

 June or September; see above). 



Further, the area of maximum concentration 

 of zooplankton is regularly in the southern, 

 southwestern, or western parts of the Gulf, 

 and the same is broadly true of yellowfin in 

 the winter and spring months. In the quarter 

 November- January the average yellowfin catch 

 per day's fishing is greater in the extreme 

 southwest of the area, and in February- April 

 (the height of the fishing season) it is greatest 

 to the west of the thermal ridge. It is probable 

 that the zooplankton is generally distributed 

 in about the sanne way at the same seasons, 

 for reasons given elsewhere in the paper. 

 These observations support the idea of aggre- 

 gation of yellowfin on biota in their food chain. 



The matter is discussed in the paper "An 

 oceanographic study of the Gulf of Tehuan- 

 tepec," referred to under "Physical features 

 and processes in the ocean." 



Oceanic Islands 



One weakness of the hypothesis that tuna 

 aggregate in times and places of abundant 

 food has been the observation that tiina are 

 generally more abundant around ocean islands 

 than in the neighboring ocean, whereas this 

 is not always true of zooplankton. Such a 

 situation was found by Tuna Commission 

 workers at Clarion Island in May and June 

 1957 (Bennett and Schaefer, I960). Zooplankton 

 standing crop was uniformly low fronn the 

 offshore oceam through the fishing area to 

 the shore of the island. 



On Expedition SCOT zooplankton hauls were 

 made around each of three ocean islands in 

 the following way: about half a mile from 

 the island and about 11 miles from it on 

 opposite sides, one such series in daylight 

 and an identical one in the dark. The three 

 islands (Clarion, Clipperton, Cocos) are shown 

 in figure 1. The zooplankton distribution at 

 each island was the same in the night as in 

 the day series of hauls. At Clarion Island 

 both offshore stations yielded nnore zooplankton 

 than the inshore station; at the other two 

 islands the inshore station yielded more thaui 

 either offshore station. 



41 



