OBSERVATIONS ON THE ENVIRONMENT 



In conjunction with the experimental fishing, 

 extensive observations were made on the sea 

 temperature. In the long-term approach these 

 observations are intended to help explain sea- 

 sonal and annual fluctuations in catch rates of 

 tuna. Tester (1956) points out a high positive 

 correlation between sea temperatures and yellow- 

 fin catch. He does not regard the temperature 

 change as the direct cause of the change in tuna 

 abundance; rather, that temperature indicates 

 the age of the water subsequent to upwelling. 

 Since upwelling brings to the surface water low 

 in temperature and high in nutrients, he regards 

 the coolest water as newly upwelled and warmer 

 water as water that has been longer at the sur- 

 face and has remained in the area sufficiently 

 long to permit tuna forage to develop in it. 



Since November 1953 the U. S. Fish and 

 Wildlife Service has obtained weather andhydro- 

 graphic data from the weather station on Christ- 

 mas Island. At that time instruments and aid 

 were given to improve the station maintained by 

 a plantation weather observer at the village of 

 London on the lee (west) side of the island. 

 Differences between observations made at the 



island station and in the open ocean are being 

 examined. At present it appears that on-shore 

 water temperatures are consistently lower than 

 those off-shore, but they may provide an index 

 of oceanic conditions. 



Surface temperatures obtainedat the Christ- 

 mas Island station during 1954 show a trend 

 generally similar to the trend in the catch rates 

 of yellowfin (fig. 18). Both sea surface tem- 

 peratures and yellowfin catch rates were 

 relatively high early in the year and both were 

 relatively low during the late summer and fall 

 months. 



Tester (1956, fig. 3) used yellowfin catches 

 and temperatures collected in the equatorial 

 Pacific over several years to define the 

 temperature -yellowfin relationship. In this re- 

 port we are dealing with a large number of 

 observations taken within one year and all from 

 the Line Islands area. Since the same inter- 

 pretation can be given to the two somewhat dis- 

 similar lots of data, the hypothesis that there is 

 a significant relationship between the tempera- 

 ture of the water and the abundance of yellowfin 

 tuna appears reliable. 



83 



82 



a: 



3 



< 80 



tr 



o 

 < 

 Li- 

 ar 



to 78 



77 



re 



Figure 18. --Surface temperatures at Christmas Island and Line Islands yellowfin 

 catches during 1954 by 5-day periods. /Only single weekly temperature obser- 

 vations are available through March 12. / 



