APPENDIX I 
BATHYTHERMOGRAPH OBSERVATIONS 
-by Robert Pyle 
Bathythermograph (BT) casts were taken at U-hour intervals in sectors 
A, B, and G of the cruise* except when in port or anchored off islands. 
BT instrument number KK09297 was lost during the night of November 5-6; 
number KK09299 was used for the remainder of the cruise. The calibrated 
grid supplied with the instrument was used to determine temperature values 
at various depths. From these* a running plot was kept of the thermal 
structure of the sea along the cruise track. The BT casts regularly reached 
200 meters depth* and frequently reached 250 meters. 
The surface layer of the ocean normally is well-mixed and isothermal* 
or nearly so. This mixed layer usually has a sharply defined base below 
which is the thermocline where the temperature cools abruptly with depth. 
When leaving Johnston Island (near 17°N) November 11* the base of the 
mixed layer was found at 75 meters depth. As the ship proceeded southwest- 
ward* the mixed layer became shallower. Its base rose gradually but steadily 
to a depth of only 30 meters between 10 and 9 north latitude. As the cold 
water below the mixed layer rose closer to the surface* the temperature at 
150 meters depth cooled steadily from 22.5°C near Johnston to 13.5°at 10°N. 
(see Figure l). This region where the base of the surface mixed layer 
tilts upward toward the south, and the underlying water cools toward the 
south, is the region of the westwardly flowing North Equatorial Current (NEC). 
At about 10°N* the ship passed out of the NEC into the eastwardly 
flowing Equatorial Countercurrent (ECG). The surface mixed layer immediately 
began to thicken. Its base sank steadily to 180 meters depth* and the 
tempgrature at 150 meters rose correspondingly* reaching a high of 27*2° 
at h N (Figure 1). 
At this latitude* the ship'prossed the southern boundary of the ECC 
into the westwardly flowing South Equatorial Current (SEC). The temperature 
at 150 meters began to cool again* the deeper portions of the mixed layer 
were replaced by cooler water* and a new* base to the surface mixing formed 
near 75 meters depth, gradually rising to 50 meters as the ship entered the 
Southern Grid at 2°N. 
Figure 1 also shows a plot of total birds sighted each day, for 
comparison with sea temperature at the surface and at 150 meters depth. Bird 
numbers were moderate from Johnston Island to lU N* then dropped significantly 
in the NEC and the northern edge of the ECC. However* the birds then increased 
southward to a peak value near the southern boundary of the ECC, and dropped 
off again as the ship passed into the SEC and approached the Southern Grid. 
Bird numbers show a very good correlation with the 150 meter temperature, 
but do not correlate well with surface temperature. The 150 meter temperature 
itself is not the property of significance to the birds. Rather, the 
temperature at this depth is probably a good indicator of differing water 
masses* whose differences in food content at the surface are the more likely 
reason why birds are concentrated in favored zones. 
The temperature plots in Figure 1 are based on a preliminary analysis 
of the BT data performed at sea. A refined analysis will be performed by 
the Honolulu Laboratory* Bureau of Commercial Fisheries* which may be 
expected to modify the temperature data negligibly if at all. 
