Figure 7. — Oceanic sightings between 140^. 

 longitude and 170°W. longitude summarized by 

 latitude. The height of the bar represents the 

 average bird count per day, and the shaded por- 

 tion the average number of fish schools per day, 

 whether or not they were accompanied by birds. 

 The number of days' observations Is shown below 

 each bar. The zone between the Equator and 10° 

 N. has been considered In two portions because 

 the Countercurrent lies between 5° and 10°N. 

 North of the Countercurrent Is the North Equa- 

 torial Current and south of It flows the South 

 Equatorial Current. 



moderate In the Counter- 

 cur]?ent, greatest In the 

 South Equatorial Current be- 

 tween the Equator and 5°N. , 

 where surface fish schools 

 were scarce, and relatively 

 low south of the Equator. 



The difference between 

 surface sightings and long- 

 line catches may be related 

 to the difference In the 

 species of fish Involved, for 

 the longllne takes mostly 

 yellowfln and to a lesser ex- 

 tent blgeye tuna and the sur- 

 face sightings are predomi- 

 nantly skipjack. These spe- 

 cies undoubtedly have dif- 

 ferent ecological require- 

 ments, and so there is little 

 reason to expect their dis- 

 tributions to be coincidental. 



TROLLING 



Trolling represents a 

 method of directly sampling 

 the surface populations of 

 fishes. It is a direct 

 measure inasmuch as the lo- 

 cation of fish is not so de- 

 pendent on some associated 

 phenomenon such as flocks of 

 birds. For this reason it 

 potentially provides an inde- 

 pendent check on the relative 

 abundance of fish as deter- 

 mined by sightings. There 

 were two types of trolling 

 carried out on POFI vessels, 

 secondary and primary 

 trolling. 



Secondary trolling is conducted as a routine activity whenever vessels are 

 under way during daylight hours unless it would interfere with more Important 

 missions. The speed of the vessel is not adjusted to facilitate trolling, and no 

 attempt is made to fish in favorable locations or to work the schools of fish en- 

 countered. The usual cruising speed of the POFI vessels (7 to 10 knots) does not 

 preclude hooking fish, but there have been occasions when hooked fish were lost be- 

 cause of the difficulty of handling them from a rapidly moving vessel. After 

 January 1952 the method of surface trolling was standardized. Two 25-fathora lines 

 were fished, one from each side of the stern. One line was equipped with a 6-ounce 

 leadhead jig with white and Plymouth Rock feathers and a 9/0 double hook. The 

 second line had a red bone jig with a 9/0 double hook. Before that time two lines 

 of varying lengths and a variety of lures had been used, but we doubt that these 

 variations had a significant effect on the catch. 



On a number of occasions the primary type of trolling has been done, particu- 

 larly in the vicinity of islands. In contrast to secondary trolling, the ship's 

 speed is brought down to about 6 knots and attempts are made to work favorable 

 areas and the schools of fish encountered. Usually 4 to 6 lines were fished from 

 the stern of the vessel, and various lures were used. These lures were of the same 

 general type and size as those used in secondary trolling. (For details of primary 

 trolling see Bates 1950.) 



11 



