HORIZONTAL DISTRIBUTION OF DEEP=SWIMMING TUNAS 



Two of the cruises upon which this report is based were 

 designed to ascertain the abundance of tunas in the equatorial 

 region in the general vicinity of 150" to 160° VJ", longitudeo The 

 third was designed to ascertain the abundance of deep-'swimming tunas 

 in the vicinity of a small oceanic island„ They are considered as 

 a unit because they took place during a "season" when meteorological 

 and presumably hydrographic conditions were similar, with southeast 

 trades predominating over the Equator. The precise location of each 

 station of the three cruises is shown in figure 9„ The catches 

 corresponding to these stations are indicated graphically in figure 

 10 and are listed in tables 1 to S^/o 



The distribution of the catches indicates that the pattern 

 of variation was not a random one but rather shows a concentration 

 of yellowfin tuna between l" and 6° N, latitude. Generally a con- 

 centration like this arises because the fish are congregated in a 

 particularly favorable environmental situation as far as the immediate 

 necessities of life are concerned, or they may be congregated in a 

 particularly favorable location for spawning. 



The possibility that the abundant yellowfin tuna north of 

 the Equator are a spawning concentration seems rather remote. Data 

 presented by various workers (Schaefer and Marr 1948, Schaefer 1948) 

 do not indicate well-delineated spawning areas and seasons for yellow- 

 fin tuna, without which a marked spawning concentration seems 

 improbable, and field observations on yellowfin ovaries in the area 

 under consideration indicate spawning is taking place during all 

 months of the year. Spawning concentrations cannot be completely 

 discounted, as June 0.953) has shown that the period of high yellow- 

 fin catch in the Hawaiian Islands area coincides with the period of 

 maximum spawning activity. He suggests that this fishery is based 

 on a "spawning run." 



There is, however, considerable indirect evidence that the 

 concentration of yellowfin tuns is a response to a more favorable 

 food supply, though direct evidence such as the demonstration that 

 the tuna are better fed where they are more abundant is lacking. 

 The available evidence centers around interpretation of the hydro- 

 graphic features and variations in the supply of plankton in this 

 section of the oceano 



A brief sketch of the environment during the three, 

 equatorial fishing sections is afforded by figure 11 to IZ^ . 



^See appendix for complete catch records by station. 



Z/ Comp i 



^Comparable data for stations not covered in the figures are given 

 in the appendix. Wind direction and force for each station are 

 also given in the appendix. 



13 



