centered off the mouth of the Gulf of California and 

 the other is the offshore area bounded by lat. 

 15°-20°N and long. 107°-115°W. 



FISHING CONDITIONS 



IN RELATION TO 

 THE THERMOCLINE 



Figure 8 shows the monthly thermocline topog- 

 raphy (depth to the top of the thermocline) for the 

 eastern tropical Pacific as described by Robinson 

 and Bauer (1971). The depth to the top of the ther- 

 mocline is in general relatively shallow in the east- 

 ern tropical Pacific (Cromwell. 1958; Forsbergh 

 and Broenkow, 1965: Sund and Renner. 1959). As 

 shown in Figure 7, the shallow thermocline area 

 begins to extend seaward beginning in June, ex- 

 tends farthest seaward from July through Sep- 

 tember, and begins contracting after October. 



This pattern of expansion and contraction of the 

 shallow thermocline area coincides fairly closely 

 with the pattern of expansion and contraction of 

 good fishing grounds as shown in Figure 5. It is 

 noted that the areas of good fishing begin expanding 

 after June in correspondence with the expansion of 

 the shallow thermocline area. From July through 

 September, when the shallow thermocline area is 

 most extensive, the areas of good fishing are also 

 most extensive. In November, when the shallow 

 thermocline area is contracted, so is the area of 

 good fishing. Between December and March, when 

 the shallow thermocline area is narrowest and con- 

 fined to the region around the mouth of the Gulf of 

 California, the area of good fishing is also confined 

 to the same small area. For example, the 100-ft con- 

 tour in thermocline topography is recessed shore- 

 ward at about lat. 23°-25°N and long. 1 17°W in Sep- 

 tember and lat. 21°N and long. 112°W in October. 

 In these same general areas good fishing grounds 

 are found in a similar pattern. In November, the 

 100-ft contour is noticeably recessed shoreward at 

 about lat. 20°-23°N and long. 106°W and a good fish- 

 ing ground is also found with this same shape. 



In order to clarify this relationship between 

 thermocline depth and good fishing grounds, the 

 areas with depths to the top of thermoclines shal- 

 lower than 100 ft were calculated for subarea S 

 (subareas SE and SW combined) and plotted along 

 with average monthly catch rates in Figure 9. It is 

 seen that the monthly catch rates increase as the 

 index of shallow thermocline area increases. Both 

 the catch rates and index are highest from July 



through October. The catch rates are also some- 

 what high from December through January when 

 the index of shallow thermocline area is low. This 

 phenomenon is caused by the fact that fishing is 

 conducted around the mouth of the Gulf of Califor- 

 nia where the thermocline is shallow during these 

 months. 



The relationship between the depth of the ther- 

 mocline and the distribution of tunas has been dis- 

 cussed by several workers (Brandhorst, 1958; 

 Blackburn, 1965; Green, 1967; Suda, Kume, and 

 Shiohama, 1969; and Kawai, 1969). According to 

 Brandhorst ( 1958), an area with a high standing crop 

 of zooplankton is generally also a region with a shal- 

 low thermocline, while an area with poor standing 

 crop would, in general, tend to correspond with a 

 deeper thermocline. Laevastu and Rosa (1963) sug- 

 gested that thermocline ridges seem to be favorable 

 for aggregation of tunas. As one of the factors re- 

 lated to the above, it is considered that a high stand- 

 ing crop of zooplankton would have the effect of 

 attracting small forage organisms which in turn re- 

 sults in aggregating tunas. 



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Figure 9. — Index of extent of surface areas with thermo- 

 clines shallower than 100 feet plotted against the monthly 

 average catch rates of striped marlin for area S (includes 

 subareas SW and SE in Fig. 1). 



306 



