sources were generally similar and are not dis- 

 tinguished in this paper. Data were available on 

 the amounts of tuna captured at each of the 

 charted positions, but they are not given here be- 

 cause size of catch is not necessarily a measure of 

 abundance. Catch per unit of fishing effort can 

 be considered as a measure of abundance, but it 

 would be difRcidt to establish the amount of elfort 

 expended at each of the individual points shown 

 on the charts. For this study, any commercial catch 

 of tuna indicates an aggregation of the fish, and 

 the main interest lies in the general distribution 

 of the aggregations. 



In areas where no occurrences are shown, there 

 was either no significant fishing effort (hence 

 tuna distribution unknown) or no fish, at the 

 period of the cruise. Data showing absence of fish 

 are few; they refer to combinations of 1° squares 

 and months in which at least 5 boat-days of fishing 

 effort yielded no catches. These examples of un- 

 productive effort are mentioned later for the 

 individual cruises. 



A few catches and observations of yellowfin and 

 skipjack tunas made from the oceanographic ves- 

 sels have been included in the charts. 



RESULTS OF INDIVIDUAL CRUISES 



In the following sections, six cruises are dis- 

 cussed in the order that best represents the typical 

 seasonal change during the period of the year when 

 yellowfin and skipjack tunas occur west of Baja 

 California. The distributions of surface tempera- 

 ture, surface chlorophyll (7, red crabs, and tunas 

 are described and explained for each cruise, with 

 special reference to their relations with the sea- 

 sonal cycle of coastal upwelling. 



CRUISE TO-64-1 



This cruise showed environmental conditions 

 that are probably typical of the beginning of a 

 yellowfin and skipjack tuna season off western 

 Baja California (see fig. 1). Figure 3 shows the 

 track and station positions, omitting individual 

 positions of two clustei's of stations that were 

 occupied for special purposes. Figure 4 gives the 

 essential features of the distributions of the prop- 

 erties previously discussed ; this and similar figures 

 show only selected isograms of surface tempera- 

 ture and surface chlorophyll a. The isotherms in 

 figure 4 refer only to the temperature distribution 



on part 1 of the cruise, June 9-15, 1964. At the few 

 stations that were re-occupied on part 2 of the 

 cruise, temperatures averaged about 2° C. higher 

 than on part 1. Figure 4 includes only those tuna 

 occurrences that were recorded for the period 

 June 9-15. The charted distributions of chloro- 

 phyll a and red crabs are based on all available 

 data fi'om the entire cruise, as they are also for 

 the other cruises. The red crab data are given in 

 table 1. 



Along the Baja California coast below Point 

 San Eugenio the coastal upwelling typically be- 

 gins about April, is strong from April through 

 June, and ceases about August (Keid, Roden, and 

 Wyllie, 1958; Wyllie, 1961; Lynn, 1967). 

 CalCOFI stations 123.37 and 143.26, which are the 

 same as stations 2 and 18 in figure 3, are con- 

 sidered to be the centers of the two principal 

 coastal upwelling areas (Lynn, 1967). These 

 areas, one along the coast south of Point San 

 Eugenio and the other south of Cape San Lazaro, 

 are indicated by the 15^ C. surface isotherm in 

 figure 4. In the northern area, surface tempera- 

 tures below 13° C. were recorded off San Pablo 

 Point. The 16°, 17°, and 18° C. isotherms lie gen- 

 erally parallel to the coast, rather than in the wavy 

 configurations described later for certain offshore 

 isotherms on subsequent cruises. This parallel 

 orientation apjjears to be normal in June 

 (Anonymous, 1963). 



Off Cape San Lucas a well-defined tempera- 

 ture front existed at the sea surface, oriented in 

 much the same way as in May 1960 and April 1961 

 (Griffitlis, 1963, 1965). Surface water below 17° 

 C, in part below 16° C, lay in a narrow band just 

 west of Cape San Lucas ; it was probably upwelled 

 farther north, where temperatures were similar. 

 Eastward from this cold area along the south 

 coast of the peninsula, surface temperatures rose 

 to above 25° C. The 17° to 21° C. isotherms trended 

 more or less to the southeast from Cape San Lucas, 

 whereas the 22° to 25° C. isotherms trended paral- 

 lel to the south coast of Baja California. The 

 front, therefore, was very strong close to Cape San 

 Lucas but split about 15 nautical miles (28 km.) 

 offshore into two weaker parts. Some 60 miles (110 

 km.) southwest of Cape San Lucas, a tongue of 

 water over 19° C, mostly over 20° C. (the two 

 isotherms are close together), protruded north- 



DISTRIBUTION OF TROPICAL TUNAS OFF WESTERN BAJA CALIFORNIA 



155 



