found that the northern range limits of the species 

 agreed in position with tlie 21° C. (70° F.) surface 

 isotherm at almost any time. Broadhead and 

 Barrett (1964) showed a similar agreement of 

 both northern and southern range limits with the 

 20° C. (68° F.) surface isotherm. 



Figure 1 shows the kind of data that Blackburn 

 and associates (1962) presented for the period 

 1951-59, together with similar data for 1960-65. 

 It gives the approximate latitudinal position, in 

 each montli of 1951-65, of the northern limit of 

 commercially caught yellowHn tuna, the northern 

 limit of commercially caught skipjack tuna, and 

 the 21° C. surface isotherm. The latitudes shown 

 are those along the entire west coast of Baja 

 California and the southern part of the coast of 

 California. Tuna range limits are based on 

 lATTC catch data that are grouped by 1° squares. 

 They have been graphed at the midpoint of the 

 1° range of latitude at which the most northern 

 commercial catch (regardless of its amount) was 

 made during the montli. Temperatures for 1951-59 

 are from the series of monthly temperature charts 

 published by Eber, Saur, and Sette (1968), except 

 for August 1952 when CalCOFI cruise data 

 showed a more northward penetration of the 

 21° C. isotherm. Temperatures for 1960-65 are 

 from monthly temperature charts published by 

 the BCF (Bureau of Commercial Fisheries) Bio- 

 logical Laboratory, San Diego, Calif. Because the 

 BCF temperature data were contoured in inter- 

 vals of 5° F., including 70° F. (21° C), it was 

 more convenient to compare tuna limits with 

 21° C. than with 20° C. 



As mentioned jji-eviously, figure 1 illustrates 

 the seasonal movement of yellowfin and skipjack 

 tunas along the west coast of Baja California 

 (and in some yeai"s, California) . It is not a spawn- 

 ing migration for either species (Orange, 1961; 

 Klawe, 1963). The timas appear west of the south- 

 em tip of Baja California (lat. 23° N.) in late 

 spring or early summer, extend their ranges north- 

 ward during the summer, contract their ranges 

 southward during the autumn and early winter, 

 and leave the area (except in 1958) during the 

 late winter and early spring. The 21° C. surface 

 isotherm changes position in the same way. The 

 positions of the isotherm and the range limits 

 vary from year to year in the same month, but 

 they generally agree closely with one another in 



any particular month. It is because of this agree- 

 ment that I think temperature determines the 

 range limits. Figure 1 shows a few disagreements 

 which could have resulted from temperature data 

 or tuna data that were unrepresentative of con- 

 ditions within a month. The two kinds of data 

 were not collected together. All temperature data 

 ajjply to the sea surface or the upper 10 m., where 

 yellowfin and skipjack tunas are generally seen 

 and caught. 



The range of skipjack tuna appears to be limited 

 also by a high temperature, about 28° C, and the 

 same temperature or a higher one may possibly 

 limit the range of yellowfin tuna (Blackburn, 

 1965, and references there). Such temperatures 

 seldom occur in extensive areas off western Baja 

 California, however. 



Nothing indicates that temperature plays any 

 direct part in determining the patchy distribution 

 of the tunas within their range limits (Blackburn, 

 1965). According to the hypothesis being tested, 

 food supply is responsible for this aspect of the 

 distribution. The principal food organism of trop- 

 ical tunas off western Baja California is red crab, 

 as mentioned above. Alverson (1963) sorted stom- 

 ach contents of 567 yellowfin and 151 .skipjack 

 tunas taken off the west coast of Baja California 

 and the coast of California. The comj^osition 

 of the stomach contents of yellowfin tuna by vol- 

 ume was 78 percent red crab, 10 percent northern 

 anchovy, Engraidis mordax^ and 12 percent other 

 animals (euphausiids absent). The composition of 

 the skipjack tuna stomach contents was 37 percent 

 red crab, 28 percent northern anchovy, 19 percent 

 euphausiids, and 16 percent other animals. North- 

 ern anchovies are much more abundant off Cali- 

 fornia and northern Baja California than off 

 southern Baja California, however (Baxter, 1967; 

 Ahlstrom, 1967) ; in the latter area red crabs are, 

 therefore, probably a larger component of the 

 tunas' diets than Alvereon showed. Northern an- 

 chovies were very seldom taken on the cruises de- 

 scribed in this paper, jjrobably because they were 

 not common in the area. Euphausiids were taken 

 on these cruises, but it was decided not to attempt 

 to study their distribution in relation to that of 

 the tunas, for the following reasons. Euphausiids 

 are more difficult to sort and measure \olumetri- 

 cally than red crabs are; they are not routinely 

 catchable or observable by some of the methods 



150 



U.S. FISH AND WILDLIFE SERVICE 



