comparisons between occurrences of tuna and dis- 

 tributions of properties as observed in the ocean. 

 Many of these comparisons have been based on 

 noneontemporaneous data. Distributions of tuna 

 prey have been inferred from other property dis- 

 tributions more often than they have been ob- 

 served. Different kinds of data on tuna occurrence, 

 some of better quality than others, have been used. 

 Tlie tendency has existed to compare tuna data 

 with environmental data on broad scales of space 

 and time, a procedure which is more suitable for 

 generating hypotheses than for testing them. 



The hyixjthesis must survive tests of detailed 

 close comparison between tuna and environment 

 on narrow scales of space and time if it is to be 

 accepted, but very few suitable tests have been 

 made. The need for a good test became particularly 

 evident about 5 years ago, when plans were being 

 made for a series of oceanographic surveys of the 

 eastern tropical Pacific Ocean (the EASTROPAC 

 Expedition, 1967-68). One of the purposes of the 

 expedition was to identify areas, outside the limits 

 of existing fisheries, in which skipjack tuna, 

 Euthynmus ■peJ<i'mts, might be abundant. It was 

 necessary to know whether that purpose would be 

 served by measuring standing stock of tuna prey 

 routinely on the expedition. 



The STOR (Scripps Tuna Oceanography Re- 

 search) Program made a series of cruises off the 

 west coast of southern Baja California to test the 

 hypothesis. This paper gives the results of the test. 

 The area off southern Baja California was se- 

 lected for the following reasons: 



1. It is the area closest to the Scripps Institu- 

 tion of Oceanography in which both yellowfin 

 tuna, Thunnus alhacar'es, and skipjack tuna occur 

 regularly in abundance. These are the principal 

 species taken in American tima fisheries in the 

 eastern tropical Pa^^ific. The most detailed pre- 

 vious study of tuna ecology in the eastern tropical 

 Pacific dealt particularly with yellowfin tiuia 

 (Blackburn, 1962, 1963), but the need is now press- 

 ing for work on skipjack tuna as well. 



2. Yellowfin and skipjack tuna move seasonally 

 from the tropics into the area west of Baja Cali- 

 fornia and then back to the tropics. This shift has 

 been shown by tagging experiments (Schaefer, 

 Cliatwin, and Broadhead, 1961 ; Fink, unpub- 

 lished). The northern range limits of both species 

 are generally in the area or south of it (fig. 1). 

 Property distributions in the area should, there- 



fore, show which properties correspond with range 

 limits and which do not. 



3. The food chain of yellowfin and skipjack 

 tuna is shorter and simpler in the Baja California 

 area than elsewhere in the eastern Pacific in ways 

 that were expected to facilitate the study. In the 

 other areas both tuna species consume a great vari- 

 ety of fishes, crustaceans, and cephalopods, most 

 of whicli are presumably carnivores (Alverson, 

 1963). Off the west coast of Baja California, liow- 

 ever, both tuna species feed mainly on the gala- 

 theid crab Pleuroncodes planipes (Alverson, 

 1963), which is a facultative herbivore and some- 

 times the dominant herbivore in the area (Long- 

 hurst, Lorenzen, and Thomas, 1967). P. planipes, 

 frequently called red crab, is commonly pelagic, 

 easy to collect in nets from a research sliip, and 

 conspicuous (easy to sort) in net catches. It is the 

 principal species of the micronekton — about 80 

 percent of the total, by volume — in the area west 

 of southern Baja California (Blackburn, 1968). 

 Boyd (1967) recently published a good illustra- 

 tion. 



CalCOFI (California Cooperative Oceanic 

 Fisheries Investigations) has provided much 

 oceanographic information about waters west of 

 Baja California, through a program of seasonally 

 repetitive cruises that began in 1949. lATTC 

 (Inter- American Tropical Tuna Commission) has 

 been recording commercial catches of yellowfin 

 and skipjack tunas in the same area and elsewhere 

 since the beginning of 1951. Tlie lATTC tuna 

 data were suitable for the )>roposed stiuly, but the 

 CalCOFI oceanographic data were not very suit- 

 able because they had been obtained for other pur- 

 poses. The CalCOFI cruises generally coA-ered 

 only about half the area that yellowfin and skip- 

 jack occupy off Baja California (see example in 

 fig. 7) ; they frequently missed the period when the 

 tunas were most widespread in the area ; and they 

 seldom provided any information on phytoplank- 

 ton or on animals eaten by tuna. It was necessary, 

 therefore, to make special cruises for tuna ecology 

 studies. Five such cruises, together with one Cal- 

 COFI cruise that was equipped to serve the same 

 purpose, were made. They covered most of the 

 period of the year (in different years) when tropi- 

 cal tunas occur off Baja California. 



The hypothesis to be tested was that yellowfin 

 and skipjack tunas generally do not occur in wa- 

 ters of surface temperature below 20° C, even 



148 



U.S. FISH AND WILDLIFE SERVICE 



