Table 1. — Concentrations of Pleuroncodes planipes, adults and juveniles {not larvae), in ml. /IC^m.^ of water strained, on 



cruise TO-64-1 



ILetters under kind of observation signify: M, micronekton tiaul; Z, zooplankton liaul; S, seen in tlie water. Where concentrations were measured or estimated by 

 more than one metiiod, the highest concentration, corresponding to the first letter, is listed] 



that were use.ful with red crabs (high-speed net 

 catches, and observations at the sea surface) ; and 

 yellowfin tuna do not eat them. In any event, north- 

 ern anchovies and some euphausiids are faculta- 

 tively herbivorous like red crabs and miglit be 

 expected to have a similar distribution. 



I decided to distinguish between concentrations 

 of red crabs that were greater or less than 40 

 ml./l,000 m.', as mentioned above. Concentrations 

 above zero ranged from 0.1 to 5,238 ml./l,000 m.' 

 (tables 1-6), with median 36 ml./l,000 m.^ It is 

 implicit in the hypothesis being tested that tunas, 

 which are highly mobile, encounter and aggregate 

 around the highest concentrations of food in any 

 area which they enter. It was, therefore, desirable, 

 on the one hand, to use some value at least as high 

 as the median to distinguish high concentrations 

 from low. On the other liand, the frequencies at 

 successive intei"vals of concentration declined 

 sharply above the median, to the extent that chart- 

 ing would have be«n difficult if a value appreciably 

 over 100 ml./l,000 m.^ had been used. Trial made 

 it evident that the projiei-ty charts presented later, 

 and the conclusions of tlie study, were much the 

 same at a value of 40 (just above the median) and 

 at 100 ml./l,000 m.^ Therefore, the former value, 

 wliich made more data available for drawing iso- 

 grams of abundance of red crabs on the charts, was 

 chosen. 



MATERIAL AND METHODS 



The following sections give information about 

 the cruises and stations from which data were ob- 

 tained; the methods used for measuring surface 

 temperature, surface chlorophyll a, and concentra- 



tion of red crabs ; and the kinds and sources of con- 

 temporaneous data on tunas. 



CRUISES AND STATIONS 



The oceanographic data for this study were ob- 

 tained in six cruises — one made in 1959, which gen- 

 erated the hypothesis mentioned above, and five 

 made in 1964-66. The area of study was restricted 

 to south of lat. 28° N., where the tunas (fig. 1) and 

 red crabs (Longhurst, 1967) occur each year ; these 

 species appear farther north in some years, but not 

 in all. Figure 2 identifies localities and topograpliic 

 features to which I refer in this paper. The princi- 

 pal banks (underwater elevations of the bottom) 

 within and beyond the 100-fathom (183-m.) line, 

 and the small islands known as Alijos Rocks, are 

 shown. Tuna fishermen consider that these fea- 

 tures rej^resent good fishing areas, a matter which 

 is discussed later. 



The tracks and station positions for the six 

 cruises are shown in figures 3, 5, 7, 9, 11, and 13, 

 which appear in later sections of this report. They 

 were based on tlie CalCOFI basic station plan 

 (since 1950) , and most of the stations can be iden- 

 tified easily with CalCOFI station positions 

 (Anonymous, 1963), although they have not been 

 numbered in the CalCOFI way. Two of the cruises 

 were devoted entirely to occupying a series of Cal- 

 COFI stations (figs. 7 and 9) . On each of the other 

 four cruises (figs. 3, 5, 11, and 13) a series of Cal- 

 COFI stations was occupied first (part 1 of the 

 cruise, terminating in the southern part of the 

 area) and the ship then returned northward by a 

 different route, occupying special stations in areas 

 of particular interest (part 2 of the cruise). The 



DISTRIBUTION OF TROPICAL TUNAS OFF WESTERN BAJA CALIFORNIA 



151 



