(1) the physical, chemical, and biological prop- 

 erties of the entrance to the Gulf of California 

 at the time of year when tuna are normally 

 abundant there (Alverson, 1960; Griffiths, 

 I960); and (2) any oceanic fronts found there, 

 California Cooperative Oceanic Fisheries In- 

 vestigation (CalCOFl) cruise 6004 (M. V. Black 

 Douglas) which ended at Cape San Lucas, 

 Lower California, on 30 April 1960, provides 

 supplementary data nearly contemporaneous 

 with those of TO-60-1, inthe area imnnediately 

 north of Cape San Lucas, off western Cali- 

 fornia. Further supplementary data were ob- 

 tained from several CalCOFIand other cruises 

 to the area around Cape San Lucas (or nearby 

 Cape Falso). The references of the data re- 

 ports of these cruises are given in the bibliog- 

 raphy. 



The fronts discovered on TO-60-1 in May 

 1960 were numbered 1, 2, 3, and 4; they formed 

 a valuable basis for the more intensive study 

 of one front (no, 5) at the end of STOR cruise 

 TO-61-1 in April 1961. The results of this 

 latter study form the body of this paper. 



The term "front" apparently is a meteoro- 

 logical term that has been widely adopted by 

 oceanographers, but it has no precise defini- 

 tion in oceanography. Cromwell and Reid ( 1956) 

 say, "A front will mean a band along the sea 

 surface across which density changes ab- 

 ruptly." This definition does not specify the 

 cause of the density change, which may be due 

 to a change in either salinity or temperature, 

 or both. Accordingly, this leads to the use of 

 such terms as "temperature fronts" or 

 "salinity fronts." 



Uda (1959) says that a front (siome) is a 

 boundary between two different kinds of water: 

 "Then the boundaries of the water mass are in 

 a region where the gradients of the property 

 are maxinnum (i.e., the Polar Front, etc.)." 



La Fond (1961a) states: "The leading edge of 

 a border separating unlike water masses is 

 called a front. Fronts can occur not only 

 between water masses of different salinity but 

 also between those differing in other prop- 

 erties, such as temperature." 



Undoubtedly the term "water mass" is used 

 loosely in these definitions and does not imply 

 the specific definition given by Sverdrup, 

 Johnson, and Fleming (1942). Fronts often are 

 found, for example, between upwelled and 

 nonupwelled water, though both kinds arefronn 

 one water mass in the sense of Sverdrup et al. 



It is difficult to achieve a precise numerical 

 definition of a front. Uda (1959) specifies a 

 front, in terms of a temperature gradient, as 

 ranging from 0.5° C./lO miles to 5.0° C./lO 

 miles, which illustrates the lack of precision 

 in defining a front. 



I propose to use the term "frontal system" 

 for the whole boundary between two kinds of 

 water, reserving the word "front" for those 

 small parts of the system that actually are 

 investigated. Convergent and divergent fronts 



are terms occasionally encountered and are 

 derived from the terms convergence and 

 divergence as commonly used in oceanography. 



Uda (1959) and Griffiths (1963) reviewed the 

 research on the major frontal systems of the 

 world oceans. 



It is widely believed that convergent fronts 

 mechanically aggregate plankton. Uda (1938) 

 described the accumulation of flotsann at siome 

 (fronts), and Uda and Ishino (1958) indicated 

 that convergent fronts show a line of biological 

 demarcation owing to aggregation of biota. Hela 

 and Laevastu (1962), without quoting specific 

 references, illustrate plankton accumulated in 

 a front. La Fond's (196la) definition, stated 

 above, continues: "These fronts raise water 

 of higher nutrient content to the surface. In 

 addition, the fronts tend to concentrate floating 

 fish food in adjacent areas." This raises the 

 question of whether a front manifests a higher 

 standing crop of plankton than does water on 

 either sid^, either by aggregating the biota or 

 by causing conditions specially favorable to 

 the growth of a higher standing crop. This, in 

 turn, probably depends on whether the front 

 is convergent, in which case aggregation might 

 be expected, or divergent, in which case 

 nutrient-rich water might be brought to the 

 surface to increase plankton growth. Uda ( 1959) 

 suggested that the latter occurs at siome, 

 though those fronts he discusses apparently are 

 convergent. Evidently, there is a conflict of 

 possibilities. Siomina (1958) stated that phyto- 

 plankton "blooms" are a feature of the area of 

 mixing between the Kuroshio and Oyashio, but 

 he does not say whether these "blooms" are a 

 result of in situ production or of mechanical 

 aggregation; perhaps both are the cause. 



King and Hida (1957) show, on a relatively 

 large scale, a relation between high standing 

 crop of zooplankton and the frontal system in 

 the central equatorial Pacific, without, how- 

 ever, establishing the actual cause of the high 

 plankton values. They showed peak values 

 slightly south of the three fronts studied. They 

 also showed a somewhat lower quantity of zoo- 

 plankton in the convergence zone (2° N.- 4° N.) 

 than in the divergence zone (2° S. - 2° N.). 



Beklennishev and Burkov (1958) showed that 

 plankton is more abundant in the zone of con- 

 tact between the Kuroshio and Oyashio than to 

 either side and that this plankton comprises 

 forms from both biotopes ((1) cool, low- 

 salinity; (2) warmi, high- salinity). The zone of 

 mixing, however, is about four hundred miles 

 wide in the area they studied. They say further, 

 "The quantity of species inthe zone of mingling 

 of water masses sometimes turns out to be 

 greater than each separately (Siomina, 1957)." 



^In their bibliography these authors do not refer to a 

 1957 publication by Siomina, but do refer to his 1958 publi- 

 cation, which seems to be appropriate to their statement, 

 consequently, his 1958paper is listed in the literature cited 

 section of this paper. 



