A Study of Oceanl^c Fronts off Cape San Lucas, 

 Lower California ^ 



By 



RAYMOND C. GRIFFITHS 

 Assistant Specialist in Biology- 

 University of California 

 Institute of Marine Resources, Scripps Institution of Oceanography 



La Jolla, Calif. 



ABSTRACT 



The general concept of a front is discussed, with some emphasis on the problem 

 of strict definition and on the probable effects of a front on the biota. A note is made 

 of the lack of precise studies, particularly biological studies. 



Some of the difficulties in obtaining observations at five fronts off Cape San 

 Lucas are mentioned. 



Data from two cruises in April and May I960 are used to depict the main 

 oceanographic features of the area at large. 



The most easily studied, sharpest, and most stable front was found off Cape 

 San Lucas on a cruise in April 1961. Here warm, saline Gulf of California water 

 moved slowly, inshore, while cool, less saline California Current water moved slowly 

 eastward offshore, the front being maintained between them. Profiles of all the main 

 oceanographic variables across the best studied front were drawn from data of two 

 triplets of hydrocasts, a day apart, across the front. The vertical temperature and 

 salinity distributions showed the interface between the two water masses to be 

 Z-shaped. 



Animal and plant plankton and nekton were measured in relation to this front, but 

 the evidence for accumulation of biota at the front is not conclusive. 



Four other fronts were studied, though not as thoroughly; the data from them are 

 used to illustrate certain difficulties in studying fronts. 



INTRODUCTION 



Since June 1957, the Scripps Tuna Ocean- 

 ography Research (STOR) Program of the 

 Scripps Institution of Oceanography (SIO) has 

 been concerned with "An oceanographic in- 

 vestigation of causes, mechanisms, and pre- 

 dictability of changes in availability of tuna 

 in the eastern tropical Pacific." The work is 

 supported by the U.S. Fish and Wildlife Serv- 

 ice, Bureau of Commercial Fisheries. This 

 paper shows results of oceanographic studies 

 related to this research. Yellowfin tuna, in 

 particular, and possibly skipjack, migrate in 

 the spring or early summer across the entrance 



The study is a contribution from the Scripps Institute 

 of Oceanography, University of California, San Diego. 



Note. — This work was financed by the Bureau of Com- 

 mercial Fisheries under Contract Nos. 14-19-008-9354, 

 14-17-307-1, 14-17-0007-28 and 14-17-0007-70, with 

 funds made available under the Act of July 1, 1954 (68 

 Stat. 376), commonly known as the Saltonstall-Kennedy 

 Act. 



of the Gulf of California from the waters off 

 western Mexico to waters off western Lower 

 California (Schaefer, Chatwin, and Broadhead, 

 1961). The most prominent oceanographic 

 features of the Gulf's entrance at that time are 

 oceanic fronts. This raised the question of 

 whether the occurrence of these fronts are 

 related to the nnovements of the tuna,^ either 

 directly and physically or indirectly due to 

 the effects of the fronts on the biotic regime 

 in the area. 



To answer this question we first need to 

 know something of the structure of, and 

 distribution of properties in and about, these 

 fronts, including the kinds of water forming 

 them. The main purpose of this paper is to 

 present the results of STOR's preliminary 

 investigations of these phenomena. 



STOR cruise TO-60-1 (30 April to 26 May 

 1960), with M. V. Hugh M. Smith, investigated: 



Henceforth the word "tuna" will be used when referring 

 generally to both species; the common names, yellowfin 

 and skipjack, will be used when referring specifically to 

 one or the other. 



