42 



Roden and Groves state that temperatures at La Paz average from 19.8° 

 to 21.1°C., with a range of 9.3"", which is quite similar to the temperature 

 distribution at Mazatlan, and typical of a true tropical environment. 

 Figures are also given for Cabo San Lucas, which is slightly warmer, 

 having only 8.5° C. difference between winter and summer. Cape San 

 Lucas seems to have the narrowest range of temperature in the Gulf, 

 being comparable to oceanic equatorial Pacific surface temperatures. This 

 may explain why a number of Indo-Pacific invertebrates and fish can be 

 found only in this region (Walker, 1960, and Rosenblatt, 1959). These 

 animals or their larvae must be transported by oceanic currents from the 

 central Pacific islands to this small cape where suitable ecological con- 

 ditions, consisting of a small coral reef and warm clear waters, can be found. 

 In many respects. Cape San Lucas has an environment similar to a tropical 

 Pacific island, and thus may be the closest habitat of this kind to be met 

 with along the coast of Middle America. The faunal breaks along the west 

 side of the Gulf (fig. 12) can also be closely correlated with various shallow 

 water temperature regimes and other physical barriers. 



Salinity Distribution in the Gulf of California. 



No independent study of the salinities in the Gulf was carried out by 

 this investigator. It was observed from earlier studies that salinity differ- 

 ences from one end of the Gulf to the other, and from surface to bottom, 

 were of such small magnitude that they were assumed to be relatively 

 unimportant in influencing the distribution of most benthic animals. 

 Roden and Groves (1959, p. 15) show a difference of only one part per 

 thousand salinity from winter to summer, and from Punta Peiiasco to 

 Mazatlan. The isohalines at 10 meters depth for April and August are 

 reproduced on figs. 13a and b. Likewise, from surface to 1800 meters 

 depth, salinity deviates from only 367oo to 34.6%o, a difference too small 

 to be of significance to most marine animals. 



There are certain inshore regions of the Gulf of California, however, 

 which are characterized by salinity changes large enough to be significant 

 to animal distribution. These areas are the lagoons and bays along the east 

 coast of the Gulf from the Costa do Hermosillo to San Bias, Nayarit. The 

 salinities in these lagoons and estuaries are influenced both by prevailing 

 climate and by seasonal climatic changes. Those lagoons situated north 

 of Los Mochis in the arid to semi-arid Sonoran coastal climate, seldom 

 receive enough runoff or river discharge to make them brackish, but often 

 the extreme high summer temperatures and high evaporation rate may 

 bring about long-period hypersaline conditions (Nichols, 1962 and 1963). 



