Silber Phocoena sinus in the northern Gulf of California 



345 



untested, I suggest that vaquita occur in the upper Gulf 

 year-round, and although the majority of the popula- 

 tion may have receded to the upper Gulf following a 

 relatively recent decline in population size, some in- 

 dividuals are probably scattered throughout the Gulf 

 as separate subpopulations or a sparsely dispersed 

 single population. Reported sightings of P. sinus out- 

 side the northern Gulf (Scammon 1874, Norris and 

 McFarland 1958, Norris and Prescott 1961, Villa-R. 

 1976; presented here) suggest the possibility of a 

 greater historical range. However, extensive field work 

 on cetaceans near the Midriff Islands, Guaymas, and 

 Kino Bay areas has not yielded sightings of vaquita (L. 

 Ballance, Dep. Biol., Univ. Calif., Los Angeles, CA 

 90024, pers. commun.. May 1989; L.T. Findley, Escu- 

 ela de Ciencias Man'timas y Alimentarias, Institute 

 Tecnologico y de Estudios Superiores de Monterrey, 

 Unidad Guaymas, A. P. 484, Guaymas, Sonora, 85400 

 Mexico, pers. commun., Nov. 1985; B. Tershy, Dep. 

 Biol., Cornell Univ., Ithaca, NY 14853, pers. commun., 

 Sept. 1987). Most phocoenids are cold-temperate water 

 animals (Gaskin 1982), with ranges restricted to 

 <20°C. Northern Gulf of California surface water 

 temperatures in the summer and fall can exceed 28 °C 

 (Hendrickson 1973, Robinson 1973). If the porpoise is 

 limited to this region, it deviates remarkably from other 

 phocoenids in its ability to tolerate seasonal water 

 temperature fluctuations. 



Management recommendations 



Exposure of the depleted P. sinus population to cur- 

 rent levels of human impact may result in the demise 

 of the species. Habitat degradation and pesticide con- 

 tamination of the northern Gulf may contribute to the 

 decline of P. sinus. However, vaquita mortality in 

 gillnets (Brownell 1983, Robles et al. 1986, Silber 1988) 

 may represent the most direct threat to the population, 

 and increased efforts to reduce porpoise entanglement 

 is imperative. Nets set for totoaba probably account 

 for the highest percentage of incidental porpoise 

 deaths. The totoaba is an endangered species and can- 

 not be legally caught, sold, or exported. Nonetheless, 

 illegal fishing continues due to the inability to enforce 

 restrictions. Additional study is needed to quantify 

 incidental mortality rates and to explore possible 

 modifications of fishing amounts, timing, and tech- 

 nique. Vaquita sightings occurred predominantly in 

 discrete locations and water depths. If widespread 

 fishing bans are not possible, emphasis should be placed 

 on excluding fishing from these areas. However, 

 because maximum catch of totaba also occurs in spring 

 (Flanagan and Hendrickson 1976), there will be difficul- 

 ty achieving compliance by fishermen. Rigorous sanc- 

 tions on importation of totoaba bound for U.S. markets 



are required. Additional boat and aircraft census 

 surveys should be conducted to determine the vaquita 

 population size, and to enhance current knowledge of 

 the ecology of the porpoise. Large areas of the northern 

 Gulf have been recommended for consideration as a 

 Marine Sanctuary (B. Villa-R., Inst. Biol., Univ. Nal. 

 Auton. Mexico, A.P. 70-153, Mexico, D.F. 04510, pers. 

 commun., Feb. 1986), and these plans should be en- 

 dorsed. Mexican and U.S. scientists should engage in 

 joint efforts to monitor changes in the vaquita popula- 

 tion size. Action by Mexican and U.S. governments, 

 fishing cooperatives, cetacean biologists, and resource 

 management personnel is necessary to the preserve the 

 species. 



Acknowledgments 



This study would not have been possible without im- 

 portant contributions by the following field assistants: 

 T. Silber, M. Newcomer. G. Ellis, H. Perez-Cortes, 

 G. Barros, D. Breese, T. Jefferson, L. Torrez-M., 



A. Velazquez-R., and A. Robles. These people endured 

 the rigors of field work and high expectations of a 

 demanding captain. 1 extend thanks to K. Norris, 



B. Villa-R., and R. Wells who provided encouragement 

 and advice. I am grateful to D. Aurioles and H. Perez- 

 Cortes for interviewing fishermen in La Paz and San 

 Felipe, respectively, and for observations provided by 

 G. Notarbartolo di Sciara. The project received finan- 

 cial support from the Nature Conservancy, the Center 

 for Marine Conservation, and the American Cetacean 

 Society (Los Angeles Chapter), and logistical support 

 from the Center for the Study of Deserts and Oceans, 

 Project Lighthawk, and West Coast Whale Research 

 Foundation. The study was conducted under Scientific 

 Research Permits 301856, 300422, and 400036 issued 

 by the Secretaria de Pesca, Mexico. The paper was 

 improved by comments from K. Norris, B. Wursig, 

 R. Wells, M. Newcomer, and T. Jefferson. This paper 

 is dedicated to my wife Trish. 



Citations 



Barlow, J. 



1986 Factors affecting the recovery of Phocoena sinus, the 

 vaquita or Gulf of CaUfornia harbor porpoise. Adm. Rep. 

 LJ-86-37, Southwest Fish. Cent., Natl. Mar. Fish. Serv., 

 NOAA. P.O. Box 271, La .Jolla, CA 92038, 19 p. 

 Brownell, R.L., Jr. 



1983 Phocoena sinus. Mamm. Species 198:1-3. 



1986 Distribution of the vaquita. Phocoena sinus, in Mexican 

 waters. Mar. Mammal Sci. 2:299-305. 



