180 
Entanglement of California sea lions, 
Zalophus californianus californianus, 
in fishing gear in the central-northern 
part of the Gulf of California, Mexico 
Alfredo Zavala-Gonzalez 
Eric MeSSiok 
Departamento de Ecologia, Centro de Investigacion Cientifica y 
de Educacion Superior de Ensenada 
Apdo. Postal 2732, Ensenada, Baja California, Mexico 
U S. mailing address: CICESE; PO. Box 434844, San Diego, CA 92 1 43 
E-mail address: alzago@cicese.mx 
The range of the California sea lion, 
Zalophus californianus califor- 
nianus, extends from British Co- 
lumbia south to Mazatlan, Mexico, 
and includes the Gulf of California. 
The population of sea lions in 
Mexico has been estimated at 
74,467 individuals along the Pacific 
coast (Lowry et al. 1 ), 28,220 in the 
Gulf of California (Zavala, 1990). 
Little is known about the Pacific 
coast population, but there are 
probably 8 breeding colonies (Lowry 
et al., 1992). In the Gulf there are 
40 rookeries: 13 breeding colonies 
and 27 haulouts (Zavala et al., in 
press). Strong tidal forces cause a 
constant upwelling condition in the 
central-northern part of the Gulf of 
California that sustains high nutri- 
ent and phytoplankton concentra- 
tions, especially around the Midriff 
Islands in the central Gulf (Al- 
varez-Borrego, 1983; Alvarez-Bor- 
rego and Lara-Lara, 1991). This 
upwelling condition allows the exist- 
ence of large populations of fish, 
marine mammals, and marine birds. 
Between the 1960’s and the 
1980’s, the population at some 
breeding colonies of California sea 
lions in the Gulf of California in- 
creased 30% (Le Boeuf et al., 1983). 
During the 1980’s and early 1990’s, 
the yearly increase in those popu- 
lations was between 2% (Morales, 
1990; Zavala et al., in press) and 
4.7% (Aurioles and Arizpe 2 ). After 
1991 some populations experienced 
a slight reduction in size and later 
a partial recovery (Heath et al., 
1994; Zavala et al. 3 ). 
Since 1985, we have censused 
annually 10 of the 11 reproductive 
sea lion colonies that account for 
94.9% of all sea lions in the Gulf of 
California (Fig. 1 in Aurioles and 
Zavala, 1994). In 1991 we com- 
menced seeing more sea lions with 
pieces of fishing gear entangled 
around their head and neck than 
we had remembered seeing during 
previous years (Zavala and Garcia 4 ) 
and began documenting the inci- 
dence of entanglement. We report 
the numbers of entangled sea lions 
observed between 1991 and 1995 in 
the central-northern part of the 
Gulf of California and comment on 
the effect this may have on the con- 
servation of the species. 
Methods 
Ten of 11 breeding colonies in the 
central-northern Gulf of California 
were studied (Fig. 1). Only Roca 
Consag (31°12'N, 114°29'W) was 
excluded. Between 1991 and 1995, 
we made eight cruises to the 10 
breeding colonies: 16 Jun-19 Jul 
1991, 8 Jul-4 Aug 1992, 16-25 Jun 
1993, 10-29 Jul 1993, 16-25 Jun 
1994, 11-20 Jul 1994, 1-4 Aug 
1994, and 15-28 Jun 1995. San 
Jorge and El Coloradito were vis- 
ited only once each year whereas 
Los Cantiles and San Pedro Martir 
were not surveyed in 1991 and 
1992, respectively. All other islands 
were surveyed on every trip. 
All cruises were made on patrol 
ships of the Mexican Navy, leaving 
from Guaymas, Sonora. Surveys 
around the islands were made 
aboard small (7 m in length) fiber- 
glass boats with 35-55 hp outboard 
motors. We cruised at about 2 
knots, 30-50 m from the coast, to 
census the animals. They were clas- 
sified as adult males, subadult 
males, females, juveniles, or pups 
(sensu LeBoeuf et al., 1983; Auri- 
oles and Zavala, 1994). 
Entanglement frequencies were 
calculated by dividing the total 
number of entangled animals (those 
animals with pieces of fishing gear 
around head and neck) by the total 
number of adult, subadult, and 
1 Lowry, M. S., P. Boveng, R. J. DeLong, Ch. 
W. Oliver, B. S. Stewart, H. DeAnda, and 
J. Barlow. 1992. Status of California 
sea lion (Zalophus californianus cali- 
fornianus) population in 1992. Admin. 
Rep. LJ-92-32, 35 p. Southwest Fisher- 
ies Science Center, NMFS, NOAA, P.O. 
Box 271, La Jolla, CA 92038. 
2 Aurioles, D., and O. Arizpe. 1989. 
Unpubl. data. Departamento de Pes- 
querias y Biologia Marina. Centro Inter- 
disciplinario de Ciencias Marinas. Apdo. 
Postal 592, La Paz, B.C.S., Mexico. 
3 Zavala, A., H. de la Cueva, and E. 
Mellink. 1991. Unpubl. data. Departa- 
mento de Ecologia, Centro de Investi- 
gacion Cientifica y Educacion Superior de 
Ensenada, Apdo. Postal 2732, Ensenada, 
B.C., Mexico. 
4 Zavala, A., and M. C. Garcia. 1991. De- 
partamento de Ecologia, Centro de Inves- 
tigacion Cientifica y Educacion Superior 
de Ensenada, Apdo. Postal 2732, Ensen- 
ada, Baha California, Mexico. Personal 
obs. 
Manuscript accepted 4 September 1996. 
Fishery Bulletin 95:180-184 (1997). 
