182 
Fishery Bulletin 95 ( 1 ), 1997 
Table 1 
Number of entangled and total (in parentheses, including entangled) California sea lions, except pups, in 10 breeding colonies in 
the central-northern Gulf of California, 1991-95. 
Rookeries 
Locations 
Year 
Lat. 
Long. 
1991 
1992 
1993 
1994 
1995 
San Jorge 
31°01'N 
113°15'W 
8 (4,536) 
14 (2,915) 
10 (2,183) 
4 (2,208) 
24 (3,200) 
Coloradito 
30°03'N 
114°29'W 
4 (2,100) 
2 (1,610) 
7 (1,662) 
3 (1,749) 
13 (1,688) 
Granito 
29°34'N 
113°33'W 
2 (609) 
1 (603) 
6 (390) 
2 (923) 
4 (631) 
Los Cantiles 
29°32'N 
113°29'W 
— 
1 (712) 
3 (620) 
1 (602) 
6 (916) 
Los Machos 
29°18'N 
113°31'W 
0 (718) 
0 (601) 
1 (718) 
4 (659) 
1 (512) 
El Partido 
28°53'N 
113°02’W 
5 (463) 
2 (524) 
4 (798) 
3 (311) 
1 (402) 
El Rasito 
28°49'N 
113°00'W 
1 (353) 
0 (326) 
1 (101) 
5 (223) 
0 (198) 
San Esteban 
28°43'N 
112°35'W 
10 (4,758) 
4 (3,135) 
10 (2,610) 
20 (3,859) 
14 (3,396) 
S.P. Martir 
28°23'N 
112°20'W 
1 (1,379) 
— 
11 (676) 
8 (770) 
7 (937) 
S.P. Nolasco 
27°58'N 
111°23'W 
3 (1,009) 
0 (338) 
1 (517) 
3 (340) 
2 (358) 
Total 
34 (15,925) 
24 (10,764) 
54 (10,275) 
53 (11,644) 
72 (12,238) 
effects in the Gulf (Hamman et al., 1995). Entangle- 
ment rate did not exhibit any tendency through time 
(n=5, r 2 = 0.44, F=2.35, P= 0.223), and the ANOVA de- 
tected only 1992 as statistically inferior to 1993 and 
1994, whereas 1991 and 1995 were not different from 
any other year (F=4.20, P=0.007). 
San Jorge and San Esteban exhibited the largest 
overall numbers of sea lions (>2,000 California sea 
lions) (Table 1); El Coloradito had intermediate val- 
ues (>1500, <2200), and the other colonies <1400 sea 
lions. The ANOVA indicated that the first two sites 
had statistically more entangled sea lions than did 
El Partido, Granito, Cantiles, San Pedro Nolasco, El 
Rasito, and Los Machos, whereas San Pedro Martir 
and El Coloradito were not different from any other 
site (F= 7.67, P<0.001). This pattern corresponds to 
differences in the size of the colonies; a regression 
linking total number of entangled animals at each 
colony and size of the different colonies was highly 
significant (n=10, r 2 =0.92, F=89.71, P<0.001). 
We have no detailed records of differences in the 
fishing effort throughout the study area, although it 
seems to be larger in the Midriff region than in the 
northern Gulf (E. Mellink, personal obs. [1995]). The 
ANOVA did not show differences in the entanglement 
rates between colonies (F=0.64, P=0.76), and the 
number of incidents seemed to be more a function of 
the size of the colony than of local and yearly varia- 
tions in fishing effort, although, as suggested by the 
1992 data, these effects cannot be neglected. 
Entanglement rates in our region varied between 
0% and 2.24%. These values are substantially lower 
than the 3. 9-7. 9% detected in Los Islotes, Bahia de 
La Paz, in the southern Gulf of California (24°35'N, 
110°23'W; Harcourt et ah, 1994). In the latter local- 
ity, high values may have been due to the proximity 
of Los Islotes to a moderate-size city (La Paz, approx. 
250,000 inhabitants) and to abundant sport and com- 
mercial fishing. However, our entanglement values 
are higher than those at California islands (0.08%, 
Stewart and Yochem, 1987) and, again, this could be 
due to differences in the intensity and type of fish- 
ing practiced. 
The main fishing gear involved in sea lion entangle- 
ment in the study area were nets and, to a lesser 
degree, lines and ropes. The nets included monofila- 
ment, purse seine, and gill nets (with stretched mesh 
sizes 3”, 3.2”, 4,5”, 5”, and 8”), cotton gill nets (1.5” 
and 5.1” mesh size), and trammel nets (either cotton 
or nylon monofilament with 14” to 16” mesh size). 
These nets originally measure 120-180 fathoms long 
and 7 fathoms high. The lines involved were all ny- 
lon of different thickness and, in most cases, were 
found tied around the animal. In only one case did 
we see a hook in a sea lion’s mouth, or a line coming 
out of it. 
Most entanglement occurs during fishing, either 
when the net and the catch are hauled out or when a 
net is deployed during 24-48 hr periods. In other 
regions of the North Pacific, in addition to entangle- 
ment during events involving active fishing, entangle- 
ment occurs because marine mammals encounter drift- 
ing debris, especially when they are foraging or mi- 
grating (Fowler, 1987). In the central-northern Gulf 
of California it is rare to encounter fishing gear de- 
bris drifting in the water. Artesanal fishermen, who 
carry out most of the fishing in the area, cannot af- 
ford to lose nets; therefore nets are usually fixed, not 
drifted. When part of a net or a complete net is no 
longer usable, it is usually discarded in a local gar- 
