271 
Estimates of marine mammal, turtle, 
and seabird mortality for two 
California gillnet fisheries: 1 990-1 995 
Abstract .—Incidental kills of ma- 
rine mammals, turtles, and seabirds are 
estimated for the California drift gillnet 
fishery for broadbill swordfish, Xiphias 
gladius, common thresher shark, 
Alopias vulpinus, and shortfin mako 
shark, Isurus oxyrinchus, and the set 
gillnet fishery for California halibut, 
Paralichthys californicus, and Pacific 
angel shark, Squatina californica, for 
the period July 1990 through Decem- 
ber 1995. Estimates were based on ob- 
servations made by National Marine 
Fisheries Service observers placed 
aboard commercial fishing vessels. 
Yearly observer coverage varied be- 
tween 4% and 18% of estimated total 
effort. Total fishing effort-days per Cali- 
fornia Department of Fish and Game 
fishing block was used as the measure 
of effort for the drift and set gillnet fish- 
eries. Incidental kill was estimated 
from observed data and estimates of 
total effort by using mean-per-unit and 
ratio estimators. Additional bycatch 
data collected by NMFS observers were 
used to derive kill estimates of marine 
turtles and seabirds. 
In the drift gillnet fishery, seven out 
of 387 mammals observed entangled 
were released alive. In the set gillnet 
fishery, five out of 1,263 mammals ob- 
served entangled were released alive. 
Estimates of incidental kill are pre- 
sented along with estimates of en- 
tanglement for species that were ob- 
served to be released alive. For the pe- 
riod under consideration, the estimated 
mortality for the drift gillnet fishery 
was over 450 marine mammals each 
year. A total of 20 turtles and 3 seabirds 
were observed entangled during the 
entire period. The most frequently en- 
tangled species in this fishery were 
common dolphins, Delphinus spp., and 
northern elephant seals, Mirounga 
angustirostris. Estimated cetacean 
mortality in the driftnet fishery de- 
creased from 650 in 1991 to 417 in 1995; 
pinniped mortality decreased from 173 
in 1991 to 116 in 1995. Estimated ceta- 
cean mortality in the set gillnet fish- 
ery ranged from a high of 38 in 1991 to 
a low 14 in 1993; pinniped mortality 
rose to a high of 4,777 in 1992 and then 
decreased to 1,016 in 1995. We postu- 
late that there has been a decline in the 
number of pinnipeds and cetaceans in 
the setnet fishery owing to area closure. 
No similar proposal can be made for the 
driftnet fishery. The most frequently 
entangled mammals in the setnet fish- 
ery were California sea lions, Zalophus 
californianus, and harbor seals, Phoca 
vitulina. Six turtles and 1,018 seabirds 
were estimated entangled in this fishery 
during the NMFS Observer Program 
from July 1990 to December 1995. 
Manuscript accepted 25 June 1997. 
Fishery Bulletin 96:271-284 (1998). 
Fred Julian 
1 42 Sierra Way 
Chula Vista, California 91911 
E-mail address: fjulian@osiris.ucsd.edu 
Marilyn Beeson 
California Department of Fish and Game 
330 Golden Shore, Suite 50 
Long Beach, California 90802 
Two major gillnet fisheries in Cali- 
fornia are known to kill marine 
mammals, turtles, and seabirds in- 
cidentally: the drift gillnet fishery 
for broadbill swordfish, Xiphias 
gladius, common thresher shark, 
Alopias vulpinus, and shortfin mako 
shark, Isurus oxyrinchus, and the 
set gillnet fishery for California 
halibut, Paralichthys californicus, 
and Pacific angel shark, Squatina 
californica. Historically, concern 
was focused on incidental kill of sea- 
birds, sea otters, Enhydra lutris, 
harbor porpoise, Phocoena phocoena, 
harbor seals, Phoca vitulina, and 
California sea lions, Zalophus 
californianus, in the setnet fishery 
(Salzman, 1989; Jefferson et al., 
1994; Diamond and Hanan 1 2 3 ; Hanan 
et al. 2,3 ; Hanan and Diamond 4 ). In 
recent years the driftnet fishery has 
received more attention because it 
interacts with more cetaceans (Bar- 
low et al., 1994; Lennert et al., 
1994). Estimation for the driftnet 
fishery has been possible because, 
in July 1990, the National Marine 
Fisheries Service (NMFS) imple- 
mented an observer program to 
monitor the marine mammal by- 
catch. Complementing the observer 
program was a project by the Cali- 
fornia Department of Fish and 
Game (CDFG) to develop estimates 
of total effort in the drift and set 
gillnet fisheries. Results from these 
programs were used to estimate 
incidental kill with stratified ratio 
and mean-per-unit estimation 
1 Diamond, S. L., and D. A. Hanan. 1986. 
An estimate of harbor porpoise mortality 
in California set net fisheries, April 1. 1983 
through March 31, 1984. NOAA/NMFS 
SWR Admin. Rep. SWR-86-15, 40 p. [Avail- 
able from Southwest Fisheries Science 
Center, National Marine Fisheries Service, 
PO. Box 271, La Jolla, CA 92038.] 
2 Hanan, D. A., S. L. Diamond, and J. P. 
Scholl. 1986. An estimate of harbor por- 
poise mortality in California set net fish- 
eries, April 1, 1984 through March 31, 
1985. NOAA/NMFS SWR Admin. Rep. 
SWR-86-16, 38p. [Available from South- 
west Fisheries Science Center, National 
Marine Fisheries Service, PO. Box 271, La 
Jolla, CA 92038.] 
3 Hanan, D. A., S. L. Diamond, and J. P. 
Scholl. 1988. Estimates of sea lion and 
harbor seal mortalities in California set 
net fisheries for 1983, 1984, and 1985. Fi- 
nal Rep. NA-86-ABH00018 submitted to 
NOAA Fisheries, SWR, 10 p. [Available 
from Southwest Fisheries Science Center, 
National Marine Fisheries Service, PO. 
Box 271, La Jolla, CA 92038 ] 
4 Hanan, D. A., and S. L. Diamond. 1989. 
Estimates of sea lion, harbor seal, and har- 
bor porpoise mortalities in California set 
net fisheries for the 1986-87 fishing 
year. Final Rep. NA-86-ABH00018 sub- 
mitted to NOAA Fisheries, SWR, 10 p. 
[Available from Southwest Fisheries Sci- 
ence Center, National Marine Fisheries 
Service, PO. Box 271, La Jolla, CA 92038.] 
