Julian and Beeson: Estimates of marine mammal, turtle, and seabird mortality 
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Table 1 
Observed net characteristics for the driftnet and setnet fisheries taken from nets characterized by one set of characteristics only 
(some nets may consist of two or more panels with differing characteristics), n = number of sets observed. 
Characteristic 
Driftnet fishery (n=2,932) 
Setnet fishery (n= 7,994) 
Net type 
All 2,932 were drift nets. 
1,592 set nets and 6,278 1-panel trammel nets. 
Net material 
2,838 (97%) were multifilament nets. 
7,520 (94%) were monofilament and 439 were 
multifilament. 
Net strength and 
strength code 
Twine size of 24 was used for 25% of the 
nets; size 27 was used for 36%, and size 
30 was used for 22% of the nets. 
3,203 (40%) nets with twine size 66, 10% with twine size 
55, and 19.4% nets with unrecorded data. 
Net length (m) 
mean=l,784.9; SD=55.4; mode=l,828.8 
mean=468.9, SD=164.7; mode=457.2 
Net depth (meshes) 
mean=128; SD=24; mode=130 
mean=23.7; SD=8.9; mode=20 (n=7,880) 
Mesh size (cm) 
mean=52.1; SD=3.9; mode=53.34 
mean=21.2 ; SD=2.2; mode=21.6 (n=7,968) 
Extender length (m) 
mean=11.48; SD=4.37; mode=11.30 
Extenders not typically used; 98.4% of the nets did not 
use them. 
Hanging line 
material 
2,809 (95.8%) nets were of synthetic 
fiber. 
7,628 (95.4%) nets were of synthetic fiber. 
Percent slack 
mean=45%; SD=5.4%; mode=50% 
(n=2,609). 
No slack indicated for 38% of the nets. For nets with 
slack, mean=57%; SD=11%; mode=50% (n =4,986). 
Meshes hanging 
60% had 2 and 35% had 1 mesh 
hanging between knots tied to the cork 
line. 
42% had 6, 26% had 4, and 17% had 6 meshes hanging 
between knots tied to the cork line. 
Hanging length (cm) 
mean=50.7; SD=14.6; mode=60.0 for 
distance between knots on the cork line. 
mean = 38.4; SD=6.5; mode=38.1 for distance between 
knots on the cork line. 
and those records were submitted to CDFG twice 
monthly. Landing receipts included information on 
species landed, weight by species, price, gear, area 
fished, and vessel and permit identification. Land- 
ing information was entered into a database and 
checked for accuracy. A target fish species was as- 
signed to each entry. Logbook data, landing informa- 
tion, and NMFS observer data (date, set position, 
gear, and catch) were subsequently used in estimat- 
ing effort. 
Estimation of total effort 
Fishing effort in both fisheries is an unknown quan- 
tity and absolute determination is impractical. Con- 
sequently, estimates of total fishing effort in each fish- 
ery were used to estimate incidental kill. These esti- 
mates were based on the combination of observer 
records, logbook data, and landing receipts. Effort was 
measured in “effort-days” which was defined as one 
day of fishing for one vessel. In the driftnet fishery, 
one effort-day was considered equivalent to setting 
and retrieving one net, generally 1828.8 m (1,000 fm) 
in length. (One vessel, targeting thresher shark, 
made two sets per day.) In the setnet fishery, typi- 
cally two to four net settings, each of about 457.2 m 
(250 fm) in length, made up one effort-day. Days ac- 
tually fished was used as the measure of total effort 
in each fishery because previous exploratory analy- 
sis determined that number of days of effort and gen- 
eral location of effort were significant factors in esti- 
mation of mortality (Perkins, et al. 5 ). For some spe- 
cies, quarter of the year was also determined to be 
significant. These factors were available for all ef- 
fort through the California Fish and Game Depart- 
ment and although other approaches to estimation 
(e.g. a modeling approach) can be developed, the cur- 
rent analysis is based on these factors. Data on other 
variables, such as total number of nets fished, total 
length of nets fished, or tons of target fish caught, 
were not readily available or contained additional 
variability due to nonsampling errors. Nonsampling 
