estimates of marine mammal, turtle, and seabird mortality for two
TRANSCRIPT
~fi.: AbstraCt.-Incidental kills. of mai';i cinemammals, turtles, and seabIrds are; ° stimated for the California drift gillnet
~shery for broadbill swordfish, Xiphias:'W,igladius, common thresher shark,~i<iAlopias vulpinus, and shortfin makoiiM!{O, shark, Isurus oxyrinchus, and the settlll'J gillnet fishery f~r Ca.lifornia halib.ut,iii::; Paralichthys cahformcus, and PacIficti,', ,angel shark, Squatina californica, forf¥«' the period July 1990 through Decem:/o,!ber1995.Estimates were based on obI;;':servations made by National Marinei;@:Jo'FisheriesService observers placed; °aboard commercial fishing vessels.
'early observer coverage varied between 4% and 18% of estimated totaleffort.Total fishing effort-days per California Department of Fish and Game°fishingblock was used as the measureofeffortfor the drift and set gillnet fish"eries.Incidental kill was estimated
, from observed data and estimates of~t~~totaleffort by using mean-per-unit and~(;jtrratioestimators. Additional bycatchr,;q!,idatacollectedby NMFS observers were
00 'usedto derive kill estimates of marineurtles and seabirds.
In the drift gillnet fishery, seven out"V'I of 387 mammals observed entangled~&\;'\Verereleased alive. In the set gillnet~;;"fishery,five out of 1,263 mammals obV~~:"servedentangled were released alive.~~~;iEstimatesof incidental kill are preI~'~ented along with estimates of en~tii"'itanglementfor species that were ob~";'servedto be released alive. For the pered'}'" riodunder consideration, the estimated~j\imortality for the drift gillnet fishery;iIt""'wasover 450 marine mammals each,;!/:""t',."" iyear.Atotal of 20 turtles and 3 seabirds
i~ere observed entangled during theentire period. The most frequently entangled species in this fishery werecommondolphins, Delphinus spp., and
,~~§;'northernelephant seals, Mirounga
\!~it'r;ngustirostris. Estimated cetacean!!j;~IIlortalityin the driftnet fishery de-
creasedfrom 650in 1991to 417 in 1995;pinnipedmortality decreased from 173in 1991to 116in 1995. Estimated cetacean mortality in the set gillnet fisheryranged from a high of 38 in 1991 toa low 14 in 1993; pinniped mortalityroseto a high of 4,777 in 1992 and thendecreased to 1,016 in 1995. We postuI~tethat there has been a decline in thenumber of pinnipeds and cetaceans inthesetnet fishery owingto area closure.Nosimilar proposal can be made for theriftnet fishery. The most frequentlyntangled mammals in the setnet fish
eryWereCalifornia sea lions, Zalophus'~lifornianus, and harbor seals, Phocavltulina. Six turtles and 1018 seabirds
er~estimated entangled in this fishery~unng the NMFS Observer ProgramromJuly 1990 to December 1995.
I Diamond, S. L., and D. A. Hanan. 1986.An estimate of harbor porpoise mortalityin California set net fisheries, Aprill. 1983through March 31, 1984. NOAA/NMFSSWRAdmin. Rep. SWR-86-15,40 p. [Available from Southwest Fisheries ScienceCenter, National Marine Fisheries Service,P.O. Box 271, La Jolla, CA92038.]
2 Hanan, D. A., S. L. Diamond, and J. P.Scholl. 1986. An estimate ofharbor porpoise mortality in California set net fisheries, April 1, 1984 through March 31,1985. NOAA/NMFS SWR Admin. Rep.SWR-86-16, 38p. [Available from Southwest Fisheries Science Center, NationalMarine Fisheries Service, P.O.Box 271, LaJolla, CA92038.]
3 Hanan, D. A., S. L. Diamond, and J. P.Scholl. 1988. Estimates of sea lion andharbor seal mortalities in California setnet fisheries for 1983, 1984, and 1985. Final Rep. NA-86-ABHOOOI8submitted toNOAA Fisheries, SWR, 10 p. [Availablefrom Southwest Fisheries Science Center,National Marine Fisheries Service, P.O.Box 271, La Jolla, CA 92038.]
4 Hanan, D. A., and S. L. Diamond. 1989.Estimates ofsea lion, harbor seal, and harbor porpoise mortalities in California setnet fisheries for the 1986-87 fishingyear. Final Rep. NA-86-ABH00018 submitted to NOAA Fisheries, SWR, 10 p.[Available from Southwest Fisheries Science Center, National Marine FisheriesService, P.O. Box 271, La Jolla, CA92038.]
Game (CDFG) to develop estimatesof total effort in the drift and setgillnet fisheries. Results from theseprograms were used to estimateincidental kill with stratified ratioand mean-per-unit estimation
Marilyn BeesonCalifornia Department of Fish and Game330 Golden Shore. Suite 50
Long Beach, California 90802
Estimates of marine mammal, turtle,and seabird mortality for twoCalifornia gillnet fisheries: 1990-1995Fred JulianJ 42 Sierra WayChula Vista. California 9 I 91 I
E-mail address: [email protected]
271-
Two major gillnet fisheries in California are known to kill marinemammals, turtles, and seabirds incidentally: the drift gillnet fisheryfor broadbill swordfish, Xiphiasgladius, common thresher shark,Alopias vulpinus, and shortfin makoshark, Isurus oxyrinchus, and theset gillnet fishery for Californiahalibut, Paralichthys californicus,and Pacific angel shark, Squatinacalifornica. Historically, concernwas focused on incidental kill of seabirds, sea otters, Enhydra ° lutris,harbor porpoise,Phocoenaphocoena,harbor seals, Phoca vitulina, andCalifornia sea lions, Zalophuscalifornianus, in the setnet fishery(Salzman, 1989; Jefferson et aI.,1994;Diamond and HananI; Hananet ap,3; Hanan and Diamond4). Inrecent years the driftnet fishery hasreceived more attention because itinteracts with more cetaceans (Barlow et aI., 1994; Lennert et aI.,1994). Estimation for the driftnetfishery has been possible because,in July 1990, the National MarineFisheries Service (NMFS) implemented an observer program tomonitor the marine mammal bycatch. Complementing the observerprogram was a project by the California Department of Fish and
anuscript accepted 25 June 1997.SheryBulletin 96:271-284 (1998).
272
methods (Lennert et aI., 1994; Perkins et aI.5;Julian6, 7, 8). Separate estimates ofentanglement areprovided for species that had individuals releasedalive. This paper documents incidental marine mammal, turtle, and seabird kill estimates in these twoCalifornia gillnet fisheries, based on data from theNMFS observer program and CDFG effort estimatesfor the period July 1990 through December 1994, anddocuments the process and methods leading to theseestimates.
Methods
Data collection
National Marine Fisheries Service observer data,daily logbooks of commercial gillnet fishermen, andreceipts of landed fish sales were used in marinemammal mortality estimation. NMFS observer datawere collected by trained technicians aboard commercial gillnet fishing boats that had a Marine Mammal Protection Act (MMPA)Exemption Permit andthat met minimum U.S. Coast Guard safety standards. There were two general observation categories: observation of randomly selected trips and observation of approximately every fifth vessel trip. Inthe setnet fishery, systematically selected trips werefurther divided. Notification prior to the setting ofnets resulted in a preset, systematic observationrather than a postset, systematic observation wherenotification to the vessel was given after the nets wereset (see "Discussion" section). NMFS observers recorded data on location, date, marine mammal entanglements, including location of mammals in the
5 Perkins, P., J. Barlow, and M. Beeson. 1992. Pinniped andcetacean mortality in California gillnet fisheries: 1991. International Whaling Commission Scientific Committee workingpaper SC/44/SM14. [Available from Southwest Fisheries Sci-
. ence Center, National Marine Fisheries Service, P.O. Box 271,La Jolla, CA92038.]
6 Julian, F. 1993. Pinniped and cetacean mortality in California gillnet fisheries: preliminary estimates for 1992, rev. 2/94. International Whaling Commission Scientific Committeeworking paper SC/45/022. [Available from Southwest Fisheries Science Center, National Marine Fisheries Service, P.O. Box271, La Jolla, CA92038.]
7 Julian, F. 1994. Pinniped and cetacean mortality in California gillnet fisheries: preliminary estimates for 1993. International Whaling Commission Scientific Committee working paper SC/46/0n. [Available from Southwest Fisheries ScienceCenter, National Marine Fisheries Service, P.O. Box 271, LaJolla, CA92038.]
8 Julian, F. 1995. Cetacean and pinniped mortality in California gillnet fisheries: preliminary estimates for 1994. International Whaling Commission Scientific Committee working paper SC/47/05. [Available from Southwest Fisheries ScienceCenter, National Marine Fisheries Service, P.O. Box 271, LaJolla, CA92038.]
Fishery Bulletin 96(2), 1998
net (bythirds ofthe net-vertically and horizontally),gear, bycatch, and target species catch for each netpull observed during a trip (Lennert et aI., 1994).
Observers recorded twelve net-related parametersfor drift and set nets. They were net type (set, drift,float, or trammel), net material (monofilament, multifilament, or a combination), net strength (pounds testor twine size depending on strength code), strengthcode, net length (fathoms), net depth (number ofmeshes), stretched mesh size (inches), extenderlength (feet, float, and drift nets only), hanging linematerial (synthetic or natural fiber), percent slackin net, number of meshes hanging (between knots tothe cork line), and hanging length (distance betweenknots on the cork line in inches). Not infrequently, adrift or set net will consist of panels of varying characteristics. In this case, observers would record characteristics on up to 5 different panels. Net characteristics for both fisheries are summarized in Table 1.Although the variability in these characteristics contributes somewhat to the variability in mortality estimates, the significant factors for mortality estimation are the amount of effort and the generallocation of the effort. For some species, e.g. pinnipeds,quarter ofthe year is also significant (Perkins et aI.5).
Collected data were entered into a database file,checked for accuracy, and tabulated for mortalityestimation (Tables 2-5). After an initial six monthperiod, this observation method continued unchangedin both fisheries. Realized observation rates variedbetween 4.4% and 17.9% yearly, but observation rateswere more variable if stratified by area and quarter.Observation in the driftnet fishery continued throughDecember 1995, whereas observation in the setnetfishery terminated by July 1994 because of a significant decrease in fishing effort in that fishery (due toregulations that restricted areas open to gillnet fishing). Observer data were complemented by information from vessel logbooks and landing receipts (i.e.receipts from sales of landed fish).
Vessel logbooks were submitted monthly to CDFGand constituted the major source of information for$;ii'J:'estimation of total effort. Data for each logbook en<~ltry included date, vessel and permit identification?area fished by CDFG block number (Lennert et aL1994), gear, number of sets made, and number andspecies offish caught (CDFG blocks are typically 10'square; larger blocks are defined for areas furtherfrom shore). Logbookinformation was entered into adatabase by technicians and checked for accuracy b
biologists. Fish species targeted for catch by the fisermen were determined and assigned to each daentry by CDFG personnel according to fish caughgear used, and other pertinent factors. PurchaseSlanded fish by commercial fish buyers were recorde
Julian and Beeson: Estimates of marine mammal, turtle, and seabird mortality-- 273
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
Net type
Net material
Net strength andstrength code
Net length (m)
Net depth (meshes)
Mesh size (cm)
Extender length (m)
Hanging linematerial
Percent slack
Meshes hanging
Hanging length (cm)
Driftnet fishery (n=2,932)
All 2,932 were drift nets.
2,838 (97%) were multifilament nets.
Twine size of 24 was used for 25% of the
nets; size 27 was used for 36%, and size30 was used for 22% of the nets.
mean=1,784.9; SD=55.4; mode=1,828.8
mean=128; SD=24; mode=130
mean=52.1; SD=3.9; mode=53.34
mean=11.48; SD=4.37; mode=11.30
2,809 (95.8%) nets were of syntheticfiber.
mean=45%; SD=5.4%; mode=50%(n=2,609).
60% had 2 and 35% had 1 mesh
hanging between knots tied to the corkline.
mean=50.7; SD=14.6; mode=60.0 fordistance between knots on the cork line.
Setnet fishery (n=7,994)
1,592 set nets and 6,278 I-panel trammel nets.
7,520 (94%) were monofilament and 439 weremultifilament.
3,203 (40%) nets with twine size 66, 10% with twine size55, and 19.4% nets with unrecorded data.
mean=468.9, SD=164.7; mode=457.2
mean=23.7; SD=8.9; mode=20 (n=7,880)
mean=21.2 ; SD=2.2; mode=21.6 (n=7,968)
Extenders not typically used; 98.4% of the nets did notuse them.
7,628 (95.4%) nets were of synthetic fiber.
No slack indicated for 38% of the nets. For nets with
slack, mean=57%; SD=l1%; mode=50% (n=4,986).
42% had 6, 26% had 4, and 17% had 6 meshes hangingbetween knots tied to the cork line.
mean = 38.4; SD=6.5; mode=38.1 for distance betweenknots on the cork line.
and those records were submitted to CDFG twicemonthly. Landing receipts included information onspecies landed, weight by species, price, gear, areafished, and vessel and permit identification. Landing information was entered into a database andchecked for accuracy. A target fish species was assigned to each entry. Logbook data, landing information, and NMFS observer data (date, set position,gear, and catch) were subsequently used in estimating effort.
Estimation of total effort
Fishing effort in both fisheries is an unknown quantity and absolute determination is impractical. Consequently, estimates oftotal fishing effort in each fishery were used to estimate incidental kill. These estimates were based on the combination of observerrecords, logbookdata, and landing receipts. Effort wasmeasured in "effort-days" which was defined as oneday of fishing for one vessel. In the driftnet fishery,
one effort-day was considered equivalent to settingand 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, typically two to four net settings, each of about 457.2 m(250 fm) in length, made up one effort-day. Days actually fished was used as the measure of total effortin each fishery because previous exploratory analysis determined that number of days of effort and generallocation of effort were significant factors in estimation of mortality (Perkins, et al.5). For some species, quarter of the year was also determined to besignificant. These factors were available for all effort through the California Fish and Game Department and although other approaches to estimation(e.g. a modeling approach) can be developed, the current analysis is based on these factors. Data on othervariables, such as total number of nets fished, totallength of nets fished, or tons of target fish caught,were not readily available or contained additionalvariability due to nonsampling errors. Nonsampling
274 Fishery Bulletin 96(2), 1998
Table 2
Observed (obs) and (est) estimated cetacean, pinniped, turtle, and seabird mortality, stratified by year, in the California sword-fish and shark drift gillnet fishery during the NMFS Observer Program, July 1990-December 1995. Estimates of total mortalityare reported to the nearest individual. Estimated coefficients of variation (CV) are included in parentheses; (-) indicates CVwasundefined. Effort and estimates for 1990 pertain to the third and fourth quarters only.Year
199019911992199319941995
Estimated days effort
4,0784,7784,3795,4424,2483,673Observed days effort
178470596728759572
Percent observer coverage4.4%9.8%13.6%13.4%17.9%15.6%
Observed trips effort
54889710713497
obs est
CV obs estCV obs estCV obs estCV obs estCVobs est CV
Dall's porpoise
123(0.95) 220(0.67)17 (0.92) 967(0.44) 211 . (0.64)16 (0.92)
Pacific white-sided dolphin
369(0.56) 551(0.63)322 (0.70) 215(0.66) 317 (0.67)16 (0.92)
Risso's dolphin
00(-) 551(0.50)537 (0.48)752(0.51)16 (0.91)639 (0.57)
Bottlenose dolphin
00(-) 00(-)322 (0.93) 00(-) 00(-)00(-)Striped dolphin
00(-) 00(-)00(-) 00(-)16 (0.90)00(-)Common dolphin (unknown stock)
492(0.79) 771(0.70)537 (0.40) 430(0.57)16 (0.91)00(-)Common dolphin (long beak)
00(-) 00(-)215 (0.92) 00(-)16 (0.91)639 (0.65)
Common dolphin (short beak)
492(0.47) 37 376(0.21) 39 287 (0.21) 24 179(0.26) 25 140 (0.18) 36 231 (0.29)
Northern right whale dolphin
00(-) 771(0.41)215 (0.65) 752(0.39)739 (0.42)958 (0.59)Killer whale
00(-) 00(-)00(-) 00(-) 00(-)16 (0.92)
Short-finned pilot whale
123(0.95) 00(-)17 (0.92) 860(0.54) 00(-)00(-)Baird's beaked whale
00(-) 00(-)00(-) 00(-)16(0.90)00(-)Stejneger's beaked whale
00(-) 00(-)00(-) 00(-)16 (0.91)00(-)Hubbs' beaked whale
00(-) 00(-)322 (0.53) 00(-) 211(0.64)00(-)Mesoplodontbeaked whale
123(0.97) 00(-)17 (0.93) 00(-) 00(-)00(-)Cuvier's beaked whale
00(-) 00(-)644 (0.36) 322(0.53) 634 (0.36)532 (0.40)Unidentified beaked whale
00(-) 00(-)215 (0.65) 00(-)16 (0.90)00(-)Sperm whale
00(-) 00(-)17 (0.94) 215(0.66) 00(-)00(-)Pygmy sperm whale
00(-) 00(-)00(-)17(0.93) 0 0(-)00(-)Unidentified Kogia
00(-) 00(-)17(0.92) 0 0(-) 00(-)00(-)Minke whale
00(-) 00(-)00(-) 00(-)16 (0.91)00(-)Unidentified cetacean
00(-) 110(0.95)17(0.93) 0 0(-) 00(-)00(-)Unidentified dolphin
00(-) 00(-)17 (0.93) 00(-) 00(-)00(-)Unidentified whale
00(-) 00(-)00(-)17(0.93) 0 0(-)00(-)Steller sea lion
00(-) 00(-)17 (0.92) 00(-)16 (0.91)00(-)California sea lion
246(0.99) 441(0.58)966 (0.34) 1182(0.42) 528 (0.40)426 (0.45)
Unidentified sea lion
246(0.97) 00(-)00(-) 00(-) 00(-)00(-)Harbor seal
123(0.95) 00(-)00(-) 00(-) 00(-)00(-)Northern elephant seal
5 115(0.44) 13 132(0.25) 15 110 (0.24) 14 105(0.26) 22 123(0.23) 1490 (0.25)
Loggerhead turtle
00(-) 00(-)17(0.93) 0 0(-) 00(-)00(-)Leatherback turtle
123(0.97) 00(-)215(0.65) 215(0.66) 00(-)426 (0.55)
Unidentified turtle
00(-) 00(-)00(-)17(0.93) 0 0(-)00(-)Seabirds (all unidentified)
123(0.98) 00(-)00(-) 00(-)16 (0.90)00(-)~}
errors, those not due to the sampling design, include,
made. The occurrence ofthis type of errorbut are not limited to
that estimates of mortality are biased loweractual mortality levels.1
Effort that is not recorded. This type of error may 2Incorrect reporting of effort location by fishermen.'"occur for several reasons, namely the times when This type of error may bias estimated mortalitno marketable target fish are caught during the either higher or lower.entire day of effort and when a log entry is not
.-, Table 3...
Observed(obs)and estimated (est) cetacean, pinniped, turtle, and seabird entanglement, stratified byyear, in the California swordfishand shark drift gillnet fishery during the NMFS Observer Program, July1990-December 1995. Estimates of entanglement are"reported to the nearest individual. Estimated coefficients of variation (CV) are included in parentheses; (-) indicates CV was
undefined. Effort and estimates for 1990 pertain to the third and fourth quarters only.Year
199019911992199319941995
obs
estCV obs estCV obs estCV obs estCV obs estCVobsestCV
Common dolphin(unknown stock)
492(0.79) 771(0.70)644 (0.36)430(0.57)16 (0.91)00(-)Cuvier's beaked whale
00(-) 00(-)644 (0.36)322(0.53)634 (0.36)639 (0.36)
Sperm whale
00(-) 00(-)322 (0.94)322(0.69)00(-)00(-)Humpback whale
00(-) 00(-)00(-)00(-)16 (0.91)00(-)California sea lion
246(0.99) 441(0.58)966 (0.34) 1290(0.39)528 (0.40)532 (OAO)
Unidentified sea lion
246(0.97) 110(0.95)00(-)00(-)00(-)00(-)Loggerhead turtle
00(-) 00(-)215 (0.66)537(0.49)00(-)00(-)Leatherback turtle
123(0.97) 110(0.94)429 (0.46)322(0.53)16 (0.91)532 (0.47)Unidentified turtle
00(-) 00(-)00(-)322(0.93)00(-)00(-)Seabirds (all unidentified)
123(0.98) 00(-)17 (0.93)00(-)16 (0.90)00(-)
Julian and Beeson: Estimates of marine mammal, turtle, and seabird mortality- 275
3 Underestimation of effort from landing receipts.This type of error may occur when a landing receipt, in absence of additional information, is assumed to represent one effort-day (default-value).
A reliable determination of the magnitude of biasthese errors cause in estimates of mortality rates hasnot been made; however, their characteristics indicate that estimates of mortality may be lower thanactual values.
Once quarterly effort data were collected, computerprograms developed by CDFG were used to assigntarget species to landing receipts on the basis of information provided by logbooks and observations(Beeson and Hanan9). Landing data were then confirmed or modified on the basis of logbook and observer data for the same target species, date, andvessel number. After all three data sources were compared, a day of effort was tallied for each record witha logbook entry or observer record. Landing receiptswithout corresponding logbook or observer entriesthree days before and after the receipt date wereassumed to represent one day of effort. The numbers of days fished in each CDFG block were thentallied and the resultant data represented estimatedtotal effort. Total effort was estimated quarterly andyearly. Delayed submission of data to CDFG was theprimary reason that estimates ofyearly effort differedfrom the sum of the quarterly estimates of effort.
9 Beeson, M., and D. Hanan. 1996. Manuscript submitted toCalifornia Fish and Game. [Available from the authors at California Department ofFish and Game, 330 Golden Shore, Suite50, Long Beach, CA90802.]
Mortality estimation
Sampling design The NMFS observer programbegan in July 1990. Initially, the plan was to sampleevery fifth trip made by a drift gillnet vessel according to an assignment schedule. The order in whichvessels with MMPA exemption certificates werescheduled to be observed was randomly selected. Itbecame evident during the 1990 season that thisscheme would not work because oflogistical difficulties in adhering to the sampling plan.
Beginning in January 1991, gillnet vessel tripswere selected according to the targeted coverage rate(20%),the availability of observer personnel, call-ins(fishermen called in prior to departure), and the ability to notify fishermen of their obligation to carry anobserver. (Occasionally fishermen did not call theObserver Program administrator for possible observer assignment.) In addition, the NMFS Fisheries Observer Branch began monitoring vessel activity (arrivals and departures) to estimate observercoverage for placement purposes. If estimated observer coverage dropped below 20% for a vessel, theowner was notified of the obligation to carry an ob'server. In the setnet fishery, most fishermen werenotified after their nets were set whether they wouldbe required to carry an observer. As the programevolved, setnet fishermen began to expect an observerabout every fifth trip. "
Mortality estimation in the drift gillnet fishery Inthe swordfish and shark drift gillnet fishery, vesselsmade trips lasting from one to about 15 days so that
276 Fishery Bulletin 96(2), 1998
Table 4Observed (obs) and estimated (est) cetacean, pinniped, turtle, and seabird mortality, stratified by year, in the California halibutand angel shark set gillnet fishery during the NMFS Observer Program, July 1990-December 1995. Estimates oftotal mortalityare reported to the nearest individual. Estimated coefficients of variation (CV)are included in parentheses; (-) indicates CVwasundefined. Effort and estimates for 1990 pertain to the third and fourth quarters only.
Year
1990199119921993199419951---Estimated days effort3,0417,1715,5775,6801,9432,257
Observed days effort
1587066988751500
Percent observer coverage
5.2%9.8%12.5%15.4%7.7%0%
Observed trips effort
4062,2332,1232,6425470
obs
est CVobsestCVobsestCVobsestCVobs estCVobs estCV
Harbor porpoise
437 (0.56)538 (0.47)648 (0.46)213 (0.64)114(0.96) -14 (0.64)
Common dolphin (unknown stock).00(-)00(-)215 (0.65)00(-)00(-) --(-)
Unidentified cetacean
00(-)00(-)18 (0.92)00(-)00(-) --(-)California sea lion
67867 (0.22) 142 1,842 (0.16) 338 3,418 (0.28) 237 1,942 (0.13) 109 905(0.15) - 724 (0.08)
Unidentified sea lion
123 (0.96)6109 (0.53)754 (0.34)00(-)00(-) --(-)Harbor seal
30411 (0.23)42601 (0.23)90 1,204 (0.47)71475 (0.13)23 227(0.33) - 228 (0.13)
Northern elephant seal
13119 (0.40)330 (0.55)751 (0.35)1170 (0.27)216(0.66) -47 (0.29)
Unidentified pinniped
242 (0.79)330 (0.55)750 (0.39)732 (0.90)18(0.94) - 17 (0.83)
Sea otter
327 (0.53)00(-)00(-)00(-)00(-) --(-)Greenlblack turtle
00(-)00(-)18 (0.92)16 (0.90)00(-) -2 (0.61)
Loggerhead turtle
00(-)00(-)18 (0.92)00(-)00(-) --(-)Leatherback turtle
00(-)00(-)00(-)00(-)18(0.94) - -(-)Unidentified turtle
00(-)00(-)00(-)16 (0.90)00(-) -2 (0.61)
Pacific loon
00(-)113 (0.94)00(-)00(-)00(-) --(-)Commonloon
00(-)222 (0.68)17 (0.92)00(-)00(-) --(-)Unidentified loon
123 (0.96)448 (0.48)00(-)00(-)00(-) --(-)
Western grebe
00(-)18 (0.92)323 (0.70)16 (0.90)00(-) -2 (0.61)
Unidentified grebe
00(-)00(-)431 (0.92)16 (0.90)00(-) -3 (0.83)Double-crested cormorant
218 (0.93)00(-)17 (0.92)00(-)18(0.94) - -(-)Brandt's cormorant
241 (0.78)36409 (0.44)14279 (0.67)313 (0.64)216(0.66) - 3 (0.43)
Pelagic cormorant
133 (0.98)18 (0.92)00(-)00(-)00(-) --(-)Unidentified cormorant
9132 (0.45)15450 (0.92)968 (0.30)532 (0.40)00(-) -10 (0.35)
Commonmurre
142 1,300 (0.21) 289 2,201 (0.27) 292 2,333 (0.28) 137879 (0.32)20 284(0.29) - 967 (0.32)
Unidentified alcid
19 (0.93)00(-)00(-)00(-)00(-) --(-)Unid. seabird
00(-)222 (0.68)323 (0.53)16 (0.90)00(-) -3 (0.83)
1 Estimates for 1995 were based on stratified rates from 1993 results.
when a trip was chosen to be observed, the NMFStechnician observed all net pulls during the trip. Asingle net per day was set at dusk and retrieved before dawn. Net pulls and effort-days were equivalent units for this fishery. For estimation ofincidental kill, the collection of observed trips during a yearwas treated as a random sample (an approximation)and a ratio estimator was used. Trips were treatedas sampling units and the number of days per tripwas treated as an auxiliary variable. Stratification byquarter of year or set location was not used for yearlyestimates because previous exploratory analysis hadnot found this type ofstratification to be significantly
related to incidental kill. Yearly estimates, 1991-95,were calculated for each species observed entangled("Results" section). Estimates for 1990 correspondonly to the last two quarters of that year. Formulaefrom Cochran (1977) were used for estimating killrate, r, total incidental kill, m, and variances:
277
(5)
(7)
(6)
lands, Ventura, and central California. Central California included all effort south of Bodega Bay andnorth of Point Conception, 34°30'N (CDFG blocks437-650). There was no legal coastal setnet fishingnorth of Bodega Bay. The Channel Islands stratumincluded all effort within CDFG blocks containingpart of any Channel Islands (CDFG blocks 684-690,707-713,760-762,765,806-807,813-814,829,849850, and 867). The southern California stratum included all effort south of 33°50' N (blocks 718-918),excluding CDFG block 776 and the Channel Islandsstratum. The Ventura stratum contained the remaining area. Incidental kill estimation for sea lions andharbor seals included additional stratification byquarter of the year for the southern California andVentura areas because for these species, overstratification was not a problem.
Yearly estimates ofincidental kill in the setnet fisheries were calculated for all species of marine mammals observed entangled (Tables 4 and 5). Estimatesfor turtles and seabirds were calculated by the samemethod. For each species, kill rate, rs' and total incidental kill, ms' were estimated for each area by using a mean-per-unit (MPU) estimator with effortdays as the sampling unit (Diamond and Hanan1).Formulae for MPU estimators and their estimatedvariances for each area are (Cochran, 1977)
(3)
(4)
Table 5
Observed and estimated cetacean, pinniped, turtle, and seabird entanglement, stratified by year, in the California halibut and. angel shark set gillnet fishery during the NMFS Observer Program, July 1990-December 1995. Estimates of entanglement arereported to the nearest individual. Estimated coefficients of variation CCV) are included in parentheses; (-) indicates CV wasundefined. Effort and estimates for 1990 pertain to the third and fourth quarters only,- 1990199119921993199419951
Unidentified sea lion
67867(0.22)143 1,850(0.16) 341 3,438(0,28) 239 1,977 (0.13) 109905(0.15)-729(0.08)
Harbor seal
30411(0.23)43615(0.23)90 1,204(0.47)71475 (0.13)23227(0.33)-228(0,13)
Unidentified turtle
00(-)00(-)00(-)213 (0.64)00(-)-5(0.59)
Common loon
00(-)448(0.60)17(0,92) 00(-)00(-)--(-)Western grebe
00(-)222(0.68)323(0,70)16 (0.90)00(-)-2(0,61)
Brandt's cormorant
241(0.78)41494(0.37)20321(0.58)525 (0.45)216(0.66)-9(0.40)
Unidentified seabird
00(-)222(0.68)537(0.41)16 (0.90)00(-)-3(0.83)
1 Estimates for 1995 were based on stratified rates from 1993 results.
]lian and Beeson: Estimates of marine mamma.1. turtle, and seabird mortality....-
Variables ki and di represent the observed kill andnumber of days for the ith trip; davg is the sampledmean number of days per trip; a-J, a-Z, and a-d,k arethe sample variances and covariance of di and ki ; dand n are the observed number of days and trips,and D represents the total number of days of effort.The finite population correction factor, (1 - n /N),where N is the total number of trips, was approximated by using (1 - d / D) because the total numberof driftnet trips was not determined during the estimation of effort.
Mortality estimation in the set gillnet fishery Inthe setnet fishery, a trip was selected and all net pullsduring the trip were observed. Because greater than99% of all observed set gillnet trips were one day inlength, a trip was considered equivalent to an effortday. Net pulls were not randomly sampled and totalnumber of net pulls per trip was not recorded forunobserved trips, therefore a trip was considered thesampling unit. For observed trips, the overall average number of net pulls per trip (or effort-day) was3.08 (CV=0.43), and the overall average number ofmarine mammal entanglements per trip was 0.48(CV=3.10) during the NMFS Observer Program.
Researchers had previously shown (Perkins et al.5)
that quarter of year and set location were significant predictors of sea lion and harbor seal entanglement. Consequently, estimation of incidental kill inthe setnet fishery was stratified by area for all mammal species. The geographic area fished was dividedinto four strata: southern California, Channel Is-
278 Fishery Bulletin 96(2), 1998-
During the observed period, thedriftnet fishery was subject toarea and time restrictions inside 370.4 km (200 nauticalmiles [n miDfor effort that tookplace largely (80% of seasonal effort) from 15Augustto 31 January. Between 1February and 30April, fishing effort was restricted to locations farther than370.4 km offshore. From 1 May through 14 August(15 July for 1990-92) effort was required to be located farther than 138.9 km (75 n mi) offshore. Forthe remainder ofthe year, fishing was allowed within138.9 km of shore. Since the beginning of the NMFSobserver program in 1990, estimated total effort-daysfor the fourth quarter increased each year except for1994. Combined effort for third and fourth quarterswas consistently over 3,500 sets each year (Table 2).Even though there was effort along the entire California coast, the highest concentration of observedsets was south of33°N (Fig. 1). Observer coverage in
117W120
.......i.
123
this fishery typically ranged between 10% and 15%of estimated total yearly effort for most areas andquarters. (Exceptions as low as 2% and as high as25% were noted for some quarters and areas.) Biasof a ratio estimate is typically a problem in the caseof small sample sizes such as may occur in a perfishing block situation. Because sample size was generally quite large and stratification was not used,estimates were not corrected for bias. During thestudy period, July 1990-December 1995, nine (2%)ofthe 472 marine mammals observed entangled werereleased alive (one unidentified sea lion, one common dolphin, Delphinus spp., two California sea lions, one humpback whale, Megaptera novaeangliae,one Cuvier's beaked whale, Ziphius cavirostris, and
126129
Figure 1Approximate location of observed sets in the driftnet fishery during the period July1990-December 1995.
41
33
37
39
35
43
45N
(8)
r = 'LsD/s , (9)D
D2 A2
A2 'Ls s(Jrs (10)a = ' , r D2
m=Dr,
(11)
A 2 _ D2 A 2
(12)(Jm - (Jr '
The variable k. represents ob-',Sserved kill for the itk observedday in stratum s, and a-L is thesample variance of the observed kill. Variables ds andDsare observed and total numberof days of effort in the stratum,respectively. Estimates ofoverall kill rate, r, and total incidental kill, m, across all strata, andvariances, are then weightedaverages:
Results
Driftnet fishery
where D is total number of daysof effort during the year.
rJ""a~ and Beemno ""'mate' of ma,'ne mammal, turtle, and ,eab'm mortality- 279
three sperm whales, Physeter macrocephalus). Observed mammal bycatch in this fishery consisted predominantly of common dolphins (196). Northern elephant seals, Mirounga angustirostris, were the second most commonly entangled mammal (83). Thewide variety of mammal species incidentally entangled, over 25, is notable.
There was no statistically discernible trend in theestimates of total effort during the study period. Coefficients of variation for estimates of marine mammal incidental kill ranged from 0.18 to 0.95. The highest observed incidental kill in a single CDFG fishingblockwas in block 918, the southernmost block, whichhad 10%of the total observed mammal bycatch. Waters off central California west of 122°W longitude
Figure 2Approximate location of observed incidental marine mammal kills in the driftnetfishery during the period July 1990-December 1995, all species.
+
:I:
~++
also had 10% of the observedmammal bycatch. Locations ofmarine mammal mortality fromobserver records indicate thatmortality was concentrated inareas of heavy effort but geographically unrestricted (Figs. 12). On a quarterly basis, observedmammal bycatch and swordfishcatch were correlated (r=0.8), buton a per set basis this correlationwas not evident (r=O.O). Thischange in correlation is perhapsdue to the inclusion of substantial effort (quarterly) versus perset consideration, i.e. both catchand bycatch are related to effort.
Observed seabird mortalitywas very low in the driftnet fishery (three kills observed duringthe period). Marine turtle entanglement was more common,with 25 observed (13 releasedalive). All but one ofthe 15 leatherback entanglements occurrednorth of 35°N latitude. Estimatedturtle entanglement increasedtrom 23 for 1990to 81 for 1993andthen fell to 32 for 1995.
Setnet fishery
117W
Mortality estimates in the setnetfishery pertain to fishing efforttargeting halibut or angel shark.By law, effort targeting these species required gillnets withstretched mesh sizes of 21.6 cm(8.5 inches) or greater. This resulted in the elimination of 665
out of 8,614 observer records (net pulls) with meshsize less than 21.6 cm from the original data becausethey pertained to effort in another fishery. There werean additional 179 observed net pulls with no position data. This effort was assigned to geographicstrata based on the specific vessel's known fishinggrounds or was apportioned out to strata accordingto fishing effort from NMFS observer records withknown positions.
From 1990 through 1993, estimated effort in thehalibut and angel shark fishery was fairly stable withheavy setnet fishing effort along the southern California coast up to Pt. Conception (excluding the region from Rocky Pt. to Pt. Dume, where there wasno commercial fishing, and an area around Newport
120
____ 0. _
123126
;
+ +!+
+ +..,r )t+--- ---I ~~;; i --.-
++ -ii+ .ft"+ !
+ +!.•..
129
31
43
45N
41
33
37
35
39
280 Fishery Bulletin 96(2), 1998
Beach; Fig. 3). There was also effort in the MorroBay and Monterey Bay areas, as well as in some areas around the Channel Islands. Estimated effort for1994 decreased sharply because gillnet fishing wasbanned inside the area within 5.6 kIn (3 n mi) ofshorefrom Pt. Arguello south to the U.S.-Mexico border.During each year, observed mammal bycatch peakedduring the second quarter, as did estimated total effort (Tables 4 and 5). California sea lions were themost frequently killed mammal (899), followed byharbor seals (257), northern elephant seals (36), andthen harbor porpoises (18). Fourteen of the eighteenobserved harbor porpoise mortalities occurred in thenorthern portion ofMonterey Bay; three others wereobserved in the Monterey Bay area, and the remaining mortality was in the Morro Bay area (none wasreleased alive). There were 1,025 seabird mortali-
37N
36
ties: 1 Pacific loon, Gavia pacifica, 5 common loon,Gavia immer, 5 unidentified loon, Gavia spp" 6 western grebe, Aechmophorus occidentalis, 5 unidentifiedgrebe, Podiceps spp., 4 double-crested cormorant,Phalacrocorax auritus, 70 Brandt's cormorant,Phalacrocorax penicillatu, 2 pelagic cormorant,Phalacrocorax pelagicus, 38 unidentified cormorant,Phalacrocorax spp., 880 were common murre, Uriaaalge, 1 unidentified alcid, Alcidae, and 8 unidentified seabirds, Aves. Of the common murre mortalities, 99% occurred north ofPt. Conception-predominantly in the Morro Bay area (Fig. 4). All other birdspecies were entangled south of Pt. Conception withfew (12)exceptions (Tables 4 and 5). The six observedmarine turtle kills were separated either in time orlocation, but four occurred offshore of Ventura. Locations of observer-recorded effort and bycatch indi-
cated that no fished area wasentirely free ofmammal mortality due to gillnets (Figs. 3 and5). Estimated total marinemammal mortalities for thesetnet fishery rose from approximately 1,500in 1990, to 4,800 in1992, and fell to 1,300 in 1994.
Discussion
Figure 3Approximate location of observed sets in the setnet fishery during the period July1990-December 1995.
Channel Islands Stratum
(includes all islands)!
. D1'0·1
121
Although many resources wereused to obtain accurate estimates of mortality and fishingeffort, several significant difficulties were encountered indata collection and estimationof total effort. Difficulties inimplementing the samplingplan included availability ofobservers and type of notificationgiven to fishermen. Generally,observers were hired by the Program according to how muchfishing effort was expected. Because this aspect ofthe programwas well monitored, little seasonal bias was expected. Previous exploratory analysis has indicated that pre- and postnotification is a significant factor for the setnet fishery (Perkins et al.5), In this fishery, outof 2,588 days of observations,
117W
Data collection and effortestimation
118119120
Southern CaliforniaStratum
122
, i\) Morro Bay
Central California''""'] "stratum"";,'
34
33
35
32
Julian and Beeson: Estimates of marine mammal, turtle, and seabird mortality- 281
Figure 4Approximate location of observed incidental seabird kills in the setnet fishery duringthe period July 1990-December 1995, all species.
2 Absent or imprecise reporting of location-Eventhough location data were inaccurate, a record of
117W
Mexico
118119120
effort was tallied. Thus, because estimates werenot stratified by area in the driftnet fishery, thistype of error had no impact on the determinationof estimates of total effort or mortality for thisfishery. (This type of error may be important inmore detailed studies.) In the setnet fishery thistype of error would not have produced an error inan estimate oftotal effort but may have producederrors in effort estimates for individual geographicstrata. For purposes of mortality estimation, thesetnet fishery was divided into four geographicstrata, and effort without a recorded location wasassigned, with good accuracy, to a stratum basedon the vessel's known activity. If no informationon a vessel's behavior was known, effort was divided among strata according to the proportionof effort in known locations. No additional error
121122
32
33
34
37N
35
36
only74 (3%)were classified aspreset notification. In thedriftnet fishery, out of 3,338observations, only 29 werecategorized as postset notification. Because these data aresounbalanced in terms oftypeof notification, further analysis involving type of notification was not pursued. Apartfrom the overall difficulty ofimplementing an appropriatesampling plan, perhaps themost common difficulty waswith accurate determinationof location. Frequently, navigational equipment would notbe "working" or was not available to the observer. In somecases, location was incorrectlyrelayed to the observer andunder other circumstances ob-servers were not able to deter-mine location from majorlandmarks on shore owing toinclement weather. A seconddifficulty was that observersmay not have been able to de-termine accurately the totalnumber of animals or the species of the animals entangledin gillnets owing to obstruction of view by gear or crew.Also, mammals may havedropped out of a gill net priorto observation. Events of thistype were considered nonsampling errors and may havebiased estimates of incidental kill.
Estimates oftotal effort were also subject to error,although they were treated as accurate for the purpose of bycatch estimation. They contained inaccuracies due to at least three nonsampling errors:
1 Completely unrecorded effort -A typical exampleof this was when no marketable target fish werecaught during a trip and a logbook entry was notmade. It was not possible to determine the size ofthis error; however, given that there were threechances for effort to be monitored (logbooks, observer records, and landing receipts), this errorwas probably small.
282 Fishery Bulletin 96(2). 1998
was included in variance estimates because stratawere large enough and vessels predictable enoughthat "assignment error" was most likely negligiblecompared with other assumptions. There was noway to determine accurately the direction of biasin effort estimates for each area.
3 Underestimation of effort from landing receiptsUnderestimation of effort from landing receiptsoccurred when no effort was logged by fishermenor recorded by a NMFS observer for three days priorand subsequent to the date of the landing receipt.In these cases, a landing receipt was assumed torepresent one day offishingeffort.Ofthe three typesof nonsampling errors examined, this was potentially significantfor the driftnet fishery because tripsin this fisherymay last overtwoweeks. For example,
in 1993, the median length of a driftnet trip was 7days; the mean was 6.8(SE=3.3).Consequently, thenumber of times this approximation occurred wasimportant. In the setnet fishery,with shorter trips,this assumption had less impact. It applied to multiple-day trips around the Channel Islands area,offthe coast north of Santa Barbara, or when a fisherman held catch for more than one effort-day.Setnet trips were seldommore than a day in lengthbecause of catch storage and preservation limitations. In both the drift and setnet situations, theminimum effort representing a landing receipt wasused, namely one effort-day,because a more appropriate approximation would have required a caseby-case investigation. The assumption contributedtoward a possible negative bias ofestimates oftotaleffort and incidental kill.
Figure 5Approximate location of observed incidental marine mammal kills in the setnet fishery during the period July 1990-December 1995, all species.
.Ft
------ :----- - ----------- ¢ ~.~.
Infrequent entanglement ofmarine mammals, turtles,and seabirds makes accurateestimation of species-specificincidental kill difficult. Inspite of our good intentions,estimates may have been confounded by distribution offishing effort or movement ofspecies. In the future, thevariability ofestimates can bedecreased by more strictlyimplementing a samplingplan. For some species, aposthoc analysis may be productive as information onmovement and abundance ofstocks becomes available. Forinfrequently entangled species, only increased observercoverage will improve estimation of incidental entanglement and kill rates.
117W
Mortality
Combined, these three difficulties in estimation oftotaleffort may have produced anegative bias in estimates oftotal effort and, consequently,in estimates of incidental kill(i.e. current estimates of totaleffort may reasonably be considered a minimum). An accurate estimate of bias would bedifficult and costly to obtain.
118
+
119120121122
34
33
32
37N
36
35
Julian and Beeson: Estimates of marine mammal, turtle, and seabird mortality- 283
In the driftnet fishery, the wide variety of cetacean species killed is attributable to the large geographic range ofthe mammals, nonselectivity ofgear,and the amount and location of fishing effort. Cetacean bycatch in the driftnet fishery is greater andmore diverse than for the setnet fishery because areaof driftnet effort contains more diverse habitat thanthe area of setnet fishery. Data from recent yearsshow that effort is increasing off the coasts of northern California and Oregon, and beginning in 1995,the state of Oregon allowed 10 driftnet fishing vessels to land fish in a limited number of Oregon ports(landings were not permitted prior to 1995). Yearlyeffort is expected to continue at 3,500-4,500 sets peryear. The mortality rate averaged 0.11 (SD=0.25)mammals/set (or day) during the observer program,from January 1991 through December 1995; initialeffort in 1990 was omitted because of low coveragerate. Incidental seabird catch has been almost nonexistent and there is currently no reason to expectan increase. The amount ofseabird bycatch contrastswith the Japanese driftnet fishery in which manyseabirds are incidentally caught (Ito et aI., 1993), inpart, because swordfish drift nets are extended belowthe surface, typically 11m. The Japanese salmondrift nets, set at the surface, would naturally entangle more seabirds. Migration of seabirds in relation with the fishing season and effort location mayplaya role in seabird bycatch. No trend is evident inthe estimated yearly turtle entanglements. Regarding cetaceans, at this time there are 10 species thatare being entangled at a rate greater than potentialbiological removal (PBR) according to the U.S. Pacific Marine Mammal Stock Assessment Report(Barlow et aI., 1995). The impact of driftnet mortality on individual cetacean stocks is examined in thisreport. In order to reduce entanglement of cetaceans,NMFS, in conjunction with driftnet fishermen andthe California Department of Fish and Game, haveimplemented a Pacific Cetacean Take ReductionTeam whose goal is to decrease cetacean bycatch.
Over the past five years, estimated effort in thehalibut and angel shark setnet fishery was fairlystable until 1994, when a restrictive California voterproposition banned gillnet fishing within three milesofthe coast from Pt. Arguello south to the US-Mexicoborder. Before 1994, effort in this fishery varied substantially during the year. Since early 1994, gillnetfishing has occurred mainly offVentura, Morro Bay,and Bodega Bay (Fig. 3). There has also been someeffort off Imperial Beach, near the US-Mexico border. Effort north ofPt. Arguello has not been affectedby the 1994 resolution and is not expected to change.
Pinniped mortality in the setnet fishery was substantial during 1990-95. More pinnipeds were ob-
served entangled in the setnet fishery than thedriftnet fishery because effort coincided substantiallywith pinniped habitat. In particular, setnet effortaround the Channel Islands and near the ports SanPedro, Ventura, and Santa Barbara frequently entangled pinnipeds. Effort in the driftnet fishery results in less overall pinniped entanglement by virtue of this effort being placed farther from shore. Itis interesting to note, however, that entanglement ofnorthern elephant seals in the driftnet fishery is substantially greater than for the setnet fishery becausethese mammals typically range greater distancesfrom shore than do sea lions and harbor seals. Estimates of California sea lion and harbor seal mortality are expected to decline and then stabilize becauseof reduced effort south of Pt. Arguello.
Similarly, we expect the number of seabirds killedsouth of Pt. Conception to decline, because setneteffort was been pushed offshore 5.6 km (3 n mi) byregulation. There has been more bycatch of birds inthe set fishery because the entangled species are typically resident and their habitat overlaps substantially with setnet effort. Driftnet effort results in fewseabird entanglements because drift nets are retrieved after an overnight soak (set nets typicallysoak 24 hours) and drift nets are typically extendedover 11 m below the surface. In addition, mesh sizefor drift nets is almost three times greater than meshsize for set nets, resulting in fewer entanglements.The mortality of common murres entangled in setnets is expected to continue at the same level. Murrewere primarily observed entangled in the MontereyBay area. The nets in this area are set on the bottomin typically 55.8 m (30.5 fm, SE=0.2 fm) of water.Using a chi-square (X2) test, we determined that thereis some dependency between entanglement and soaktime ofthe net W<0.05). Odds of entanglement morethan doubled for an increase from 1 to 2 days of soaktime (reasonable checking for confounding factorswas done). Turtles will continue to be entangled inthe setnet fishery, but the number and rate of entanglement will be unknown because observer coverage in this fishery has been eliminated.
Acknowledgments
Special thanks are given to AI Jackson and RandRasmussen of NMFS for assistance with observerdata and to NOAAobserver-technicians and port coordinators who carry out the California gillnet monitoring programs. Tim Price kindly provided the sampling design description. Jay Barlow, Karin Forney,Doyle Hanan, Mark Lowry, and Peter Perkins reviewed this report and provided valuable input. Help-
284
ful and constructive comments were kindly providedby the reviewers. Finally, the cooperation of California gillnet fishermen, who allowed observers accessto their boats, is sincerely appreciated.
literature cited
Barlow, J., R. W. Baird, J. E. Heyning, K.Wynne,A. M. Manville II, L. F.Lowry, D. Hanan, J. Sease, andV.N. Burkanov.
1994. A review of cetacean and pinniped mortality in coastalfisheries along the west coast of the USA and Canada andthe east coast of the Russian Federation. Rep. Int. Whaling Comm., special issue 15:405-426.
Barlow, J., R. L. Brownell, D. P. DeMaster, K. A. Forney,M. S. Lowry, S. Osmek, T. J., Ragen, R. R. Reeves,R. J. Small.
1995. U.S. Pacific marine mammal stock assessments.
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U.S. Dep. Commer., Southwest Fisheries Science CenterTech. Memo. NOAA-TM-NMFS-SWFSC-219, 162 p.
Cochran, W. G.1977. Sampling techniques. John Wiley and Sons, New
York, NY,428 p.Ito, J., W. Shaw, and R. L. Burgner (eds.).
1993. Symposium on biology,distribution and stock of species caught in the high seas driftnet fisheries in the northPacific ocean. Int. North Pacific Fisheries Comm., no. 53,vol. III, 576 p.
Jefferson, T. A., B. E. Curry, and N. A. Black.1994. Harbor porpoise mortality in the Monterey Bay hali
but gillnet fishery, 1989. Rep. Int. Whaling Comm., special issue 15:445-448.
Lennert, C., S. Cruse, M. Beeson, and J. Barlow.1994. Estimates of incidental marine mammal bycatch in
California fisheries forJuly through December,1990. Rep.Int. Whaling Comm., special issue 15:449-461.
Salzman, J. E.1989. Scientists as advocates: the Point Reyes Bird Obser
vatory and gill netting in central California. Conserv.BioI.3(2):170-80.