~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: fjulian@osiris.ucsd.edu

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.

Fishery Bulletin 96(2), 1998

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.