Observations of Harbor Porpoise in the Vicinity of Acoustic Alarms on a Set Gill Net

43 Pages·2000·0.49 MB·English

NOAA Technical Memorandum NMFS-AFSC-84

Observations of Harbor Porpoise
in the Vicinity of Acoustic Alarms
on a Set Gill Net
by
J. Laake, D. Rugh, and L. Baraff
U.S. DEPARTMENT OF COMMERCE
National Oceanic and Atmospheric Administration
National Marine Fisheries Service
Alaska Fisheries Science Center
January 1998
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Technical Memorandum NMFS-AFSC-84

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TCARTSBA
Large and small-scale fishery experiments have demonstrated that attaching acoustic
devices (pingers) on gill nets reduces harbor porpoise (Phocoenaphocoena) entanglement and
mortality. However, the mechanism for mortality reduction is unknown. We conducted shore-
based observations of a set gill net that was alternately set with and without alarms for 2- to 5-day
periods during 27 days between 11 July - 6 August 1996. When the acoustic alarms were
attached to the net, harbor porpoise were displaced by an acoustic buffer with a radius of at least
12am5r ountdh nee (t<P O .Ol)w,h icdhe monstratetdh eef fectivenesospf i ngerisrn e ducing
.tnemelgnatne
iii
S
TNETNOC
tcartsbA
iii
Introduction. 1
Methods 2
ydutS aerA noitpircseD 2
Net Array 2
latnemirepxE yevruS/ngiseD locotorP 3
Analysis 5
R e s u l t s 8
Discussion 10
stnemgdelwonkcA 11
C i t a t i o n s 13
F i g u r e s 15
Tables 18
xidneppA A - sliateD fo thg ineohi-tfaftiulpCmoC 21
Appendix B - Field Procedure Details 29
xidneppA C - sisylanA sliateD 35
V
INTRODUCTION
Pingers have dramatically reduced incidental harbor porpoise (Phocoenpah ocoena)
ytilatrom nsit ndeemticruedpn xohectiw eht knis llig ten yrehsif ni eht fluG fo eniaM suarK( te .la
)7991 dna eht hakaM labirT tes llig ten yreh sniifraeG( te .la )6991 ffo eht tsaoc f onotgnihsaW
State. However, the mechanism for mortality reduction is unknown. The report of the Workshop
on Acoustic Deterrence (Reeves et al. 1996) concludes:
Although it is clear that the pingers significantly reduced the harbor
porpoise bycatch in the [Gulf of Maine] experiment, it is not clear why they
worked. For example, it is not clear whether porpoises were alerted or
repelled by the sounds made by the pingers; their prey responded by moving
away from the sounds and the porpoises followed; or other factors were
involved.
The workshop recommended conducting “studies of the fine-scale distribution of porpoises in
areas where pingers are used, to look for signs of displacement and other effects”.
To address the workshop recommendation, we repeated the work of Gearin et al. (1996)
taos sestsh eef fectivenesospf i ngerasrt e ducinign cidentamlo rtalitayn idn cludetdh feo llowing
additional objectives: 1)m easuret hee ffectso fp ingerso nt hef ine-scaled istributiono fp orpoise
by observing surfacing harbor porpoise in relation to a set gill net that was alternately set with
pingers (alarmed) and without pingers (unalarmed); and 2) determine signal to noise ratios of the
pinger frequencies in the region surrounding the net by recording acoustic measurements when
thneew taa sl armeadnu dn alarmed. We report here on the study of the effects of pingers on the
fine-scdailset ribhuatorpifboo ornrp oise.
METHODS
Study Area Description
The Spike Rock Fishery Area, seaward of Shi Shi Beach, is within the Olympic Coast
NationalM arineS anctuaryi nt heP acificO ceana tt hen orthwesterne dgeo ft heO lympicN ational
Park in Washington State (Fig. 1). Water depths do not exceed 20 m within the study area at
mean tide levels. During the survey period, 11 July to 6 August, daily differences between low
anhdi gthi dersa ngefdr oalmsi ttla1ut4(es.mpo.mf 6 4 r t+ o-.mo3m10 .m 4) .
snoitavresbO fo eht ekipS kcoR yrehsiF aerA erew edam morf a etis no na desopxe ffulb
northeast of Shi Shi Beach (48°6.5’N, 124°40.7’W). The observation site was located on the
MakahI ndianR eservation,a djacentt ot heO lympicN ationalP ark. Vegetation at the observation
site limited the field of view of the Spike Rock Fishery area to 56° (magnetic 202.5° to 259.2° for
the inshore observer, and 197.2° to 253.5° for the offshore observer; magnetic declination was
+21.3°). The site altitude, 46.8 m (149 A) above sea level at a 0 m tide, was determined by using
triangulation with a pair of targets at sea level separated by a known distance (Appendix A).
Distancesb asedo nv erticala nglesf romt heh orizono rh orizontalu sedt he4 6.8m h eighta djusted
for tide height at the time of the measurement, which was interpolated between extremes from a
edit .elbat
teN yarrA
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2
(183 m long by 16 m deep) placed opposite the shore-based observers (Fig. 1, Table 1). The ends
of each net were marked by orange buoys attached to the cork line. Theodolite bearings to each
buoy were recorded at low and high tides each day, providing a record of net locations relative to
porpoise sightings. Daily measurements were used to construct an average position of each net.
The theodolite (Nikon NT2A) was accurate to 0.006° with a 19 power lens and an optical field of
weiv fo .°05.1 Vertirceaald iwndegerste e rmirnepelrdae atc tiiovs leee lvye bluebdb hloer;i zontal
bearings were determined relative to magnetic north. A distinctive notch on Spike Rock, at the
raf dne fo eht yevru s,aera saw desu o tetarbil a eclhat tsngonziirrao ehtbes( . )t”aOO’OO°112 youB
locations were also determined using a Global Positioning System (GPS), aboard the fishing
vessel that attended the nets.
Experimental Design/Survey Protocol
Eleven pingers, built to the specifications of John Lien (Fullilove 1994) were placed on
the corkline of each net at 16.6 m intervals to create an acoustic alarm (Gearin et al. 1996).
yllaiti ne Iwdednetn iot ec aslrpegnip no eno ten nihti whcae fo eht owt sriap .oN 01 - .oN 11 dna
.oN 21 - .oN .31 The pingers were to be rotated between nets within each pair every 2 days. This
design was followed for the first week of the 1996 study; however, as we collected acoustic
measurements, it became clear that the lower tonal component (3Hz) of the pings was discernible
between nets within the same pair (Bowles, unpub. data). Therefore, we modified the original
designs ucht hatb othn etsi ne achp airw eree ithera larmedo ru nalarmed. Inclement weather
sometimesp reventedc hangingt hep ingersb etweenn etse very2d ays. When this occurred, the
following period was matched in length of days to the previous observation period (e.g., a 3-day
3