Hawaiian monk seal diving behavior - SWFSC Home Page

Acta Zool. Fennico 172:129-131. 1984

Hawaiian monk seal diving behavior
R.L. DeLong, G.L. Kooyman, W.G. Gilmartin & T.R. Loughlin

DeLong, R.L., Kooyrnan, G.L., Gilmartin, W.G. & Loughlin. T.R. 1984: Hawaiian monk seal
diving behavior. - Acta Zool. Fennica 172:129-131.

Diving behavior of Hawaiian monk seals Has studied to gain an understanding of their use 01 thc
marine habitat. Radio tags and maximum-multiple-depth recorders (MMDR)were attached ulth
anklets t u seven adult male monk seals at Lisianski Isldnd. 24-h automatic monitoring of radio tag\
provided a detailed record 0 1 hauling o u t activity on the island's beaches. Over 4800 individual
dives b) six animals were recorded by the MMDR system. The majorit) (59 ' i J of dives were in the
10-40 m depth range, the remaining dives were to depths gcnerall) greater than 4tI m. with l i
diier to at least 121 m. Monk seal prey items were identified l r o m exammaticn cl seal scat and
included fish otoliths. cephalopod beaks and invertebrate ekoskeleron lragment3. The :)re)
assemblage uas benthic; the majority of prey were shallow water for:n* nnd a rmaller nunibrr
species were dcep water forms. The depth at which the preq were taken generally agrecd with the
distribution of monk seal dive depths recorded. rhii information indicuted that Hdwdiiair insrik
seals depend entirely upon the food resources tound over the reel wrfdce and slopez while at
rookery islands during the breeding season.

K . L. DeLong. Narional Marine Mammal Luboraror). Nor. M a r . Fish Ser , 76011 Sand Poinr U h j .
N E . . Searrle. WA 98/15, USA.

G.L. Kooyman, Physiological Kesearch Lobururor~S, crippJ Insrirurion of Or.eunogrnpn),. Lu lollu.
C'A Y2113;. USA.

W . 6 . Gilmarrin, Honolulu Luborarory. Nar. Mar. Fish. Srr P.0.Box 3 8 W Hosoiulu. Hi W n l 2 .

LSA.

%R. Loughlin. Narronal Marine Mammal Loburator).. .Val. M a r . ) i s h S e i . . 7600 Sand Puinr Wo)

&.E., Seartle. WA 98115. USA.

1. Introduction waters. The seaward extension of the boundary of
critical habitat became a major point of contention with
The endangered Hawaiian monk seal Monachus commercial fisheries interests not wanting to be
schauinslondi has breeding populations restricted to excluded from fishing any nearshore waters and
atolls of the Northwestern Hawaiian Islands. The resource managers recognizing that the protection of
species appears to have declined in numbers by about food resources within critical habitat would be essential
one-half between 1958 and 1978 and the center of to recovery of the endangered monk seal.
abundance has shifted from the atolls in the western
portion of the range to those at the eastern extreme of O u r study was an attempt to obtain a series of dive
the breeding range (Johnson et al. 1982). Conservation records from feeding monk seals to provide a data base
efforts by the U.S. National Marine Fisheries Service from which a seaward boundary of critical habitat could
include consideration of designating critical habitat be designated. In addition we were interested in testing
under terms of the U.S. Endangered Species Act. The radio tags for determining the proportion of time
rookery beaches were to be included as part of critical animals spend ashore. This was needed to adjust census
habitat. The fact that monk seals are known to feed on figures as an estimation method for population size.
benthic or reef-dwelling fishes and invertebrates argued
for including marine habitat within the critical habitat, 2. Materials and methods
but no information existed on the depths to which monk
seals commonly dive in search of prey. The need to 1 he study was conducted at Lisianrki Island (26'U?'N, 174'04'W)
preserve an abundant prey assemblage for monk seals l r o m 4 May through I I June 1980. Adult male heals were relcctcd t o r
was countered by an intense interest in developing iii~tri~n~sntatbieocnause they remain on or ncar the rookery ibldiids
commercial fisheries in the inner reef and reef slope during most of the pupping x a a o n which cxtcnd, lroin March
through J u n e . Males normally go to \sd in the e\eiling. Iced during

130 R.L. DeLong. G.L. Kooyman, W.G. Gilmariin & T.R. Loughlin

the night. and return to t h e island in earl) to mid-morning. B! Idhle I A c t i \ i t y (11 adult mdle Iiauaiian m o n k serils inrtruinented
instrumenting only adult males we uere Jssured that s o i i i e porticiii uith d i \ e recordcrs a i d rddi,) tdgs. I.iaianski I s h n d . .hfdy-June
of the dives uould reflect the depths ulierr animals search l o r loud
or were actually feeding. Futher. since the males dre lurccd to return 19x0.
to land to obtain access to lemale, ue had a high probability 01
being able to recover instruments. l;lg Ui\e Proportion :\t 5e.i D i b e s recorded
radio ot ddkliglit (da?,’
SeLen adult males uere captured h) ph)\ical restriiint uith J Ireq. record hours akhorr. nigh!’) --
can\ds restraining device, a hoop net. and black bag. l h e c d n \ d \ d.~)s
restraining device consisted o l a heavy canvas stretcher w i t h uoodcii lb4.lY2 Number Depth
poles uhich u e placed ober sleeping animais. Physicdl restrr:int uas I64 I30 2
nece>sar) only tor instrumenting the animal, and subscquenr 164.113 0.2 I U/ : IUS 10-40 m
adjustment of one anklet. 7
164 ??X 0.20 0/! ‘ f 10-40 m
Each seal was equipped w i t h J radio transmitter on o n e aiiklr and 10 10-40 I l l
a depth-of-dive recorder o n t h e o t h e r Kadio transmitters u e r e lil4.161 20 0 4 3 0/u
purchdsed Irom the Cedar Creek Biuelectronic Ldburdtor! ol t h e
Unikersiiy 01 Minnesota. l h e transmitters uere ? / S \ b c i i i Ih4.U’)’) IX 0.30 5/13: 1193 10-40 m
cylindricdl pdckage weighing dpproxiinatel) 50 g i n dir. lhey
transmitted on 164 M H z , %ere poucred b) a lithium batter) IMu j i 18 133 31-X5 in
sutticient lor one 10 tuu months hie, 2nd uere encapsulated in
electrical casting resin. Traiiamirters Herr attached b) m e i n s (it d n IY u23 2ilb2 I46Y 10-4) in
anklet ut machine belting I ? inm uide and 3 inni thick, cotcrcd uith
20 m m diameter latex surgic‘il tubing. l h e belting uilb connected 1x4 4 1 - 4 5 m
using a one-eighth inch brdss mdchine bolt uirh brass llat uashers
and a \tee1 lock uasher and nut. Ihe binictallic cornbination 0.40 1/4 Kecorder ddnidged
pro\ided certain release of the Anklet ,hould i t not be reco\cred
lrom the animal. no record

Instruments uere removed with d hnite blade set at the hdch ut a u.13 6/11 7 5 10-30 m
stainlesb steel hook attached to a 1.2 m uoodcn handle. I he point ot
the hook uas slipped under t h e dnklrt a n d tugged sharp11 cutting 935 31-Wm
both thc surgical tubing And mdchinc ucbbing. treeing the , i n l l e t .
6b9 61--1?0 ni
Ihe radio-taggeu animals were nionit;red uith d-.&lenient 1 dgi
antcniide and programmdble rcsnner-reccibcr connected tu a i i I? 121-175 ni
Esterline .4ngus strip-chart recorder. T u u antennae u c r e mountcd
on a 9-ni high mast located on t h e highest point 01 the island and ‘Nights lire tallied i t dnimdl u s s at sed lrom ddrk to midnight.
oiirnrrd along the long and short dxis o1 the island. midnight i o d.iun, ur during both period\, da!s uere tallied 11
animal u a ) d t sea 1 r o m daun t u noun or iiuon to d,irk
Mulriplc maximum depth recorders (MMDK)uere attached in the
bame manner as the radio transmitters. T h e units used ucre 2.3 c i i i in !Includes 2 absence\ ill a night. d‘iy, a n d dnot!ier night.
outside diameter by 9.3 cm long a n d weighed 95 g in air. rhe
hlhlDK basically was a series ot prcsursc tran\ducer>and a time ’Inclutca o n e abhencc 01 4 s u c c c s ~ i b ed d ) ~and 5 night,. d n d one
circuit uhich monitored depths ot d i b e s u h i c h uere processed in absenic 01 d night. d d ) diid d n o t h c r n i g h t .
eight storage registers for data readout ( K o o y m d n et 31. 1982). 4
potentiometer made i t possible to ,et the span on the pres\ure rdnge mubt swim 10-21 k m f r o m t h e island t o reach w a t e r 40
lroni 0-500 psig. The pressure ranges currespond to specitic u d x r
depth ranges. Anytime a dive exceeded the threshold ot d counter. m deep.
the dibe uas added to that rcgister’a count and stored. Ihe end result
uils a lrequency distribution of all d i b e s made b} the sed1 o\’er the R a d i o tags f u n c t i o n e d well a n d a l l o w e d location of
period 01 time the M M D R was artdchrd. The data \tored in t h e
recorder was determined by connecting the unit to a digital instrtirnented males when they were ashore. The average
processor. Each register was read separately.
time ashore during daylight hours for 6 reproductive
To assess food habits of monk seals during the study, we collected
fresh scats which were soaked for 12 to 24 h in sea water to which adult males was 16 (2 d u r i n g the h o u r following d a w n ,
was added ca. 1 g liquid detergent. The liquified scat were then run then gradually increased to 35 ‘2 b e t w e e n 1100-1200

through a set o f t w d nested testing ~ieveS’mea~urin2g.36 mm and h o u r s , a n d d e c r e a s e d to 21 l,? in the h o u r s preceding
0.71 mm. Mard parts such as fish otoliths, bone, teeth, and scales,
cephalopod beaks, and exoskeleton of lobster were removed from darkness. T h e p r o p o r t i o n of time individual
the sieves and stored in alcohol for later identification.Fish otoliths
r e p r o d u c t i v e m a l e s were a s h o r e varied f r o m 13 R to
were identified by John E. Fitch, California Department of Fish and
Game, and c e p h a b o d beaks by Clyde Roper, U.S. Museum of 40 (2 of total daylight hours. A single n o n r e p r o d u c t i v e
Natural History, Smithsonian Institution.
adult male exhibited a pattern unlike the reproductive
3. Results
males. He spent 43 % of total daylight hours ashore but

was ashore early morning and late evening and was

frequently in the water from 1100-1600 hours. We

,-o~jected~a~~ found on the island, We obtained

ldentifiabie hard parts from Of 232 scat

O t o l i t h s of 260 fishes of 1 1 families, beaks of 71
c e p h a l o p o d s all of the g e n u s Octopus a n d e x o s k e l e t o n
fragments of one lobster were identified. additional

11 otoliths could not be identified from available

reference material a n d 15 otoliths were too digested t o

allow identification.

D i v e recorders were a t t a c h e d f o r 2 t o 20 d a y s f o r a n 4. Discussion
aggregate of 94 days. The dive recorder for one animal
was inoperative. Six animals accomplished 4817 dives in Lisianski Island is located o n t h e n o r t h e n d of Neva
60 d a y o r night periods a t sea (Table 1) f o r a n a v e r a g e of Shoals - a n extensive shallow water area which extends
80 d i v d p e r i o d a t sea. T h e m a j o r i t y (59 %) o f dives primarily s o u t h of t h e island. To reach the I O m e t e r
were in t h e 10-40 or 10-30 m e t e r range, with the isobath, the minimum diving depth recorded, animals
r e m a i n i n g 41 % of dives b e i n g i n d e p t h s b e y o n d t h e 30 needed to be from Ito 13 k m from land. Beyond Neva
Shoals there a r e extensive coral flats within the 10-40
t o 40 m e t e r s u p p e r threshold o f t h e first register t o meter depth range where male monk seals were diving
depths up t o at least 121 m. Only three animals were and feeding. The 40-m isobath generally delineates the
away from the island two or more nights and these were o u t e r e d g e of t h e reef. A s u b s t a n t i a l p o r t i o n o f t h e
the o n l y seals t h a t d o v e t o d e p t h s greater t h a n 40 m . D u e r e c o r d e d dives, those beyond 30 m , reflect a n i m a l s

t o a n extensive reef system of Lisianski Island, the seals

A C T A Z O O L O G l C A F E N N I C A 172 131

diving along the reef slope. of time ashore.
Results of the scat collection indicate that the
I hcae are the f i r i t d i v e record.. lor the 11auaii.in m o n k ~ ~ " i lIsh.i.!
llawaiian monk seal ieeds heavily o n reef fishes. The
suggc,t ,I pzttern 01 genernll) rhallou dives, yt'i the d i \ z \ in e * c r s o1
fish found most frequently a n d in highest numbers were
121 in by m e dnimal arc moderately deep t o r pinniped>. Severdl
conger eels (Congridae), surgeon fish (Acanthuridae),
specicc have been recorded t o dite bevund 150 m ' d noithern l u r %a1 and wrasses (Labridae) which prefer shallow, benthic
habitats. Less numerous were brotulids (Brotulidae) and
C o l i o r h i n u ~ ursinu.5 I ~ n u l eto 19U ni ( K o o $ n i a n et :iI 19:6). .I cusk eels (Ophididae) which a r e probably found in both
s h a l l a w a n d deep, benthic h a b i t a t s , a n d lizard lish
Cali!ornt,i te'i lion Zulophus culr,forniun;r male to 2 3 1 ni (Kidzua! (Slnodidae) \sere the only prey species from a deep

1Y72). d h'cddell scal l . i , p : m i ( h o : c , ~iic&4li to 000 ni (hiio)iiiaii water, benthic habitat. The habitat depth of the
identified prey is consistent with distribution of the
1966). When compared 10 the &\e Jepihs accomplirhed b) thew monk seal dive depth records. During the breeding
season monk seals apparently depend entirely on the
species the I21 m (or grcatcr) diber recorded i n thia siudy ma! br
food resources found on the coral reefs, sandy flats, and
considerabl? less th-n the maximum d i \ i n p capability 01 t h e d e e p e r reef slopes. A designation of critical h a b i t a t
u h i c h included all waters f r o m island shorelines to the
Hauaiian monk seal. reef slope edge is justified by t h e m a r i n e h a b i t a t
utilization data obtained in our study. Setting the
The deep recurdcd i n thi, 5tudy rupport a ~ ~ p dliveing s e a u a r d b o u n d a r y of critical h a b i t a t a t 20 f a t h o m s
~ o u l dinclude m o s t of t h o s e waters a n d p r o v i d e a
record lor the hlediterranean monk >edl , M O I I ~hCu, motruchui H hiLh consistent boundary which could be easily detected b)
vessel o p e r a t o r s a p p r o a c h i n g the r o o k e r y islands of the
*a!. caught on the hook o l luiid gear \ e t . i i .i depth ut 7 5 m at Hauaiian monk seal.

Cai,iera d e Lobos, b l d e i r a in I957 (Sr.rge.int et al. 19-X). liicre had

b x n rnmc qucsiioii as t c i uhcilier that liniiiial niighi lhdic bcen

huohed while the gear was being \SI,but in light 01 the drm~mrtrated

diving cap.tbilitics 01 the H a

Mediterranean monk yedl u

Determination (31 the proportion 01 time spcnt ashore

for adult males during daylight hours 15difi.ieult becawe

of their t e n d e n c y to lay between a teniale a n d the water's

edge with their rear flippers immeisrd in the water.

W h e n the transmitter a n t e n n a is in wet s a n d oi under

water. the signal is a t t e n u a t e d aiid no signal is received.

yet the male is almost entirely ashore. 'Therefore, the

radio tag record underrepresents the actual proportion

References

J o h n s m . 4 %I , DcLong, K L , I-ixu,. C.H & ticnkon, ti b . I Y S ? . - In: MacAlister, A.G. & Pride, I.G. (Eds.). Experimental
PopuLition %itus ol the Hauaiian m o n h scai (.\tunachur Biology at Sea. Academic Press, New York.
acliauinalandi). 1Y78 - J. \lammal. r l r i presrl. Ridguay, S.H. 1972: Homeostasis in the Aquatic Environment. -
In: Ridgway, S.H: (Ed.), Mammals of the Sea: 590-747.
tiooyman, G.L. lYh6: b ~ a x i i i l i i mdiving capdci!i:h uf the .irddeii C.C. Thomas, Springfield, Ill.
real (Leptonychotes uedelli). - Science 151.IZ53-1554. Sergeant, D.E.R,onald, K., Boulva, J. & Berks, F. 1978: The recent
status of Monachus monachus, the Mediterranean monk
Kouyman. G I - . , GrnIr), R . L . & Ilrgaharr. D . W 1Y76. Sorthern l u r seal. - In: Ronald, K. & Duguy, R. (Eds.), h o c . First Int.
real diving behavior: A neu approach to i t s study. -
Conf. Mediterranean Monk Seal: 31-54, (Rhodos,
Science 191:411-412. Greece). Pergamon Press, New York.

Roo)man. G.L., Billupr, J.O. & Faruell, W . D . 1982. l u o recently
developed recorders lor monitLtringoldi\ing activity ai sea.