Tracking Coastal Sharks with Small Boats: Hammerhead Shark Pups as a Case Study

Aust. J. Mar. Freshwater Res., 1992, 43, 61-6

Tracking Coastal Sharks with Small Boats:
Hammerhead Shark Pups as a Case Study

K. N. ~ o l l a n d ! C. G. owe^, J. D. peterson* and A.          ill^
A   Hawaii Institute of Marine Biology, PO Box 1346, Kaneohe, Hawaii, USA.
    California State University, Long Beach, California, USA.
    University of Leicester, Leicester, England.

Abstract
Acoustic telemetry techniques have been adapted for use with small boats to facilitate tracking of
nearshore reef species. In addition to permitting tracking in areas where manoeuvrability and quick
responses are required, the system has modest operating costs that make tracking experiments a viable
option for a wide range of researchers. Tracking and communication equipment can be powered for
several days by a single 12-V 8D truck battery. Current topics in shark biology that are amenable to
these tracking techniques are discussed. For instance, hammerhead pups have been tracked on their
natal grounds for periods of up to 13 days. Their daytime movements appeared to be restricted to a
well defined 'core area' where a school of sharks hovered between 1 and 3 m off the lagoon floor.
At night, the sharks became more active, expanding their range of movements before returning to the
core area the next morning.

Introduction
    Sonic telemetry of the movements of fish can provide detailed information of the type
essential to the formulation of sound management and conservation policies. Phenomena
such as the size and dimensions of home range, site fidelity, diurnal changes in distribution
and behaviour, foraging patterns, and the influences of topography and physical ocean-
ography are all amenable to sonic tracking techniques (Carey and Robison 1981; Carey
et al. 1982; Klimley and Nelson 1984; McKibben and Nelson 1986; Holland et al. 1990b).
As with most scientific studies, the utility of tracking data depends on the size of the sample
(i.e. on the number of replicates obtained). However, replication can be difficult in open-
ocean telemetry, largely because of the high costs and cumbersome logistics involved with
       - of research.
this tvve
    Fortunately, recent decreases in the size and cost of electronic components have resulted
in commensurate decreases in the size and cost of tracking vessels and equipment. This
makes replicable tracking research a viable option for more researchers, allows targeting of
species that were otherwise inaccessible to large research vessels (Holland et al. 1985, 1990a;
Gruber et al. 1988), and makes feasible the tracking of fish previously thought to be too
small to carry transmitters.
    Acoustic tracking from small vessels has been utilized in the limited confines of some
freshwater systems (Kelso 1976), and small tracking vessels have previously been used by
shark biologists (Tricas et al. 1981; Gruber et al. 1988; inter alia). However, the continuing
improvements in this technology, and an increase in the number of investigators who might
take advantage of these techniques, make an update of this methodology appropriate.

K. N. Holland et al.

   In this paper, we describe in some detail the tracking equipment and techniques we have
developed for use with a small skiff. This method uses portable equipment assembled to
permit continuous tracking by a single operator. This greatly reduces the costs of logistical
support required to obtain continuous high-resolution movement data. Examples of the
type of data that can be produced by this system are taken from the preliminary results of
an ongoing study of the movements of hammerhead shark pups on their natal grounds in
Kaneohe Bay, Oahu, Hawaii.
   Shallow lagoons and estuarine areas of Oahu are pupping and mating grounds for
Sphyrna lewini. Within these areas, they are apex predators that, judging from their
apparent high densities, probably have a major impact on the energy budgets of their
habitat. On the basis of net and hook-and-line captures, Clarke (1971) hypothesized that the
predominant distribution of the pups was in the turbid waters of the southern end of the
bay, from which they made foraging excursions into the rest of the bay at night. However,
without detailed knowledge of the foraging range of individual animals, the apparent
population size can not be adjusted to account for diel movements, and the productivity
of the bay as represented by the standing biomass of the sharks can not be calculated.
Consequently, we have embarked on a programme to determine the ranges of diel move-
ments of these neonatal animals.

Materials and Methods
    A modular ultrasonic tracking system was designed for rapid, but not permanent, installation on
a 5.0-m skiff (Boston Whaler) with minimal permanent modification of the hull. A weatherproof
Plexiglass cabinet housing the ultrasonic receiver, the communication radio, and the navigation
equipment was bolted to a removable wooden plank that bridged the gunwales of the boat. All of the
tracking equipment and radios were powered by a 12-V 8D battery housed in a portable battery box.
Individual electronic components were connected to the battery via a bank of 'cigar lighter' receptacles
inside the box. Navigation lights and a spotlight were powered by a separate automotive battery housed
in its own box.
    A hydrophone mounting block, shaped to the outside contour of the hull, was secured by two
stainless-steel bolts passing through watertight plastic tubes sealed into the hull 30 cm above the
waterline. This permitted rapid installation and removal of the mounting block while maintaining the
functional integrity of the hull. The directional hydrophone (Vemco, Halifax County, Nova Scotia,
Canada) was mounted on a 1.5-m-long, 2.5-cm-0.d. galvanized-steel pipe that fitted through a guide
tube in the mounting block, allowing the hydrophone to rotate through 360' and to be retracted
completely out of the water or lowered to a variety of depths to a maximum of 10 cm below the keel
of the boat. A handle on the mounting pole allowed the hydrophone to be rotated to acquire the
transmitted signal and gave a visual indication of the direction of maximum signal strength (i.e. the
direction of the transmitter). The cable connecting the hydrophone to the receiver ran out of the top
of the mounting pipe, thereby reducing twisting of the cable caused by rotation of the hydrophone.
The hydrophone was mounted towards the stern of the boat so that tracking could be accomplished
by one person who could simultaneously swivel the hydrophone to locate the fish and steer the boat
via the tiller of the outboard motor (Fig. 1). Consequently, most tracking was performed by single
operators working 4-h shifts. Horizontal position of the boat was determined every 15 min by using
an electronic hand-held compass and visual landmarks. In more remote locations, a global positioning
system (GPS) receiver was used.
    Transmitters (8.0 x 40.0 mm) with carrier frequencies of 65.5 and 76.8 kHz were obtained from
Vemco. A two-piece transmitter with separate transducer and battery sections was developed for use
with specimens with a high likelihood of recapture. Upon recovery, the battery section can be removed
and replaced, with a commensurate reduction in cost compared with the purchase of a complete new
transmitter. Nominal battery life is approximately 15 days.
    Neonatal hammerhead sharks were captured during the daytime by using handlines with baited
recurved hooks with the barbs removed. The fish were brought aboard the tracking boat, where the
hook was removed, the animal sexed, and the fork length determined. The transmitter was inserted into
the gut by using a premeasured tube that extended into the gut region adjacent to the pelvic girdle.
The transmitter was dropped into the tube and kept in place with a ramrod inserted down the tube.

Tracking Coastal Sharks with Small Boats

Fig. 1. Small-vessel tracking system for single-person operation. The mounting block and pole for the
rotatable directional hydrophone can be seen on the side of the hull. The cable from the hydrophone
exits from the top of the pole and connects to the receiver in front of the operator.

The tube and ramrod were both removed, leaving the transmitter in the gut. The fish was then released
and tracking was initiated. Three captive animals were fitted with dummy transmitters in a similar way
in order to determine tagging trauma and evaluate regurgitation rates. Convex polygon analysis of the
size of daytime and nighttime ranges (Klimley and Nelson 1984) was performed by using an image-
analysis system that measures areas traced from a digitizing pad (ZIDAS; Carl Zeiss Co.)

Results
   The small-vessel tracking technique, with single-person tracking crews, proved to be a
viable method, allowing a three-person tracking team to acquire detailed, continuous
positional data for periods of more than 72 h and spanning 13 days. The shallow draft
and high manoeuvrability of the skiff permitted the animals to be tracked to within a few
metres of patch reefs and over sand flats. In these shallow locations, the hydrophone, still
rotatable, was retracted to a depth less than the draft of the hull (approximately 35.0 cm).
Between tracking sessions, the vessel could rapidly be made available for other, nontracking
purposes. To date, data have been analysed from three animals (fork lengths 38.0, 40.1 and
43.0 cm) caught in the same locality in southern Kaneohe Bay, and additional tracks are
currently being acquired.
   Transmitter retention times were variable. Of three captive fish, one regurgitated the
dummy unit after 5 h, but the other two retained them for 3 and 9 days, respectively. Of
the animals tracked in the wild, Shark 1 retained the transmitter for at least 12 days, and
Shark 2 regurgitated after 22 h and the transmitter was recovered from the bay floor.
The transmitter for Shark 3 was fitted with two small barbs before insertion into the gut,
and this animal was located in the daytime area 13 days after release.
   A consistent pattern of behaviour of the shark pups is emerging. During daytime,
movements of the tracked fish were restricted to a well defined core area, but at night the
animals ranged further afield, often completely leaving the daytime core area. Departure and
return to the daytime area often occurred at dusk and dawn respectively (Figs 2a and 2b).

K. N. Holland et al.

                                               Fig. 2.   (a) Movements of Shark 1 over
                                               2 4 h . Daytime core activity area (solid lines)
                                               expands to a larger nighttime activity area
                                               (dashed lines) that is apparently limited by
                                               the 9-m isobath. Maximum bay depth in
                                               this locality is uniformly about 13 m.
                                               (b) Daytime movements of Shark 2.
                                               Following nighttime excursions, the shark
                                               returns to the daytime core area (solid lines).
                                               (c) Daytime home range of Shark 3 as
                                               observed over 3 days. Days 1 and 2 were
                                               part of a continuous 48-h track, Day 3
                                               occurred 6 days after track initiation.
                                               The size of the common area (shaded) is
                                               0.12 km2, and the total area covered in
                                               3 days is 0.38 km2. C.I., Coconut Island
                                               and surrounding shallow patch reef;
                                               0, sunrise; +,   sunset.

Although they were not tracked simultaneously, the three animals tracked to date have
appeared to share the same daytime core area, continually moving within the limits of this
area. Sonar scans from a collaborating vessel, and high catch rates from lines and nets
dropped into the area, indicate that the tracked sharks may be part of a slowly moving
school milling around in the core area about 1 m off the bottom. The location of the
daytime core area appears to be stable, at least over a period of several days (Fig. 2 4 .
Convex polygon analysis of the movements of Shark 1 yields a total daytime activity area
of 2.31 km2, a total nighttime activity area of 3.1 km2, and a total combined activity area
of 3.5 km2.

Discussion
  Replication transforms interesting data into data that can be useful in making manage-
ment decisions about a species. The small-vessel techniques described in this paper can

Tracking Coastal Sharks with Small Boats

produce high-quality, low-cost information about the movements of coastal or lake species
in localities where shore facilities can be reached easily to permit crew changes by a shuttle
boat or the tracking boat itself. Species inhabiting more remote or offshore areas can be
tracked by using slightly larger craft as described elsewhere (Holland et al. 1985) or by
servicing the tracking skiff from a tender or mother ship. The portable, modular design of
the tracking system allows general-purpose craft to serve as tracking vessels and then to
revert to other functions without significant changes to the hull. The costs associated with
customized or permanently dedicated vessels are therefore avoided.
    Force-feeding the transmitters to sharks has variable success. Regurgitation has its
advantages if it occurs after a reasonable time and if the transmitter can subsequently
be recovered. Otherwise, fitting the transmitters with small corrosible barbs prior to place-
ment in the gut appears to be a reliable and nonlethal way of insuring prolonged tracks.
Harpooning the transmitter into the dorsal musculature has proved to be effective for larger
specimens of a variety of species (Tricas et al. 1981; Carey et al. 1982; Holland et al. 1990a).
    The movements of the hammerhead pups p n their natal grounds have distinct similarities
to the behaviour of adult Sphyrna lewini observed around seamounts in the Gulf of
California (Klimley and Nelson 1984; Klimley et al. 1988), where they appear to use the
seamount environment as a daytime refuge area (Hamilton and Watt 1970) from which they
make foraging excursions at night. Similar diurnal patterns are emerging for a growing
number of shark and teleost species (McKibben and Nelson 1986; Klimley et al. 1988;
Holland et al. 1990b).
   There are two distinct differences between adult hammerhead aggregations and the pups
observed in the present study. First, as indicated by our capturing techniques, S. lewini
pups definitely feed in the daytime, whereas adults (in the Gulf of California, at least)
apparently do not. Second, whereas the functional significance of daytime schooling of adult
hammerheads has been difficult to explain, the refuging behaviour that may be emerging in
the present study probably assists predation avoidance, for hammerhead pups have been
found in the guts of adult male hammerheads taken from the bay (Clarke 1971). As with
the adult fish, the expanded nighttime ranges of pups observed in the present study probably
represent foraging excursions.
   The tracking techniques used to acquire these data from hammerhead pups could be
applied to a variety of other problems confronting shark biologists. For example, elucidating
the movements of sharks around recreational areas might permit more efficient beach-
meshing techniques as well as determination of the extent of net avoidance and the survival
rates of sharks released from entanglement. Similarly, the efficacy of putative shark
repellents could be ascertained by tracking the movements of sharks before and after
exposure to the stimulus. Where shark nursery grounds are used for other fishery uses (as,
for instance, with the school shark, Galeorhinus galeus, in Tasmania; Williams and Schaap
1992), precisely the type of data acquired from hammerhead pups with a small boat as
described above could result in decreased fishing-gear conflict in these areas.

Acknowledgments
   This work was supported by the Edwin S; Pauly Foundation Grant to the Summer
Graduate Research Program at Coconut Island and by the State of Hawaii Department of
Land and Natural Resources, Division of Aquatic Resources. Hawaii Institute of Marine
Biology contribution No. 862.

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Manuscript received 3 April 1991, revised and accepted 24 June 1991