Acoustic Monitoring of a Previously Unstudied Whale Shark Aggregation in the
Red Sea
Thesis by
Jesse Cochran
In Partial Fulfillment of the Requirements
For the Degree of
Master of Science
King Abdullah University of Science and Technology, Thuwal,
Kingdom of Saudi Arabia
Awaiting Approval
2
The thesis of Jesse Cochran is approved by the examination committee.
Committee Chairperson: Michael Berumen
Committee Member: Stein Kaartvedt
Committee Member: Greg Skomal
3
ABSTRACT
Acoustic Monitoring of a Previously Unstudied Whale Shark Aggregation in the
Red Sea
Jesse Cochran
The whale shark (Rhincodon, typus), is a large, pelagic, filter feeder for which the
available information is limited. The Red Sea populations in particular are
practically unstudied. An aggregation site was recently discovered off the
western coast of Saudi Arabia. We report the use of passive acoustic monitoring
to assess the spatial and temporal behavior patterns of whale sharks in this new
site. The aggregation occurs in the spring and peaks in April/ May. Whale sharks
showed a preference for a single near shore reef and even a specific area within
it. There is no evidence of sexual segregation as the genders were present in
roughly equal proportion and used the same habitat at similar times. This
information can be used to guide future studies in the area and to inform local
management. Continued study will add to the collective knowledge on Red Sea
whale sharks, including the population dynamics within the region and how they
interact with the global whale shark community.
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TABLE OF CONTENTS
Committee Approval Page…….…………………………………….2
Abstract..………………………………………………………………..3
Table of Contents……………………………………………………..4
Chapter 1
Literature Review……………………………………………………..5
Introduction…………………………………………………………..25
Methods and Materials……………………………………………...27
Results…………………………………………………………………32
Discussion…………………………………………………………….42
Work Cited……………………………………………………………..47
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Chapter 1
Review of the Biology of the Whale Shark
The whale shark, Rhincodon typus, is a large pelagic filter feeder. This species is
poorly understood, with only limited data on many aspects of its biology. Hard
data, when it exists, is often preliminary as many studies are restricted to small
sample sizes and specific regions. The available information, limited though it
may be, provides a valuable foundation for this and other whale shark studies.
The known biology of the whale shark—including its systematics, biogeography,
feeding ecology, and life history—is reviewed below. Aggregation sites are
examined separately because of their value to research, potentially integral role
in whale shark ecology, and their fundamental importance to this thesis.
Systematics
Taxonomy
Rhincodon typus is derived from a misspelling of the Greek rhine (file), the Greek
odont (tooth), and the Latin typus (type). The generic name is believed to be the
result of a printer error in which the “e” in “Rhineodon” was mistaken for a “c”
(Castro, 2011). Known synonyms include: Rhineodon typus, Rhiniodon typus,
Rhinodon typicus, Rhinodon pentalineatus, and Micristodus punctatus. The
official name was the subject of some debate until 1984 when the International
Commission on Zoological Nomenclature placed Rhincodon on the Official List of
Generic Names in Zoology (Castro, 2011).
6
Phylogeny
R. typus is a modern shark, meaning that it belongs to the class Chondrichthyes,
the subclass Elasmobranchii, the super order Selachamorpha, and the subcohort Neoselachii. It is the only pelagic member of the Orectolobiformes (carpet
sharks), an order otherwise characterized by benthic foragers and ambush
predators (Compagno, 1999). Within this order, it is most closely related to the
nurse sharks (Ginglymostomatidae) and the zebra sharks (Stegastomatidae)
(Musick et al, 2004). It is the only member of its family, Rhincodontidae, which is
believed to have diverged sometime during the Paleocene period, 65.5-56 million
years ago (Musick et al, 2004).
Description/Identification
Figure 1: A free swimming whale shark accompanied by pilot fish and remoras. Photo credited to
Pedro de la Torre.
Most authors describe the whale shark as “unmistakable” (Castro, 2011)
(Campagno, 2005). Its size, shape, and coloration easily distinguish it from all
other taxa. It is a huge species, with individuals commonly attaining sizes of ten
7
meters and the largest specimens growing in excess of twelve (Castro, 2011). It
has a broad, flattened head with a terminal mouth containing 300 rows of
vestigial teeth. The body is fusiform and possesses prominent lateral ridges
which run along its dorsal surface. Dorsal coloration is a grey-green or greybrown overlaid by an intricate pattern of white spots and bars, giving the shark its
unique “checkerboard” appearance. This patterning fades to a solid white on the
shark's ventral surface.
Biogeography
Range and Distribution
Figure 2: Global whale shark distribution (Norman, 2005).
The whale shark’s pelagic lifestyle has allowed it to range further than its
demersal kin. It is cosmopolitan in all tropical and warm temperate seas except
for the Mediterranean (see Figure 2). Though the whale shark is capable of
inhabiting any area within its range, it is not evenly distributed throughout. Most
8
regions report whale shark sightings only at certain times of the year (Rowat,
2007).Even then, occurrences are unpredictable, making encounters with whale
sharks haphazard (Castro, 2011). The dozen or so known aggregation sites,
areas in which large numbers of whale sharks gather on a predictable seasonal
basis, are an exception to this (see Figure X). These events can range in size
from a half dozen to over a hundred individuals and are usually associated with
local increases in zooplankton abundance. Aggregations are examined in greater
detail later in this review.
Population Structure
With such a wide spread distribution, it might be expected for different
populations of whale sharks to become geographically isolated and diverge into
corresponding genetic lineages. Such patterns have occurred in other
circumtropical fish, in at least one case producing a ring species (Bowen et al,
2001). So far the genetic work conducted on the whale shark has produced
mixed results. A study examining mitochondrial DNA has found significant
differences between whale sharks sampled in the Atlantic and the Indian/Pacific
(Castro et al, 2007). Research using microsatellites, however, has found no such
differences (Schmidt et al, 2009). Mitochondrial DNA is inherited on a matrilineal
basis; microsatellites, on the other hand, are part of the nuclear DNA inherited
from both parents. The uniformity of microsatellites coupled with the differences
in mitochondrial DNA suggests that while female whale sharks may be
geographically isolated, there is still significant male-mediated gene flow between
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ocean basins(Schmidt et al, 2009). Like much of the available data on the whale
shark, the information above is preliminary. Both the microsatellite and
mitochondrial studies point out that the sample sizes used for their analysis were
small (n < 100). Still, whale sharks would not be the first shark species to exhibit
this pattern. Research on the great white (Carcharodon, carcarias) populations of
South Africa and Australia has yielded similar results (Pardini et al, 2000).
Movement
The evidence of gene flow between geographically disparate populations
suggests that whale sharks are capable of traveling significant distances. This
has been corroborated by several tagging studies. Satellite tags attached to
whale sharks routinely report movements thousands of kilometers in length, the
longest being nearly 13000 km (Eckert and Stewart, 2001). This shark was
tagged inside the Gulf of California, tracked west past the Hawaiian Islands and
into the central Pacific (Eckert and Stewart, 2001). Even relatively small sharks
are capable of long distance migrations. For instance, a 3m whale shark tagged
north of Mindanao in the Philippines, traveled over 4500 km in 70 days. It
crossed the entire Sulu Sea and last reported 280 km south of Vietnam (Eckert et
al 2002). Six sharks tagged on Ningaloo reef in Australia recorded journeys
ranging from 1200-6600 km, often moving against the prevailing currents
(Sleeman et al, 2010).
The capability of whale sharks to undertake large-scale horizontal movements
makes connectivity of the global population possible, yet vertical movement
10
through the water column may be just as important. Several studies have used
pressure sensor equipped tags to monitor dive behavior. In shallow coastal
areas, whale sharks use the entire water column, regularly diving close to the
bottom and then rising back near the surface (Gunn et al, 1999). Whale sharks in
open water regularly make dives ranging to 500m, punctuated with the
occasional dive in excess of a kilometer (Brunnschweiler, 2009). Most authors
attribute movement through the water column to foraging behavior
(Brunnschweiler, 2009). The sharks are generally shallow at night and deeper
during the day, and this correlates with the diel vertical migrations of many
zooplankton species (Graham et al, 2006). In Belize, whale shark diving behavior
has been correlated to the lunar-driven spawning cycles of snapper (Graham et
al, 2006). During the new moon, when the snapper are not spawning, the sharks
dive deeper and stay at depth longer. Another possible explanation for vertical
movement behavior in whale sharks is the use of diving to optimize cost of
transport (Gleiss et al, 2011). Whale sharks may effectively “glide” horizontally as
they dive, reducing energy consumption. The long range horizontal movements
so important for the shark’s dispersal might not even be possible without the
vertical component.
Feeding Ecology
Anatomy and Mechanism of Filtration
Filter feeding evolved independently in all three shark species known to utilize it,
and each species possesses a unique feeding mechanism (Taylor et al, 1983).
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The filtration anatomy of the whale shark consists of an upper and lower filtration
pad associated with each of the sharks ten gill slits. The interior surface of these
pads is composed of a fine (average opening size = 1.2 mm) irregular mesh. The
exterior surface is lined with primary and secondary cartilaginous channels
(Motta et al, 2010). During feeding, water enters the mouth and flows through the
pads where it is filtered by the irregular mesh. The water is then directed by the
exterior channels through the shark’s gill filaments before exiting out the gill
openings (Motta et al, 2010).
Three hypotheses have been proposed for the method by which the whale shark
separates plankton from the water: dead-end sieving, hydrosol filtration, and
cross-flow filtration (Motta et al, 2010). In dead-end sieving water flows
perpendicular to the filter pads, and only particles larger than the mesh size are
captured. This seems unlikely because whale sharks have been observed
feeding on prey far smaller than 1.2 mm (Motta et al, 2010). Hydrosol filtration
implies that the mesh of live sharks is coated in mucus or some other adhesive
material. While this setup would allow the shark to capture smaller particles, it
would also clog quickly and require the shark to clear its filters more often than
has been observed (Motta et al, 2010). In cross-flow filtration, water and plankton
flow parallel to the filters at high velocities. Water is drained through the gills and
plankton is concentrated at the back of the pharyngeal cavity and then swallowed
(Motta et al, 2010). Cross-flow filtration has been observed in bony fish, and
preliminary analysis suggests that the hydrodynamics of filter feeding in whale
sharks are similar (Motta et al, 2010). While there is no direct evidence for cross-
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flow filtration in whale sharks, it lacks the shortcomings of both dead-end sieving
and hydrosol filtration.
Prey Selection
The whale shark is known to feed on a variety of planktonic prey. It has been
observed feeding on fish spawn (Heyman et al, 2001), copepods (Clark and
Nelson, 1997), and crustacean larvae (Springer, 1957). The one in depth
analysis of whale shark stomach contents comes from a single specimen
captured off of India (Silas and Rajagopalan, 1963). A subsample of its stomach
contents contained 87% zooplankton; 12% Sand, shell bits, and plant material
(all possibly ingested during capture); and 1% nektonic fish, mollusks, and
crustaceans. The zooplankton component was further subdivided into 70.4%
crustaceans, 0.3 % mollusks, and 29.3% chaetagnaths (Silas and Rajagopalan,
1963). While several authors have reported whale sharks deliberately feeding on
schools of small fishes and even tuna (Springer, 1957), others have suggested
that these are mistaken accounts in which the sharks and the fish schools are
both filter feeding in close proximity (Nelson and Eckert, 2007).
Foraging Behavior
The feeding apparatus of the whale shark is not capable of the same through-put
as that of the basking shark but it is thought to be more versatile (Taylor et al,
1983). While all three filter-feeding sharks are capable of ram feeding (using
forward motion to force water over their gills), only the whale shark is known to
employ suction feeding (gulping water independent of forward motion) (Taylor et
13
al, 1983). The ability to use both has given rise to three distinct foraging
behaviors: passive feeding, vertical feeding, and active feeding. Passive feeding
is a ram-type technique in which the shark swims slowly beneath the surface with
its mouth open (Taylor, 2007). During vertical feeding, the shark remains
stationary, with its head near the surface. This is a suction type behavior as the
shark gulps and expels water without the use of forward motion (Nelson and
Eckert, 2007). While active feeding, the shark swims with its mouth partially
breaking the surface, frequently turning its head and making sudden changes in
direction (Clarke and Nelson, 1997). The shark simultaneously uses forward
motion and suction to force water over its gills. During all of these behaviors the
shark must close its mouth in order to swallow accumulated plankton, and also
“cough” infrequently to clear its filter pads (Nelson and Eckert, 2007).
While several studies have described the foraging behavior of whale sharks, only
one (Nelson and Eckert, 2007) has investigated the relationship between the
available prey and the mode of predation. This study found that the feeding
strategy was determined by prey abundance and, to a lesser extent, prey
composition. Passive feeding was observed in the lowest plankton
concentrations and might be the “default” behavior adopted by the shark as it
searches for higher prey densities. A few zooplankton taxa were present in
greater proportion during passive feeding when compared to the other feeding
modes, but overall abundance was considered too low for this to be driving the
shark’s behavior (Nelson and Eckert, 2007). Vertical feeding occurred in medium
densities of mixed zooplankton assemblages (48% copepod, 10% shrimp, 15%
14
amphipod, and 11% other) (Nelson and Eckert, 2007). This stationary method
may be more energy conservative than active feeding while still being effective at
filtering moderate concentrations of plankton. Active feeding was observed in
zooplankton concentrations four times higher than those in which sharks were
feeding vertically. In addition, plankton composition was dominated by copepods
(96.6%) (Nelson and Eckert, 2007). The vigorous swimming associated with
active feeding is energetically costly which may limit its use to areas of high
plankton density.
The shark’s ability to locate increased concentrations of plankton remains a
mystery. One proposed method is the use of olfaction to home in on dimethyl
sulfide, a chemical released by phytoplankton when grazed by zooplankton
(Martin, 2007). Other animals are known to use dimethyl sulfide as a feeding cue
(Nevitt et al, 1995) and the whale shark’s widely spaced, terminal nostrils and
well developed olfactory capsules suggest that it is adapted to foraging by smell
(Martin, 2007). It has also been suggested that whale sharks use hearing to
detect the presence of schooling planktivorous fish (Nelson and Eckert, 2007).
The whale shark possesses the largest inner ear in the animal kingdom (Martin,
2007) and has been observed feeding in association with such schools (Nelson
and Eckert, 2007). Until additional data becomes available, both hypotheses are
little more than speculation.
Life History
Early Development
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The available data on early development of the whale shark consists of 15
reported encounters with neonates (Aca and Schmidt, 2011), a single aborted
egg case containing a live embryo (Baughman, 1955), and the dissection of a
single pregnant female containing 300 pups (Joung et al, 1996). In the latter
case, two late term embryos were birthed alive and kept in captivity (Joung et al,
1996). Whale sharks are born at 46 to 64 cm total length. This is based on the
largest embryos found in the pregnant female (58-64 cm) (Joung et al, 1996) and
the smallest free swimming whale shark yet recorded (46 cm) (Aca and Schmidt,
2011). They are apparently born with significant stores of energy as one of the
surviving embryos swam continuously without eating for the first seventeen days
following birth (Chang et al, 1997). This is further supported by a dissected whale
shark embryo which was found to contain ample stores of absorbed yolk
(Garrick, 1964). The few neonate whale sharks successfully kept in captivity
have demonstrated a remarkable growth rate. A 60 cm individual weighing one
kg grew to 139 cm and 20.4 kg in 120 days (Chang et al, 1997). Another
specimen, kept for 3.2 years, grew from 70 to 369 cm and from 0.8 to 151.25 kg
(Yang and Horike, 1998 (Cited in Castro, 2011)). If wild growth rates are
anywhere near those observed in captive specimens, then it would make the
whale shark one of the fastest growing of all shark species.
The rapid somatic growth rate and large litter size suggest that neonate whale
sharks are subject to high mortality in the wild (Martin, 2007). The fact that two
neonates have been discovered as part of the stomach contents of pelagic
predators corroborates this (Kukuyev, 1995) (Coleman, 1997). Poor swimming
16
ability in young whale sharks may contribute to their vulnerability (Martin, 2007).
Neonatal and juvenile whale sharks (46-93 cm) have proportionally smaller fins
and a more strongly heterocercal caudal fin with a lower angle of thrust than their
adolescent and adult counterparts (4-10m) (Martin, 2007). These traits are
similar to those of neonatal tiger sharks (Galeocerdo cuvier) which utilize an
inefficient anguilliform swimming stroke (Branstetter et al, 1987). The
morphological similarity makes it likely that neonate whale sharks employ a
similar and equally ineffective mode of transportation (Martin, 2007).
Lacking an effective means to flee from predators suggests that young whale
sharks employ other strategies to avoid predation. One hypothesis is that the
shark’s countershading, along with the disruptive properties of its patterning,
serve as camouflage in the pelagic environment (Wilson and Martin, 2003). The
countershading allows the shark to blend with the water when viewed from above
or below, and the pattern of bright spots and bars serve both to obscure the
sharks outline and to mimic the mottled effect of sunlight passing through surface
waves (Wilson and Martin, 2003). These are not unique adaptations. All known
Orectolobiform sharks possess disruptive color patterns at birth, and most
epipelagic sharks display some form of countershading. It is interesting to note
that neonate tiger sharks, which share the weak swimming capabilities of
neonate whale sharks, also possess both countershading and disruptive
coloration. Unlike other Orectolobiform sharks, and also unlike the tiger shark,
the whale shark’s coloration remains unchanged throughout its life (Wilson and
Martin, 2003). The reasons for this are unclear though crypsis may remain
17
important to the whale shark as even adults can fall prey to orca whales and
possibly great whites (O’Sullivan, 2000) (Fitzpatrick et al 2006).
There is a distinct lack of information on whale sharks ranging from one to three
meters. The available data consists of a single 1.4m specimen from the
Philippines, the two birthed embryos which grew to this size range in captivity, a
school of fourteen whale sharks estimated at 1m from India, and an unconfirmed
report of a stranded 2m specimen from Western Australia. Whether animals in
this size range remain at depth, at least diurnally, or occupy some other
inaccessible habitat is unknown (Martin, 2007). A deep-at-day shallow-at-night
lifestyle would make sense as it would allow the shark to follow vertically
migrating zooplankton and use darkness to avoid visual predators. Given the
rapid growth rate, whale sharks may occupy this size range for only a limited
period of time. Regardless of the reason for these individuals’ scarcity, more data
is needed on whale sharks at this stage of development in order to understand
the ontogenetic shifts that occur between the shark’s juvenile and adolescent
forms.
Age and Growth
Over the course of its life, a whale shark will grow from a 46-64 cm neonate to
over 12 m in adulthood (Aca and Schmidt, 2011) (Castro, 2011). How quickly
growth occurs and how growth rate changes with age is poorly understood and
has received only limited research attention. There have been few studies of
whale shark growth rates using captive specimens, including those on neonatal
18
growth mentioned earlier in this review. At the Osaka Aquarium, a female whale
shark grew 369 cm in eight years (46.125 cm/year) and a male grew 106 cm in
three (35.3 cm/year) (Kitafuji and Yamamoto, 1998). In another study, growth
rates were measured for three specimens, a female and two males. The female
grew 166 cm in 5.6 years (29.5 cm/year), one male grew 61 cm in 2.8 years
(21.6 cm/year), and the other grew 32 cm in 1.2 years (25.5 cm/year) (Uchida et
al, 2000). Unlike the research conducted on neonate growth, the sharks from
these studies were of unknown age. It is therefore difficult to incorporate this
growth data into an understanding of whale shark life history.
The most common method for age determination in sharks is counting the
translucent/opaque band pairs (also known as growth rings) in the animal’s
vertebrae. There is only one published work which has attempted to correlate
growth with vertebral ring counts in whale sharks (Wintner, 2000). This study
compared the size data and vertebra of 17 specimens (14 from beachstrandings, 3 in captivity). The analysis yielded a linear relationship between
growth ring count and length (Wintner, 2000). Analysis of a single captive
juvenile whale shark suggests that one growth ring is deposited every year in this
species (Calliet et al, 1986). If this applies to wild sharks and holds through
adulthood, then the linear relationship between length and growth ring count
could be translated to a linear rate of growth. Like many aspects of whale sharks
life history, more data is required for these findings to be confirmed. Validated
age studies on larger, wild sharks are required to verify the deposition of a single
ring per year and to determine the growth rate at latter stages of whale shark
19
development. Unfortunately, the collection and analysis of vertebrae requires
lethal sampling, which limits available specimens to those that have died in
aquaria, been brought in by fisheries, or stranded on beaches.
Of the seventeen sharks examined in Wintner’s age and growth study, three
males with total lengths of 9-9.5m were judged to be mature (Wintner 2000).
Another study of stranded whale sharks found two mature males with total
lengths of 9 and 9.3 m, and also two immature males with lengths of 9.1 and 9.2
m (Beckley et al, 1997). Based on these findings maturity in male whale sharks
occurs at lengths between 9 and 9.5 meters. There is insufficient data to assess
size of maturity in females. In most sharks females reach maturity at larger sizes
than males, and the one recorded pregnant female was 10.6 m. In the absence
of further information, size at maturity for female whale sharks can be estimated
as occurring between 9.5 and 10 meters (Castro, 2011).
Reproduction
Whale sharks have never been observed in copulation. There is one record of
suspected courtship between two sharks from Ningaloo Reef. The interaction
consisted of “circling one another at variable distances, broken sporadically by
periods of close follow and parallel swim-by behaviours” (Martin, 2007). The
classification of this as courtship is tenuous, as neither coitus nor even the
genders of the specimens involved were confirmed (Martin, 2007). Mating
behavior has been observed in the closely related nurse shark (Ginglymostoma
cirratum) (Carrier et al, 1994). In this species, the male bites one of the female’s
20
pectoral fins and positions her before inserting a clasper into her cloaca. The
male’s ability to maintain a hold on the female until copulation has been
completed serves as a test of his fitness (Pratt and Carrier, 2001). Similar
behaviors may or may not occur in the whale shark, and only confirmed
observations of its courtship and mating will yield anything more than
speculation.
There is no data on the length of gestation in whale sharks, though the discovery
and dissection of a pregnant female in Taiwan confirmed that the species is a
lecithotrophic livebearer capable of supporting litters of at least 300 pups (Joung
et al, 1996). In this case the pups were at different stages of development with
some ready to be born and others not yet hatched from their egg cases. This,
along with paternity analysis suggesting that all of the pups shared the same
father, indicates that female whale sharks are capable of storing sperm and
fertilizing their eggs continuously (Schmidt et al, 2010).
Parturition in whale sharks may occur as a single massive birthing event in which
the entire litter is discharged, or it may happen over a period of weeks in which
fetuses are birthed as they develop. Pupping grounds for the whale shark are
unknown and may not exist. As neonate whale sharks are thought to be poor
swimmers (Martin, 2007), it stands to reason that the few known neonatal
specimens were discovered near their place of birth. The locations of these
encounters seem more indicative of the circumtropical distribution of the species
than of any specific birthing ground. If pupping grounds do not exist, then multiple
21
birthing events would be advantageous (Schmidt et al, 2010). Separate births
occurring at different times could also occur at different places, reducing the
likelihood of the entire litter being deposited in a dangerous or unsuitable habitat
(Schmidt et al, 2010).
There are two records of juvenile whale sharks echelon swimming with a larger
conspecific (Taylor, 1994) (Pillai 2001). This is unlikely to be an example of
parental care, both because such behaviors are unknown in sharks and because
the larger individual in one of these records was judged to be 5.5 m in length, far
below the suspected size of maturity (Castro, 2011). It does suggest schooling
behavior in juvenile and possibly neonatal whale sharks. This would allow them
to cooperatively spot and avoid predators, and also spread the chance of
predation throughout the school, reducing each shark’s individual risk. Schooling
behavior might favor a single birth event as the neonatal sharks would be born
into an environment rich in suitable conspecifics (their siblings). This would also
fit with recent research suggesting that the closely related nurse shark retains
developed embryos until the entire litter is ready for birth (Wilson and Martin,
2003).
Aggregations
Known Aggregation Sites
Aggregation sites are areas in which whale sharks gather on a predictable basis.
They are given special attention here because of their continued utility to ongoing
research, and because a previously unknown aggregation is the subject of this
22
thesis. Prior to this writing, approximately a dozen aggregation sites have been
documented. Each of these has received varying degrees of study and much of
the work cited above would not have been possible without the relatively large
sample sizes available in these locations.
Figure 3: Global whale shark distribution (dark grey) with known aggregations: 1=Gulf of Baja,
2=Galapagos, 3=Holbox, Mexico, 4=Belize, 5=Mozambique, 6=Maldives, 7=Gujarat, India,
8=Philippines, 9=Ningaloo, western Australia, D=Djibouti, S=Seychelles (Rowat et al 2011).
The distribution of the various aggregation sites is shown in Figure 3. Though
there are some differences in the seasonality, size, and composition of these
aggregations, there are also traits which most or all of them share. Most
aggregations have been shown to occur in conjunction with local increases in
zooplankton abundance. In Ningaloo Reef, the arrival of whale sharks
corresponds to coral spawning (Taylor, 1996). In Belize, the aggregation has
been associated with fish spawning events (Heyman et al, 2001). In the Gulf of
Baja, aggregating whale sharks have been shown to feed on copepod blooms
(Clark and Nelson, 1997). Rich zooplankton biomass has been associated with
23
the seasonal abundance of whale sharks in Djibouti (Rowat et al, 2011), and the
Seychelles aggregation appears to coincide with summer plankton blooms
(Gifford, 2002).
Many aggregation sites also show evidence of sexual segregation, with most
having gender ratios skewed to male. Out of 360 whale shark observations made
over 3 years at Ningaloo reef, 85% were of males (Norman and Stevens, 2007).
In Belize, out of 162 shark encounters 154 (94%) were males (Graham and
Roberts, 2007). In the Seychelles 82 % of identified sharks were male, 85% in
Djibouti (Rowat et al, 2011). The Maldives reported 95% males out of the 63
identified specimens (Riley et al, 2010), and in Mozambique 81% of 300
individuals were male (Pierce et al, 2008). Holbox, Mexico was more weakly
skewed to male with an average 72% over 6 years (de la Parra, unpublished
data). Gujarat, the Gulf of Baja, and the Philippines were the exceptions; all
having gender ratios close to one to one (Silas, 1986) (Ramirez-Macias et al,
2006) (Quiros, 2008). In the case of Gujarat this may be an artifact of small
sample size (27) (Silas, 1986). Until more gender data is obtained from this
region, it is impossible to say anything conclusive. This leaves the Gulf of Baja
and the Philippines, and the high occurrence of females at these sites is still
unexplained, as is the segregation that occurs in other areas.
Another common characteristic is the predominance of immature whale sharks
within these sites. This has been shown for all aggregations in which it has been
assessed. Again the Gulf of Baja is interesting, not only because it reports
24
mature individuals, but also because these larger sharks segregate themselves
from their smaller counterparts (Eckert and Stewart, 2001). Immature sharks
have been shown to occupy the northern, inner portions of the gulf, while mature
specimens are found in the southern reaches near the open ocean (Eckert and
Stewart, 2001). It is also noteworthy that the mature individuals are almost
exclusively female, while the immature sharks tend toward male (RamirezMacias et al, 2007). The purpose of this population structure is unknown, and
The Gulf of Baja certainly warrants further study.
Taken in combination, the male dominated gender ratios and low abundance of
mature individuals means that none of the currently known aggregations
represents a functional population. Too few mature individuals are using these
sites for them to be considered breeding aggregations, meaning that feeding may
be the sole motivation for these gatherings. A large subset of the breeding
population must either be staying at depth, offshore, or both. If this is the case,
then we may never gain a complete understanding of whale shark ecology from
only studying aggregation sites. Still, the information available at these sites has
hardly been exhausted and the known aggregations require additional research.
Case in point, the Philippines, Gujarat, and the Galapagos aggregations are
largely unstudied. Work in the Philippines has focused on the effects of the
regions lucrative ecotourism industry (Quiros 2005) (Quiros 2007), the several
targeted whale shark fisheries scattered around the islands (Alava et al, 1997),
and more recently on a few confirmed neonate whale sharks (Aca and Schmidt,
25
2011). The Gujarat aggregation is known almost entirely from fisheries records
(Silas, 1986). Research on Galapagos whale sharks has begun only recently and
remains unfinished. In addition to these known but understudied sites, there may
also be aggregations which are still unknown to science, as evidenced by the
present study.
Conclusion
As the feeding apparatus of the whale shark is dependent on high prey
availability, aggregation sites may serve as essential temporary habitats for this
species. Many aggregations are primarily composed of immature specimens, and
the abundance of food in these areas may help fuel their rapid and apparently
linear growth. Furthermore, as demonstrated by numerous studies, the
predictable concentration of normally diffuse whale shark populations at these
sites is invaluable for ongoing science. Therefore, an understanding of the spatial
and temporal characteristics of these aggregations is vital to guiding future
research and management efforts for this species.
Introduction
One potential way to assess both the temporal and spatial characteristics of a
whale shark aggregation is passive acoustic monitoring. This technique involves
the placement of acoustic receivers in strategic locations throughout the area of
interest. Target animals are tagged with acoustic transmitters which “ping” an
individually coded, ultrasonic signal at regular intervals. The receivers are
capable of detecting and recording these pings at ranges of about 500 meters. In
26
this way fine scale presence/absence data can be accumulated over long periods
of time.
This technique has been used on sharks in the past, including studies of juvenile
blacktips (Carcharhinus limbatus) (Heupel and Heuter, 2001, 2002), scalloped
hammerheads (Sphyrna leweni) (Klimley et al. 1988), and great whites
(Carcharodon carcarias) (Klimley, 2001). Klimley’s work on hammerheads and
great whites is particularly relevant here as both species were capable of moving
beyond the range of his arrays. The hammerheads, for instance, were monitored
by Klimley’s array as they swam around a Sea Mount in the Gulf of Baja. They
left the sea mount, and the detection radius of the receivers, at dusk and
returned at dawn. While the migration cycles of hammerheads show a daily
pattern as opposed to the seasonal movements of whale sharks, the principle of
monitoring known gathering sites remains the same. In the case of the great
whites, Klimley used receivers modified for extremely high spatial resolution.
These receivers could only cover a small area, but they could triangulate the
position of a tag to within one meter. This is important because, like the whale
shark, the great white is capable of large scale transoceanic migrations. This
study demonstrated that monitoring sites of heavy use can be effective, even if
the target area is only a tiny fraction of the animals’ total range.
In 2009 a previously unknown aggregation site was discovered in the southern
Saudi Arabian Red Sea. While data is limited on whale sharks in general, the
Red Sea populations are practically unstudied. Only a single published article
27
exists on Red Sea whale sharks and this is a dubious account of the fatal
ramming of four sharks by shipping vessels. By using passive acoustic
monitoring to evaluate this new aggregation, we can verify the technique in this
context while simultaneously collecting valuable data on an understudied portion
of the whale shark population.
Methods and Materials
Receiver Setup and Deployment
Mooring Construction
A total of 32 vemco VR2W acoustic receivers were deployed in the region. They
were secured to the bottom using either cinder block moorings or tire moorings.
Cinder block moorings were constructed by shackling two cinderblocks together
with steel chain. A half meter length of standard garden hosing was looped
through the chain and rope was threaded through the hosing and spliced into
itself. Subsurface floats were attached to rope at three meters depth. Tire
moorings were constructed by filling the “doughnut hole” of the tire with cement.
A three meter long PVC pipe was fixed into the cement before drying. The free
end of the PVC had a hole drilled into it through which hosing could be looped
and rope threaded. In both cases the receiver was secured to the line using
cable ties.
Cinderblock moorings were used on reef topography and at shallow depths. They
are lighter and less stable than tire moorings, so where possible a diver would
28
wedge them into reef crevices in order to secure them. Tire moorings were used
on soft substrate and in deeper waters. They are effectively immovable once
submerged which prevented repositioning after deployment. Each receiver site
was assessed specifically to determine which method was most advantageous
for that location.
Receiver Array
Figure 4: WS Reef with the receiver sites marked.
Twelve receivers were placed around the perimeter of a single reef known locally
as “Shib Habil” (“Rope Reef”). This reef has been associated with the majority of
recorded whale shark sightings in the Al Lith area, so “Whale Shark Reef” has
also become common. For the purposes of this paper we refer to it as “WS
29
Reef.” It is approximately 2.5 kilometers from shore and 3.5 kilometers from Al
Lith. The reef itself is “Г” shaped, with the horizontal line running 3km east to
west and the vertical line running 5km northwest to south east. Two years of
observation and photo identification have recorded 107 unique individuals on this
site, with a near equal ratio of males and females (Berumen, unpublished data).
Figure 5: Total Array
The remaining twenty receivers are scattered throughout other regions of interest
in the area. Two are north of WS reef, and two are shoreward of it. Seven are in
a southern, inshore area riddled with various mangrove mouths and patch reefs.
Two are on a northern inshore reef and three are located on offshore patch reefs.
30
The final three are located around Abu Latt, the largest island in the area. Only
four whale sharks sighting have been recorded away from WS reef. Three were
in open water and one was sighted near Abu Latt. One of the goals of this project
is to determine if these reflect transient behavior or if the sharks are making
frequent undetected use of areas outside WS reef.
Transmitter Setup and Deployment
Tag Rigging
Figure 6: A: Tagging dart, B: Heat shrink wrap, C: Crimps, D: 1.5mm wire, E: Vemco V16P acoustic transmitter, F:
Fully rigged tag.
We used externally cased Vemco V16P individually coded acoustic transmitters
which were programed to ping every two minutes plus or minus thirty seconds.
The tagging attachment was rigged by looping stainless steel wire through a
tagging dart and crimping the loop closed. The free end of the wire was looped
31
through the V16’s external casing opposite the transmitter then crimped. Both
crimps and the wire were covered in heat shrink wrap to prevent rust and to keep
the metal from abrading the shark’s skin.
Deployment
Figure 7: A: Tag attached to spear gun, B: Shark being tagged, C: Correctly tagged shark.
Sharks were approached by 2-4 snorkelers. Pictures of both sides were taken for
photo identification. When possible, sex was determined by swimming under the
shark and observing the presence/absence of claspers. Tagging was done with a
Hawaiian sling style spear gun and transmitters were placed immediately ventral
to the first dorsal fin. Total length was estimated and used along with clasper
morphology to determine maturity. All tagging was done in March, April, or May
32
of 2010 and 2011. Acoustic tags were successfully attached to 37 sharks in 2010
and 28 sharks in 2011.
Results
Overview
Seasonality: The array has logged 13685 detections from 34 of 37 sharks
tagged in 2010, and 6110 detections from 23 of 28 sharks tagged in 2011. The
data collected so far confirms spring seasonality for the aggregation with 90% of
all detections occurring from March through June. From July through December
the array registering only a few, short lived periods of whale shark presence.
Activity increased by approximately one order of magnitude during January and
February, and by another order of magnitude during the season peak in April
and/or May (see Figure 8).
10000
Figure 8: 2010-2011 Monthly Detections
8000
6000
4000
Total Detections
2010 Cohort
2000
0
Habitat Use: Comparing the number of detections logged by each receiver has
shown that whale sharks strongly prefer WS Reef to the other areas in which
receivers are present. Over 91% of the 19,795 whale shark detections have been
33
logged by receivers stationed around WS Reef, which account for only 37.5% of
total receiver effort. Even within WS reef, detections are not evenly distributed.
Detections across the reef seem to increase with proximity to the reef’s northwest
corner. The three closest receivers have logged a total of 9257 received
transmissions, which amounts to 51% of WS Reef detections and 45% of
detections in general. Conversely, the three most isolated from the northwest
corner have only recorded 585 detections, which is 3% of those from WS Reef
and 1% of detections in general (Figure 9). Vertical Distribution favored shallow
waters with 95% of detections occurring in the 10 shallowest depth bins (Figure
10).
Figure 9: WS Reef Detections
4000
North Reef Edge
3000
West Reef Edge
2000
East Reef Edge
1000
8000
WS Line
24
WS Line
25
WS Line
18
WS Line
17
WS Line 5
WS Line 6
WS Line 7
WS Line 8
WS Line 4
WS Line 2
WS Line 1
WS Line 3
0
Figure 10: Depth Distribution
6000
4000
2000
0
4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30
Depth Bins
34
Sexual Segregation: There is no evidence of either temporal of spatial
segregation of the sexes on a broad scale. There are no significant differences in
the seasonality for male and female sharks, as both aggregate in the spring and
disperse throughout the rest of the year (Figure 11). In addition, they are utilizing
the same habitat over similar time scales. Both show a strong overall preference
for the northwest corner of WS Reef (Figure 12), and spend most of their time in
shallow waters (Figure 13).
Figure 11: Monthly Detections By Gender
5000
Males
Females
4000
3000
2000
1000
0
Figure 12: WS Reef Detections by Gender
1500
Female
1000
Male
500
WS Line
25
WS Line
24
WS Line
18
WS Line
17
WS Line 5
WS Line 6
WS Line 7
WS Line 8
WS Line 4
WS Line 2
WS Line 1
WS Line 3
0
35
Figure 13: Depth by Gender
4000
3000
Females
2000
Males
1000
0
4
5
6
7
8
9
10
11 12 13
Depth Bins
14
15
16
17
18
19
Seasonal Differences
2010 Season Structure: Acoustic activity over the 2010 season was erratic, generating
multiple peaks and valleys (Figure 14). Some of this volatility may be due to the
convoluted levels of transmitter/receiver effort used during the 2010 season. Transmitter
effort increased incrementally over the season as additional sharks were encountered
and tagged. More importantly, the receiver array was also being set up, dramatically
increasing effort with each receiver added. Under these conditions, it is difficult to draw
any meaningful conclusions about the structure of 2010 aggregation season.
Figure 14: Spring 2010 Daily Detections
Number of Detections
800
600
400
200
0
Mar/01
Apr/01
May/01
Jun/01
2010 Habitat Use: Analyzing the distribution of detections is less complicated by the
shifts in effort. As all tagged sharks are detectable by all receivers, increases in
36
transmitter effort affect the entire array equally. Therefore, tagging additional sharks
should not have any impact on the distribution of detections. Changes in receiver effort,
on the other hand, alter the distribution because older receiver stations have had more
time to accumulate detections. Fortunately, this can be easily accounted for by simply
comparing each receiver’s rate of detections as opposed to the absolute number. This
data shows a strong preference for the western areas of the reef, and a somewhat
weaker preference for the north. The eastern areas of the reef were almost totally
inactive, accounting for only 1% of daily detections (Figure 15). Vertical distribution is
nearly identical to that found in overall data (Figure 10), and 95% of detections occur in
the shallowest ten depth bins.
Figure 15: 2010 WS Reef Detection Frequency
15.00
North WS
Reef
10.00
West WS
Reef
East WS Reef
5.00
WS Line
24
WS Line
25
WS Line
18
WS Line
17
WS Line 5
WS Line 6
WS Line 7
WS Line 8
WS Line 4
WS Line 2
WS Line 1
0.00
WS Line 3
Detections/Day
20.00
2010 Sexual Segregation: Even under closer examination, there is still no evidence of
temporal segregation of the sexes. There are some peaks in activity of one gender
which correspond to valleys in the activity of the others. Examples include the 18th and
the 27th of April. However these incidents are usually composed of only a few individuals
(2 male and one female on the 18th, 1 male and 3 female on the 27th) and even though
the detection ratio is skewed to one of the genders, there are still points of close
interaction between sharks of differing sex. There are also peaks which are shared by
37
both genders, which includes the 7th and 12th of May. These are also composed of few
individuals (2 males and 2 females on the 7th, 3 males and 2 females on the 12th). Still,
as both sexes were frequently detected at the same times, temporal segregation of the
sexes did not occur during this season (Figure 16).
Figure 16: 2010 Detections By Gender
500
400
300
Females
Males
200
100
0
01/Mar
01/Apr
01/May
01/Jun
Habitat use is still similar between the sexes at this scale, as is distribution through
depth. West WS Reef accounted for 71% of male detections/day and nearly 80% of
female detections/day. East whale shark reef accounted for only 1.1 and 1.3%
respectively (Figure 17). Depth detections were predominantly shallow, with 96% of
female and 90% of male detections occurring in the shallowest 10 depth bins, and
46/36% occurring at or near the surface.
Figure 17: 2010 Detection Frequency by Gender
Females
Males
10
8
6
4
2
WS Line
24
WS Line
25
WS Line
18
WS Line
17
WS Line
5
WS Line
6
WS Line
7
WS Line
8
WS Line
4
WS Line
2
WS Line
1
0
WS Line
3
Detections/Day
12
38
Off Season: There is no real structure to the off season (July-December). There are
only a few scattered patches of whale shark activity during this period. These always
involve a single individual and last no longer than a few days. What makes the off
season worth mentioning is the distribution of detections. The preference for WS Reef
vanishes during this period, with only 25% of off season detections occurring on WS
Reef receivers. Two individuals were never detected in both spatial and temporal
proximity to one and other, as such it is impossible to judge sexual segregation over this
period.
January and February: January and February saw whale activity increase by
approximately an order of magnitude. Ten sharks from the 2010 season (5 males, 4
females, 1 undetermined) returned to record a total of 886 detections during this period.
The sharks did show a preference for WS Reef (64% of total detections), though less
than that observed during the aggregation season. There was no evidence of sexual
segregation during this period.
2011 Season Structure: The 2011 season was characterized by several small peaks
and valleys in activity which increased in magnitude as the season progressed. This was
punctuated by a single massive spike in detections which occurred from April 23-26. The
spike accounted for 43% of all detections during this season, and included 18 of the
season’s 36 sharks to log acoustic transmissions (seven male, seven female, four
undetermined). Receiver effort was standardized throughout the 2011 season, and 75%
of the detections composing the spike were recorded from animals tagged more than a
week prior. After the 26th, whale shark activity declined by more than order of magnitude
and remained low for the rest of the season (Figure 18).
Number of Detections
39
1500
Figure 18: Spring 2011 Total Daily Detections
1000
Total Detections
500
0
01/Mar
01/Apr
01/May
01/Jun
Thirteen whale sharks which were tagged in 2010 returned during the 2011 season,
including five males, three females, and five undetermined. They recorded 5180
transmissions throughout the season. Six of these were present during the April 23-26
spike, logging 2242 detections. They included two males, two females, and two
undetermined.
2011 Habitat Use: The distribution of whale sharks throughout WS reef was very
different in 2011 as opposed 2010. Preference for the western edge of the reef was
absent, though preference for the north remained. Activity in East WS Reef was 30 times
higher than in 2010 (Figure 19). Much of the difference is due to an abrupt change in
habitat use during the April 23-26 spike. This period accounts for over 75% of activity on
East WS reef and was also characterized by reduced activity in the western regions
(Figure 20). If this four day period is removed, then the distribution of daily detections
becomes much more similar to that seen in 2010 (Figure 21). Depth distribution was
shallow, both for the season in general and for the spike in particular.
Figure 19: 2011 WS Reef Detections/Day
30
North WS Reef
West WS Reef
20
East WS Reef
10
WS Line
24
WS Line
25
WS Line
18
WS Line
17
WS Line
5
WS Line
6
WS Line
7
WS Line
8
WS Line
4
WS Line
2
WS Line
1
0
WS Line
3
Detections/Day
40
Detections/Day
Figure 20: April 23-26 2011 Detections/Day
400
North WS Reef
300
West WS Reef
200
East WS Reef
100
WS Line
24
WS Line
25
WS Line
18
WS Line
17
WS Line
5
WS Line
6
WS Line
7
WS Line
8
WS Line
4
WS Line
2
WS Line
1
Figure 21: 2011 Detections/Day Without April Spike
North WS Reef
West WS Reef
WS Line
24
WS Line
25
WS Line
18
WS Line
17
WS Line
5
WS Line
6
WS Line
7
WS Line
8
WS Line
4
WS Line
2
East WS Reef
WS Line
1
14
12
10
8
6
4
2
0
WS Line
3
Detections/Day
WS Line
3
0
2011 Sexual segregation: Sexual segregation did not occur during the 2011 season.
Sharks of both genders were present at similar times and utilized similar habitat (Figures
22 and 23). This is true both for the season in general and for the spike in activity which
occurred in late April (Figure 24).
WS Line
24
150
WS Line
24
WS Line
25
WS Line
18
8
WS Line
25
WS Line
18
WS Line
17
01/May
WS Line
17
WS Line 5
WS Line 6
400
WS Line 5
WS Line 6
WS Line 7
WS Line 8
WS Line 4
01/Apr
WS Line 7
WS Line 8
WS Line 4
WS Line 2
0
01/Mar
WS Line 2
200
WS Line 1
WS Line 3
Detections/Day
10
WS Line 1
WS Line 3
Detections/Day
41
800
FIgure 22: 2011 Detections by Gender
600
Males
Females
200
01/Jun
FIgure 23: 2011 Detection Frequency by Gender
Females
6
Males
4
2
0
FIgure 24: April 23-26 Detection Frequency by Gender
Females
Males
100
50
0
42
Discussion
Seasonality and Habitat Use
This study represents the first assessment of the Red Sea whale shark
aggregation. Analysis of broad patterns of whale shark presence/absences has
revealed that the whale sharks aggregate in the spring, and that they prefer WS
reef and the area immediately surrounding it. The story becomes more
complicated when the data is analyzed at a finer scale. Detections were not
evenly distributed through either of the aggregation seasons, or during the
moderate levels of activity observed in January and February. Many of the
detections during these periods were part of short-lived but intense spikes in
whale shark presence. This was also true of the offseason, though the spikes
were less intense, composed of detections from lone individuals, fewer, and
further between. The spatial distribution also changed with time. The preference
for WS Reef was strong in the 2010 season, absent in the off season, and
intermediate during January and February. Data is still being gathered on the
2011 season, but the available information suggests some level of fidelity of WS
Reef during this period.
At finer spatial scales, it becomes apparent that not only do aggregating sharks
prefer WS Reef over the surrounding regions, but they also prefer certain parts of
the reef over others. Overall, receivers installed in on western side of the reef
recorded more activity than their eastern counterparts. This was particularly true
during the 2010 season in which the eastern receivers were almost completely
43
inactive. This is contrasted by the 2011 season which had much less difference
between the eastern and western regions. Part of the difference was due to the
dramatic change in whale shark behavior observed from April 23rd to the 26th
2011. Detections in the offseason were too few and too sporadic for fine-scale
habitat use to be assessed for this period. Vertical habitat was predominantly
shallow across all seasons and for both genders which correlates with other
studies (Wilson et al, 2006).
Sexual Segregation
Both the broad and fine scale analysis failed to find any evidence of sexual
segregation occurring within this aggregation. The gender ratio of observed
sharks is effectively one to one. In addition, tagged sharks of differing genders
have often been detected around the same receiver, at the same time. This
complete lack of gender segregation is very nearly unique among studied
aggregations, which are usually dominated by males (Norman and Stevens,
2007)(Graham and Roberts, 2007)(Rowat et al, 2011)(Riley et al, 2010)(Pierce et
al, 2008)(de la Parra, unpublished data). While the Gulf of Baha (RamirezMacias et al, 2006) and the Philippines (Quiros, 2008) aggregations also have
gender ratios close to even, neither has been scrutinized at this fine a scale with
regard to gender separation. Male and female sharks at these sites may occur at
different times, and possibly use different areas within the aggregation.
Site Fidelity
44
The array did find evidence of phylopatry. Sixteen of the thirty-seven the sharks
tagged in the spring of 2010 returned to the area at some point in 2011. Three
additional sharks which were sighted and photographed in 2010 were resighted
and tagged in 2011. One of these, WS 103, had also been sighted in 2009 when
the aggregation was first discovered. This brings our total detected phylopatry to
nineteen sharks. WS 025, a shark tagged in 2010, lost its acoustic tag at some
point between February 13th 2011 (the last time its old tag was detected) and
April 20th 2011 (when it was retagged.) Fortunately it could be identified using
photo analysis of the spot pattern. Still this case confirms that whale sharks are
capable of shedding tags possibly causing us to underestimate the number of
returning sharks. Other aggregations have reported returning individuals, and
only further investigation will allow for comparisons in site fidelity to be made
(Rowat et al, 2011).
Importance of this Study
As this aggregation was previously unstudied, one of the most important aspects
of this project was to highlight areas of interest which warrant further research.
The causes of the aggregation in general and of the short, intense activity spikes
in particular remain unclear. Many aggregations have been linked to seasonal
increases in food availability (Taylor, 1996) (Heyman et al, 2001) (Clark and
Nelson, 1997) (Rowat et al, 2011), (Gifford, 2002), and most of the sharks tagged
for this study were discovered while feeding. It remains unlikely that this is a
breeding aggregation as all individuals were believed to be immature. The
45
clasper morphology of all observed males was undeveloped and not yet useable
for mating. Additionally, all encountered whale sharks were judged to be smaller
than 7 meters, far below suspected sizes of maturity (Norman and Stevens,
2007). If breeding is not occurring, then why is the sexual segregation so
prevalent in other aggregations absent here? What causes some sharks to return
in subsequent years while others do not?
Answering these questions should be a priority for future shark work in the area.
Plankton sampling could be used in conjunction with continued acoustic
monitoring to determine if food is whale sharks’ primary motivation behind
aggregating. Measurement and comparison of environmental factors between
this site and segregated sites could give insights into the causes behind these
behaviors. Photo and tag identification over long time periods could be used to
determine how long individuals remain loyal to the site. If cameras are equipped
for laser photogrammetry then accurate size estimates could also be obtained
and growth rate determined over multiple seasons. The current data set is hardly
exhausted, and more can be learned about this aggregation without the need of
additional field work. Finer temporal analysis could be used to determine if there
are diel patterns to the behavior of aggregating sharks, and individual variation
between sharks also needs to be assessed.
In addition to the potential impact on research, the information provided here is
also relevant to conservation efforts. The whale shark is considered vulnerable
by the IUCN, and the discovery and analysis of unique and potentially vital
46
habitat is of significant value. Additionally, knowledge of the seasonality and
distribution of the aggregation can be beneficial to local management as
potentially harmful gears such as gill nets can be seasonally or regionally
banned.
Summation
This is the first use of acoustic monitoring to analyze a whale shark aggregation.
It confirms that this technique is capable of determining the temporal and spatial
characteristics of such sites and producing data which is relevant to ongoing
research and conservation efforts. The information gained from this study could
serve as a foundation for future whale shark work in the area, and contribute both
to the understanding of the dynamics of the local populations and to how they fit
in to the global whale shark community.
47
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