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Shallow water tidal flat use and associated specialized foraging behavior of the great hammerhead shark ( Sphyrna mokarran )

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Evidence suggests the great hammerhead shark, Sphyrna mokarran, is vulnerable to a variety of anthropogenic stressors, and is an understudied species of shark due to its cryptic nature and wideranging movements. While recognized as both a pelagic-coastal and a highly mobile predator, minimal anecdotal evidence exist describing shallow water habitat use by this species. This report describes six cases in which a great hammerhead shark utilizes an inshore shallow water flats environment (<1.5 m in depth), five of which involve prey capture. These observations permitted identification of two novel behaviors that may allow great hammerheads to inhabit these shallow habitats: a (1) prey-capture technique termed ‘grasp-turning’ that involves burst swimming at tight turning angles while grasping prey and (2) a post-predation recovery period whereby the shark maintains head-first orientation into the current that may facilitate respiration and prey consumption. These behavioral observations provide insights into the natural history of this species.
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Marine and Freshwater Behaviour and Physiology
ISSN: 1023-6244 (Print) 1029-0362 (Online) Journal homepage:
Shallow water tidal flat use and associated
specialized foraging behavior of the great
hammerhead shark (Sphyrna mokarran)
Robert P. Roemer, Austin J. Gallagher & Neil Hammerschlag
To cite this article: Robert P. Roemer, Austin J. Gallagher & Neil Hammerschlag (2016):
Shallow water tidal flat use and associated specialized foraging behavior of the great
hammerhead shark (Sphyrna mokarran), Marine and Freshwater Behaviour and Physiology,
DOI: 10.1080/10236244.2016.1168089
To link to this article:
Published online: 29 Apr 2016.
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© 2016 Informa UK Limited, trading as Taylor & Francis Group
Shallow water tidal at use and associated specialized
foraging behavior of the great hammerhead shark (Sphyrna
Robert P. Roemera,c, Austin J. Gallaghera,c,d and Neil Hammerschlaga,b,c
aRosentiel School for Marine and Atmospheric Science, University of Miami, Miami, FL, USA; bLeonard and
Jayne Abess Center for Ecosystem Science and Policy, University of Miami, Coral Gables, FL, USA; cShark
Research and Conservation Program, University of Miami, Miami, FL, USA; dBeneath The Waves Incorporated,
Syracuse, NY, USA
Evidence suggests the great hammerhead shark, Sphyrna mokarran,
is vulnerable to a variety of anthropogenic stressors, and is an
understudied species of shark due to its cryptic nature and wide-
ranging movements. While recognized as both a pelagic-coastal and
a highly mobile predator, minimal anecdotal evidence exist describing
shallow water habitat use by this species. This report describes six
cases in which a great hammerhead shark utilizes an inshore shallow
water ats environment (<1.5 m in depth), ve of which involve prey
capture. These observations permitted identication of two novel
behaviors that may allow great hammerheads to inhabit these
shallow habitats: a (1) prey-capture technique termed ‘grasp-turning’
that involves burst swimming at tight turning angles while grasping
prey and (2) a post-predation recovery period whereby the shark
maintains head-rst orientation into the current that may facilitate
respiration and prey consumption. These behavioral observations
provide insights into the natural history of this species.
Understanding the habitat use and movements of large and highly mobile marine preda-
tors is inherently challenging due to their wide-ranging behaviors, the concealing nature
of the environment and their increasing rarity due to human exploitation (Nelson 1977).
However, these types of data can provide information for initiating eective conservation
and management of threatened species (Green et al. 2009; Sims 2010, Dulvy et al. 2014).
Biotelemetry approaches are commonly employed to investigate movements and behav-
iors of many dierent shark species, with most studies designed to determine high-use areas
and environmental preferences (Donaldson et al. 2008; Sims 2010; Hammerschlag et al.
2011a; Papastamatiou & Lowe 2012). While these studies can reveal new information on
the habitat utilization and movements of large shark species (e.g. Bonl et al. 2005; Skomal
et al. 2009), their success is predicated on several key conditions: locating individuals,
Shark; hammerhead; habitat
use; behavior; predation;
Received 2 September 2015
Accepted 29 February 2016
CONTACT Robert P. Roemer
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generating a suitable sample size, tag retention and functionality, community or stakeholder
acceptance, as well as ethical issues surrounding tagging impacts on animal health and
survival (Hammerschlag et al. 2014). While increased use and rapid advances in aquatic
biotelemetry are expanding our ability to document the behaviors of large sharks (Hussey
et al. 2015), other more traditional natural history approaches to studying shark behavior
(e.g. observation) can be overlooked and underutilized, possibly leading to incomplete
understanding of animal biology. Natural history approaches have long been viewed as
valuable for advancing the knowledge base of elasmobranch ecology and continue to pro-
vide important insights on species which are otherwise cryptic or rarely observed (e.g.
Strong et al. 1990; Klimley et al. 1992; Martin et al. 2005; Fallows et al. 2013). In particular,
natural history data gathered from local stakeholders or traditional ecological knowledge
may provide insights that could otherwise go undetected, since these individuals spend the
most time on the water and thus most likely to observe and interpret rare animal behaviors
(Huntington 2000; Drew 2005; Silvano and Valbo-Jørgensen 2008).
Large hammerhead sharks (family Sphyrnidae) are among the most specialized of extant
shark species, behaviorally (Gallagher et al. 2014a, 2014b), physiologically (Kajiura & Holland
2002; Mello 2009; Tricas et al. 2009; Gallagher et al. 2014a), and morphologically (Nakaya 1995;
Kajiura 2001; Kajiura et al. 2003; McComb et al. 2009). Although there was a petition to list
the great hammerhead on the US Endangered Species List, listing was not found to be war-
ranted (Miller et al. 2014). On the other hand, their vulnerability to anthropogenic stressors
such as sheries bycatch (Dudley & Simpfendorfer 2006; Zeeberg et al. 2006) and reported
wide-spread population declines, led to an ocial global listing of ‘Endangered’ by the IUCN
Red List (Denham et al. 2007). Despite their widespread distribution, relatively little is known
about their migratory patterns or habitat use across their life history. Presently, the vast major-
ity of what is known about hammerhead shark biology and ecology is based on studies on the
scalloped hammerhead shark (Sphyrna lewini). is is probably due to its gregarious behavior
at a limited number of predictable locations worldwide (Klimley & Nelson 1981; Klimley et al.
1988; Hearn et al. 2010; Hoyos-Padilla et al. 2014; Ketchum et al. 2014). Less is known about the
great hammerhead (Sphyrna mokarran), the largest of the hammerhead species which reaches
a maximum length of over six meters.
e great hammerhead is considered a nomadic and migratory coastal-pelagic/semi-
oceanic species (Compagno 1984; Queiroz et al. 2016). ere is consequently a paucity
of information on its habitat utilization and behavior. To our knowledge, there have only
been three published telemetry-tracking studies focusing on the movement of this species
(Hammerschlag et al. 2011b; Graham et al. 2016; Queiroz et al. 2016). Hammerschlag etal.
(2011b) documented a range extension for this species in the Atlantic Ocean based on a
female shark that migrated from the Florida Keys to north of the mid-Atlantic. In addition
to providing data corroborating this range extension, satellite tracking of 18 great hammer-
heads tagged in Florida by Graham et al. (2016) revealed all of their Core Habitat Use Area
(CHUA) fell within the combined waters of the Florida and US Exclusive Economic Zones
(EEZ). However, Queiroz et al. (2016) found that this species makes repeated movements
into the Atlantic Ocean associated with use of frontal zones where they are targeted by
commercial longline sheries. Much remains to be learned on the habitat use of this species,
especially in shallow inshore environments.
While this species is primarily found over continental shelves, island terraces, and deep coral
reefs (10–30 m, Compagno 1984; Compagno et al. 2005), it is known to occupy in-shore habitats
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to feed on elasmobranchs such as stingrays. Strong et al. (1990) documented a natural predation
by S. mokarran on a southern stingray (Dasyatis americana) in 6 m depth of water, located 12km
east of North Bimini Island, Bahamas. Similarly, Chapman and Gruber (2002) documented
predation by a great hammerhead on a spotted eagle ray (Aetobatus narinari). is event took
place within a pass between two of the northernmost islets of small cays approximately 5km
south of South Bimini Island, Bahamas. Although the pass was fringed with re coral, creating
depths of less than 1 m, the predation event was conned to the pass, which dropped steeply
to a uniform depth of 3 m. Our own observations, combined with anecdotal evidence suggest
that great hammerheads may enter even shallower inshore habitats (<1.5 m depth) such as tidal
ats to feed, but no published data exist to substantiate these behaviors. It has been documented
that it may be benecial for elasmobranchs to forage in warm waters but return to cooler waters
to rest (Bernal et al. 2012). However, to our knowledge, there are no published reports of great
hammerhead predatory behavior within shallow tidal at ecosystems. Further investigations into
shallow water use by otherwise oshore species and potential mechanisms driving such behavior
could augment our understanding of how species tolerate environments with potentially lower
dissolved oxygen, higher temperatures, as well as persistent shing pressure that could render
sharks vulnerable to stranding, exhaustion, and capture.
In the present study, we used a combination of direct observation, discussions with
shers/stakeholders, and analysis of digital media generated by local shers/stakeholders
to describe new aspects of the natural history of the great hammerhead with a focus on
foraging within inshore shallow water at environments (i.e. seagrass ats and back reef
ats, <1.5 m depth). e information adds to our knowledge of the behavioral ecology of
this species and potentially provides avenues leading to future research.
Methods and results
Below, we describe six instances of inshore shallow water at (<1.5 m) habitat use by
S. mokarran, composed of our own personal observations, rst-hand reports gathered
from various stakeholders, and analysis of video obtained from local shing guides and
their clients. We used various forms of communicative instruments including social media
outlets, and in-person open-ended interviews with various stakeholder groups that spend
large amounts of time within tropical and sub-tropical inshore at ecosystems. Once a case
was found to be potentially pertinent to this study, we contacted the individuals who wit-
nessed the events, subsequently conducting more interviews to obtain supplementary data
on environmental parameters and details of the respective ats habitat. e data presented
below was based on opportunistic observations derived from dierent sources. e level of
detail and available information therefore varies by case. Additionally, more information
was attainable post hoc from our analyses of video compared with still photographic images.
Case 1
On 6 May 2012 at 14:30, study author Robert Roemer (RR) observed from a ~150 m dis-
tance while wading and shing for bonesh (Albula vulpes), a great hammerhead shark
inside a shallow water at on the east side of Rock Sound, Eleuthera Island, Bahamas (near
Poison Point, 24°490.01N −76°1159.99W). e single individual was identied by its
distinctive sickle-shaped large dorsal n protruding entirely out of the water column. It
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had an estimated total length (TL) of 2.5 m. e water depth the shark was occupying was
~0.7 m. is location contained shallow areas such that bonesh were observed with their
caudal ns and dorsal ns partially-to-fully exposed while likely feeding on benthic prey
items (‘tailing, Crabtree et al. 1998). RR observed the shark move sinuously forward with
wide sweeps of its cephalofoil. is calm, non-erratic behavior was repeated within the at
habitat, for a total distance of roughly 15 m, and a period of 10min until the shark was
presumably alarmed and darted away, not to be seen again (a distance of ~80 m between
RR and the shark). At the time of observation, the at environment was large and relatively
uniform with water depth varying from 0.2 to 0.9 m during a low incoming tide. e ben-
thic habitat comprised various hard and so coral species, including rose coral, Manicina
aerolata and shallow-water starlet coral, Siderastrea radians. e following species of tele-
osts and elasmobranchs were also identied in the area both before and aer the shark was
observed: scrawled cowsh (Acanthostracion quadricornis), bonesh, yellow n mojarra
(Gerrescinereus), neonate and juvenile lemon sharks (Negaprion brevirostris), and several
species of stingrays (Dasatidae spp.).
Case 2
On 10 March 2014 at 10:22, a great hammerhead roughly 2.5 m TL was observed at a dis-
tance of less than ~8 m by Matthew Glaze, and was rst spotted at an approximate distance
of 270 m. Wave action was less than 0.3 m, on a sunny day with prevailing winds less than
5 knots and water clarity of 16 to 18 m. e lone hammerhead was occupying a shallow
back-reef in the Republic of Palau, on the south side of German and Lighthouse Channels
(7°1715.7N 134°2746.9E) during a low incoming tide. e approximate depth of the
occurrence and back-reef habitat was between 0.8 and 1.3 m with uniform benthic contour
for 450 m. e benthic substrate consists of various hard coral rubble created from constant
wave action and as a result is quite barren. It has minimal ora or fauna with the exception
of a fairly common presence of bubble algae, (Valonia spp.) and nominal reef teleosts. e
shark was rst sighted when a bait ball composed of sardines (genus Sardinella or possibly
Sardinops) was being actively predated on by a giant trevally, Caranx ignobilis (Figure 1(A)–
(C)). is feeding activity appeared to attract the great hammerhead from a distance of ~20
m. As it moved through the shallow reef at habitat, it performed erratic movements, and
employed rapid surges of speed in its attempts to feed on the sardine bait ball. e right
ank of the shark had numerous scarring/wounds. Also present in close proximity to the
bait ball were two nurse sharks (Ginglymostoma cirratum) and a spotted eagle ray (Aetobatus
narinari). Aer a total time of 15min, the hammerhead moved slowly into a deeper nat-
ural channel on the reef at (Figure 1(D)) and eventually into deeper waters o the reef
edge aer which it was not observed again. Other teleosts present on the reef at habitat
were houndsh (Tylosurus crocodilus), parrotsh (Scaridae spp.), wrasse (Labridae spp.),
butterysh (Chaetodontidae spp.), and angelsh (Pomacanthidae spp.). While the number
of anthropogenic disturbances is low on the reef at habitat, this area is subject to large,
direct discharges of raw sewage from Palau’s largest city, Koror. Information from this case
and associated photo documentation was provided to Matthew Glaze aer RR performed
a search on social media for professional photographers that have recorded hammerheads
in shallow water environments.
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Case 3
On 30 January, 2015 at 16:15 during high-slack tide, Lorna Scribner (assistant lab manager
of the Bimini Biological Field Station) and John Rayeld (scientic volunteer) witnessed a
great hammerhead in South Bimini Island, Bahamas (25°4151.4N 79°1728.4W) within a
shallow water at environment. e backwater at was lined with mangroves and relatively
sheltered with an estimated average depth of 1.2 m and a substrate consisting mainly of
turtle grass (alassia testudinum). e male shark had a TL of approximately 2.5 m and
was rst observed from a distance of 10 m. e shark was seen swimming erratically with its
dorsal n protruding from the water column (Figure 2 (A)–(B)). It was probably in pursuit
of a spotted eagle ray, which breached out of the water column most likely to escape the
shark trailing close behind. e air temperature during observation was 21.6°C and the
sky was overcast. is evidence was acquired from John Rayeld aer several photographs
were posted and identied on his social media site. We subsequently followed up with the
observer to gather information relating to this case, including photographic documentation.
Case 4
On February 11th, 2011 at 10:45, a great hammerhead (estimated 2.0 m TL) was docu-
mented in South Andros Island, Bahamas, near Leaf Cay (23°4431.1N 77°5110.8W) on
the western end of the island. e observation was made by Andy Dober who was on a
guided y-shing ats expedition for bonesh. is shark was observed in a shallow water
Figure 1.(Colour online) A great hammerhead in the Republic of Palau: (A) lateral view of animal skimming
coral substrate; (B) dorsal fin of the same individual protruding almost entirely out of the water, fin may
appear dried due to extended heat exposure; (C) top-down view of animal turning on minimal radius; (D)
underwater view of animal showing ventral proximity to the back-reef habitat. Published with permission
of the copyright holder, Matthew Glaze.
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sand at with an estimated depth of less than 0.9 m. e great hammerhead was initially
attracted by disturbances caused by a struggling bonesh which was hooked on y rod/reel
gear (Figure 3(A)). A lemon shark was also actively pursuing the hooked bonesh. Aer
17-s into the video evidence, the hammerhead had isolated the lemon shark, which was
still following the hooked bonesh. While in pursuit of the lemon shark, the hammerhead
exhibited rapid bursts of speed while performing several successive tight turns. At the 22-s
mark in the video, the great hammerhead executed a fast, straightforward motion in an
attempt to close with the lemon shark (Figure 3(B)–(D)). At this point, the hammerhead
made contact with the vessel, which appeared to cause it to abort its pursuit of the lemon
shark and slowly move o the at to deeper waters. Andy Dober provided video and envi-
ronmental information of this event aer RR interviewed several ats shing guides in the
Bahamas, then contacted AD.
Case 5
During the week of November, 25th 2014, y-shing guide William Benson witnessed and
recorded a great hammerhead while guiding a shing client. e hammerhead was situated
on a shallow at comprised mostly of so sediments o the island of Key West, Florida,
in close proximity to the Northwest Channel (24°3529.8N 81°5617.8W). e estimated
depth of the at was 0.9 m while the shark had an approximate TL of 2.1 m. e shark was
rst noticed because of the considerable amount of sediment it had disturbed during its
predation on a school of permit, Trachinotus falcatus, which were using the shallow at
to feed (Figure 4(A)–(D)). During the observed sequence, the shark exhibited rapid burst
swimming while also displaying several successive tight turns uctuating between one-half
and one-third of TL (Figure 5(E)–(H)). is was similar to the behavior observed in the
great hammerhead within Case 4. ese burst swimming events were rst observed 07-s
aer initial detection of the shark and continued throughout the entire observed episode
(24-s). Video evidence and environmental conditions were compiled and found to be per-
tinent following contact, a meeting, and an interview with William Benson.
Figure 2.(Colour online) (A and B). A single great hammerhead in South Bimini, Bahamas, exploiting a
mangrove lined seagrass flat environment approximately 1.2 m in depth while hunting elasmobranch
species. Published with permission of the copyright holder, John Rayfield.
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Figure 3.(Colour online) Still frames pulled from video showing a hammerhead in pursuit of a lemon shark,
Negaprion brevirostris, in South Andros Island, Bahamas. (A) The individual great hammerhead orientates
towards the lemon shark. (B-D) The great hammerhead attacks the lemon shark. Arrows illustrate the
lemon shark during predation event. Published with permission of the copyright holder,Andy Dober.
Figure 4.(Colour online) Frames taken from video depicting a great hammerhead using a shallow water
flat in order to prey upon a school of permit, Trachinotus falcatus. (A) Initial observation of hammerhead.
(B) Sediment cloud in relation to individual hammerhead. Arrow details fin orientation at time of
initial observation in relation to size of sediment cloud. Published with permission of the copyright
holder,William Benson.
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Case 6
On 22 March 2010 at 12:00, a great hammerhead shark of an estimated 3.65 m TL was
observed exercising erratic burst swimming behavior on a shallow at habitat with a mainly
sand substrate (Figure 6(A)) o the northwest corner of the Marquesas Islands
(24°3528.1N 82°0900.1W). William Benson was guiding a shing client (Gannon
Dudlar) and documented the entire sequence, which was provided to RR. e at habitat
was approximately 1.2 m deep and prevailing wind speeds averaged 15 knots. e tide was
outgoing, almost low-slack and the day sunny. e shark was swimming in a straight line at
the time the vessel approached. It exhibited a sudden and rapid burst of speed that propelled
it nearly 3.35 m (3min into the video observation). e shark then located and attacked a
nurse shark (~0.9 m in total length), situated on the benthic substrate (Figure 6(B)–(D)).
During the attack, the shark exhibited 22 circular turns at high speed within a tight radius
(Figure 7(A)–(B)). e great hammerhead continued to display tight rotations with limited
radii, (estimated to be one-half of TL) with the nurse shark still largely protruding from its
jaws (Figure 7(D)). Aer the nurse shark was rmly oriented within the jaws of the shark, the
great hammerhead established itself so that it was facing anteriorly into the current (Figure
8(A)–(B)). e hammerhead remained facing steadily into the current with the caudal n
of the nurse shark still projecting from its jaws. e shark propelled itself minimally so as
to remain stationary and did so for a span of 15min, undisturbed by the close proximity
of Benson and the vessel. Aer this period, the hammerhead moved slowly away from the
vessel. William Benson and Gannon Dudlar provided environmental conditions and video
evidence to RR and in interviews with WB and correspondence with GD.
Figure 5.(Colour online) Frames (E-H) detailing example of tight turning radius exhibited by the feeding
activity of the great hammerhead. Hammerhead dorsal fin labeled ‘i.’, and caudal fin labeled ‘ii.’ to help
deduce orientation. Published with permission of the copyright holder,William Benson.
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Figure 6.(Colour online) (A) The initial observation of the estimated 3.65 m great hammerhead before a
predation event in Marquesas Islands. (B-D) The predation event on prey later identified as a nurse shark,
Ginglymostoma cirratum. Published with permission of the copyright holder, William Benson.
Figure 7.(Colour online) (A and B) Tight turn radius (roughly one-half of total body length) utilized by
a hammerhead in order to capture prey. (C) Distinguishable caudal fin of a nurse shark, Ginglymostoma
cirratum protruding from the jaws of the hammerhead. (D) ‘Grasp-Turning’ technique of hammerhead
with prey still protruding from its jaws. Dorsal fin labeled ‘i.’ caudal fin labeled ‘ii.’ and nurse shark labeled
‘iii’, respectively. Published with permission of the copyright holders, Gannon Dudlar and William Benson.
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Here we have documented and described six instances of inshore, shallow water (<1.5
m depth) habitat use by great hammerhead sharks. ese occurred on tidal ats within
various tropical and sub-tropical locations across the globe. ese six cases appear to be
instances of prey searching and hunting. Five of these cases involved erratic behaviors by a
single individual in pursuit of elasmobranch or teleost prey. An exception to this was Case
1, where the observed hammerhead was calm, using wide sweeps of its cephalofoil. We
suggest that this represents prey searching behavior before the shark had located a prey
item. e observations are noteworthy for the environment in which they occurred because
this species is characterized as a coastal-pelagic species, primarily occupying oshore areas
(Compagno et al. 2005).
Inshore shallow water habitats in tropical and sub-tropical marine ecosystems contain
high species richness and abundance and are oen regarded as important to the devel-
opment of numerous small and medium-sized teleost shes and elasmobranchs (Chong
et al. 1990; Blaber et al. 1995; Beck et al. 2001). Tidal at ecosystems such as tropical and
sub-tropical inshore shallow water ats are protected, provide shelter, and, are more dicult
for larger predators to access. ey have consequently been considered to be successful
nursery grounds (Reise 2012). ese ecological features can make inshore habitats valuable
hunting and foraging grounds for large predatory shes such as sharks (Hammerschlag
etal. 2010). Entrance into these habitats may, however, incur several costs to predators
due to tidal uctuations that can result in temporary periodicity of low dissolved oxygen
levels (Sakamaki et al. 2006) and other water-quality stressors within inshore tropical shal-
low waters (e.g. raw sewage euent in Case 2). As a result, predators using these habitats
may balance trade-os between potential environmental stressors and resource abundance,
thereby requiring specialized foraging or swimming techniques to exploit these environ-
ments. Two behaviors were identied during our video analyses that may be specializations
permitting great hammerheads to balance these trade-os and use shallow tidal ats in
pursuit of prey. ese are: (a) prey-handling behavior (Case 5, 6); and (b) a post-predation
energetic recovery behavior (Case 6).
Specialized prey handling behavior of a great hammerhead was rst described in detail by
Strong et al. (1990). During this observation, a great hammerhead (~3.0 m TL) utilized its
Figure 8.(Colour online) (A and B) Details the great hammerhead positioning itself, anteriorly into the
current defining a possible ‘recovery period’ of the shark post-predation. The nurse shark is labeled ‘i.’ for
easier perspective. Published with permission of the copyright holder,William Benson.
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cephalofoil to pin a southern stingray, Dasyatis americana to the substrate (termed ‘pin and
pivot’). A similar behavior was observed by Chapman and Gruber (2002) of a single great
hammerhead (~3.6 m) capturing and consuming a spotted eagle ray. While both predation
events occurred in deeper water than reported in our study, they support the interpretation
that our sequences represent examples of specialized foraging behavior.
We documented ve cases where a great hammerhead grasped or attempted to grasp
teleost and/or elasmobranch prey in its jaws. In three of the documented cases, the ham-
merhead made multiple (up to 22) tight turns before the prey was consumed. We term this
behavior ‘grasp-turning.’ is may be a technique that allows hammerheads to use the force
exacted by the surrounding water to help keep prey within their mouths and maneuver
prey within their jaws to facilitate consumption (i.e. head down swallow). Performance of
grasp-turning’ behavior presumably provides the predatory shark with a tactical advantage
over the prey within the vertically restricted space of a tidal at.
e use of shallow water and performance of grasp-turning behaviors incur metabolic
and homeostatic costs for a large-bodied elasmobranch. Previous research has demonstrated
a rapid onset of anaerobic acidosis in exercise-stressed (sheries capture) great hammerhead
sharks (Gallagher et al. 2014c). While these behaviors may also be occurring in deeper
waters, metabolic costs and challenges to respiration from increased activity are likely higher
in shallow inshore waters that are oen warmer, more saline, and possess lower dissolved
oxygen concentrations (Belding 1929; Kitheka 1997; Meyer-Reil and Köster, 2000; Diaz
2001; Ridd & Stieglitz 2002; Hodoki & Murakami 2006). Our observations suggest, how-
ever, a potential mechanism for recovery from exercise stress in a sub-tropical inshore at
environment. e great hammerhead from the Marquesas (Case 6) re-positioned itself into
a strong, incoming current while propelling itself at a minimal rate to remain stationary for
15min. is likely maximizes oxygen uptake in the gills and promotes recovery from energy
expenditure and anaerobic acidosis during prey pursuit (Figure 8(A)–(B)). is action may
also help to further facilitate consumption: the high velocity water current keeping the nurse
shark situated within the jaws of the great hammerhead while consumption continues.
During exercise in teleosts, increased water ow over the gills is proportional to oxygen
uptake, even increasing in low-oxygen environments such as those of shallow tropical and
sub-tropical inshore waters (Randall 1982). Aer exercise, gill ventilation and water ow
interactions with the gills have eects on the blood respiratory and blood acid–base status
(Perry & Wood 1989). Teleosts remaining in low oxygen, elevated temperature environments
(like those of inshore tidal at ecosystems) are hindered from the respiratory gill ventila-
tor method and exhibit impaired recovery of tissue ATP and plasma glucose (Suski etal.
2006; Shultz et al. 2011). ey would therefore experience more acute need for recovery
from exercise. Benthic tropical elasmobranchs capable of stationary buccal pumping (i.e.
epaulette shark, H. ocellatum) can tolerate mild hypoxic environments and have a well-de-
veloped capacity for anaerobic metabolism (Wise et al. 1998). Bonnethead sharks (Sphyrna
tiburo) and blacknose sharks (C. acronotus) increase mouth gape, swim speed, and oxygen
consumption rates during hypoxic conditions (Carlson & Parsons 2001, 2003). e ham-
merhead behavior described in Case 6 may have served a similar function as those found
by Carlson and Parsons (2001, 2003). Although water quality parameters were not collected
in Case 6, it is likely great hammerheads occupying warm shallow waters will encounter
relatively lower dissolved oxygen by comparison with colder oshore waters. An animal’s
behavior is directly driven by its physiology (Ricklefs & Wikelski 2002), which is in turn
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driven by its environment. Based on the metabolic responses of great hammerheads during
exercise as well as water quality characteristics typical of shallow tropical and sub-tropical
tidal ecosystems, the observed great hammerhead behavior that we attributed to post-pre-
dation recovery is perhaps predictable.
Hammerhead sharks are a functionally, physiologically, and behaviorally specialized
group of species (Gallagher et al. 2014b). Here we provide a series of detailed opportunis-
tic observational accounts that provide insights into the habitat use and foraging behavior
of this species. We speculate that use of shallow tidal ats by great hammerhead sharks is
likely common and an important aspect of their overall feeding ecology. We also describe
several behaviors that we suggest are specializations that allow them to compensate for the
costs of foraging in shallow environments. is study also highlights the value of utilizing
natural history, social media, observational science, and local knowledge as a means for
advancing our understanding of a species that is dicult to study.
e authors would like to thank William Benson, Andy Dober, Gannon Dudlar, Matthew Glaze, John
Rayeld, and Lorna Scribner for their assistance in acquiring observational evidence. e authors
would also like to thank Rob and Susan Roemer for assistance with this publication.
Disclosure statement
No potential conict of interest was reported by the authors.
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... Sphyrna mokarran can reach a maximum size of 6.0 m total length (TL) and in situ observations imply it predominantly feeds on rays 14-16 but, like most of its apex congeners also consumes teleosts and other sharks 12,13,17,18 . Nevertheless, data are limited and in a recent review Gallagher and Klimley 19 stated that S. mokarran feeding ecology requires further assessment. ...
... The data collected here represent the first efforts at metabarcoding the stomach contents of S. mokarran and C. limbatus off eastern Australia. Our results support the general trends in the literature describing the diet of S. mokarran as being dominated by rays [14][15][16] , but also including teleosts and other sharks 12,13,17,18 , and reports of C. limbatus predominantly feeding on teleosts [20][21][22] . By 'Russian-dolling' 39 the stomach contents of these marine predators and their prey, we have also quantified some of the issues encountered when attempting to reconstruct trophic interactions from metabarcoding stomach contents. ...
... In this study, metabarcoding assays provided some insights into the dietary preferences of S. mokarran and C. limbatus off eastern Australia. These species appear to have some dietary overlap, but with consistency among prey species identified in studies of their feeding ecology from across their broader ranges [12][13][14][15][16][17][18][20][21][22] . Sphyrna mokkaran fed predominantly on Myliobatiformes and Rajiformes, but also teleosts, whereas C. limbatus fed predominantly on teleosts, which is consistent with its smaller body size and lack of cephalofoil that allows specialized feeding of benthic prey. ...
Full-text available
Increasing fishing effort, including bycatch and discard practices, are impacting marine biodiversity, particularly among slow-to-reproduce taxa such as elasmobranchs, and specifically sharks. While some fisheries involving sharks are sustainably managed, collateral mortalities continue, contributing towards > 35% of species being threatened with extinction. To effectively manage shark stocks, life-history information, including resource use and feeding ecologies is pivotal, especially among those species with wide-ranging distributions. Two cosmopolitan sharks bycaught off eastern Australia are the common blacktip shark (Carcharhinus limbatus; globally classified as Near Threatened) and great hammerhead (Sphyrna mokarran; Critically Endangered). We opportunistically sampled the digestive tracts of these two species (and also any whole prey; termed the ‘Russian-doll’ approach), caught in bather-protection gillnets off northern New South Wales, to investigate the capacity for DNA metabarcoding to simultaneously determine predator and prey regional feeding ecologies. While sample sizes were small, S. mokkaran fed predominantly on stingrays and skates (Myliobatiformes and Rajiformes), but also teleosts, while C. limbatus mostly consumed teleosts. Metabarcoding assays showed extensive intermixing of taxa from the digestive tracts of predators and their whole prey, likely via the predator’s stomach chyme, negating the opportunity to distinguish between primary and secondary predation. This Russian-doll effect requires further investigation in DNA metabarcoding studies focussing on dietary preferences and implies that any outcomes will need to be interpreted concomitant with traditional visual approaches.
... While the species may rely on coastal and shallow habitats to target elasmobranch prey (Chapman et al., 2002;Doan & Kajiura, 2020;Roemer et al., 2016), they regularly venture into pelagic waters off the oceanic shelf (Queiroz et al., 2016;Raoult et al., 2020), and movement studies in the western North Atlantic have revealed complex patterns. Queiroz et al. (2016) showed that S. mokarran used oceanic shelf waters characterized by high productivity and steep temperature gradients. ...
... The extensive use of shallow depths could reflect habitat use as part of the species' foraging behavior. Small-bodied carcharhinid sharks, rays and M. atlanticus represent important prey for S. mokarran (Chapman & Gruber, 2002;Roemer et al., 2016;Raoult et al., 2019;Doan & Kajiura, 2020;Griffin et al., 2022). The vertical range of PSAT-equipped whitespotted eagle rays Aetobatus narinari (Euphrasén, 1790) was limited to depths of 26 m, with rays primarily occupying depths < 10 m (Brewster et al., 2021). ...
The great hammerhead (Sphyrna mokarran) is a highly mobile, large‐bodied shark primarily found in coastal‐pelagic and semi‐oceanic waters across a circumtropical range. It is a target or bycatch species in multiple fisheries, and as a result, rapid population declines have occurred in many regions. These declines have contributed to the species being assessed as globally Critically Endangered on the IUCN Red List. While conservation and management measures have yielded promising results in some regions, such as the United States, high levels of at‐vessel and post‐release mortality remain a major concern to the species population recovery. We examined the vertical space use and thermal range of Pop‐Off Archival Satellite tagged S. mokarran in the western North Atlantic Ocean, expanding our understanding of the ecological niche of this species and providing insight into bycatch mitigation strategies for fisheries managers. Our results showed that S. mokarran predominantly used shallow depths (75% of records < 30 m) and have a narrow temperature range (89% of records between 23 and 28 °C). Individual differences in depth use were apparent and a strong diel cycle was observed, with sharks occupying significantly deeper depths during the daytime. Furthermore, two individuals were confirmed pregnant with one migrating from the Bahamas to South Carolina, USA providing further evidence of regional connectivity and parturition off the U.S. east coast. Our findings suggest that S. mokarran may be vulnerable to incidental capture in the western North Atlantic commercial longline fisheries due to substantial vertical overlap between the species and the gear. Our results can be incorporated into conservation and management efforts to develop and/or refine mitigation measures focused on reducing the bycatch and associated mortality of this species, which can ultimately aide S. mokarran population recovery in areas with poor conservation status.
... However, Roemer et al. (2016) shows that adult great hammerhead sharks venture into shallow waters to feed on small sharks or eagle rays. This suggests that large sharks-as vagrant predators occupying a top-predatory position Navia et al., 2016)-may use shallow habitats like the intertidal opportunistically, but spend the large majority of time in (adjacent) subtidal waters ( Figure 1c). ...
Full-text available
Intertidal habitats (i.e. marine habitats that are (partially) exposed during low tide) have traditionally been studied from a shorebird‐centred perspective. We show that these habitats are accessible and important to marine predators such as elasmobranchs (i.e. sharks and rays). Our synthesis shows that at least 43 shark and 45 ray species, of which 54.5% are currently threatened, use intertidal habitats. Elasmobranchs use intertidal habitats mostly for feeding and as refugia, but also for parturition and thermoregulation. However, the motivation of intertidal habitat use remains unclear due to limitations to observe elasmobranch behaviour in these dynamic habitats. We argue that elasmobranch predators can play an important role in intertidal food webs by feeding on shared resources during high tide (i.e. ‘high‐tide predators’), which are accessible and also consumed by terrestrial or avian predators during low tide (i.e. ‘low‐tide predators’). In addition, elasmobranchs are able to change the bio‐geomorphology of intertidal habitats by increasing habitat heterogeneity due to feeding activities and may also alter resource availability for other consumers. We discuss how the ecological role of elasmobranchs in intertidal habitats is being affected by the continued overexploitation of these species, and conversely, how the global loss of intertidal areas poses an additional threat to an already vulnerable taxonomic group. We conclude that studies on intertidal ecology should include both low‐tide (e.g. shorebirds) and high‐tide (e.g. elasmobranchs) predatory guilds and their ecological interactions. The global loss of elasmobranch predatory species and intertidal habitat provides additional compelling arguments for the conservation of these areas.
... Not only seagrass but also terrestrial cockroach ingestion was found in another stomach; therefore, it can be inferred that some individuals frequented shallower and more coastal waters as their habitat. In other regions, great hammerheads were also found to use inshore, flat shallow water environments (<1.5 m), and young-of-the-year S. mokarran used nearshore, even highly human-impacted marine habitats as its nursery ground [41][42][43][44]. ...
Full-text available
The stomach contents of 30 male and 43 female (age < 3 years; 74–236 cm total length) juvenile great hammerhead sharks (Sphyrna mokarran (Rüppell, 1837)) obtained from commercial fisheries operating in Saudi Arabian waters of the Arabian Gulf were analyzed for the first time. After exclusion of parasites and abiotics, a total of 31 prey items, including the remains of cephalopods, fish, crustaceans, and bivalve mollusks, were identified in the stomachs of 59 great hammerheads. Based on the index of relative importance, teleosts were their main prey, and Platycephalus indicus (Linnaeus, 1758) was the most important prey at the species level. Significant age-related dietary differences were noted (F = 1.57, p = 0.026), indicating that the prey of the hammerheads aged 0–3 years shifted from Platycephalidae to Myliobatidae. Levin’s niche overlap index was low (0.05–0.21), indicating that <3-year-old juvenile great hammerheads are specialized predators. The estimated trophic level was 4.40–5.01 (mean ± SD, 4.66 ± 0.45), indicating that the great hammerhead is a tertiary consumer.
... Unlike other shark species, cephalopods are not an important prey item for this species (Smale and Cliff 1998). Interestingly, Roemer et al. (2016) reported this species to be an opportunistic feeder in shallow tidal flats. ...
Full-text available
Sharks are important part of coastal and offshore pelagic ecosystems and being caught mainly as bycatch of tuna gillnet fishing operations. There are 12 species of pelagic sharks caught in Pakistan which belongs to 5 families and 7 genera. Silky shark (Carcharhinus falciformis) is the most dominating pelagic shark followed by shortfin mako (Isurus oxyrhinchus) and pelagic thresher shark (Alopias pelagicus). Blue shark (Prionace glauca) is the rarest pelagic shark that is seldom caught by tuna gillnet vessels. There is general concern regarding over-exploitation of pelagic sharks globally as well as in Pakistan, as some species including scalloped hammerhead (Sphyrna lewini) are disappearing very fast and it is feared that they may become extinct in near future. Although most pelagic sharks are included in the Appendix-II of CITES which restricts their global trade as well as there is a ban on their catching, landing, marketing and trade has been imposed through national fisheries legislations, however, there is no effective implementation mechanism in place for ensuring these restrictions in Pakistan. Exploitation of pelagic sharks, therefore, continue unabated in Pakistan as well as some other regional countries which may lead to their disappearance from commercial catches or may ends up in regional or global extinction.
... This species is a generalist mesopredator, feeding on a variety of small teleosts, crusta ceans, and mollusks (Castro 2000). The great hammer head Sphyrna mokarran is a large-bodied species that utilizes a variety of habitats, spanning inshore flats (Roemer et al. 2016), coral reefs (Guttridge et al. 2017), and pelagic environments (Hammerschlag et al. 2011). This species is a specialized apex predator (Gallagher et al. 2014a) that may selectively feed on elasmobranchs such as rays (Raoult et al. 2019). ...
Full-text available
Understanding and ultimately predicting how marine organisms will respond to urbanization is central for effective wildlife conservation and management in the Anthropocene. Sharks are upper trophic level predators in virtually all marine environments, but if and how their behaviors are influenced by coastal urbanization remains understudied. Here, we examined space use and residency patterns of 14 great hammerheads Sphyrna mokarran , 13 bull sharks Carcharhinus leucas , and 25 nurse sharks Ginglymostoma cirratum in proximity to the coastal metropolis of Miami, Florida, using passive acoustic telemetry. Based on the terrestrial urban carnivore literature, we predicted sharks would exhibit avoidance behaviors of areas close to Miami, with residency patterns in these urban areas increasing during periods of lower human activity, such as during nocturnal hours and weekdays, and that dietary specialists (great hammerhead) would exhibit comparatively lower affinity towards highly urbanized areas relative to dietary generalists (bull and nurse shark). However, we did not find empirical support for these predictions. Space use patterns of tracked sharks were consistent with that of ‘urban adapters’ (species that exhibit partial use of urban areas). Modeling also revealed that an unmeasured spatial variable was driving considerable shark residency in areas exposed to high urbanization. We propose several hypotheses that could explain our findings, including food provisioning from shore-based activities that could be attracting sharks to urban areas. Ultimately, the lack of avoidance of urban areas by sharks documented here, as compared to terrestrial carnivores, should motivate future research in the growing field of urban ecology.
... While both tarpon and permit are considered natural prey for these shark species (A. J. Adams, unpublished data;Castro, 2010;Roemer et al., 2016), recreational angling has exacerbated predation events when fish are exposed to capture stress, including extended fight times (Ault et al., 2007;Guindon, 2011). A common time for anglers to target both tarpon and permit in the Florida Keys is before and during spawning events when they aggregate in large schools ranging from hundreds to thousands of individuals in the spring and summer (tarpon: Luo et al., 2020;Griffin et al., 2022;permit: Brownscombe, Griffin, Morley, et al., 2019). ...
Interspecific interactions can play an essential role in shaping wildlife populations and communities. To date, assessments of interspecific interactions, and more specifically predator–prey dynamics, in aquatic systems over broad spatial and temporal scales (i.e., hundreds of km and multiple years) are rare due to constraints on our abilities to measure effectively at those scales. We applied new methods to identify space use overlap and potential predation risk to Atlantic tarpon (Megalops atlanticus) and permit (Trachinotus falcatus) from two known predators, great hammerhead (Sphyrna mokarran) and bull (Carcharhinus leucas) sharks over a three year period using acoustic telemetry in the coastal region of the Florida Keys (USA). By examining spatial‐temporal overlap, as well as the timing and order of arrival at specific locations compared to random chance, we show that potential predation risk from great hammerhead and bull sharks to Atlantic tarpon and permit are heterogeneous across the Florida Keys. Additionally, we found that predator encounter rates with these game fishes are elevated at specific locations and times, including a pre‐spawning aggregation site in the case of Atlantic tarpon. Further, using machine learning algorithms, we identify environmental variability in overlap between predators and their potential prey, including location, habitat, time of year, lunar cycle, depth, and water temperature. These predator–prey landscapes provide insights into fundamental ecosystem function and biological conservation, especially in the context of emerging fisheries‐related depredation issues in coastal marine ecosystems.
... Considering the Brownian and resident movements documented here are associated with different resource distributions (widespread and localized distributions, respectively; Schtickzelle et al. 2007, Humphries et al. 2010, Reyna-Hurtado et al. 2012, our findings are consistent with previous hypotheses suggesting that instead of sex or life stage, great hammerhead movements may be related to high prey availability in coastal shallow habitats (Guttridge et al. 2017). Indeed, this species has been observed entering coastal areas less than 1m in depth to forage (Roemer et al. 2016). Furthermore, the great hammerhead high-use regions identified here (i.e. ...
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Animals follow specific movement patterns and search strategies to maximize encounters with essential resources (e.g. prey, favourable habitat) while minimizing exposures to suboptimal conditions (e.g. competitors, predators). While describing spatiotemporal patterns in animal movement from tracking data is common, understanding the associated search strategies employed continues to be a key challenge in ecology. Moreover, studies in marine ecology commonly focus on singular aspects of species' movements, however using multiple analytical approaches can further enable researchers to identify ecological phenomena and resolve fundamental ecological questions relating to movement. Here, we used a set of statistical physics-based methods to analyze satellite tracking data from three co-occurring apex predators (tiger, great hammerhead and bull sharks) that predominantly inhabit productive coastal regions of the northwest Atlantic Ocean and Gulf of Mexico. We analyzed data from 96 sharks and calculated a range of metrics, including each species' displacements, turning angles, dispersion, space-use and community-wide movement patterns to characterize each species' movements and identify potential search strategies. Our comprehensive approach revealed high interspecific variability in shark movement patterns and search strategies. Tiger sharks displayed near-random movements consistent with a Brownian strategy commonly associated with movements through resource-rich habitats. Great hammerheads showed a mixed-movement strategy including Brownian and resident-type movements, suggesting adaptation to widespread and localized high resource availability. Bull sharks followed a resident movement strategy with restricted movements indicating localized high resource availability. We hypothesize that the species-specific search strategies identified here may help foster the co-existence of these sympatric apex predators. Following this comprehensive approach provided novel insights into spatial ecology and assisted with identifying unique movement and search strategies. Similar future studies of animal movement will help characterize movement patterns and also enable the identification of search strategies to help elucidate the ecological drivers of movement and to understand species' responses to environmental change.
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Increasing fishing effort, including bycatch and discard practices, are impacting marine biodiversity, particularly among slow-to-reproduce taxa such as elasmobranchs, and specifically sharks. While some fisheries involving sharks are sustainably managed, collateral mortalities continue, contributing towards >35% of species being threatened with extinction. To effectively manage shark stocks, life-history information, including resource use/feeding ecologies is pivotal, especially among those species with wide-ranging distributions and habitats. Two cosmopolitan sharks bycaught off eastern Australia are the common blacktip shark (Carcharhinus limbatus; globally classified as Near Threatened) and great hammerhead (Sphyrna mokarran; Critically Endangered). We opportunistically sampled the digestive tracts of these two species and also any whole prey; (termed the ‘Russian-doll’ approach) caught in bather-protection gillnets off northern New South Wales to investigate their regional feeding ecologies and the capacity for DNA metabarcoding to delineate trophic interactions. Sphyrna mokkaran fed predominantly on Myliobatiformes and Rajiformes, but also teleosts, while C. limbatus mostly consumed teleosts, with some inter-specific dietary overlap of prey items. Extensive cross-contamination of predator and prey digestive tracts, likely via the predator’s stomach chyme, was evident from the metabarcoding assays limiting the opportunity to delineate trophic interactions from these data. This Russian-doll effect requires further investigation in DNA metabarcoding studies focused on dietary preferences, but implies any outcomes will need to be interpreted concomitant with traditional visual approaches.
Coastal elasmobranchs tend to be upper-level predators, which may exert top-down impacts on the systems they inhabit; but there remains much to learn about their trophic ecology. In this chapter, we update our knowledge on the trophic interactions of coastal elasmobranchs as prey, predators, and competitors. We also explore factors that affect these relationships and elasmobranch interactions within key coastal habitats.
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The rapid expansion of human activities threatens ocean-wide biodiversity. Numerous marine animal populations have declined, yet it remains unclear whether these trends are symptomatic of a chronic accumulation of global marine extinction risk. We present the first systematic analysis of threat for a globally distributed lineage of 1,041 chondrichthyan fishes-sharks, rays, and chimaeras. We estimate that one-quarter are threatened according to IUCN Red List criteria due to overfishing (targeted and incidental). Large-bodied, shallow-water species are at greatest risk and five out of the seven most threatened families are rays. Overall chondrichthyan extinction risk is substantially higher than for most other vertebrates, and only one-third of species are considered safe. Population depletion has occurred throughout the world's ice-free waters, but is particularly prevalent in the Indo-Pacific Biodiversity Triangle and Mediterranean Sea. Improved management of fisheries and trade is urgently needed to avoid extinctions and promote population recovery.
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Study aim and location: Many populations of highly mobile marine fishes, including large sharks, are experiencing declines. The benefits of spatial management zones, such as marine protected areas (MPAs), for such animals are unclear. To help fill this knowledge gap, we examined core habitat use areas (CHUAs) for bull (Carcharhinus leucas), great hammerhead (Sphyrna mokarran) and tiger sharks (Galeocerdo cuvier) in relation to specific MPAs and exclusive economic zones (EEZs) in the western North Atlantic Ocean. Methods: Bull, great hammerhead and tiger sharks (N = 86 total) were satellite tagged and tracked in southern Florida and the northern Bahamas between 2010 and 2013. Filtered and regularized positions from Argos locations of tag transmissions were used to generate CHUAs for these sharks. Overlaps of CHUAs with regional protected areas and exclusive economic management zones were quantified to determine the proportion of each tracked shark’s CHUA under spatial protection from exploitation. Results: A total of 0%, 17.9% and 34.7% of the regional CHUAs for tracked bull, great hammerhead and tiger sharks, respectively, were fully protected from exploitation in the study area. Main conclusions: Expansion of protected areas to include U.S. territorial waters would effectively protect 100% of the CHUAs for all tracked sharks in the study area. This finding is particularly significant for great hammerhead sharks, which are currently overfished, vulnerable to bycatch mortality and are the focus of strident regional conservation efforts. These findings also provide a means to inform decision makers and marine conservation planning efforts as to the types of management actions available and potential efficacy of spatial protections for these marine predators.
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Significance Shark populations are declining worldwide because of overexploitation by fisheries with unknown consequences for ecosystems. Although the harvest of oceanic sharks remains largely unregulated, knowing precisely where they interact with fishing vessels will better aid their conservation. We satellite track six species of shark and two entire longline fishing vessel fleets across the North Atlantic over multiple years. Sharks actively select and aggregate in space-use “hotspots” characterized by thermal fronts and high productivity. However, longline fishing vessels also target these habitats and efficiently track shark movements seasonally, leading to an 80% spatial overlap. Areas of highest overlap between sharks and fishing vessels show persistence between years, suggesting current hotspots are at risk, and arguing for introduction of international catch limits.
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The distribution and interactions of aquatic organisms across space and time structure our marine, freshwater, and estuarine ecosystems. Over the past decade, technological advances in telemetry have transformed our ability to observe aquatic animal behavior and movement. These advances are now providing unprecedented ecological insights by connecting animal movements with measures of their physiology and environment. These developments are revolutionizing the scope and scale of questions that can be asked about the causes and consequences of movement and are redefining how we view and manage individuals, populations, and entire ecosystems. The next advance in aquatic telemetry will be the development of a global collaborative effort to facilitate infrastructure and data sharing and management over scales not previously possible. Copyright © 2015, American Association for the Advancement of Science.
Sharks are important living resources to human societies globally in cultural, economic, health, biodiversity, and conservation contexts. They can play a key role in the functioning of marine ecosystems and may provide higher trophic-level indication of ocean ecosystem state (Sims and Quayle 1998; Myers et al. 2007). However, rapid declines in some large pelagic sharks are occurring on a worldwide scale due to overfishing (Baum et al. 2003; Myers et al. 2007). There is particular concern that target and by-catch fisheries that have been well developed for at least the past century in the Atlantic Ocean, for example, are depleting shark populations below sustainable levels where recovery may not be possible, or at best may be very slow, even if fishing pressure is removed (Pauly et al. 2002; Clarke et al. 2006). Although some fisheries assessments indicate less pronounced declines for large pelagic (Sibert et al. 2006) and coastal sharks (Burgess et al. 2005), undoubtedly they are particularly susceptible to overharvesting on account of slow growth rates, late age at sexual maturity, and relatively low fecundity. During the twentieth century some large skate species were eliminated from areas where they were once very common and have not returned (Brander 1981; Casey and Myers 1998), suggesting large sharks, having at least some similarities in life history to skates, are also likely to be at risk of regional extinction (Chapter 17). Many pelagic sharks are now redlisted by the International Union for Conservation of Nature (IUCN), with some now at a fraction of their historical biomass (Dulvy et al. 2008). In the face of these apparently dramatic declines and the need for prompt action aimed at securing the future of species and populations, biologists have developed over the last 30 years or so numerous techniques for tracking and analyzing the movements of individual sharks in the natural environment. But how is this linked to broader issues in understanding animal ecology, and how can this help in applied settings, such as in shark fisheries management and species conservation?.
Advocates of Traditional Ecological Knowledge (TEK) have promoted its use in scientific research, impact assessment, and ecological understanding. While several examples illustrate the utility of applying TEK in these contexts, wider application of TEK-derived information remains elusive. In part, this is due to continued inertia in favor of established scientific practices and the need to describe TEK in Western scientific terms. In part, it is also due to the difficulty of accessing TEK, which is rarely written down and must in most cases be documented as a project on its own prior to its incorporation into another scientific undertaking. This formidable practical obstacle is exacerbated by the need to use social science methods to gather biological data, so that TEK research and application becomes a multidisciplinary undertaking. By examining case studies involving bowhead whales, beluga whales, and herring, this paper describes some of the benefits of using TEK in scientific and management contexts. It also reviews some of the methods that are available to do so, including semi-directive interviews, questionnaires, facilitated workshops, and collaborative field projects.
Spatial and temporal records of 146 predatory attacks by white sharks (Carcharodon carcharias) on four species of pinnipeds, one bird, and one human at the South Farallon Islands, Central California, from late Aug. to early Dec. 1986-89 are presented. During each 3.5-mo period, attacks were (1) unevenly distributed in bouts separated by hiatuses in predation, (2) paired temporally within the same day, (3) at similar times and locations on consecutive days, and (4) all during daylight hours. Predation was observed most often within 450 m of shore, with a decrease in attack frequency with increasing depth. Within this high-risk zone, predation was concentrated near coastal departure and entry points of pinnipeds, and the predatory attack positions formed linear patterns leading away from the island. Consecutive predatory attacks were often near each other, yet at times alternated between localities on either side of the island.