Catching and tracking the world's largest Zambezi (bull) shark Carcharhinus leucas in the Breede Estuary, South Africa: The first 43 hours

Abstract

The Zambezi or bull shark Carcharhinus leucas is a large, predatory shark that occurs in warm-temperate, tropical and subtropical coastal and estuarine systems worldwide. To confirm reports of Zambezi sharks in the Breede Estuary on the south-west coast of South Africa, a survey was undertaken during 20-26 January 2009. On 24 January, a large female Zambezi shark was caught on rod and reel. Measuring 400 cm total length and 320 cm precaudal length, it is the largest recorded Zambezi shark. Furthermore, its occurrence in the Breede Estuary is the southernmost record of the species, extending its previously documented range by 366 km. The shark was tagged with a continuous acoustic tag and tracked for 43 consecutive hours. During that period, it swam as far as 20 km upstream, but it also briefly exited the estuary and travelled 2 km out to sea. Most of the time (24%) was spent 11-13 km upstream, where it actively inspected boats and shore-anglers, a behaviour considered to be an opportunistic foraging strategy. Estuaries appear to represent critical habitats in the life history of Zambezi sharks.

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African Journal of Marine Science 2009, 31(1): 107–111
Printed in South Africa — All rights reserved
Copyright © NISC (Pty) Ltd
AFRICAN JOURNAL OF
MARINE SCIENCE
ISSN 1814–232X EISSN 1814–2338
doi: 10.2989/AJMS.2009.31.1.11.782
Short Communication
Catching and tracking the world’s largest Zambezi (bull) shark Carcharhinus
leucas in the Breede Estuary, South Africa: the first 43 hours
ME McCord1* and SJ Lamberth2,3
1 South African Shark Conservancy, Old Harbour Museum, Hermanus 7200, South Africa
2 Marine and Coastal Management, Department of Environmental Affairs and Tourism, Private Bag X2, Rogge Bay 8012,
South Africa
3 South African Institute for Aquatic Biodiversity (SAIAB), Private Bag 1015, Grahamstown 6140, South Africa
* Corresponding author, e-mail: meag@sharkconservancy.org
Manuscript received February 2009; accepted March 2009
The Zambezi or bull shark Carcharhinus leucas is a large, predatory shark that occurs in warm-
temperate, tropical and subtropical coastal and estuarine systems worldwide. To confirm reports of
Zambezi sharks in the Breede Estuary on the south-west coast of South Africa, a survey was undertaken
during 20–26 January 2009. On 24 January, a large female Zambezi shark was caught on rod and reel.
Measuring 400 cm total length and 320 cm precaudal length, it is the largest recorded Zambezi shark.
Furthermore, its occurrence in the Breede Estuary is the southernmost record of the species, extending
its previously documented range by 366 km. The shark was tagged with a continuous acoustic tag and
tracked for 43 consecutive hours. During that period, it swam as far as 20 km upstream, but it also briefly
exited the estuary and travelled 2 km out to sea. Most of the time (24%) was spent 11–13 km upstream,
where it actively inspected boats and shore-anglers, a behaviour considered to be an opportunistic
foraging strategy. Estuaries appear to represent critical habitats in the life history of Zambezi sharks.
Keywords: acoustic tracking, Carcharhinus leucas, estuary, freshwater, largest size recorded, nursery
Introduction
Zambezi (bull) sharks Carcharhinus leucas are large
carcharhinid sharks that commonly occur in the turbid
waters of lagoons, river mouths and shallow coastal
waters in tropical and subtropical seas (Sadowsky 1971,
Snelson et al. 1984, Compagno et al. 2005, Simpfendorfer
et al. 2005). Spending much of their time in estuaries, it
is the only shark species that readily occurs in both salt-
and freshwater. Zambezi sharks can breed in freshwater,
although most breeding likely occurs in the high reaches
of warm-water estuaries and rivers (Montoya and Thorson
1982, Cliff and Dudley 1991, Compagno et al. 2005, Pillans
2006).
In South Africa, distribution records indicate that the
species occurs from the Moçambican border to Cape St
Francis in the Eastern Cape (Compagno et al. 1989).
Recently, however, increased anecdotal reports of Zambezi
sharks in both the southern Eastern Cape and the Western
Cape provinces, particularly in the Breede Estuary, suggest
a possible southward shift in their distribution and/or a
re-establishment of populations that existed beyond collec-
tive memory.
According to Whitfield (1996, 1998), the Zambezi shark
is partially estuarine-dependent, using estuaries as pupping
and nursery grounds. As such, they would be particularly
susceptible to habitat degradation, alteration and environ-
mental change, and that ever-increasing human utilisation
and concurrent degradation of estuaries will likely result in
population declines. Zambezi sharks are currently targeted or
taken as bycatch in several South African fisheries, including
the drum-line fisheries, pelagic and demersal longline
fisheries, the recreational shark fishery and the handline
fishery, and they are commonly caught in shark-protection
nets off KwaZulu-Natal (Dudley 2000).
Listed as Near-Threatened on the IUCN Red List (IUCN
2008), Zambezi sharks are considered a species of global
conservation concern. The systematic removal of these
predators from either a coastal or estuarine system will
undoubtedly have negative cascade effects on the health
and integrity of these systems (Baum et al. 2003, Myers and
Worm 2003). Thus, it is imperative that further research into
the ecology and life history of this species, and the role of
estuaries in sustaining populations, be conducted.

McCord and Lamberth
108
To determine whether reports of Zambezi sharks in the
Breede Estuary could be confirmed, a research expedition
was undertaken, with a secondary objective of acoustically
tagging an individual to examine its movement patterns in
the estuary.
Materials and methods
Study area
The Breede River is 322 km long with a draining catchment
area of 12 600 km2. It enters the Indian Ocean in Sebastian
Bay and falls within the warm-temperate Agulhas biogeo-
graphical region (Figure 1) (Emmanuel et al. 1992). The
total surface area of the estuary is approximately 455 ha and
extends 50 km upstream to a series of rocky sills where tidal
influence ceases (Taljaard et al. 2001). The estuary falls
within the winter/bimodal rainfall transition zone, but most of
the catchment falls in the winter rainfall area and flows are
strongly seasonal with high flows and floods during winter.
However, the combination of the two rainfall zones adds
to physical variability, ultimately accounting for multiple
behavioural signals and the high diversity of the ichthyo-
fauna in the estuary (Harrison 1999, Lamberth et al. 2008).
The Breede Estuary is permanently open, has a mean depth
of 4.6 m, and high flows and strong currents are able to
completely flush and reset the system within a single tidal
cycle (Taljaard et al. 2001).
Shark capture and tracking
Attempts to capture a Zambezi shark in the Breede Estuary
were conducted during 20–26 January 2009. Fishing
methods included handline and rod-and-reel using live
spotted grunter Pomadasys commersonnii and dusky kob
Argyrosomus japonicus as bait. Post-capture stress was
minimised by using tow straps around the torso and caudal
areas to reduce pressure on the spinal column and body
cavities. Body measurements taken were total length (TL,
cm), precaudal length (PCL, cm) and interdorsal length
(IDL, cm). Genetic samples were taken and stored in
alcohol for later processing. Sex and stage of maturity were
also recorded. The shark was injected with oxytetracycline
for age validation purposes.
Three types of tags were used; a conventional plastic dart
tag of 14 cm long, which was inserted with a 4 mm outside
diameter applicator needle (Hallprint), a VEMCO-coded
acoustic transmitter (69 kHz R69K), and a VEMCO contin-
uous acoustic transmitter (V16 54 kHz), anchored with
umbrella darts (VEMCO — Division of AMIRIX Systems Inc.,
Halifax, Nova Scotia). All tags were inserted into the dorsal
muscle of the first dorsal fin. An antibiotic (propolis) was used
to wash each tag insertion to prevent possible infection.
Three tracking teams working in 4 h shifts continuously
tracked the Zambezi shark using a VEMCO VR100 acoustic
receiver, beginning at 15:30 on 24 January and ending at
10:00 on 26 January. Positions at the start and end of each
tracking shift were marked on a Garmin handheld GPS. To
ensure integrity of the track, GPS positions were manually
logged every 5–10 min on the VR100. Changes in shark
behaviour near boats, around fishing activities and at tidal
changes were noted throughout the 43 h tracking period.
Temperature, salinity, turbidity and oxygen data were
collected at 18 sites along the length of the estuary, from the
mouth to 35 km upstream, using a YSI 6600 Multi-parameter
Environmental Monitoring System. The ‘salinity run’ started
an hour before high tide on 25 January 2009 in order to
obtain a measure of maximum salinity penetration into the
system. The run was completed once water column salinity
stabilised at 1.0.
Results
The catch
The Zambezi shark (Figure 2) was hooked 5.2 km upriver
at 13:30 on 24 January using rod and reel with live spotted
grunter as bait. It was landed 7.7 km upriver at around 14:50.
The shark measured 400 cm TL and 320 cm PCL with an IDL
of 85 cm. The weight was estimated to be in excess of 600 kg,
using a length–weight relationship provided by Wintner et al.
(2002). However, given the large size of the shark, this derived
weight fell well outside the upper range of the curve and thus
is considered a rough estimate. The shark was female and
appeared pregnant due to the distension of its abdomen.
The capture of a Zambezi shark in the Breede Estuary
represents a range extension south-westwards of 366 km
from Cape St Francis in the Eastern Cape, where it was
previously documented (Compagno et al. 2005). However,
anecdotal and photographic evidence shows that the
presence of this species in the Breede Estuary dates back to
at least the 1960s.
Tracking
A total of 205 GPS positions were manually logged on
the VR100 receiver when signal strength indicated the
presence of the shark close to the observation boat. During
the first five hours of tracking (15:00–20:00), the shark
appeared to be resting against the edge of a mudbank
drop-off, spending most of its time within 500 m of where
it was released (Figure 3). From 22:00, it started to move
Mzimvubu
Estuary
Cape St Francis
34° S
32° S
20° E 24° E 28° E 32° E
St Lucia
Estuary
KwaZulu-
Natal
Western Cape
Eastern Cape
Breede
Estuary
AFRICA
South
Africa
INDIAN OCEAN
Figure 1: Map of South Africa showing the Breede Estuary and
other localities mentioned in the text

African Journal of Marine Science 2009, 31(1): 107–111 109
.
Figure 2: The Zambezi shark that was caught, tagged and released with acoustic tags in the Breede Estuary on 24 January 2009 (photo:
Alison Towner)
Figure 3: Google Earth image of the Breede River showing the track of the tagged Zambezi shark over a 43 h period during 24–26 January
2009. The positions shown (white dots) are those logged at 15 min intervals. Date, time and direction of major movements are indicated by
grey arrows. The hooked and landed positions are shown
St Sebastian
Bay

McCord and Lamberth
110
slowly downstream, inspecting some fishing boats on the
way. At 3:00 the following morning (25 January), it exited
the mouth and swam approximately 2 km out to sea —
it was then 15 km from its release position. At 06:00, the
shark returned to the estuary and spent the next 4 h resting
just inside the surf-zone adjacent to the entrance channel.
From 09:00 to 10:00, it inspected a group of shore-anglers
and cast-netters at the mouth of the estuary, after which it
made a 5 km burst upstream. From there, it swam slowly
farther upstream and spent most of the next 24 h moving
among boats and shore-anglers, possibly in anticipation of
easy prey. The shark spent 24% of its time between 11 km
and 13 km upstream (Figure 3).
The estuary was well mixed at the time of the study, with
barely discernible vertical stratification. Saltwater intrusion
extended 35 km upstream at high tide, but throughout the
tracking period the shark remained within the 15–35 salinity
ranges in the lower 20 km of the estuary. Water tempera-
tures were fairly high, ranging from 20 °C at the mouth to
24 °C upstream. Turbidity at the mouth was low at 5 NTU
but increased to >20 NTU in the upper reaches. The shark
remained within the 10–15 NTU range throughout most of
the daylight hours and forays into the clearer, lower reaches
of the estuary were almost exclusively confined to the night
and early morning. The water column was well oxygenated
throughout the estuary. All physico-chemical parameters
measured were typical of the Breede system at the time of
sampling (Lamberth et al. 2008).
Behavioural notes
Although the shark’s behaviour was not quantified during
this exploratory survey, descriptive notes on its behaviour
near boats and around fishing activities were recorded
during the night when such activities peaked.
On 24 January, boat-based night fishing was concen-
trated between 11 km and 13 km upstream, where the shark
appeared to spend the majority of its time. From 19:40 to
21:00, the shark swam between three boats that were fishing
for grunter and kob. It explored an area beneath each vessel
for several minutes before moving to another boat.
During the final tracking shift (06:00–10:00 on 26 January),
the shark moved from 11 km to 15 km upriver, following
boat-based fishing activities. It moved between seven
boats, all with fishing lines in the water, spending several
minutes near each one before moving away. At 07:23, the
shark moved toward a boat while it pulled anchor and began
following the boat when it trolled downstream with four
rapalla fishing lures in the water. The shark spent approxi-
mately 35 min following the trolling boat.
At 08:00 on 26 January, as tidal flow slacked (low tide
was at 09:20) and the majority of fishing boats returned
to shore, the shark moved into the shallower bank waters
of the river and appeared to drift downstream (at a rate of
approx. 3.5 km h–1), during which time it inspected shore-
and boat-anglers for a few minutes at a time, and frequently
entering shallow waters <1.5 m deep.
Discussion
Zambezi sharks commonly occur in shallow coastal waters
and estuaries of tropical and subtropical seas throughout the
world (Compagno et al. 2005, Pillans 2006). In South Africa,
they are found in several estuaries and coastal waters from
KwaZulu-Natal to Cape St Francis in the Eastern Cape
(Compagno et al. 1989). This study is the first confirmed
record of Zambezi sharks in the Breede Estuary and
represents the southernmost occurrence for the species,
extending its previously known range by 366 km.
Anecdotal reports and photographs indicate that Zambezi
sharks inhabited the Breede Estuary in the 1960s and 1970s.
A notable reduction in anecdotal reports from the late 1970s
to early 2000s suggests that direct hunting of these sharks
during earlier years (as documented in the photographs)
may have decimated the population of these sharks in
that estuary. Increased reports of Zambezi sharks in the
Breede Estuary since 2003 may mark their local reappear-
ance. Possible reasons for this could be the degradation
and closure of other estuaries — and possible nursery and
breeding grounds — along the South African coastline (Cliff
and Dudley 1991, Turpie et al. 2002).
Previously thought to reach a maximum total length
of 340–350 cm (Bass et al. 1973, Cliff and Dudley 1991,
Dudley 2000, Compagno et al. 2005), the Zambezi shark
under study represents a 50–60 cm increase in the
maximum known size of the species.
The shark spent most of its time between 5 km and
11 km upstream, where boat-based fishing activities were
most concentrated. This may represent conditioning to
these activities, which could result in lower levels of energy
expenditure while foraging. The shark appeared to move with
the tidal flow, swimming down the estuary on slack and low
tide and up the estuary during high tide. Similar movements
have been shown for estuarine fish, whereby they follow
the tidal cycle to minimise energetic output (Ackerman et al.
2000, Heupel et al. 2006, Collins et al. 2007, Childs et al.
2008). Alternatively, the shark may have been following prey
such as grunter and kob, which exhibit synchronised tidal
movements (Cowley et al. 2008). This behavioural adapta-
tion may also allow the shark to opportunistically prey on fish
by either moving on or off the shallow intertidal areas during
the flood and ebb tides. Further tracking studies on this
Zambezi shark will examine its fine-scale movement patterns
and behaviour in the estuary.
As a partially estuarine-dependent elasmobranch species,
Zambezi sharks use estuaries as pupping and nursery
grounds (Whitfield 1998, Pillans 2006). The shark under
study appeared to be pregnant at the time of capture,
suggesting that the Breede Estuary may serve as a nursery
ground for this species. The species was previously thought
to pup only farther north, in the St Lucia Estuary in KwaZulu-
Natal (Bass et al. 1973, Cliff and Dudley 1991). Exploratory
surveys similarly to this study indicate that the Umzimvubu
Estuary, some 800 km north of the Breede Estuary (Figure 1),
may also be a (previously undocumented) nursery ground
for Zambezi sharks; juveniles bearing distinct umbilical scars
were captured in that estuary in February 2008 (S Wintner,
Natal Sharks Board, pers. comm.).
It is uncertain whether this record of a Zambezi shark
in the Breede Estuary represents a single or a population
range extension for the species. The Breede River has ideal
nursery conditions for Zambezi sharks: a permanently open
warm-temperate estuarine system, is highly productive, has

African Journal of Marine Science 2009, 31(1): 107–111 111
few potential predators, favourable salinity and temperature
gradients, and the estuary is relatively large (Whitfield 1996,
Pillans 2006, Wiley and Simpfendorfer 2007, Lamberth et
al. 2008).
Although the residency and abundance of these sharks in
the Breede Estuary is yet to be determined, the discovery
of the largest recorded Zambezi shark there indicates that
the estuary provides favourable habitat for a species listed
globally as Near Threatened (IUCN 2008). Future studies
should focus on the role of these vulnerable predators in the
Breede Estuary, including abundance, residency, population
dynamics and habitat utilisation.
Acknowledgements — We thank those who made the expedition
possible: Hennie Papenfuss and the Big Fish Safari team, Alison
Towner (Marine Dynamics), Stiaan Conradie, Nick Scholtz and Rupert
Bellairs (Lower Breede River Conservancy), Corne Erasmus, Ricardo
Williamson, Mike Meyer and Sarika Singh (Marine and Coastal
Management [MCM]), Warren Swanapoel and Steve Smuts (South
African Shark Conservancy), Alison Kock (Save Our Seas Shark
Centre), Lucy Scott (Agulhas Large Marine Ecosystem), Jean Duplessis
and Enrico Menezies (Cape Nature), Dave McCord and Mike Gray. Lara
van Niekerk (CSIR), Sven Kerwath and Rob Tarr (MCM) are thanked for
their contribution towards the data plots and illustrations. We acknowl-
edge the generous donations and sponsorship from White Shark
Projects, Dyer Island Conservation Trust, Gateway Fruit and Veg. City,
Van Blommestein Slaghuis, Quest Seafood Distributors, Hennie Visser,
BS Diver’s, and Hilary Smith and Tim Arnett (Mudlark Guestlodge). Paul
Cowley and Dave Ebert are thanked for their constructive reviews of the
manuscript.
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