Patterns of human–crocodile conflict in Queensland: a review of historical estuarine crocodile (Crocodylus porosus) management

Abstract

Context
In Queensland, the management of estuarine crocodiles (Crocodylus porosus) by the government is important for ensuring public safety, especially along the populated east coast, where there is a large human population.

Aims
The present study aimed to determine historical, temporal and spatial patterns of human–crocodile conflict in Queensland.

Methods
The study used Queensland Government records of estuarine crocodile attacks (1971–2015), sightings by the general public (2003–2015), and removals and relocations for management purposes (1985–2015) to develop General Linear Models describing historical, temporal and spatial patterns.

Key results
The highest number of attacks, sightings, removals and relocations occurred along the populated east coast between Townsville and the Daintree during wet season months (November–February). There have been 35 crocodile attacks in Queensland since 1971 (total 0.8 per year; fatal 0.3 per year), mostly involving local people or regular visitors (77.1%), specifically adult males (71.4%; mean age 44). There has been an increase in the rate of crocodile attacks over time, with an average of 1.3 per year since 1996, most of which were non-fatal (84%). The number of crocodile sightings has been increasing annually (with a mean of 348 per year since 2011), while the number of crocodiles removed or relocated for management purposes (n = 608) has fluctuating widely each year (range 1–57).

Conclusions
The level of human–crocodile conflict in Queensland is increasing, and this is likely to be a consequence of increasing human and crocodile populations. While conflict is highest during the wet season, estuarine crocodiles pose a threat to public safety year round.

Implications
With the increase in conflict, the ongoing management of estuarine crocodiles, through targeted removals in and around areas of higher human habitation and through education, is essential for ensuring public safety into the future.

Full text

Patterns of human–crocodile conflict in Queensland: a review
of historical estuarine crocodile (Crocodylus porosus) management
M. L. Brien
A,D
, C. M. Gienger
B
, C. A. Browne
A
, M. A. Read
C
, M. J. Joyce
A
and S. Sullivan
A
A
Department of Environment and Heritage Protection, PO Box 375, Garbutt East LPO, Qld 4870, Australia.
B
Department of Biology and Center of Excellence for Field Biology, Austin Peay State University,
Clarksville, Tennessee 37044, USA.
C
Great Barrier Reef Marine Park Authority, PO Box 1379, Townsville, Qld 4810, Australia.
D
Corresponding author. Email: matt.brien@ehp.qld.gov.au
Abstract
Context. In Queensland, the management of estuarine crocodiles (Crocodylus porosus) by the government is important
for ensuring public safety, especially along the populated east coast, where there is a large human population.
Aims. The present study aimed to determine historical, temporal and spatial patterns of human–crocodile conflict in
Queensland.
Methods. The study used Queensland Government records of estuarine crocodile attacks (1971–2015), sightings by the
general public (2003–2015), and removals and relocations for management purposes (1985–2015) to develop General
Linear Models describing historical, temporal and spatial patterns.
Key results. The highest number of attacks, sightings, removals and relocations occurred along the populated east coast
between Townsville and the Daintree during wet season months (November–February). There have been 35 crocodile
attacks in Queensland since 1971 (total 0.8 per year; fatal 0.3 per year), mostly involving local people or regular visitors
(77.1%), specifically adult males (71.4%; mean age 44). There has been an increase in the rate of crocodile attacks over
time, with an average of 1.3 per year since 1996, most of which were non-fatal (84%). The number of crocodile sightings
has been increasing annually (with a mean of 348 per year since 2011), while the number of crocodiles removed or relocated
for management purposes (n= 608) has fluctuating widely each year (range 1–57).
Conclusions. The level of human–crocodile conflict in Queensland is increasing, and this is likely to be a consequence of
increasing human and crocodile populations. While conflict is highest during the wet season, estuarine crocodiles pose a
threat to public safety year round.
Implications. With the increase in conflict, the ongoing management of estuarine crocodiles, through targeted
removals in and around areas of higher human habitation and through education, is essential for ensuring public safety
into the future.
Additional keywords: attacks, removals, sightings.
Received 9 February 2017, accepted 24 June 2017, published online 29 August 2017
Introduction
The estuarine crocodile (Crocodylus porosus) is the largest (up to
6 m) and most widely distributed crocodilian, and is found in
coastal waterways from Sri Lanka in the west, throughout south-
east Asia to the Caroline Islands in the east, and down to northern
Australia (Webb and Manolis 1989). This species is considered
the most territorial and least tolerant of conspecifics (Lang
1987; Brien et al.2013a), and along with the Nile crocodile
(C. niloticus), accounts for the highest number of crocodilian
attacks on humans each year (Caldicott et al.2005; CrocBITE –
The Worldwide Crocodilian Attack Data-Base, available at
http://www.crocodile-attack.info, accessed 12 September 2015).
As such, effective management of estuarine crocodile populations
is critical in places where range and habitat use overlap.
The worldwide status of estuarine crocodiles is listed as Low
Risk and of Least Concern (2009 IUCN Red List), with large
stable populations in Australia, Papua New Guinea and Indonesia
(Webb et al.2010). In Australia, populations in the Northern
Territory and Western Australia are large and increasing, while
recovery in Queensland has been more modest and patchy
since implementation of protective measures (Webb et al.
2010). Densities in Queensland are generally low but variable
across the state when compared with the Northern Territory,
with the highest densities in rivers along north-west Cape York
Peninsula and Lakefield National Park (Taplin 1987; Read et al.
2004).
Estuarine crocodiles are found in coastal waterways and
associated wetlands throughout north Queensland, including
CSIRO PUBLISHING
Wildlife Research,2017, 44, 281–290
https://doi.org/10.1071/WR17011
Journal compilation CSIRO 2017 www.publish.csiro.au/journals/wr

offshore islands and up to 100 km inland (Taplin 1987). Breeding
populations are known from the Fitzroy River, south of
Rockhampton, on the east coast, north to the Torres Strait
and into the Gulf of Carpentaria (Taplin 1987; Messel et al.
1981). However, individuals are also known in waterways as
far south as the Mary River on the east coast, and are not
uncommon on offshore islands (Miller and Bell 1997). Along
the populated east coast, significant habitat alteration for
agriculture and urban development has occurred, and it is here
where most human–crocodile conflict occurs (Taplin 1987;
Kofron 2004).
Management of estuarine crocodiles in Queensland is the
responsibility of the Government (currently Department of
Environment and Heritage Protection) under the Nature
Conservation Act of 1992 and currently the Nature Conservation
(Estuarine Crocodile)Plan of 2007. Historically, unregulated
hunting of estuarine crocodiles occurred until they were
protected in 1974 (Taplin 1987), with the species currently
state listed as Vulnerable, due to the impacts of threatening
processes such as urban and rural development. Estuarine
crocodiles that pose a threat to human safety have been
captured and removed by the Government since the mid-late
1980s. However, crocodile management policies in Queensland
have changed over time in response to both changes of
government and community and political pressure resulting
from increased crocodile sightings and/or attacks.
In this study, we reviewed historical records of estuarine
crocodile attacks, sightings, removals and relocations for
management purposes to determine patterns of human–crocodile
conflict in Queensland. The main objective was to determine the
effect of time, location and season on the frequency of estuarine
crocodile attacks, sightings, removals and relocations to inform
management programs.
Materials and methods
Distribution and habitat
The study area included all coastal waterways estuarine
crocodiles are known to inhabit in Queensland, from the Mary
River near Bundaberg on the south-east coast (2530043.1900S
15245028.5000E), north to the tip of Cape York Peninsula,
including the Torres Strait (1011057.0600S 1421602.1400 E),
and down through the Gulf of Carpentaria to the Northern
Territory border (1632033.3000S137
59041.0000E), including all
offshore islands (Fig. 1). However, breeding populations are
only known as far south on the east coast as the Fitzroy River
near Rockhampton (Taplin 1987; Fig. 1). For the purposes of
analysis, the coast of Queensland north of the Mary River
Zone 1
Zone 10
Zone 11
Zone 3
Zone 2
Zone 4
Zone 5
Zone 6
Zone 7
Zone 8
Zone 9
Torres Strait
Wenlock R.
Mitchell
R.
Norman
R.
Gulf of
Carpentaria
Weipa
Escape R.
Lockhart R.
Mackay
Boyne R.
Endeavour R.
Daintree R.
Barron R.
Russel R.
Johnstone R.
Hull R.
Proserpine R.
Fitzroy R.
0250 500 Km
N
Rockhampton
Bundaberg
Brisbane
Lakefield Id National Park
Cooktown
South Pacific Ocean
Cairns
Townsville
Breeding distribution
Fig. 1. Breeding distribution of estuarine crocodiles in Queensland (Taplin 1987). The state is
divided into 11 broadly defined management zones.
282 Wildlife Research M. L. Brien et al.

around to the Northern Territory border was divided into 11
broadly defined management areas, based on the main estuarial
river systems and associated urban centres (Fig. 1). Historically,
this is also how the coastline has broadly been delineated
with regards to crocodile management. These distances refer to
approximate linear coastline and do not include lengths of inland
waterways. However, management of crocodiles extends inland,
generally up to 20 m above sea level where the majority of the
crocodile populations exist.
Area 1: Wide Bay (Fraser Island,Mary River, Maryborough,
Great Sandy Strait, Hervey Bay, Bundaberg) 150 km.
Area 2: Fitzroy (Benaraby, Tannum Sands, Gladstone,
Rockhampton, Emu Park, Yeppoon, Byfield) 140 km.
Area 3: Mackay (Carmila, Sarina, Mackay, Eimeo, Seaforth,
Lethebrook, Proserpine, Airlie Beach, Bowen) 330 km.
Area 4: Townsville (Home Hill, Giru, Barrata Creek,
Alligator Creek, Magnetic Island, Ross River, Bohle
River, Bushland beach, Toolakea, Bluewater, Toomulla,
Balgal Beach) 200 km.
Area 5: Hinchinbrook (Forrest Beach, Taylors Beach,
Halifax, Macknade, Lucinda, Hinchinbrook, Cardwell,
Edmund Kennedy NP) 80 km.
Area 6: Innisfail (Tully, Mission Beach, Wongaling, Bingil
bay, Garners Beach, Kurrimine Beach, Cowley Beach,
Mourilyan, Etty Bay, Johnstone, Flying Fish Point) 70 km.
Area 7: Cairns (Bramston Beach, Eubanangee Swamp,
Babinda, Deeral, Russel/Mulgrave, Edmonton, Cairns,
Northern beaches, Ellis Beach) 100 km.
Area 8: Port Douglas (Mowbray, Port Douglas, Cooya
Beach, Newell Beach, Wonga beach, Mossman, Daintree,
Cow Bay) 50 km.
Area 9: Cape Tribulation-Cooktown (Cape Tribulation,
Wujal, Bloomfield, Archer Point, Cooktown, Hopevale)
80 km.
Area 10: East Cape-Torres Strait (Hopevale-Crystal Creek
including the Torres Strait) 1100 km.
Area 11: WestCape-Gulf (SkardonRiver-NorthernTerritory
border) 1000 km .
Within the broad latitudinal range that encompasses
estuarine crocodile habitat in Queensland, eight separate
bioregions have been identified (Taplin 1987). These include
the low-lying, arid Gulf Plains (Southern and Northern), hot
and wet Cape York Peninsula (North-west and North-east),
large wetland systems of Lakefield National Park, coastal
fringes of the East Coast Plains, from the tropical northern
reaches (Cape Melville to Cooktown and Cooktown to Ayr),
to the sub-tropical and arid areas in the south (Ayr–Gladstone).
All of these regions are characterised by a winter dry season
(April–October), defined by low rainfall and cooler temperatures
and a summer wet season (November–March), during which
conditions are hot, humid and most precipitation occurs. Annual
rainfall is highly variable, ranging from 700–1000 mm in parts
of the south-western Gulf to 3000–4000 mm in tropical areas
of the East Coast Plains (e.g. Innisfail). The summer wet
season also corresponds with the breeding season for
C. porosus, with nesting triggered by large rainfall events
(Webb et al.1977).
Human population
Although there are large tracts of pristine coastal habitat north
of Cooktown and around the Cape into the Gulf, there has been
intensive urban and agricultural (e.g. sugarcane) development
along the East Coast Plains south of Cooktown. According to the
Queensland Government Statisticians Office (http://statistics.
qgso.qld.gov.au/qld-regional-profiles?; accessed 30 June 2016),
populations in the major local government areas along the East
Coast Plains in 2015 are estimated as (from south to north):
Bundaberg (94 453); Rockhampton (81 589); Mackay (117 703);
Townsville (192 058); Hinchinbrook (10 990); Cassowary Coast
(29 396); Cairns (162 451); Douglas (11 997); and Cook (4 424).
A few studies have suggested that human disturbance in
catchment areas may be the dominant factor limiting crocodile
population recovery along the east coast (Taplin 1987; Kofron
and Smith 2001; Read et al.2004). However, Fukuda et al.(2007)
concluded that habitat availability was the dominant influence on
successful recovery of the species in Queensland from the impact
of commercial hunting (protection began in 1974). Queensland
essentially has far less suitable nesting habitat available for
estuarine crocodiles than the Northern Territory (Fukuda et al.
2007).
Cultural attitudes to crocodiles in Queensland vary among
Aboriginal and Torres Strait Islander groups, with some
traditionally hunting crocodiles and their eggs for food while
others regard the crocodile as a totem animal or a spirit of
ancestors. The rights of indigenous people to continue to hunt
or use estuarine crocodiles as they have traditionally done so are
recognised. With regards to managing problem crocodiles,
cultural beliefs are taken into consideration when making
decisions. For example, if a problem crocodile is captured that
is larger than 4 m, it is recognised as an ‘icon’crocodile and the
department is required to formally consult with the traditional
owners and enter into an agreement as to the fate of the animal.
Estuarine crocodile population
The population of estuarine crocodiles in Queensland has
increased since protection in 1974, but this increase has been
lower than in the Northern Territory and Western Australia
(Taplin 1987;Readet al.2004). The most recent comprehensive
survey of the state occurred 1994–2000 and included 103
waterways (4174.3 km) between Gladstone, Rockhampton and
the Northern Territory border (Read et al.2004). The results
indicated that crocodile numbers were generally low and highly
variable across the state, with 91% of all animals sighted being
less than the minimum breeding size for the species (Read et al.
2004). The population had undergone a limited recovery, with
the highest numbers recorded from waterways in north-west
Cape York Peninsula and Lakefield National Park, and low
densities along the populated east coast. However, trends in
population abundance and biomass since these surveys in the
late 1990s and early 2000s are not well understood due to a lack
of survey data. Freshwater crocodiles (C. johnstoni) also inhabit
many of the same river systems as C. porosus in Queensland,
although they are often found further inland (Taplin 1987),
and considered a consequence of interspecific aggression with
C. porosus (Brien et al.2013b). Freshwater crocodiles were not
Estuarine crocodile management in Queensland Wildlife Research 283

included in this study. Due to their smaller size, they pose little
threat to humans, with very few attacks recorded.
History of estuarine crocodile management
Crocodile management in Queensland has generally involved
the removal of crocodiles (>2 m) deemed to be a threat to safety
of humans, livestock or working dogs. Crocodiles have been
captured using either a trap (floating, gate) or with a harpoon, and
either removed and placed at a suitable facility (farm, zoo) for
breeding stock, or translocated to other sections of the river or
to other systems such as Lakefield National Park (1995–97).
However, this practice was discontinued in 1999 because
estuarine crocodiles are known to ‘home’back to their capture
site, often travelling long distances (Walsh and Whitehead 1993;
Kay 2004; Read et al.2007). Intensive removal programs
have also been in place along the populated east coast from:
1987–1991; 1998–2006; and 2012–15.
In late 1987, the East Coast Crocodile Management
Program (ECCMP: 1987–1991) was introduced state-wide in
response to community concerns that there had been an increase
in crocodile sightings, along with several attacks (1980–1986:
five fatal, one non-fatal). Three zones were established that
dictated the type of management response: (1) all crocodiles
removed; (2) all crocodiles >1.2 m removed; and (3) removed if
posing a threat. During this period private contractors were also
called on by the government to remove crocodiles in designated
areas.
In 1998, the Trial Intense Management Area for Crocodiles
(TIMAC) (Read et al.2004) was established in the Cairns region
(Trinity Inlet north to Wonga beach), again in response to
community concerns about an increase in crocodile sightings,
along with several attacks (1993–97: one fatal, two non-fatal).
All crocodiles and crocodile eggs found in the designated
TIMAC waterways were removed during the trial (1998–2001).
The plan also included the removal of all crocodiles from areas
of the Russell, Mulgrave, North Johnstone and South Johnstone
rivers west of the Bruce Highway. In 2001, the program was
extended as the Intensive Management Area for Crocodiles
(IMAC) program (2001–06). The ‘Croc-wise’educational
campaign was also introduced by the Government in 2001,
which involved the delivery of educational talks to the public,
with a focus on schools.
In 2007, the Nature Conservation (Estuarine Crocodile)
Conservation Plan 2007 and Management program 2007–17
came into effect. In 2009, it was also decided that any
crocodiles south of the Boyne River should be removed. In
2011, the Government began reporting all crocodile sightings
and declared ‘crocodiles of concern’on its website (www.ehp.
qld.gov.au), which was regularly updated. In 2012, Crocodile
Urban Management Areas (CUMAs) were introduced in
Gladstone, Mackay, and Rockhampton, in which all crocodiles
>2 m in length were to be removed. In 2013, trial Crocodile
Management Plans (CMPs) were introduced for the
Cairns, Townsville, Hinchinbrook and Cassowary Coast local
government areas. These areas were chosen because they
experience a higher level of potential crocodile–human conflict
(sightings, removals, attacks) than other population centres on
the east coast, due to the larger size of both the human and
crocodile populations. The plans set out a risk-based approach
to crocodile management consisting of three different zones:
Zone 1: prevent crocodiles from entering the zone and
remove all crocodiles that enter into it; Zone 2: remove all
crocodiles 2 m or any crocodile displaying aggressive
behaviour; and Zone 3: remove crocodiles of concern.
This current approach to crocodile management in
Queensland was largely derived from the Northern Territory
model, in which crocodiles are more intensively managed and
actively targeted for removal in and around urban centres (e.g.
Darwin harbour). Along the populated east coast of Queensland
between Cairns and Rockhampton, crocodiles 2 m are targeted
for removal in and around several urban centres, with all
crocodiles proactively removed from the Cairns area. The
main difference between the Northern Territory and Queensland
is the larger human population located in areas with healthy
populations of crocodiles (Kofron 2004).
Data reduction and analyses
We used Queensland Government records to develop General
Linear Models describing historical, temporal and spatial patterns
of recorded crocodile attacks, sightings, removals and relocations
in Queensland. We assigned each record to one of 11 management
areas based on location of observation (Fig. 1) and collated data
for year, month, sex (if known), crocodile body size (estimated
from sightings, measured for captures), and crocodile attack
outcome (fatal or non-fatal attack). As time of day was difficult
to determine accurately for the majority of attacks, we did not
attempt to examine or report on it. In comparing sex ratios of
captured crocodiles we used chi-square tests. Recorded estuarine
crocodile attacks on humans (non-fatal and fatal) have been
reliably recorded by the government since 1971, and analyses
included all recorded attacks from 1971 to 2015. We also
confirmed records and cross-referenced information on attacks
from The Worldwide Crocodilian Attack Data Base, CrocBITE.
Estuarine crocodile sightings have been reported to the
government since the 1980s, but before 2003 reports were not
always filed and the facility for the public to report sightings
was limited. Therefore, only sighting data from 2003 to 2015
were included in the analyses. Crocodile removals (including
euthanised animals) and relocations have been recorded by the
government since 1985, and analyses included all data from
1985 to 2015. Minimum monthly air temperature data for
Cairns (Cairns AERO station 031011; 16.87S; 145.75E;
Bureau of Meteorology 2016) was compared with monthly
attack, sighting and capture data.
Results
Attacks
There have been 35 crocodile attacks recorded in Queensland
since 1971 (mean 0.8 per year), 12 of which were fatal (34.3%;
0.3 per year). In one instance, a second person was also injured
by the crocodile attempting to help the victim. Total length
was recorded for 24 of the 35 attacking crocodiles (68.6%),
with the mean size of crocodiles involved in fatal attacks
(4.1 0.22 m s.e.; range 2.7–5.0 m; n= 9) being significantly
larger (F
1,21
= 23.14; P= 0.0001) than those involved in non-
fatal attacks (2.5 0.21 m s.e.; range 1.2–4.2 m; n= 15; Fig. 2a).
284 Wildlife Research M. L. Brien et al.

Most attacks involved local people or people from north
Queensland who regularly visited the area (n= 27; 77.1%),
with two non-locals and six unknown. Adult males (mean age
44.2 3.33 s.e.; range 24–75; n= 25) were the most common
victims (71.4%), with four attacks on children (two males, two
females; ages 5–11), three on teenagers (all males; ages 15–19),
and three on adult females (ages 31–48; Fig. 2b). Only two
victims were identified as aboriginal, although race was not
identified for seven victims. Most victims were attacked in the
water or on the edge (88.6%), with two attacks on people in
canoes and two on people in tents. Most people were engaged in
some form of recreational activity at the time. Attacks occurred
between Innisfail on the east coast through to Normanton in the
Gulf. The highest number of attacks occurred in and around
rivers on the populated east coast between Cairns (n= 7; 1 fatal)
and the Port Douglas area (n= 6; 2 fatal; Fig. 3). Several attacks
also occurred in Weipa (n= 4; 1 fatal) and on offshore islands,
including the Torres Strait (n= 4; non-fatal; Mt Adolphus Island
2006, 2007, MacArthur Islands 1999 and Thursday Island
2004) and Lizard Island near Cooktown (n= 2; 2009, 2015).
There has been a significant increase (r
2
= 0.61; P<0.0001)
in the overall rate of crocodile attacks recorded over time with a
mean of 1.3 per year since 1996, most of which were non-fatal
(84%; Fig. 4). Prior to 1996, the mean rate of attack was 0.4
per year (1971–1995), with most attacks being fatal (80%; Fig. 4).
The mean size of crocodiles responsible for attacks has not
changed significantly over time (r
2
= 0.48; P= 0.47).
The distribution of attacks was not significantly different
across months (x
2
= 8.30; d.f. = 10; P= 0.60), but attacks were
more frequent December through February (Fig. 5a). The mean
size of crocodiles responsible for attacks was not different
across months of the year (F
10,12
= 0.88; P= 0.57).
Sightings
In total, 3419 crocodile sightings were reported to the
government from 2003 to 2015, with location recorded for
3223. Most sightings originated from the populated east coast
between Ayr and Cape Tribulation (Areas 4–8: 73.6%; Fig. 3).
The number of reports differed significantly among areas
(F
10,995
= 33.4; P<0.0001) and was highest for Cairns (Area
7: 888; 27.6%; 68 per year), Innisfail (Area 6: 428; 13.3%; 33 per
year), Port Douglas (Area 8: 394; 12.2%; 30 per year), Townsville
(Area 4: 371; 11.5%; 29 per year) and Hinchinbrook (Area 5:
290; 9.0%; 22 per year), with 90 reports from offshore islands
(Fig. 3).
Along the east coast, estuarine crocodiles have been reported
as far south as Inskip Point, south of Fraser Island (Fig. 3).
However, the southern-most sightings, confirmed by the
government, were from the Mary River, Maryborough (Fig. 3).
Some individuals have been reported up to 300 km inland near
Georgetown, and at elevations up to 200 m in the upper reaches
of the Burdekin and North Johnstone Rivers. However, the
highest confirmed sighting of an estuarine crocodile (~2 m) is
from Adeline Creek near the Daintree at 470 m above sea level
(169021.4000S 1454057.2500 E; 15 May 2012; C. Hoskins). To
travel to this location, the crocodile had to traverse several steep
sections of river, including waterfalls.
The number of crocodile sightings has been increasing
significantly each year (F
1,11
= 29.44; r
2
= 0.72; P= 0.0002),
with a mean of 347.6 12.6 s.e. since 2011, and a maximum
of 426 in 2015 (Fig. 6). Despite high variability, this trend was
consistent across most locations, with the greatest increase in
sightings occurring from around Cairns (Area 7).
Sightings have been reported throughout the year with
significant differences among months (F
11,995
=2.51; P=0.004).
Sightings were highest during wet season months (October–
February), when mean air temperatures were warmer, and
lowest during dry season months (May–August; Fig. 5b).
Removals and relocations
In total, 608 estuarine crocodiles have been captured and either
removed and placed at a facility (n= 552), relocated or released
in-situ (n= 46) or euthanised (n= 10) since 1985. Crocodiles
were captured from coastal areas from the Mary River in the
south along the populated east coast and Cape York through
to Normanton in the Gulf, including the Torres Strait (Fig. 2).
However, the number of crocodiles captured differed significantly
among areas (F
10,135
= 2.61; P= 0.006; location recorded for
597), with most captured from waterways between Hinchinbrook
and Port Douglas (Areas 5–8; 75%; Fig. 2). Crocodile captures
were highest for Cairns (Area 7: 181; 5.8 per year; 30.3%),
Innisfail (Area 6: 116; 3.7 per year; 19.4%), Hinchinbrook
(Area 5: 85; 2.7 per year; 14.2%), and Port Douglas (Area 8:
66; 2.1 per year; 11.1%; Fig. 2).
10
8
6
4
2
0
1–2.0
<13 14–19 20–30 31–40 41–50 51–60 >60
2.1–3.0 3.1–4.0 4.1–5.0
Fatal
Non-fatal
10
8
6
4
2
0
Number of crocodile attack victims
Total length of crocodile (metres)
(a)
(b)
A
g
e of victims (
y
ears)
Female
Male
Fig. 2. Size of estuarine crocodiles involved in attacks on people in
Queensland, and the age and sex of victims (1971–2015). The number of
victims according to (a) total length of estuarine crocodiles (metres) involved
in non-fatal and fatal attacks and (b) age and sex of victims of crocodile attack.
Estuarine crocodile management in Queensland Wildlife Research 285

A mean of 19.6 2.3 s.e. crocodiles have been captured
per year but this has been highly variable, fluctuating between
one per year (1993) up to 57 per year (2014; Fig. 7). However,
capture rates did not differ significantly among years
(F
30,115
= 0.87; P= 0.65).
The mean number of crocodiles removed and relocated did
not differ significantly across months of the year (F
11,325
= 0.84;
P= 0.60), but the highest numbers were recorded between
November–January (breeding or wet season), when mean air
temperatures were warmer, with fewer captures in July and
August (non-breeding or dry season) (Fig. 5c).
Of the total number of crocodiles captured (n= 608), sex was
determined for 455 individuals (74.8%), with a significantly
(x
2
= 20.12; P= 0.043) higher proportion of males captured
(75.6%). However, the percentage of crocodiles for which sex
was determined has declined over time from 95.3% (1985–
2003) to 50% (2004–15). The mean size of males captured
(mean 2948.6 46.94 mm TL; range 890–4800 mm TL; n= 344)
was significantly larger than for females captured (mean
2124.1 53.51 mm TL; range 310–3400 mm TL; n= 111;
F
1,362
= 48.71; P<0.0001). Of the crocodiles captured, 32.6%
of females were of breeding size (>2.3 m) (Webb and Manolis
1989), 39.2% of males were of breeding size (>3.3 m; Webb
and Manolis 1989) and only 13.1% were >4 m in length.
Discussion
Most recorded estuarine crocodile attacks, sightings and
captures for management purposes have occurred on the
populated east coast of Queensland between Townsville and
Zone 1
Zone 10
Zone 11
Zone 3
Zone 2
Zone 4
Zone 5
Zone 6
Zone 7
Zone 8
Zone 9
Torres Strait
Gulf of
Carpentaria
Weipa
Mackay
0250 500 Km
N
Rockhampton
Bundaberg
Brisbane
Cooktown
South Pacific Ocean
Cairns
Townsville
Fatal attacks
Non-fatal attacks
Captures
Sightings
Normanton
Fig. 3. Distribution of recorded estuarine crocodile attacks, sightings and removals and relocations
in Queensland. Attacks: 1971–2015, sightings: 2003–15, and removals and relocations: 1985–2015.
2.0 Fatal
Non-fatal
Regression line
1.6
1.2
0.8
0.4
0
1971–1975
1976–1980
1981–1985
1986–1990
1991–1995
1996–2000
2001–2005
2006–2010
2011–2015
Mean number of crocodile
attack victims
Year
Fig. 4. Mean number of crocodile attacks (fatal and non-fatal) recorded in
Queensland (1971–2015). Attacks are grouped into 5-year intervals.
Regression line refers to all attacks.
286 Wildlife Research M. L. Brien et al.

the Port Douglas area. These locations have large human
populations (e.g. Cairns: 160 285; Queensland Government
Statisticians Office 2016) in the middle of known crocodile
habitat, which is why management efforts have been focused
in these areas. In Florida (USA), a similar situation exists
where the highest number of complaints and bites from
American alligators (Alligator mississippiensis) occurs in
areas where the highest human populations exist (Woodward
et al.2014).
The highest number of recorded estuarine crocodile attacks,
sightings and captures occurred during the summer wet season
(November–February), which also corresponds with the breeding
season for estuarine crocodiles (Webb et al.1977; Webb and
Manolis 1989). During summer months, ambient temperatures
8
6
4
2
0
35
24
22
20
18
16
24
22
20
18
16
24
22
20
18
16
30
25
20
15
10
5
5
5678
Month
(a)
(b)
(c)
9101112
4
4
3
3
2
2
1
1
0
Number of crocodile
attack victims
Mean number of crocodile
sightings (±s.e.)
Mean min. air temperature (°C)
Mean number of crocodile
removals (±s.e.)
Fig. 5. Recorded estuarine crocodile attacks, sightings and removals and relocations throughout
the year in Queensland. Monthly estuarine crocodile (a) attacks (1971–2015), (b) mean sightings
(s.e.; 2003–15) and (c) mean captures (s.e.; 1985–2015) in Queensland compared with mean
minimum air temperature for Cairns, Queensland (Bureau of Meteorology 2016).
Estuarine crocodile management in Queensland Wildlife Research 287

are warmer, water levels are higher and individuals are dispersing
out onto flood plains hunting prey (Webb et al.1977; Webb and
Manolis 1989). It is also a time when adults tend to be more mobile
in pursuit of breeding partners, and can also become more
aggressive, especially females in defence of a nest or young
(Webb and Manolis 1989). During the summer wet season, people
are coincidentally frequenting waterways for the purposes of
cooling off, or for other recreational activities such as boating
and fishing, bringing them into closer proximity with crocodiles.
In Florida, alligator attacks also increase during the
warmer summer months when recreational water use is highest
(Woodward et al.2014), while alligator complaints and
removals in Louisiana (USA) also peak during spring–summer
and decline considerably in winter (Hines and Woodward 1980;
Boundy 2004). In South Africa and Swaziland, 90% of attacks
by Nile crocodiles have also occurred during the summer
breeding season (1949–2014; Pooley 2014). In the Northern
Territory the pattern is slightly different, with most crocodiles
captured and removed during the beginning (September–
December) and end (March–April) of the wet season, which is
also when most attacks occur (Fukuda et al. 2014).
There have been 35 crocodile attacks recorded in Queensland
since 1971 (0.8 per year), with 34.3% fatal (0.3 per year) and
the mean size of crocodiles responsible for fatal attacks (4.1 m)
larger than that for non-fatal (2.5 m). This proportion of fatalities
is consistent with what has been reported for C. porosus
throughout the rest of Australia, but much lower than has been
reported between 2007–14 in Timor-Leste (83% fatal), Papua
New Guinea (76% fatal) and Solomon Islands (73% fatal;
Manolis and Webb 2014). While this could be a consequence
of higher water use for essential daily activities (e.g. washing),
putting human residents at greater risk, it could also be that
non-fatal attacks are under-reported or undocumented in these
countries (Manolis and Webb 2014).
Most attacks in Queensland have involved local people or
people from north Queensland who regularly visited the area
(77.1%), specifically adult men (mean age 44), and almost all
occurred in the water or on the edge (88.6%). Local adult men
have also been the most common victims of estuarine crocodile
attacks recorded in the Northern Territory (Fukuda et al.2014),
and of alligator bites in Florida (81.8%) (Woodward et al.2014).
The predominance of local adult male victims in Australia and
the USA is likely related to higher recreational water use and
risk-taking behaviour. Males also account for the highest
number of drownings in Australia each year (~80%), mostly
between the ages of 18–54 (Royal Life Saving Society Australia
2014). In contrast, the most common victim of Nile crocodile
(C. niloticus) attacks in South Africa and Swaziland have been
children and teens <20 years old (1949–2014; Pooley 2014).
There has been an increase in the rate of crocodile attacks
recorded in Queensland over time, with an average of 1.3 per year
since 1996, most of which were non-fatal (84%). In the Northern
Territory, the rate of attacks has also increased over time,
particularly non-fatal cases (1971–2013: 18 fatal and 45
non-fatal) with an average of 1.5 per year (Fukuda et al.
2014). This increase in the Northern Territory has been related
2002
500
400
300
200
100
2004 2006 2008 2010 2012 2014 2016
Year
Number of crocodile sightings
Fig. 6. Estuarine crocodile sightings reported to the government each year
(2003–15).
60 Relocated
Removed
50
40
30
20
10
0
1985 1987 1989 1991 1993 1995 1997 1999 2001
Year
Number of crocodiles
2003 2005 2007 2009 2011 2013 2015
Fig. 7. Number of crocodiles removed (captured, euthanised) and relocated each year by the government
(1985–2015).
288 Wildlife Research M. L. Brien et al.

to the increasing crocodile and human populations, and to the size
of individual crocodiles capable of attack (Fukuda et al.2014).
In Queensland, while crocodile numbers are lower and the
rate of recovery slower than in the Northern Territory, the human
population is considerably larger and steadily increasing, with
human populations in the two largest cities in northern
Queensland (Townsville: 193 946; Cairns: 160 285) increasing
by 25% between 2005 and 2015 (Queensland Government
Statisticians Office 2016). People in both Queensland and the
Northern Territory are also increasingly able to access remote
areas for the purposes of recreation. Increases in the human
population, recreational water use and access to remote areas
have also been attributed to the recent increase in non-fatal shark
attacks in Australia (1990–2000: 6.5 per year; 2001–11: 15
per year; West 2011).
It is important to put the number of deaths by crocodiles in
Australia each year into perspective. Most recorded crocodile
attacks have occurred in the Northern Territory (63%), followed
by Queensland (24%) and Western Australia (13%), with a
national increase in non-fatal cases from 0.1 per year (1971–
1980) to 3.3 per year (2001–04) but no increase in fatal cases,
which have remained at 0.5 per year (Caldicott et al.2005). This
is relatively low when compared with the number of shark
attacks in Australia per year (15 per year; 1.1 per year fatal:
2001–10) (West 2011) and the number of deaths through beach
drownings in Australia each year (48 per year nationally; nine
per year in Queensland: 2003–13; Royal Life Saving Society
Australia 2014). Regardless, fear of being eaten tends to drive
public concerns and attitudes towards species such as crocodiles.
Confirmed sightings of estuarine crocodiles have been
reported as far south as the Mary River near Hervey Bay on
the east coast and as early as the 1990s, with unconfirmed
sightings further south in the Great Sandy Strait down to Tin
Can Bay (~50 km south). This area represents the southern extent
for the species, with historical records indicating crocodiles
have always been present here but in low numbers, probably
due to climate constraints.
The presence of large estuarine crocodiles (2–4 m) in upper
freshwater sections of rivers (up to 470 m above sea level)
along the north-east coast and on or near offshore islands,
where there have been six recorded attacks, is of particular
concern for management. These areas are considered marginal
habitat for crocodiles, but are commonly used by people for
swimming and other water-based recreational activities, some of
whom may be unaware that estuarine crocodiles can be present.
While upstream movements by estuarine crocodiles have
been linked to social pressure from an increasing crocodile
population in the Northern Territory (Letnic and Connors
2006), the population of estuarine crocodiles along the east
coast of Queensland remains low. Therefore, it is unclear why
some individuals venture this far upstream in Queensland (up
to 470 m above sea level) as it requires significant energy
expenditure to navigate the shallow, rocky and steep sections
of river, temperatures are cooler, food is limited and the clear
water can hinder hunting success. Estuarine crocodiles in the
Northern Territory have only been reported up to 126 m above
sea level (Letnic and Connors 2006).
The increase in crocodile sightings in Queensland over
time may indicate an increase in either human or crocodile
populations, or both. However, the situation is complicated
by several factors: improved reporting and data management
by the government; the location of the sighting; whether it is a
public place; the size of the crocodile reported; and the level
and type of media coverage. Sightings cannot, therefore, be
considered a good indicator of population size or change, and
as a consequence, it is not possible to draw any conclusions as
to whether there has been a change in crocodile numbers
across the state, whether more crocodiles are occurring on
offshore islands, or if the population is increasing or expanding
further south.
In total, 562 estuarine crocodiles have been removed
(captured, euthanised) from Queensland waterways for
management purposes since 1985, most coming from the
populated east coast. This is a relatively low number of
crocodiles removed each year (18 per year), which appears to
have been easily compensated by recruitment in most areas. In
comparison, 5792 estuarine crocodiles were removed from the
Northern Territory between 1977 and 2013 (Fukuda et al.
2014), ~161 per year, with most removed from Darwin
Harbour. This difference reflects the larger crocodile population
in the Northern Territory, which is considered to be at or above
carrying capacity (Fukuda et al.2014).
The majority of Queensland crocodiles removed between
1985 and 2015 were male (75.6%), and most of these were
less than breeding size (<3.3 m: 60.8%) with only 13.1%
larger than 4 m. Similarly, in the Northern Territory, 69% of
problem estuarine crocodiles removed were male (1977–2013;
Fukuda et al.2014), and in the USA, ~75% of problem
A. mississippiensis removals and attacks involved males (King
and Elsey 2014; Woodward et al.2014). Subadult male
crocodilians are often nomadic, highly mobile (Tucker et al.
1998; Campbell et al.2013) and more likely to come into
conflict with people. As female crocodilians are known to
mate with multiple partners and store sperm (Lewis et al.
2013), the removal of subadult males is unlikely to be a
significant threat to the overall breeding potential of most
crocodilian populations.
The aim of any management program involving estuarine
crocodiles is to strike a balance between conservation and
management. The overall purpose is to minimise the chance of
human–crocodile conflict while not negatively impacting
upon the crocodile population as a whole. This is achieved by
reducing the number of crocodiles in and around areas where
there are high human populations, while at the same time
preserving crocodile numbers in sparsely populated remote
areas, coupled with public education. The current program in
Queensland largely achieves this goal.
Conflicts of interest
The authors declare no conflicts of interest.
Acknowledgements
This study used data collected by the Queensland Government, or those
contracted by the government, for the purposes of estuarine crocodile
(Crocodylus porosus) management. All management activities were
authorised under the Nature Conservation Act 1992 and strictly adhered to
the Queensland Government’sCode of Practice –taking,handling,and
transportation of crocodiles (including previous versions). We would like to
Estuarine crocodile management in Queensland Wildlife Research 289

acknowledge the assistance of various government staff over the years in the
collection and entry of data, in particular Lucy Evans and Carly Williams.
Grahame Webb, Yusuke Fukuda, Laurence Taplin and Amanda O’Malley
provided valuable feedback on early drafts of the manuscript. We would also
like to thank the Department of Environment and Heritage for their support
and encouragement in producing this manuscript, and Conrad Hoskins for
providing the details of an estuarine crocodile at 470 m above sea level,
including a photograph.
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