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Elasmobranchs in the Fitzroy River, Western Australia
Report to the
D.C. Thorburn, D.L. Morgan, A.J. Rowland and H. Gill
Centre for Fish and Fisheries Research
February 2004
ISBN: 0 642 553343 Disclaimer The views and opinions expressed in this publication are those of the authors and do not necessarily reflect those of the Australian Government or the Minister for the Environment and Heritage.
TABLE OF CONTENTS
1 INTRODUCTION 1
1.1 Background 1
1.2 Conservation status and distribution of elasmobranchs in the
Fitzroy River 1
1.3 Current threats to elasmobranchs in the Fitzroy River 5
1.4 Objectives 6
2 METHODS 7
2.1 Sampling strategy 7
2.2 Sample sites 7
2.3 Elasmobranch migration 11
2.4 Interpretation of abundance data 11
2.5 Tagging 11
3 RESULTS 13
3.1 Overall catch composition 13
3.2 Catch composition of replicate sampling and abundance 13
3.3 Tagging Data 16
4 DISCUSSION 17
4.1 The current survey 17
4.2 Elasmobranch migration in the Fitzroy River 17
4.3 Freshwater sawfish of the west Kimberley 19
4.4 Further research 24
4.5 Recommendations 25
5 REFERENCES 27
6 APPENDIX 1 29
1 INTRODUCTION
1.1 Background
In Australia, only two rays, Pristis microdon (freshwater sawfish) and Himantura
chaophraya (freshwater whipray), and three sharks Carcharhinus leucas (bull shark),
Glyphis sp. A (speartooth shark) and Glyphis sp. C (northern river shark), occur in
oligohaline environments of the upper reaches of rivers far from the coast. While
information on the occurrence, biology and distribution of these ‘freshwater’ species
remains limited, recent surveys of rivers throughout northern Australia by Thorburn et
al. (2003), and Morgan et al. (2002, in press) documenting the inland fishes of the
Fitzroy River, have identified the Fitzroy River, Western Australia, as a significant
habitat for several of these rare species.
1.2 Conservation status and distribution of elasmobranchs in the Fitzroy
River
Both Thorburn et al. (2003) and Morgan et al. (2002) recorded three of the above
elasmobranch species in the Fitzroy River, these being P. microdon, H. chaophraya
and C. leucas, with each being found in estuarine and freshwater sites throughout the
river (Figure 1). Furthermore, a single Glyphis sp. C was collected from the nearby
marine tidal creek, Doctors Creek (Figure 1). During a concurrently run study, a total
of six Glyphis sp. C were also collected from Doctors Creek and an additional survey
site south of Derby (Thorburn et al. 2004). The significance of the Fitzroy River
elasmobranch assemblage is further illustrated by the high conservation value of these
species (Table 1). For example, P. microdon and Glyphis sp. C are listed as
vulnerable and endangered, respectively, by the Commonwealth Government under
the Environmental Protection Biodiversity and Conservation Act 1999 (EPBC Act
1999). Another listed sawfish species, Pristis clavata (dwarf sawfish) has also been
encountered in the lower reaches of the Fitzroy River and in Doctors Creek (Morgan
et al. 2002, Thorburn et al. 2003) (Figure 1). Additionally, all five of the
aforementioned species have recently been listed in a report by the IUCN Shark
Specialist Group Australia and Oceania Regional Red List (IUCN 2003).
1
WesternAustralia
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WesternAustralia
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WesternAustralia
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Pristis microdon Himantura chaophraya
Carcharhinus leucas Glyphis sp. C
Pristis clavata
Figure 1 Sites from which elasmobranchs have been recorded from the Fitzroy River
prior to this study (Morgan et al. 2002, Thorburn et al. 2003).
2
Table 1 Conservation listings of elasmbranchs encountered in the Fitzroy River
Species EPBC Act 1999 IUCN (2003)
Pristis microdon Vulnerable Endangered
Himantura chaophraya Vulnerable
Carcharhinus leucas Near Threatened
Glyphis sp. C Endangered Critically Endangered
Pristis clavata Endangered
Although some protection of P. microdon and Glyphis sp. C is provided by the listing
of these species by the Commonwealth, this legislation only applies in
Commonwealth waters, which in Western Australia begins three nautical miles from
the low water mark (under the Offshore Constitutional Settlement 1987, 1988 and
1995). The State Government is therefore responsible for the waters between this
three nautical mile limit and the coast (see Boulter 2002). Thus, even though P.
microdon and Glyphis sp. C are protected in Commonwealth waters, they are not
listed under Schedule 2 (Protected Fish) of the Fish Resources Management
Regulations 1995, which preclude them from protection in Western Australia.
Although Part 1 of Schedule 2, detailing ‘commercially protected fish’, states that “all
freshwater aquatic organisms other than sooty grunter and catfish of the family
Ariidae” are protected from commercial practices, it is unclear if P. microdon or
Glyphis sp. C are included in this freshwater classification, as they are more
accurately described as marine opportunists. Thus, in inshore water (including rivers)
none of the aforementioned species are protected.
Pristis microdon (freshwater sawfish)
Found throughout northern Australia, P. microdon is Australia’s largest fish found in
freshwater, reputed to reach 7 m (Last and Stevens 1994). Although accurate
distributional data are limited, and the taxonomy confused, it has been known for
some time that this species is encountered in the upper reaches of rivers, often several
hundred kilometres upstream from marine waters (Whitley 1940, Herre 1955, Merrick
and Schmida 1984). In the Fitzroy River, it is known from both fresh and salt water,
and as far inland as Margaret River Gorge, approximately 400 kilometres upstream
from the mouth (Morgan et al. 2002). Thorburn et al. (2003) also noted the high
3
abundance of this species in the Fitzroy River (as compared to all others sites
surveyed in northern Australia) and described the river as “a significant ‘stronghold’
for the species”.
Himantura chaophraya (freshwater whipray)
It was not until 1989 that H. chaophraya was identified from Australia, based on a
specimen from the Daly River (Taniuchi et al. 1991). Himantura chaophraya is the
only Australian stingray to live entirely in fresh and estuarine waters (Last and
Stevens 1994) and it has not been recorded from euhaline (30-40 ppt) marine waters
anywhere within its known range (Pogonoski et al. 2002). The maximum size of this
species recorded from Australia is over 1.2 m disc width (Thorburn et al. 2003),
however, elsewhere it is reported to reach a disc width of almost 2 m and about 600
kg (Last and Stevens 1994). This species has previously been recorded from
Telegraph Pool and below Camballin Weir (barrage) in the Fitzroy River (Thorburn et
al. 2003) (Figure 1), and anecdotal evidence places this species upstream of Geikie
Gorge.
Carcharhinus leucas (bull shark)
This species has a world wide distribution in tropical and warm temperate seas with
an Australian range extending as far south as Sydney on the east coast, and Perth on
the west coast. It occurs in a wide range of habitats and although considered a marine
species, it is often described as a ‘freshwater’ species as a significant part of their
juvenile life is spent in freshwater. Additionally, this species is known to penetrate
freshwater systems for extended periods of time. Carcharhinus leucas are sometimes
confused with other coastal whaler sharks, and in particular with the very similar
pigeye shark (Carcharhinus amboinensis). The maximum known length of the bull
shark in Australia is about 3.4 m (Last and Stevens 1994). This species has been
encountered by Thorburn et al. (2003) as far inland as Camballin Weir in the Fitzroy
river, but anectdotal evidence has also placed this species further upstream near
Fitzroy Crossing (Figure 1).
Glyphis sp. C (northern river shark)
Glyphis sp. C is a medium sized whaler shark belonging to the Carcharhinidae. While
little is known regarding their biology, this species may be distinguished from Glyphis
4
sp. A, the other species occurring in Australia, by vertebral count (147-148 total
vertebral centra in sp. C cf. 217 of sp. A) (Campagno and Niem 1998). Although
known from several localities in the Northern Territory, this species is currently only
known in Western Australia from King Sound, and as yet has not been encountered in
the freshwater reaches of the Fitzroy River (Morgan et al. 2002, Thorburn et al. 2003,
Thorburn et al. 2004) (Figure 1).
Pristis clavata (dwarf sawfish)
Pristis clavata is a tropical species distributed in Australia from Cairns (Qld) to the
Kimberley (WA), in coastal and estuarine habitats. Although not included in the
‘freshwater’ category, it has been reported well inland, almost into freshwater, and is
common on mudflats in the Gulf of Carpentaria. In the Fitzroy River this species has
been encountered in the lower Telegraph Pool, and outside the river mouth in the
macrotidal mangrove systems and mudflats of King Sound (Last and Stevens 1994,
Morgan et al. 2002, Thorburn et al. 2003) (Figure 1).
1.3 Current threats to elasmobranchs in the Fitzroy River
Compared to teleosts, which may produce high numbers of eggs and develop rapidly,
sharks and rays are generally slow growing animals that mature after an extended
period and produce low numbers of well-developed offspring, thus resulting in them
being easily threatened. Furthermore, estuarine and freshwater elasmobranchs are
even more prone to population decline as they combine the above life history
strategies with all the problems associated with restricted habitats (eg. water quality,
habitat availability, dams and land uses). The popularity of the Fitzroy River by
recreational fishers, and the fact that fishes (including elasmobranchs) continue to
form part of the diets of people in the area, means that additional pressure is placed
upon these species. Thorburn et al. (2003) documented the threat to these species, as
a bycatch to the popular recreational barramundi fishery, in which they encountered
numerous dead sawfish on the banks of Telegraph Pool on two occasions. Many of
these carcasses still had fishing line attached, and several had their rostrums removed
(see Figure 12), probably taken as souvenirs.
5
1.4 Objectives
This project primarily aimed to determine the seasonal abundance of freshwater
elasmobranchs in the lower, middle and upper sections of the Fitzroy River. By
comparing these abundances, inferences relating to the movements of these species
can be made. Sampling over two periods also presented the opportunity to initiate a
sawfish tagging program, and although dependent upon the recapture of tagged
individuals, is additionally useful in monitoring movements over time.
6
2 METHODS
2.1 Sampling strategy
Originally, sampling was to be conducted to compare the seasonal abundance of
elasmobranchs in the lower, middle and upper reaches of the Fitzroy River, during the
wet and dry season. However, a small wet season over the summer of 2002/2003
meant that water levels encountered in May/June were much lower than expected, and
thus unrepresentative of wet season conditions (i.e. minimal flow rates and low water
levels). The subsequent sampling period (which was to be an exclusive dry season
sample) was therefore deferred, and provided a comparison between early dry
(May/June), and late dry conditions in November.
2.2 Sample sites
A total of 24 gill net sets, at 22 individual locations, were used to sample for
elasmobranchs during the early dry and late dry season in 2003 (Figure 2 and 3,
Appendix 1). Sampling in the Fitzroy River was conducted in the lower reach (upper
estuarine), Snag Pool and Telegraph Pool, the middle reach, below Camballin Weir,
and from the upper reach, upstream of Geikie Gorge. Longitude and latitude were
recorded at each sample site using a Global Positioning System (GPS). A sampling
site and distribution map was created using the aforementioned GPS data and
MapInfo (Mapinfo Corporation 1998). Sample site locations are provided in
Appendix 1.
Figure 4 illustrates the types of locations present at the time of sampling in all three
reaches. Figures 4a and b show Telegraph Pool in the lower reach of the Fitzroy
River in the early and late dry season, respectively. Waters in the early dry were near
fresh (0.4 ppt). Although water remains in this large pool of the lower estuary
throughout the late dry, water is not flowing from upstream into the pool, rather it is
now only supplied by incoming marine waters on large high tides. In the middle
reaches, the late dry sees a large drop in the water levels, and flow rates become close
to zero as shown by the lack of flow over Camballin Weir (Figures 4c and d). Pools
in the middle reaches have also become heavily contracted (Figures 4e and f). In
contrast, at Geikie Gorge, in the upper reach of the Fitzroy River, a large volume of
water is present in both the early and late dry season (Figures 4g and h). However,
7
Figure 2 Sites sampled for elasmobranchs in the Fitzroy River during J
WesternAustralia
Add
Rep
1
23
46
57
108
Derby
Fitzroy River
F
Middle
Lower
Upper
une 2003.
itional sample sites
licate sample sites
119
itzroy Crossing
8
Figure 3 Sites sampled for elasmobranchs in the Fitzroy River duri
2003.
WesternAustralia
Addi
Repl
Derby
Fitzroy River
Fi
12, 131415
1617
1819
222021
Middle
Lower
Upper
ng November
tional sample sites
icate sample sites
tzroy Crossing
23,24
9
a b
c d
fe
g h
Figure 4 Typical sampling sites and river morphology of the Fitzroy River during
June (left column) and November (right column) 2003. (a and b) Lower reach
(Telegraph Pool), (c and d) Camballin Weir (barrage), (e and f) middle reach (pools
below Camballin Weir) and (g and h) upper reach (Geikie Gorge).
10
although there is still a large volume of water at this site in the late dry, it no longer
flows out of Geikie Gorge (there is now a large sandbar at the lower end of the gorge
through which some water percolates), and the river above the gorge has become
heavily contracted and isolated. The size of Geikie Gorge may therefore provide a
significant refuge to these species through the dry season.
2.3 Elasmobranch migration
In an attempt to standardise sampling, 18 of the 24 gill net sets (i.e. excluding gill net
sets conducted at night) were used to compare abundances of elasmobranchs, with
three replicate samples conducted in each of the lower, middle and upper reaches of
the Fitzroy River, respectively, in both the early dry (June) and again in the late dry
(November) (Figure 2 and 3, Appendix 1). Sampling was conducted during daylight
hours using a combination of 20 m (2 m drop) monofilament gill net panels of 5, 7.5,
10, 15 and 20 cm stretched mesh, respectively. During sampling the 5 and 7.5 cm
stretched mesh were joined, as were the 10, 15 and 20 cm stretched mesh. Gill nets
were primarily anchored to run at right angles to the river bank or to block as much of
the channel as possible. Upon capture, individuals were removed from the net,
identified and released. The numbers caught and the length of the netting period was
also recorded. Additionally, all the sawfish were tagged (see below).
2.4 Interpretation of abundance data
The number of individuals captured by each 100 m replicate gill net set (Appendix 1)
was divided by the number of hours the net was fishing, thus providing a catch per
hour for the combined 100 m of net used at each sample site.
2.5 Tagging
The use of tags, such as that shown in Figure 5, is an effective way to monitor the
movements of fishes over time. Sampling in the Fitzroy River therefore presented the
opportunity to utilise tags from the Department of Fisheries Western Australia, which
are already used in elasmobranch tagging in offshore areas of the Kimberley Region.
Although dependent upon the recapture of the same individual at a later date,
information such as changes in growth and migration patterns can be collected. While
the data provided by recaptures is beyond the scope of this project, tagged individuals
may be captured and the information relayed back to the Department of Fisheries
11
Western Australia for years to come. Along with a unique number, the compact,
highly visible tags also include contact details for the aforementioned authorities. In
the case of sawfish, individuals were tagged on the pectoral fin or second dorsal fin.
Figure 5 The cattle-style tags used for tagging sawfish in the Fitzroy River.
.
12
RESULTS
3.1 Overall catch composition
A total of 68 elasmobranchs from three species were caught while sampling in the
Fitzroy River in the early and late dry season of 2003 (Appendix 1). Only two
‘freshwater’ elasmobranch species were encountered, these being P. microdon (26
individuls) and C. leucas (18 individuals). In addition, P. clavata (24 individuals)
was also captured in the lower reaches of the Fitzroy River. Of these, 39 of the
aforementioned elasmobranchs were captured during replicate sampling conducted
during daylight hours in the lower, middle and upper reaches of the Fitzroy River
(Table 2), 21 of these being P. microdon, four C. leucas and 14 P. clavata. The
remainder of the specimens (as listed in Appendix 1) were captured during night sets.
3.2 Catch composition of replicate sampling and abundance
Pristis microdon was the most abundant species encountered (21 individuals) during
replicate sampling, despite not being recorded in the upper Fitzroy River during
daylight sampling. This species was captured in both the lower and middle reaches,
during both sampling periods (Table 2). The mean number caught per net per hour, in
the lower and middle reaches during the early dry was 0.45 and 0.88 individuals,
respectively. In comparison, sampling during the late dry saw a reduction in the
abundance of this species in both the lower and middle reaches (0.20 and 0.40
individuals, respectively) being less than half that caught in the previous sample
(Figure 6).
During the early dry season, C. leucas was only encountered during daylight sampling
in the middle reach of the Fitzroy River, with an average of 0.18 individuals being
caught per net per hour (Table 2, Figure 7). While also caught in the middle reaches
during the late dry season sample, at an average rate of 0.61 individuals per net per
hour, it was additionally encountered in the lower reaches (at an average rate of 0.10
individuals per net per hour). Similar to P. microdon, most individuals were collected
from the middle reaches of Fitzroy River, in pools downstream of Camballin Weir
(see Discussion for the effect of the Camballin Weir on migration).
13
Table 2 Combined total number of elasmobranchs caught, and mean number of elasmobranchs caught per 100 m of net per hour, during
sampling from the lower, middle and upper reaches of the Fitzroy River in June (early dry) and November (late dry).
Pristis microdon Pristis clavata Carcharhinus leucas Month Season Reach Total # Mean catch/hr Total # Mean catch/hr Total # Mean catch/hr
Jun Early dry Lower 4 0.45 0 0 0 0
Jun
Early dry Middle 9 0.88 0 0 2 0.18
Jun Early dry Upper 0 0.00 0 0 0 0
Nov Late dry Lower 2 0.20 14 1.42 1 0.10
Nov Late dry Middle 6 0.40 0 0 1 0.06
Nov Late dry Upper 0 0.00 0 0 0 0
Total 21 14 4
14
Reach of the Fitzroy River
Num
ber o
f Ind
ivid
uals
Cau
ght
Per H
our
June SampleNovember Sample
Lower Middle Upper
0.2
0.4
0.6
0.8
1.0
1.2
1.4
Figure 6 The mean number of Pristis microdon captured during daylight sampling in
the early dry season (June) and late dry season (November).
Reach of the Fitzroy River
Num
ber o
f Ind
ivid
uals
Cau
ght
Per H
our
June SampleNovember Sample
Lower Middle Upper
0.2
0.4
0.6
0.8
1.0
1.2
1.4
Figure 7 The mean number of Carcharhinus leucas captured during daylight
sampling in the early dry season (June) and late dry season (November).
15
Reach of the Fitzroy River
Num
ber o
f Ind
ivid
uals
Cau
ght
Per H
our
June SampleNovember Sample
Lower Middle Upper
0.2
0.4
0.6
0.8
1.0
1.2
1.4
Figure 8 The mean number of Pristis clavata captured during daylight sampling in
the early dry season (June) and late dry season (November).
Pristis clavata was the second most abundant species encountered during this study.
As can be seen in Table 1 and Figure 8, this species was only caught in the lower
reach (upper estuary) of the Fitzroy River in the late dry season. During this period,
this species had the highest capture rate, with an average of 1.42 individuals caught
per net per hour.
3.3 Tagging Data
A total of 44 sawfish were tagged during sampling in June and November. Of these
25 were P. microdon, ranging in length from 815 to 2770 mm TL, and 19 were P.
clavata, which ranged in length from 1295 to 2255 mm TL. Of the 12 P. microdon
tagged in the early dry season, only one sawfish was recaptured, near the point of first
capture in the middle reach of the Fitzroy River in the late dry. In the 4 months
between captures, this individual had grown 30 mm, from 2150 to 2180 mm TL.
16
4 DISCUSSION
4.1 The current survey
This project aimed to identify movements of freshwater elasmobranchs in the Fitzroy
River by comparing the seasonal change in relative abundances of each species from
sampling sites in the lower, middle and upper reaches of the river. Replicate sampling
conducted during daylight hours in each reach of the Fitzroy River saw a large degree
of variation in the number of individuals caught. The naturally low abundance of
elasmobranch species thus meant catches were generally low, however inferences in
relation to the movements of elasmobranchs in the Fitzroy River can still be made
from the data collected, and observations made in the field. Notwithstanding this, 68
elasmobranchs were captured in the river.
4.2 Elasmobranch migration in the Fitzroy River
Although the contrast in river morphology between the wet and dry season is extreme,
a large degree of variation also exists between early dry and late dry conditions. In
contrast to the late dry, flow rates remain quite high in the early dry, and large pools
throughout the river are interconnected allowing migration between pools.
Additionally, freshwaters flow into the lower reaches with the salt water interface
being closer to the sea. Salt waters are only encountered in the lower reaches during
this period on very large tides experienced in King Sound (up to 12 m). Waters at
sites such as Telegraph Pool therefore have low salinities during the early dry,
however in the late dry, negligible freshwater input means that these sites become
more saline. In comparison, Geikie Gorge has the least variation in conditions
between the early and late dry.
More P. microdon were caught in both the lower and middle reaches of the Fitzroy
River during the early dry than in the late dry season sampling period. In reference to
the middle reach, the comparatively high abundance of this species may be attributed
to the presence of Camballin Weir. As water levels fall after the wet, the weir
presents a significant obstacle to individuals moving upstream, thus concentrating
sawfish below this point. This is highlighted by the recapture of one sawfish that
remained below the weir from June to November. During the early dry season
sample, gill netting was conducted a few kilometres below the weir in a series of large
interconnected pools. These sites therefore provide a refuge during the dry season,
17
but allowed individuals to move freely between these pools. Although it could be
expected that abundances may be higher in these pools during the late dry, as water
levels drop and individuals become concentrated, this was not found to be the case, as
abundances were seen to be reduced in the late dry. It is possible that the higher catch
rates encountered while netting in the early dry season were due to higher levels of
activity, and unrestricted movements between pools. In contrast, during the late dry
water levels are reduced, pools become contracted and thus the connections existing
between these pools became fewer. The lower abundances observed during sampling
in the late dry season may therefore have been ‘pool’ dependent, as the sawfish
become ‘locked in’ and are not as active in this reach. It may however also be
possible that fishers in the area reduce the numbers of animals trapped in these pools
over the months of the dry season, when sites such as the Camballin Weir are
accessible.
The contraction of the river may also be used to explain the reduction in P. microdon
numbers in the lower reaches between the early and late dry season. In the early dry,
sampling sites were of low salinity and individuals were able to move upstream from
Telegraph Pool. In the late dry however, the minimal freshwater input and
contraction of the river at this point resulted in saline conditions at these sites. As the
river fell, sawfish may therefore have moved upstream to waters of lower salinity,
prior to the late dry season sample. Additionally, sampling during this period was
conducted soon after a large +12 metre tide, thus connecting Telegraph Pool to
freshwater pools upstream for a short period. Unlike sampling in the lower and
middle reaches, where nets were often set in waters <2 m deep (thus blocking a
significant amount of the rivers channel) and the channel is comparatively narrow,
sampling in the upper reaches (Geikie Gorge) was often conducted in deeper water in
pools that stretched for >20 km, with the nets only blocking a relatively small
proportion of the channel. Although two P. microdon were captured in Geikie Gorge
at night, none were caught during daylight (replicate) sampling, this possibly being
attributed to the sampling effort conducted in the large deep water body.
Similar to P. microdon, C. leucas was also encountered in higher numbers in the
middle reaches of the Fitzroy River, below Camballin Weir. While few specimens
were collected during replicate sampling (four individuals) in the pools below
18
Camballin Weir, 10 individuals were caught at night immediately below the weir
during the early dry season, and another two in the late dry season. The concentration
of elasmobranchs below the weir strongly suggests that C. leucas and P. microdon are
restricted for up to 10 months below this impassable barrier, and subsequently become
‘locked’ in pools over the late dry. Here, predation by other sharks may be high, and
the capture of these species by fishers likely, as Camballin Weir is popular with
recreational fishers and campers.
Pristis clavata was only encountered during this study in the lower reaches of the
Fitzroy River during the late dry season sample. It is likely that the cessation of
freshwater entering the lower pools, and subsequent increase in salinity due to tidal
inflow favours this marine species.
Although H. chaophraya was not captured during this study, one imprint (often
created by rays while feeding) was observed in the sand in shallow waters
immediately below Camallin Weir during the early dry season. Similarly, Thorburn
et al. (2003) also observed (and subsequently captured) this species moving upstream
in shallow water (<0.5 m deep) at this location. Another specimen was also captured
at Telegraph Pool, in the lower Fitzroy River, with the use of a baited longline. Gill
nets are however essentially ineffective at capturing this species. Anectdotal evidence
also places this species above Geikie Gorge.
In reference to Glyphis sp. C, this species has yet to be recorded within the Fitzroy
River, and although often referred to as a ‘freshwater’ species, may favour the
macrotidal marine feeder creeks entering King Sound (see Thorburn et al. 2004).
4.3 Freshwater sawfish of the west Kimberley
Like many of the elasmobranch species occurring in northern Australia, distribution
data for P. microdon is lacking. Although anecdotal records elude to the fact that this
species was occurring in waters in the west Kimberley, it was not until a
comprehensive survey of the Fitzroy River by Morgan et al. (2002), and another study
by Thorburn et al. (2003) with the specific aim of capturing elasmobranch species,
that the significance of the Fitzroy River population was revealed. The remoteness
and difficulties in accessing many of the rivers in the west Kimberley, have therefore
19
meant that inshore areas, and rivers have received limited attention, with several
species (such as Glyphis sp. C) remaining unreported until recently.
Although some sampling by the authors has been conducted throughout King Sound,
and other rivers entering it (such as the May and Robinson Rivers), thus far P.
microdon has only been captured by the authors within the rivers, and have only been
a maximum of 2.8 m in length, despite this species being reputed to reach 7 m in
length (Figure 9 illustrates the length frequency-histograms for female and male P.
microdon caught in the west Kimberley by the authors and other researchers).
Furthermore, all the male specimens have been immature, as indicated by small non-
calcified claspers, such as that seen in Figure 10a. Similarly, dissection of several
female specimens of ~2.5 metres in length revealed they were also immature. Thus, it
is possible that the freshwaters of the Fitzroy River act as a nursery for juvenile
animals of this species, similar to the strategy used by C. leucas, where large mature
individuals remain offshore, and juveniles penetrate freshwater riverine reaches far
inland.
While there are currently no published records of P. microdon occurring offshore
(although some anectdotal evidence has been collected), this species was recently
captured during another project undertaken by Murdoch University researchers, at two
sites a significant distance south of King Sound (80 Mile Beach (Wallal) and near
Cape Keraudren). The latter two specimens were large, with one being a ~3.5 m
female, and the other a >3 m mature male (see Figure 10b illustrating the large
calcified claspers). Thus, these captures provide some support for the notion that
upon maturation, individuals move out of the rivers, and remain offshore, and may
move inshore near the mouths of rivers to pup (Figure 11).
The capture of P. microdon at Cape Keraudren is also interesting as this species has
not been recorded in the nearby De Grey River, or in any other river throughout the
Pilbara region. Futhermore, there are no large rivers between King Sound and Cape
Keraudren, as between these points the Great Sandy Dessert extends to the coast.
Furthermore, to the north of King Sound, much of the land is topographically high,
such as the Mitchell Plateau, and thus numerous rivers end in waterfalls. A short
estuary often exists, however the steepness of the land provides a barrier to upper
20
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Length-frequency histograms of female and male P. microdon captured
itzroy River during this study, from those specimens collected from the
ver (and Robinson River) and during a survey conducted by Thorburn et al.
large female collected from Cape Keraudren, and large male from 80 Mile
llal).
21
Figure 10 The male reproductive appendage (claspers) of P. microdon from a (a) 2.2
metre immature male, and (b) mature male ~ 3 m in length. Note the large calcified
claspers on the mature male.
a) b)
WesternAustralia
Fitzroy RiverFitzroy RiverFitzroy RiverFitzroy RiverFitzroy RiverFitzroy RiverFitzroy RiverFitzroy RiverFitzroy RiverFitzroy RiverFitzroy RiverFitzroy RiverFitzroy RiverFitzroy RiverFitzroy RiverFitzroy RiverFitzroy RiverFitzroy RiverFitzroy RiverFitzroy RiverFitzroy RiverFitzroy RiverFitzroy RiverFitzroy RiverFitzroy RiverFitzroy RiverFitzroy RiverFitzroy RiverFitzroy RiverFitzroy RiverFitzroy RiverFitzroy RiverFitzroy RiverFitzroy RiverFitzroy RiverFitzroy RiverFitzroy RiverFitzroy RiverFitzroy RiverFitzroy RiverFitzroy RiverFitzroy RiverFitzroy RiverFitzroy RiverFitzroy RiverFitzroy RiverFitzroy RiverFitzroy RiverFitzroy River
80 Mile Beach80 Mile Beach80 Mile Beach80 Mile Beach80 Mile Beach80 Mile Beach80 Mile Beach80 Mile Beach80 Mile Beach80 Mile Beach80 Mile Beach80 Mile Beach80 Mile Beach80 Mile Beach80 Mile Beach80 Mile Beach80 Mile Beach80 Mile Beach80 Mile Beach80 Mile Beach80 Mile Beach80 Mile Beach80 Mile Beach80 Mile Beach80 Mile Beach80 Mile Beach80 Mile Beach80 Mile Beach80 Mile Beach80 Mile Beach80 Mile Beach80 Mile Beach80 Mile Beach80 Mile Beach80 Mile Beach80 Mile Beach80 Mile Beach80 Mile Beach80 Mile Beach80 Mile Beach80 Mile Beach80 Mile Beach80 Mile Beach80 Mile Beach80 Mile Beach80 Mile Beach80 Mile Beach80 Mile Beach80 Mile Beach
DerbyDerbyDerbyDerbyDerbyDerbyDerbyDerbyDerbyDerbyDerbyDerbyDerbyDerbyDerbyDerbyDerbyDerbyDerbyDerbyDerbyDerbyDerbyDerbyDerbyDerbyDerbyDerbyDerbyDerbyDerbyDerbyDerbyDerbyDerbyDerbyDerbyDerbyDerbyDerbyDerbyDerbyDerbyDerbyDerbyDerbyDerbyDerbyDerby
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De Grey RiverDe Grey RiverDe Grey RiverDe Grey RiverDe Grey RiverDe Grey RiverDe Grey RiverDe Grey RiverDe Grey RiverDe Grey RiverDe Grey RiverDe Grey RiverDe Grey RiverDe Grey RiverDe Grey RiverDe Grey RiverDe Grey RiverDe Grey RiverDe Grey RiverDe Grey RiverDe Grey RiverDe Grey RiverDe Grey RiverDe Grey RiverDe Grey RiverDe Grey RiverDe Grey RiverDe Grey RiverDe Grey RiverDe Grey RiverDe Grey RiverDe Grey RiverDe Grey RiverDe Grey RiverDe Grey RiverDe Grey RiverDe Grey RiverDe Grey RiverDe Grey RiverDe Grey RiverDe Grey RiverDe Grey RiverDe Grey RiverDe Grey RiverDe Grey RiverDe Grey RiverDe Grey RiverDe Grey RiverDe Grey River
Fitzroy CrossingFitzroy CrossingFitzroy CrossingFitzroy CrossingFitzroy CrossingFitzroy CrossingFitzroy CrossingFitzroy CrossingFitzroy CrossingFitzroy CrossingFitzroy CrossingFitzroy CrossingFitzroy CrossingFitzroy CrossingFitzroy CrossingFitzroy CrossingFitzroy CrossingFitzroy CrossingFitzroy CrossingFitzroy CrossingFitzroy CrossingFitzroy CrossingFitzroy CrossingFitzroy CrossingFitzroy CrossingFitzroy CrossingFitzroy CrossingFitzroy CrossingFitzroy CrossingFitzroy CrossingFitzroy CrossingFitzroy CrossingFitzroy CrossingFitzroy CrossingFitzroy CrossingFitzroy CrossingFitzroy CrossingFitzroy CrossingFitzroy CrossingFitzroy CrossingFitzroy CrossingFitzroy CrossingFitzroy CrossingFitzroy CrossingFitzroy CrossingFitzroy CrossingFitzroy CrossingFitzroy CrossingFitzroy Crossing
Figure 11 The sites at which P. microdon has been recorded by the authors.
22
riverine reaches. While conducting surveys of the Drysdale and King Edward rivers
(north Kimberley), and working out of Kalumburu, the authors were unable to find
any anecdotal accounts of sawfish occurring in fresh inland waters. Thus the waters
of King Sound may provide the most significant habitat for this species in this region.
Indeed, the collection of more data may demonstrate that King Sound (and rivers that
flow into it) may act as the only nursery areas for this species in the region with
mature animals radiating out from this area following the coastline in inshore marine
waters, and perhaps returning to this locality to pup (and/or breed). The likelihood
that King Sound and its rivers are the most important nursery ground for this species
in the region may be due to the following:
• The Fitzroy River has the highest run-off of any river in Western Australia,
and largest catchment (~90 000 km2) of any river in the Kimberley, with the
river comprised of a series of large often unconnected pools and billabongs
during the dry.
• Tropical rivers are often more productive than temperate systems. This is
highlighted by the high diversity of fishes in the large deep pools of the
Fitzroy River (the richest fluvifaunulae in Western Australia (Morgan et al.
2002, in press)). These species often occur in extremely high abundances and
are potentially an important food source for many of the larger predatory
species, such as the freshwater sawfish.
• The topography of the land surrounding King Sound and the Fitzroy River is
comparatively flat (especially below Fitzroy Crossing where the floodplain
extends some 300 km to the coast), thus the entrance to this river is accessible
throughout the year. Even during the dry season, large tides connect the upper
estuary with freshwater pools, and thus individuals can move upstream into
relatively ‘safe’ waters.
• Large potential predators of sawfish, such as adult sharks and estuarine
crocodiles, rarely penetrate farther upstream than the estuary.
The length-frequency histograms (Figure 9) indicate that more females than males
were caught in the Fitzroy River, with a sex ratio of females to males of ~1.5:1 (i.e. 29
females and 19 males). The collection of more length-frequency data may produce a
more accurate sex ratio, however these discrepancies, together with the larger size
23
classes of the females, may indicate that the males mature at a smaller size, and/or
leave the river earlier, possibly after attaining maturity. For example there appears to
be three distinct modal (year) classes of females (modal lengths of 800-900, 2000 and
2400 mm TL) and males (modal lengths 800, 1450 and 2000-2150 mm TL), and that
the modal lengths of these ‘year’ classes are considerably larger for the females.
4.4 Further research
In terms of elasmobranch migration in the Fitzroy River, a more comprehensive
understanding could be achieved by conducting multiple samples in more locations in
each reach, throughout all seasons, in conjunction with utilising recapture data from
tagged individuals. Although covering a large catchment, access to the river is
limited, however the expansion of the current project should additionally include sites
further upstream of Geikie Gorge, those sites less accessible to the general public, and
during the wet when the river can be negotiated by boat. The continuation and
expansion of a tagging program will also provided data on the movement of
individuals over an extended period of time. In light of the current project, the
authors, in conjunction with the Kimberley Land Council, traditional owners and
World Wildlife Fund (WWF) Australia, have instigated a tagging program that will
involve training indigenous persons from various communities along the Fitzroy
River to tag and record movements of P. microdon. While elasmobranchs are
important as a food source to many people living within the Fitzroy catchment,
including traditional owners, anecdotal evidence suggests their numbers and
occurrences are in decline in the upper reaches of the Fitzroy River. Thus by
providing education, the management of this resource and longevity of the occurrence
P. microdon throughout the Fitzroy River will be maintained.
Satellite tag tracking is another way to monitor the movement of individuals over a
short period. This would be especially useful in tracking individuals in the wet season
when there are no barriers, such as Camballin Weir, to their movements. Data
collected from such tags also enable researchers to understand daily movements
within rivers and patterns of activity.
The continuation of fish surveys in rivers throughout the Kimberley region is essential
if accurate estimates of species distribution and population status are to be
24
determined; data that are essential for the management of these species. Furthermore,
an understanding of life-histories, and utilisation of different habitats throughout their
life, means that we will be better able to manage them, and their environment. In the
case of P. microdon, an extensive survey of inshore embayments and offshore areas of
King Sound will determine the true significance of this area, and demonstrate if in
fact, it provides an essential breeding and birthing ground.
Although only two freshwater elasmobranch species were encountered in the Fitzroy
River during this study (and an additional marine sawfish species), H. chaophraya
and Glyphis sp. C were not. While Glyphis sp. C was the focus of another study by
the authors, H. chaophraya has received little attention. Few records of this cryptic
species exist throughout Australia, however anecdotal evidence suggests the
occurrence of large numbers of this species in isolated upstream areas of the Fitzroy
River. It is possible therefore, that as is the case for P. microdon, the Fitzroy River
represents a significant habitat for this species.
4.5 Recommendations
Of immediate concern in relation to the conservation of P. microdon, P. clavata, H.
chaophraya and Glyphis sp. C is the lack of protection of these species in inshore
waters (between the low water mark and three nautical miles offshore) and in the
Fitzroy River itself. As each of these rare species is predominantly found in State
waters, immediate consideration should be given for their protection. Penalties would
subsequently exist for acts such as that witnessed by Thorburn et al. (2003), in which
numerous P. microdon were slaughtered by recreational fishers in Telegraph Pool
(one H. chaophraya and two C. leucas were also found dead).
At this point in time, and while our knowledge is still lacking, public education
regarding the rarity of freshwater sawfish, sharks and rays, and the significance of the
environment in which they exist is likely to offer the greatest benefit to the survival of
these elasmobranch species in the Fitzroy River. Signs at popular fishing spots and
access points are an effective way to illustrate to those who utilise the Fitzroy River
the uniqueness of these species, and additionally creates a sense of stewardship to
these iconic species for communities of the west Kimberley. Awareness can also be
achieved by promoting these rare species to tourists and local wildlife and fishing
25
guides. Additionally, efforts should be made to encourage people not to discard
rubbish, such as fishing line, in or near the river, as, for example, the entanglement of
debris with the rostrum of a sawfish can cause serious or fatal injury, as seen in Figure
12.
Throughout this project the effects of a barrier such as Camballin Weir on the
migration of fishes was graphically illustrated, as animals accumulated directly below
the weir and in nearby downstream pools whilst presumably heading upstream. While
the weir itself is inundated during the peak floods of the wet season, for much of the
year it is exposed. In light of this, a comparison of the fish community and
abundances of pools immediately above and below the weir would provide valuable
insight into the effects the weir has on migrating species, such as elasmobranchs,
barramundi and mullet, and thus managers would be better informed as to what action
(such as the construction of a fishway, or removal of the weir) should be taken in
regards to the dilapidated irrigation dam.
Figure 12 The effects of discarded fishing line on the rostrum of a freshwater
sawfish. NB There is no hook attached
26
5 REFERENCES
Boulter, S.L. (2002). Coast Law in Western Australia. Environmental Defenders
Office WA, Perth, Australia.
Compagno, L.J.V. and Niem, V.H. (1998). Order Carcharhinidae. pp 1312-1360. In:
Carpenter, K.E. and Niem, V.H. (eds.), FAO Species Identification Guide for
Fisheries Purposes. The living marine resources of the western central Pacific.
Volume 2. Cephalopods, crustaceans, holothurians and sharks. FAO, Rome
Herre, A.W.C.T. (1955). Sharks in freshwater. Science 122: 417.
IUCN (2003). The Conservation Status of Australian Chondrichthyans: Report to the
IUCN Shark Specialist Group Australia and Oceania Regional Red List
Workshop, Cavanagh, R.D., Kyne, P.M., Fowler, S.L., Musick, J.A., and
Bennett, M.B. (eds). The University of Queensland, School of Biomedical
Sciences, Brisbane, Australia: 170 pp.
Last, P.R and Stevens, J.D. (1994). Sharks and Rays of Australia. CSIRO Division of
Fisheries, CSIRO, Australia.
Merrick, J.R. and Schmida, G.E. (1984). Australian Freshwater Fishes. Biology and
Management. Griffin Press, Netley, South Australia.
Morgan, D., Allen, M., Bedford, P. and Horstman, M. (2002). Inland Fish Fauna of
the Fitzroy River Western Australia, including the Bunuba, Gooniyandi,
Ngarinyin, Nyikina and Walmajarri Aboriginal names. Report to the Natural
Heritage Trust: 56 pp.
Morgan, D.L., Allen, M.G., Bedford, P. and Horstman, M. (in press). Fish fauna of
the Fitzroy River in the Kimberley region of Western Australia – including the
Bunuba, Gooniyandi, Ngarinyin, Nyikina and Walmajarri Aboriginal names.
Records of the Western Australian Museum.
27
Pogonoski J.J., Pollard, D.A. and Paxton, J.R. (2002). Conservation Overview and
Action Plan for Australian Threatened and Potentially Threatened Marine and
Estuarine Fishes. Report to Environment Australia.
Thorburn, D.C., Peverell, S., Stevens, J.D., Last, P.R. and Rowland, A.J. (2003).
Status of Freshwater and Estuarine Elasmobranchs in Northern Australia.
Report to the Natural Heritage Trust: 79 pp.
Thorburn, D.C., Morgan, D. and Rowland, A.J. (2004). Search for the Northern River
Shark (Glyphis sp. C). Report to the Natural Heritage Trust: 22 pp.
Whitley, G.P. (1940). The fishes of Australia. Part 1. The sharks, rays, devilfish and
other primitive fishes of Australia and New Zealand. Australian Zoological
Handbook. Royal Zoological Society of New South Wales, Sydney.
28
APPENDIX 1 Sites sampled for elasmobranchs in the Fitzroy River, the number of hours gill nets were set (small and large mesh refers to the
combination of the 5 and 7.5 cm, and 10, 12.5 and 15cm stretched meshes, respectively), and species captured at each site.
Set # Sample Location Reach Set Latitude Longitude Small mesh (hrs set)
Large mesh (hrs set) C. leucas P. clavata P. microdon
June sample 1 Telegraph Pool Lower rep 1 17.638 123.567 2.33 3 1 2 Snag Pool Lower rep 2 17.598 123.574 1.5 2 2 3 Snag Pool Lower rep 3 17.596 123.565 2.75 2.92 1 4 Camballin Weir Middle night set 18.188 124.492 - 12 10 5 Camballin Pools Middle rep 1 18.177 124.464 3.58 3.33 5 6 Camballin Pools Middle rep 2 18.168 124.461 3.75 4.42 1 1 7 Camballin Pools Middle rep 3 18.183 124.472 3 3.17 1 3 8 Geikie Gorge Upper rep 1 18.034 125.744 5.33 5.08 9 Geikie Gorge Upper rep 2 18.021 125.759 2 2 10 Geikie Gorge Upper rep 3 18.044 125.740 2.5 2.5 11 Geikie Gorge Upper
night set
18.111 125.698 2 (2) 2 (2) November sample 812 Snag Pool Lower rep 1 17.586 123.568 4 4.25 1 13 Snag Pool Lower night set 17.586 123.568 12.5 12.75 2 10 1 14 Telegraph Pool Lower rep 2 17.636 123.567 3 3.25 1 11 15 Telegraph Pool Lower rep 3 17.638 123.567 3 3.25 2 2 16 Camballin Weir Middle night set 18.177 124.464 12 (6,8) 2 2 17 Camballin Pools Middle rep 1 18.183 124.471 5.67 5.5 1 3 18 Camballin Pools Middle rep 2 18.176 124.464 4.67 4.5 3 19 Camballin Pools Middle rep 3 18.168 124.461 4 4 20 Geikie Gorge Upper rep 1 18.024 125.758 5.5 6 21 Geikie Gorge Upper night set 18.038 125.744 - 9 2 22 Geikie Gorge Upper rep 2 18.016 125.762 5.5 6 23 Geikie Gorge Upper rep 3 18.044 125.740 3.75 7.25 24 Geikie Gorge Upper night set 18.044 125.740 1.17 13
29
30