Surveys of Shark and Fin-fish abundance on reefs within the MOU74 Box and Rowley Shoals using Baited Remote Underwater Video Systems
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Surveys of Shark and Fin-fish abundance on reefs
within the MOU74 Box and Rowley Shoals using
Baited Remote Underwater Video Systems
Mark Meekan, Mike Cappo, John Carleton and Ross Marriott
Prepared for the
Australian Government Department of the Environment and Heritage
July 2006SHARK & FIN-FISH SURVEYS MEEKAN, CAPPO, CARLETON & MARRIOTT Australian Institute of Marine Science PMB No 3 PO Box 40197 PO Box 83 Townsville Qld 4810 Casuarina NT 0811 Fremantle WA 6959 Meekan, Mark Gregory. Surveys of shark and fin-fish abundance on reefs within the MOU74 Box and Rowley Shoals using baited remote underwater video systems. Bibliography. ISBN 0 642 32291 0 1. Fish stock assessment – Western Australia. 2. Fish surveys – Western Australia. 3. Sharks – Western Australia. I. Australian Institute of Marine Science. II. Title. 333.956311 © This work is copyright. Except as permitted under the Copyright Act 1968 (Cth), no part of this publication may be reproduced by any process, electronic or otherwise, without the specific written permission of the copyright owners. Neither may information be stored electronically in any form whatsoever without such permission. 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. While reasonable efforts have been made to ensure that the contents of this publication are factually correct, the Commonwealth does not accept responsibility for the accuracy or completeness of the contents, and shall not be liable for any loss or damage that may be occasioned directly or indirectly through the use of, or reliance on, the contents of this publication. This report has been produced for the sole use of the party who requested it. The application or use of this report and of any data or information (including results of experiments, conclusions, and recommendations) contained within it shall be at the sole risk and responsibility of that party.
SHARK & FIN-FISH SURVEYS MEEKAN, CAPPO, CARLETON & MARRIOTT
CONTENTS
CONTENTS ..................................................................................................................... i
List of Figures and Tables............................................................................................... ii
Executive Summary....................................................................................................... iii
Introduction..................................................................................................................... 1
Methods............................................................................................................................ 4
Deployments ...................................................................................................................................................................4
Analysis .............................................................................................................................................................................5
Results .............................................................................................................................. 9
Sharks ...............................................................................................................................................................................9
Reef Fishes – lutjanids, lethrinids and serranids ................................................................................................ 14
Discussion ...................................................................................................................... 22
Comparison of Shark Assemblages on Reefs within the MOU74 Box with the Rowley Shoals ......... 22
Comparison of Fish Assemblages on Reefs within the MOU74 Box with the Rowley Shoals ............. 25
References Cited ........................................................................................................... 27
iSHARK & FIN-FISH SURVEYS MEEKAN, CAPPO, CARLETON & MARRIOTT
LIST OF FIGURES AND TABLES
FIGURE 1. Location map..........................................................................................................................................1
FIGURE 2. Location of deployment sites at Ashmore, Cartier, Scott Reefs (MOU74
Box) and Mermaid, Clerke and Imperieuse Reefs (Rowley Shoals). ..........................................7
FIGURE 3. A. Number of deployments of BRUVS by depth at all sites. B. Deployments
of BRUVS by time of day at all sites..................................................................................................8
FIGURE 4. Mean abundance of sharks recorded by shallow (5-30m) and deep (40-70m)
deployments of BRUVS at all reefs....................................................................................................9
FIGURE 5. Dendrogram produced by cluster analysis of mean numbers of sharks per
hour recorded by BRUVS pooled within sites, at each reef..................................................... 10
FIGURE 6. PCAs on mean number of sharks (# hr-1) recorded by BRUVS pooled within
sites at each reef................................................................................................................................. 11
FIGURE 7. Mean numbers of sharks per hour recorded by deployments of BRUVS at
fished and unfished reefs, pooled between depths. .................................................................... 12
FIGURE 8. Mean numbers of sharks per hour recorded by BRUVS, pooled within sites
at each reef.......................................................................................................................................... 13
FIGURE 9. Mean total numbers of sharks per hour recorded in deep BRUVS
deployments at each reef. ................................................................................................................ 14
FIGURE 10. Dendrogram produced by cluster analysis of mean numbers per hour of
lutjanids, lethrinids and serranids pooled within sites at each reef......................................... 15
FIGURE 11. PCA on mean abundances of lutjanids, lethrinids and serranids pooled within
sites at each reef................................................................................................................................. 15
FIGURE 12. Discriminant Analysis on mean abundance (# hr-1) of lutjanids, lethrinids and
serranids pooled within sites at each reef. ................................................................................... 17
FIGURE 13. Mean number per hour of lutjanids, lethrinids and serranids recorded by
BRUVS.. ................................................................................................................................................ 18
FIGURE 14. Mean number of each species of lutjanid recorded by BRUVS................................................ 18
FIGURE 15. Mean number of each species of lethrinid recorded by BRUVS.............................................. 19
FIGURE 16. Mean number of each species of serranid recorded by BRUVS. ............................................. 20
FIGURE 17. PCA on abundance (# hr-1) of 6 species that were the primary targets of
Indonesian fishermen......................................................................................................................... 21
TABLE 1. Summary of benthic BRUVS deployment and number of sharks sighted by
reef and species. ....................................................................................................................................6
TABLE 2. Abundance (Number hr-1 ± SE ) for all shark species pooled in shallow (4-
30m depth) deployments at all reefs.............................................................................................. 13
iiSHARK & FIN-FISH SURVEYS MEEKAN, CAPPO, CARLETON & MARRIOTT
EXECUTIVE SUMMARY
This report describes research by the Australian Institute of Marine Science (AIMS) in June
2003 and November 2004 with funding co-investment from the Australian Government
Department of the Environment and Heritage. The specific aims were as follows:
1) To use Baited Remote Underwater Video Stations (BRUVS) to sample fish and shark
assemblages at Ashmore and Cartier Reefs within the MOU74 Box (historically fished
reefs, now Marine Protected Areas - MPAs) and Clerke and Imperieuse Reefs (unfished
reefs) in the Rowley Shoals.
2) To integrate data from previous BRUVS surveys (Scott Reef- presently fished, MOU74
Box, and Mermaid Reef, MPA, Rowley Shoals) into results to provide a comprehensive
picture of the status of shark stocks within the MOU74 Box in comparison to unfished
reefs (Rowley Shoals) nearby.
3) To provide a preliminary assessment of the status of reef fishes on Ashmore and Cartier
Reefs within the MOU74 Box (historically fished reefs, now MPAs) in comparison to
Clerke and Imperieuse Reefs (unfished reefs) in the Rowley Shoals.
At all reefs, BRUVS were deployed along depth contour lines with each unit separated by
approximately 400m in the shallow (5-30m) reef crest and deeper reef slope (40-70m) on the
outside of reefs, and on the lagoon floor (20-30m) where these could be accessed by surface
vessels. Additionally, at Scott, Ashmore and Mermaid Reefs a zodiac was allowed to drift in
deep (50-300m+) water 500m off the edge of drop offs and BRUVS hung from the side at 15m
depth.
A total of 11 species of shark were sighted in BRUVS deployments at the 6 reefs. Although
similar average numbers of sharks were recorded per hour in shallow and deep sets (1.05 ±
0.24 vs. 1.06 ± 0.17 SE), there was a strong depth effect in species richness with only 4
species occurring in shallow sets, in which 2 species Carcharhinus amblyrhynchos (grey reef
shark) and Triaenodon obesus (whitetip reef shark) dominated counts. Up to 11 species were
recorded in deep sets, in which C. amblyrhynchos and C. albimarginatus (silvertip whaler shark)
were most abundant.
Multivariate analysis of BRUVS data grouped deployments on shallow unfished and all fished
reefs together, suggesting that the effects of fishing on reefs were largely expressed on species
that occurred in relatively deep water. In shallow BRUVS deployments on the reef there was
little difference in mean numbers of sharks seen at fished, historically fished or unfished reefs,
with the exception of Cartier Reef. Deep BRUVS sets displayed a consistent pattern where
sharks at unfished reefs were from 2-4 times more abundant than at all fished reefs.
iiiSHARK & FIN-FISH SURVEYS MEEKAN, CAPPO, CARLETON & MARRIOTT
Sharks were only recorded in zodiac deployments at the edges of drop-offs at Mermaid Reef,
where counts were dominated by two species, Carcharhinus amblyrhynchos (the grey reef
shark) and C. albimarginatus (silvertip whaler). No sharks were recorded in zodiac
deployments at Scott (fished) or Ashmore (historically fished) Reefs. At Mermaid Reef, the
numbers of sharks recorded by zodiac deployments averaged 5.3 ± 1.3 SE per hr, 5 times the
average seen when BRUVS were deployed on the reef.
Over-fishing is the most plausible explanation of differences in the composition and abundance
of shark assemblages between reefs in the MOU74 Box and the Rowley Shoals. Some target
species (e.g. Carcharhinus albimarginatus) were completely absent from samples at any reef in
the MOU74 Box, while hammerhead sharks (Sphyrna spp) occurred in very low numbers.
Over-fishing has seriously depleted the abundance of shark populations in the MOU74 Box
and may have led to extirpation of silvertip whalers in parts of its former range.
Our study suggests that there may have been some recovery of shark populations at Ashmore
and Cartier Reefs where fishing is now prohibited. In deep water at these reefs total numbers
of sharks are now significantly greater than those of Scott Reef, where fishing continues. At
Cartier Reef, shark numbers in deep water approach those of the reefs of the Rowley Shoals,
although this conclusion must be treated with caution due to the very limited numbers of
BRUVS deployed at this site. Recovery does not include some components of the assemblage,
such as Carcharhinus albimarginatus, the tiger shark Galeocerdo cuvier, or hammerhead sharks
(Sphyrna lewini and S. mokarran). There was no evidence for recovery of populations in open
water just beyond the reef drop off at Ashmore Reef, despite almost 20 years of protection.
Our findings suggest that management of reefs as Marine Protected Areas, combined with
enforcement of that status, can have a significant impact on the recovery of shark populations,
although that recovery will differ among species. We lack critical information on the
movement and migratory patterns of reef sharks that would allow us to speculate on the
spatial scale at which protection should occur in order to include the complete shark fauna.
Evidence from Ashmore and Cartier Reefs suggests that protection of single reefs may be
sufficient to allow populations of grey reef and whitetip sharks to recover. This does not
appear to be the case for silvertip and hammerhead sharks. For these species, protection may
need to include different reefs that encompass the likely home ranges of these species. Due to
the total lack of migration and movement pattern data, we have no idea how large an area this
might require.
Despite the lack of movement data, our results clearly show that the Marine Protected Area
status of Rowley Shoals reefs is extremely useful, since they offer a baseline against which the
effects of shark fishing on reefs throughout the northern region can be assessed. In addition,
they also allow measurement of recovery patterns and thus illustrate the efficacy of
management strategies (Meekan and Cappo 2004). However, recent sighting of Indonesian
vessels by Customs and tourist operators indicate that these shark fishermen are now
ivSHARK & FIN-FISH SURVEYS MEEKAN, CAPPO, CARLETON & MARRIOTT
beginning to target sharks on Rowley Shoals reefs. Given the importance of these reefs as
baselines, and the degraded state of shark populations due to illegal Indonesian shark fishing in
equivalent habitats throughout large areas of north Australian waters, preservation of the fish
faunas of reefs in the Rowley Shoals should be an urgent management priority.
Analysis of the BRUVS data sets could not detect effects of fishing on abundance of lutjanid,
lethrinid or serranid reef fishes at Ashmore and Cartier Reefs. Composition and abundance
varied in response to depth rather than fishing history, with the analyses detecting strong
latitudinal gradients in abundance and diversity of these families as a whole. Typically, Ashmore
Reef had a far greater diversity and abundance of species than reefs in the Rowley Shoals,
which confounded any attempt to compare reefs with different fishing histories.
To avoid this problem, it would be possible to make comparisons only between Scott and
Mermaid Reefs, where there are less confounding effects of changes in faunal composition
with latitude. Multivariate analysis of reef fish composition and abundance in archived
videotape records from BRUVS surveys completed at Scott and Mermaid Reefs (and the Sahul
and Karmt shoals) would be useful for this purpose, but must recognise depth and benthic
microhabitats in the list of explanatory variables.
vSHARK & FIN-FISH SURVEYS MEEKAN, CAPPO, CARLETON & MARRIOTT
INTRODUCTION
Indonesian fishermen have fished the coasts and offshore reefs of northern Australia for
centuries. In the early 1800’s the European explorers Flinders and Baudin encountered
Indonesian fleets of up to 60 vessels in these waters, with total complements estimated to be
up to 1000 men (Flinders 1814, Peron and Freycinet 1816, cited in Russell and Vail 1988).
These fleets principally targeted trochus, bêche-de-mer and shark for commercial sale and reef
fish for local markets and consumption (Russell and Vail 1988).
In recognition of the long standing use of marine resources of northern reefs by this small
scale, traditional fishery, the Australian and Indonesian governments negotiated a
memorandum of understanding (MOU 74) that allows access by Indonesian fishermen to an
area of 50, 000 km2 within the Australian Exclusive Economic Zone (AEEZ). Within this area
lie 6 coral reef systems (Fig. 1), the northern most of which are Ashmore and Cartier Reefs.
FIGURE 1. Location map.
Concern over the status of stocks of reef resources targeted by Indonesian fishermen led to a
ban on all fishing at Ashmore Reef in 1988 and at Cartier Reef in 2000. One small area of the
West Island lagoon of Ashmore Reef was made exempt from this restriction to allow
subsistence fishing by crews. The primary aim of this restriction was to allow over-fished
stocks of bêche-de-mer and trochus to recover. While a variety of surveys had concluded that
there was little evidence that reef fishes had been over-exploited on reefs within the MOU74
1SHARK & FIN-FISH SURVEYS MEEKAN, CAPPO, CARLETON & MARRIOTT Box there was little evidence on which the status of shark resources could be assessed (Russell and Vail 1988, Dennis et al. 2005). One of the principal reasons that the condition of shark stocks is largely unknown within the MOU74 Box is that the survey methods used to assess the status of fish resources have been inappropriate for collection of data on shark abundances. In the past, sharks have been counted using underwater visual census (UVC) techniques, a method that restricts observations to a narrow depth strata (usually
SHARK & FIN-FISH SURVEYS MEEKAN, CAPPO, CARLETON & MARRIOTT
The specific aims of the project were as follows:
1) To use BRUVS to sample fish and shark assemblages at Ashmore and Cartier Reefs within
the MOU74 Box (historically fished reefs) and Clerke and Imperieuse Reefs (unfished
reefs) in the Rowley Shoals.
2) To integrate data from previous BRUVS surveys (Scott Reef – presently fished, MOU74
Box, and Mermaid Reef, Rowley Shoals) into results to provide a comprehensive picture
of the status of shark stocks within the MOU74 Box in comparison to unfished reefs
(Rowley Shoals) nearby.
3) To provide a preliminary assessment of the status of reef fishes on Ashmore and Cartier
Reefs within the MOU74 Box (historically fished reefs) in comparison to Clerke and
Imperieuse Reefs (unfished reefs) in the Rowley Shoals.
3SHARK & FIN-FISH SURVEYS MEEKAN, CAPPO, CARLETON & MARRIOTT
METHODS
This report combines the results from 2 field studies. The first visited Mermaid (Rowley
Shoals, unfished MPA) and Scott Reef (MOU74 Box, presently fished) in June 2003. The
second visited Ashmore and Cartier Reefs (MOU74 Box, historically fished) and Clerke and
Imperieuse Reefs (Rowley Shoals, unfished MPAs) in October 2004.
Deployments
In both field studies, BRUVS were deployed in 2 modes. Firstly, at all reefs they were deployed
along depth contour lines with each unit separated by approximately 400m in the shallow (5-
30m) reef crest and deeper reef slope (40-70m) on the outside of reefs, and on the lagoon
floor (20-30m) where these could be accessed by surface vessels (Fig. 2). Secondly, at Scott,
Ashmore and Mermaid Reefs a zodiac was allowed to drift in deep (50-300m+) water 500m
off the edge of drop offs and BRUVS hung from the side at 15m depth.
The total numbers of reef deployments at each locality are shown in Table 1. Locations of
deployments are shown in Fig. 2. The majority of reef deployments occurred in deep (40-70m)
water and all deployments (including zodiac deployments) were spread throughout daylight
hours from 07.00 - 16.00hrs (Fig. 3).
In the 2003 pilot surveys at Mermaid and Scott reefs, interrogation of each tape provided the
time the BRUVS settled on the seabed and for each shark: its species; the time of first sighting;
time of first feeding at the bait; a coarse initial estimate of length; and a list of fish species. This
enabled the identification of different sharks on each tape for cumulative summaries of shark
visits to be developed for each BRUVS set. Coarse measurements of the total length of the
largest individuals of some species were made by comparing them with the scale grids on the
bait arm. These measurements could be made only when the sharks were perpendicular to
the camera and immediately next to, or between the scale grids (see Harvey et al. 2003).
In the 2004 surveys at Ashmore, Cartier, Clerke and Imperieuse reefs, interrogation of each
tape provided the time the BRUVS settled on the seabed and, for each species of shark and
fish, the time of first sighting, time of first feeding at the bait, the maximum number seen
together in any one time on the whole tape (MaxN), the time at which MaxN occurred, and
the intraspecific and interspecific behaviour. No attempts at measurement were made in these
2004 surveys, but fish and sharks were classified as “adult” or “juvenile”, based on their size. A
“reference collection” of images of each species was made from the BRUVS tapes, and
identifications based on these images were verified by taxonomists and other fish biologists.
The benthos visible in each BRUVS set was classified and an image was stored for later
reference.
4SHARK & FIN-FISH SURVEYS MEEKAN, CAPPO, CARLETON & MARRIOTT
Analysis
Data sets of sharks and reef fishes were subjected to multivariate classification and ordination
analyses. For reef fishes, the Lutjanidae, Lethrinidae and Serranidae were considered to be the
families most likely to be targeted by Indonesian fishermen given the results of creel surveys
and fishing techniques (Russell and Vail 1988). For this reason, other reef fishes were excluded
from the analysis of fish counts made on BRUVS at Ashmore, Cartier, Clerke and Imperieuse
reefs in 2004. The tapes from the 2003 surveys at Scott and Mermaid reefs have not been
interrogated yet for fish counts. To further investigate any possible effect of fishing on reef
stocks, multivariate analyses of reef fishes were repeated on 6 species of lutjanids, lethrinids
and serranids that were reported by Russell and Vail (1988) to compose up to 75% of the
catch by Indonesian perahu vessels at Ashmore Reef. These were Lethrinus obsoletus (48.5% of
catches), Lutjanus decussatus (13.4%), Lethrinus lentjan (10.9%), Lutjanus bohar (4.9%), Taeniura
lymma (4.0%), Lutjanus gibbus (2.4%) and Cephalopholis argus (1.2%).
The abundance data (Counts hr-1) for sharks and the three families of reef fish (lutjanids,
lethrinids and serranids) were subjected to cluster analyses based on Bray-Curtis dissimilarity
measures and UPGMA (unweighted group pair average) fusion strategy to produce
dendrograms. Principal Component Analysis (PCA) using the correlation matrix from the data
sets confirmed the findings of the cluster analyses. The lutjanids, lethrinids and serranids data
was also subjected to a canonical discriminant analysis on the principal coordinates from the
Bray Curtis distance matrix (Anderson 2004). Patterns in the abundance data for the six
individual demersal species targeted by Indonesian fishers were investigated with PCA. To
conform to the assumptions of the analyses, data sets for the demersal families and species
were transformed to fourth root values before analysis.
5TABLE 1. Summary of benthic BRUVS deployment and number of sharks sighted by reef and species.
Triaenodon obesus
Hemitriakis falcata
Galeocerdo cuvier
Sphyrna lewini
amblyrhynchos
albimarginatus
Carcharhinus
Effort Min Depth
Stegostoma
Hemipristis
ferrugineus
Location N sets Max Depth (m)
mokarran
(hrs of tape) (m)
fasciatum
pelagicus
elongata
Nebrius
Sphyrna
Alopias
C.
Ashmore Reef 58 58.73 12.5 58 0 0 11 0 1 1 0 0 1 2 21
Cartier Reef 6 6.25 9.5 52 0 0 7 0 0 0 0 0 0 1 4
Scott Reef 24 41.33 41.6 69 1 0 5 0 1 0 0 1 0 0 4
Clerke Reef 24 24.99 48 62 0 12 18 0 0 0 0 1 1 0 2
Imperieuse Reef 41 39.88 12 71 0 16 8 4 0 7 0 1 3 1 8
Mermaid Reef 30 43.13 5 68.7 0 5 17 2 0 1 1 0 2 1 5
6SHARK & FIN-FISH SURVEYS MEEKAN, CAPPO, CARLETON & MARRIOTT
FIGURE 2. Location of deployment sites at Ashmore, Cartier, Scott Reefs (MOU74 Box) and Mermaid,
Clerke and Imperieuse Reefs (Rowley Shoals).
7SHARK & FIN-FISH SURVEYS MEEKAN, CAPPO, CARLETON & MARRIOTT
A
12
10
8
No of obs
6
4
2
0
0 10 20 30 40 50 60 70
Depth ( m )
B
32
28
24
20
No of obs
16
12
8
4
0
7.11 8.34 9.57 10.80 12.03 13.27 14.50 15.73 16.96 18.19 19.42
Time
FIGURE 3. A. Number of deployments of BRUVS by depth at all sites.
B. Deployments of BRUVS by time of day at all sites.
8SHARK & FIN-FISH SURVEYS MEEKAN, CAPPO, CARLETON & MARRIOTT
RESULTS
Sharks
A total of 11 species were sighted in BRUVS deployments at the 6 reefs (Table 1). Although
similar average numbers of sharks were recorded per hour in shallow and deep sets (1.05 ±-
0.24 vs. 1.06 ± 0.17 SE), there was a strong depth effect in species richness (Fig. 4) with only 4
species occurring in shallow sets, in which 2 species Carcharhinus amblyrhynchos (grey reef
shark) and Triaenodon obesus (whitetip reef shark) dominated counts (97.6% of records). In
deeper sets species richness was far greater, with 11 species recorded, of which C.
amblyrhynchos and C. albimarginatus (silvertip shark) were most abundant. Both of these species
occurred in similar numbers.
Depth
0.7
0.6
0.5
0.4
Abundance ( # hr-1 + SE )
0.3
0.2
0.1
0.0
S. fasciatum
S. fasciatum
G. cuvier
G. cuvier
H. falcata
H. falcata
T. obesus
T. obesus
S. mokarran
S. mokarran
S. lewini
S. lewini
H. elongata
H. elongata
A. pelagicus
N. ferrugineus
A. pelagicus
N. ferrugineus
C. amblyrhynchos
C. amblyrhynchos
C. albimarginatus
C. albimarginatus
Shallow Deep
FIGURE 4. Mean abundance of sharks recorded by shallow (5-30m) and deep (40-70m) deployments of
BRUVS at all reefs
Classification analysis initially split the data set into predominantly shallow and deep
deployments, reflecting the changes in species richness and abundance of sharks with depth.
Shallow samples were then split into 4 groups, 3 of which were composed solely or primarily
9SHARK & FIN-FISH SURVEYS MEEKAN, CAPPO, CARLETON & MARRIOTT
of samples from fished reefs (Ashmore and Scott) with the 4th group containing samples mainly
from unfished reefs. The deep sites split into a group composed primarily of deep non-fished
sites and a mixed group (Fig. 5).
Sharks (#hr-1) by Sites within Reefs - Non-transformed, Bray-Curtis, UPGMA (beta=-0.1)
1.4
1.2
1.0
0.8
0.6
0.4
0.2
0.0
NCleDPM21
NCleDAM19
NCleDAM18
NCleDPM20
FCarDAM15
NMerDPM10
NMerDAM9
FCarSPM11
FSouDPM46
FSouDAM47
FSouDAM45
NMerSAM11
NImpDAM22
NImpDAM28
NImpDAM25
NImpDAM26
NImpDPM23
NImpDPM24
NMerSAM3
NMerSPM7
NImpSAM30
FAshDPM10
FAshDPM16
FAshDAM12
FAshDPM8
FAshDAM6
FAshDAM7
FAshDPM9
FAshSAM13
FAshSAM3
FAshSPM1
FAshSPM2
FAshSPM4
FAshSPM5
FAshSPM8
FIGURE 5. Dendrogram produced by cluster analysis of mean numbers of sharks per hour recorded by
BRUVS pooled within sites, at each reef. F – fished N – unfished Ash – Ashmore, Car – Cartier, Sou –
Scott, Mer – Mermaid, Cle – Clerke, Imp – Imperieuse. D – deep,
S – shallow, AM – morning deployment, PM afternoon deployment.
Principal component analysis confirmed the division between shallow and deep counts
identified by the classification analysis. The major species contributing to the separation of
groups in the PCA analysis were Carcharhinus albimarginatus, which was only present in deep
deployments, and Triaenodon obesus, which was seen more commonly in shallow deployments
(Fig 6A). When the data sets were analysed using the occurrence of fishing as a grouping
factor, counts separated into fished and unfished reefs, confirming that fishing had an
important influence on the composition and abundance of shark assemblages (Fig. 6B). When
both depth and fishing were combined in the same analysis, counts from deep deployments on
unfished reefs separated from the remaining counts (Fig. 6C). Importantly, deployments on
shallow unfished and fished reefs grouped together, suggesting that the effects of fishing on
reefs are largely expressed on species that occur in relatively deep water.
10SHARK & FIN-FISH SURVEYS MEEKAN, CAPPO, CARLETON & MARRIOTT
A
C. albimarginatus
C. amblyrhynchos
H. falcata
S. lewini
G. cuvier
Deep
S. mokarran
Shallow
Dim 2 24.1 %
T. obesus
Dim 1 45.57 %
B
C. albimarginatus
C. amblyrhynchos
H. falcata
S. lewini
Not Fished
S. mokarran
Fished
Dim 2 24.1 %
T. obesus
Dim 1 45.57 %
C
C. albimarginatus
C. amblyrhynchos
H. falcata
S. lewini
DN
DF SN
S. mokarran SF
Dim 2 24.1 %
T. obesus
Dim 1 45.57 %
FIGURE 6. PCAs on mean number of sharks (# hr-1) recorded by BRUVS pooled within sites at each reef.
Ellipses are 95% confidence limits for group centroids by A. Depth. B. Fishing history. C. Fishing history
by depth where SN – shallow unfished, SF – shallow fished, DN – deep unfished, DF – deep fished.
11SHARK & FIN-FISH SURVEYS MEEKAN, CAPPO, CARLETON & MARRIOTT
Plots of mean abundance per hr of sharks on fished and unfished reefs showed that these
differences were largely due to very low numbers, or complete absence, of the silver tipped
shark Carcharhinus albimarginatus, the tiger shark, Galeocerdo cuvier, hammerhead sharks,
notably the scalloped hammerhead Sphyrna lewini, and the sicklefin hound shark Hemitriakis
falcata on fished reefs (Fig. 7).
Fishing Effort
0.5
0.4
0.3
Abundance ( # hr-1 + SE )
0.2
0.1
0.0
S. fasciatum
S. fasciatum
G. cuvier
G. cuvier
H. falcata
H. falcata
T. obesus
T. obesus
S. mokarran
S. mokarran
S. lewini
S. lewini
H. elongata
H. elongata
A. pelagicus
A. pelagicus
N. ferrugineus
N. ferrugineus
C. amblyrhynchos
C. amblyrhynchos
C. albimarginatus
C. albimarginatus
Fished Not Fished
FIGURE 7. Mean numbers of sharks per hour recorded by deployments of BRUVS at fished and unfished
reefs, pooled between depths.
Comparison of mean numbers of sharks recorded by BRUVS at each reef show that Cartier
Reef differed from the remaining fished reefs by having relatively high numbers of grey reef
sharks Carcharhinus amblyrhynchos (Fig. 8). This result was principally due to BRUVS
encountering a school of juvenile grey reef sharks at one site. As few BRUVS were deployed
around the perimeter of Cartier Reef due to its very small size, this resulted in high average
numbers of this species at this location.
Sharks were only recorded in zodiac deployments at the edges of drop-offs at Mermaid Reef,
where counts were dominated by two species, C. amblyrhynchos and C. albimarginatus. No
sharks were recorded in zodiac deployments at Scott or Ashmore Reefs. At Mermaid Reef,
the numbers of sharks recorded by zodiac deployments averaged 5.3 ± 1.3 SE per hr, a value
more than 5 times the average seen when BRUVS were deployed on the reef.
12SHARK & FIN-FISH SURVEYS MEEKAN, CAPPO, CARLETON & MARRIOTT
Location
2.0
Ashmore Reef Cartier Reef South Scott Reef
1.6
1.2
0.8
0.4
0.0
S. lewini
S. lewini
S. lewini
G. cuvier
G. cuvier
G. cuvier
S. mokarran
S. mokarran
S. mokarran
A. pelagicus
A. pelagicus
A. pelagicus
H. elongata
H. elongata
H. elongata
T. obesus
T. obesus
T. obesus
N. ferrugineus
N. ferrugineus
N. ferrugineus
H. falcata
H. falcata
H. falcata
C. albimarginatus
C. albimarginatus
C. albimarginatus
S. fasciatum
S. fasciatum
S. fasciatum
C. amblyrhynchos
C. amblyrhynchos
C. amblyrhynchos
Abundance ( # hr-1 + SE )
2.0
Clerke Reef Imperieuse Reef Mermaid Reef
1.6
1.2
0.8
0.4
0.0
S. lewini
S. lewini
S. lewini
S. mokarran
S. mokarran
S. mokarran
A. pelagicus
G. cuvier
A. pelagicus
G. cuvier
A. pelagicus
G. cuvier
H. elongata
H. elongata
H. elongata
N. ferrugineus
T. obesus
N. ferrugineus
T. obesus
N. ferrugineus
T. obesus
H. falcata
H. falcata
H. falcata
C. albimarginatus
C. albimarginatus
C. albimarginatus
S. fasciatum
S. fasciatum
S. fasciatum
C. amblyrhynchos
C. amblyrhynchos
C. amblyrhynchos
FIGURE 8. Mean numbers of sharks per hour recorded by BRUVS, pooled within sites at each reef.
Upper panels – fished reefs; Lower panels - unfished reefs.
In shallow BRUVS deployments on the reef there was little difference in mean numbers of
sharks seen at fished or unfished reefs, with the exception of Cartier Reef (Table 2). Mean
abundances were much greater at this reef, however as mentioned above, this result was
principally due to a school of juvenile C. amblyrhynchos recorded in a single deployment. Total
numbers of sharks in deep BRUV sets displayed a consistent pattern where mean numbers seen
per hour at unfished reefs were from 2-4 times more abundant than at fished reefs (Fig. 9).
TABLE 2. Abundance (Number hr-1 ± SE ) for all shark species pooled in shallow (4-30m depth)
deployments at all reefs.
Location Mean Min Max SE of Mean
Ashmore 0.67 0.17 1.28 0.174
Cartier 2.88 2.88 2.88
Scott
Clerke
Imperieuse 0.57 0 1.70 0.566
Mermaid 0.98 0 1.81 0.438
13SHARK & FIN-FISH SURVEYS MEEKAN, CAPPO, CARLETON & MARRIOTT
Deep
2.0
1.6
Abundance ( # hr-1 + SE )
1.2
0.8
0.4
0.0
Ashmore Scott Imperieuse
Cartier Clerke Mermaid
FIGURE 9. Mean total numbers of sharks per hour recorded in deep BRUVS deployments at each reef.
Reef Fishes – lutjanids, lethrinids and serranids
The dendrogram produced by the classification of the reef fish data initially split a group of
shallow from deep deployments (Fig. 10). The shallow group was then split again into samples
from Ashmore Reef and samples from Imperieuse, an unfished reef. The splits in the deep
samples showed some grouping by fishing history and contained a small group of non-fished
deep deployments, however there was generally no clear division in the data sets according to
fishing effort.
PCA analysis of these data sets confirmed that there was little division of the combined family
data sets by fishing effort, with most variation accounted for by changes in the abundance and
species richness of reef fishes with depth (Fig. 11). Discriminant analyses of the data sets based
on fishing effort and depth also separated counts based on depth, with shallow deployments
showing a greater separation between fished and non-fished reefs than deep deployments, as
was the case with the classification analysis. Plots for individual families show lutjanids and
serranids to be more diverse and abundant at shallow fished sites (Fig 12A, C), while lethrinids
were more diverse and abundant at both shallow and deep sites on fished reefs than unfished
reefs (Fig 12B).
14SHARK & FIN-FISH SURVEYS MEEKAN, CAPPO, CARLETON & MARRIOTT
Demersals (#hr-1) by Sites within Reefs - Fourth Root transformed, Bray-Curtis, UPGMA (beta=-0.1)
1.2
1.0
0.8
0.6
0.4
0.2
0.0
FCarDAM15
NCleDAM19
NCleDAM18
NCleDPM21
NCleDPM20
FCarSPM11
NImpDAM26
NImpDPM24
NImpDAM22
NImpDAM28
NImpDPM23
NImpDAM25
FAshDAM12
FAshDPM16
FAshDPM10
NImpSAM26
NImpSAM30
NImpSPM27
FAshDAM6
FAshDAM7
FAshDPM9
FAshDPM8
FAshSAM13
FAshSPM1
FAshSPM8
FAshSAM3
FAshSPM2
FAshSPM5
FAshSPM4
FIGURE 10. Dendrogram produced by cluster analysis of mean numbers per hour of lutjanids, lethrinids
and serranids pooled within sites at each reef. Abbreviations as in Fig 5.
P. lori
C. leopardus L. bohar
L. erythropterus A. furca
L. olivaceus
L. decussatus
L. amboinensis
P. leopardus
Deep P. pascalus
Shallow
L. rubrioperculatus
M. grandoculis
Dim 2 12.9 %
L. semicinctus
L. ravus
L. gibbus
Dim 1 21.68 %
FIGURE 11. PCA on mean abundances of lutjanids, lethrinids and serranids pooled within sites at each
reef. Data sets fourth root transformed. Ellipses are 95% confidence limits for deep and shallow group
centroids.
15SHARK & FIN-FISH SURVEYS MEEKAN, CAPPO, CARLETON & MARRIOTT
A. Only Lutjanidae plotted
0.8
L. kasmira
L. fulviflamma Fished
0.4 L. semicinctus
L. decussatus M. niger L. vitta
L. gibbus
L. quinquelineatus
A. virescens
S. nematophorus
Axis 2
0.0 L. bohar A. rutilans
M. macularis
L. monostigma
-0.4 A. furca
Not Fished
-0.8
-0.9 -0.6 -0.3 0.0 0.3 0.6
Axis 1
B. Only Lethrinidae plotted
0.8
L. ravus
L. obsoletus
M. grandoculis
L. atkinsoni
Fished
0.4 L. semicinctus
L. nebulosus
L. rubrioperculatus
G. euanus
G. grandoculis
Axis 2
0.0
L. olivaceus
L. amboinensis
L. erythropterus Not Fished
-0.4 L. xanthochilus
L. erythracanthus
-0.8
-0.8 -0.4 0.0 0.4 0.8
Axis 1
16SHARK & FIN-FISH SURVEYS MEEKAN, CAPPO, CARLETON & MARRIOTT
C. Only Serranidae plotted
0.8
DF
DN
SF
SN G. albomarginata
C. boenak C. formosa Fished
0.4
C. sp1
V. albimarginata
C. sp
C. urodeta
E. maculatus
P. sp6
Axis 2
E. fuscoguttatus
0.0 C. argus
P. sp1
P. areolatus
C. miniata C. leopardus P. sp
P. leopardus
E. polyphekadion P. lori
E. fasciatus
-0.4 E. malabaricus
P. pascalus
V. louti
Not Fished
-0.8
-0.9 -0.6 -0.3 0.0 0.3 0.6
Axis 1
FIGURE 12. Discriminant Analysis on mean abundance (# hr-1) of lutjanids, lethrinids and serranids
pooled within sites at each reef. Data sets fourth root transformed. Ellipses are 95% confidence limits
for fished and unfished group centroids. DF – deep fished, SF – shallow fished, DN – deep unfished, SN
– shallow unfished. A. Only Lutjanidae plotted. B. Only Lethrinidae plotted. C. Only Serranidae plotted.
Total numbers of lutjanids declined significantly with depth (shallow – mean 7.38 ± 1.317 SE
deep – 2.18 ± 0.643 per hr). Richness also declined, with 17 species recorded in shallow and
only 10 in deep BRUVS deployments. There was also a decline in abundance and species
richness between fished and unfished reefs with the former having a greater abundance (5.21
± 1.314 SE) and richness (16 species) than unfished reefs (2.85 ± 0.548 SE, 11 species) (Fig.
13). Plots of means confirm the pattern shown in the multivariate analysis, with species
richness at shallow sites on fished reefs twice that of shallow sites on fished reefs or deep
sites on all reefs (Fig. 14).
17Abundance (DblSqr( # hr-1 ) + SE ) Abundance ( # hr-1 + SE )
values.
6
6
0
2
4
8
10
12
14
16
18
20
0
2
4
8
10
12
14
16
18
20
0.3
0.3
0.0
0.6
0.9
1.5
0.0
0.6
0.9
1.2
1.5
1.2 DN
DF
A. furca A. furca
A. rutilans A. rutilans
A. virescens A. virescens
L. bohar L. bohar
L. decussatus L. decussatus
SHARK & FIN-FISH SURVEYS
L. fulviflamma L. fulviflamma
Lutjanidae
Lutjanidae
L. gibbus L. gibbus
L. kasmira L. kasmira
DF
DN
L. monostigma L. monostigma
Lethrinidae
L. quinquelineatus L. quinquelineatus Lethrinidae
L. rivulatus L. rivulatus
L. semicinctus L. semicinctus
L. vitta L. vitta
Serranidae
Serranidae
M. macularis M. macularis
M. niger M. niger
M. sp M. sp
S. nematophorus S. nematophorus
18
A. furca A. furca
SF
SN
A. rutilans A. rutilans
Sampling Effort by Depth
A. virescens A. virescens
L. bohar L. bohar
L. decussatus L. decussatus
Lutjanidae - Fishing Effort by Depth
L. fulviflamma L. fulviflamma
Lutjanidae
Lutjanidae
L. gibbus L. gibbus
L. kasmira L. kasmira
fished, SF – shallow fished, DN – deep unfished, SN – shallow unfished.
SF
SN
L. monostigma L. monostigma
Lethrinidae
Lethrinidae
L. quinquelineatus L. quinquelineatus
L. rivulatus L. rivulatus
L. semicinctus L. semicinctus
Serranidae
Serranidae
L. vitta L. vitta
M. macularis M. macularis
M. niger M. niger
M. sp M. sp
S. nematophorus S. nematophorus
FIGURE 14. Mean number of each species of lutjanid recorded by BRUVS. DF – deep fished,
SF – shallow fished, DN – deep unfished, SN – shallow unfished. Data sets transformed to fourth root
FIGURE 13. Mean number per hour of lutjanids, lethrinids and serranids recorded by BRUVS. DF – deep
MEEKAN, CAPPO, CARLETON & MARRIOTTSHARK & FIN-FISH SURVEYS MEEKAN, CAPPO, CARLETON & MARRIOTT
In contrast to lutjanids, the abundance of lethrinids increased with depth (shallow 4.52 ± 1.372
SE, deep 8.45 ± 0.574 SE), although there was little change in richness (12 vs 14 species in
shallow and deep deployments respectively). Similar mean total numbers of this family were
recorded on fished and unfished reefs (7.58 ± 1.026 SE vs 6.2 ± 0.97 SE), although there was
higher richness on fished than unfished reefs (13 vs 8 species, respectively). Plots of individual
species means confirm the pattern shown in the multivariate analysis, with shallow sites at
unfished reefs having relatively low abundance and richness (Fig. 15). The dominance of
Lethrinus obsoletus and L. lentjan in earlier creel surveys by Russell and Vail (1988), and their
absence or low number in sightings made on BRUVS, may be due to mis-identifications of
lethrinids in the earlier studies. The Ambon emperor Lethrinus amboinensis and Pacific yellow-
tail emperor L. atkinsoni were very common in the BRUVS tapes and can be misidentified as L.
obsoletus. The Spot-cheek emperor L. rubrioperculatus is commonly confused with the Pink-ear
emperor L. lentjan. The small lethrinids L. rubrioperculatus, L. ravus and L. semisinctus are easily
mistaken for one another, as are the long-nosed emperors L. microdon and L. olivaceus (see
Carpenter and Allen 1989).
Lethrinidae - Fishing Effort by Depth
1.6
DF SF
1.2
0.8
0.4
0.0
Abundance (DblSqr( # hr-1 ) + SE )
sp1
sp2
sp3
sp1
sp2
sp3
G. euanus
G. euanus
G. grandoculis
L. ravus
G. grandoculis
L. ravus
L. nebulosus
L. nebulosus
L. olivaceus
M. grandoculis
L. olivaceus
M. grandoculis
L. amboinensis
L. obsoletus
L. amboinensis
L. obsoletus
L. atkinsoni
L. atkinsoni
L. rubrioperculatus
L. rubrioperculatus
L. xanthochilus
L. xanthochilus
L. erythropterus
L. erythropterus
L. erythracanthus
L. erythracanthus
L. semicinctus
L. semicinctus
1.6
DN SN
1.2
0.8
0.4
0.0
sp1
sp2
sp3
sp1
sp2
sp3
G. euanus
G. euanus
G. grandoculis
L. ravus
G. grandoculis
L. ravus
L. nebulosus
L. nebulosus
L. olivaceus
M. grandoculis
L. olivaceus
M. grandoculis
L. atkinsoni
L. atkinsoni
L. amboinensis
L. obsoletus
L. rubrioperculatus
L. amboinensis
L. obsoletus
L. rubrioperculatus
L. xanthochilus
L. xanthochilus
L. erythropterus
L. erythropterus
L. erythracanthus
L. erythracanthus
L. semicinctus
L. semicinctus
FIGURE 15. Mean number of each species of lethrinid recorded by BRUVS. Abbreviations as for Fig. 14.
Data sets transformed to fourth root values.
There was no significant change in the mean total abundance or species richness of serranids
with depth, although total numbers were greater on unfished reefs than fished reefs (17.19 ±
19Abundance (DblSqr( # hr-1 ) + SE )
0.0
0.5
1.0
1.5
2.0
0.0
0.5
1.0
1.5
2.0
DF
A. rogaa A. rogaa
DN
C. argus C. argus
C. boenak C. boenak
C. formosa C. formosa
C. leopardus C. leopardus
C. miniata C. miniata
C. sp C. sp
C. sp1 C. sp1
C. urodeta C. urodeta
SHARK & FIN-FISH SURVEYS
E. fasciatus E. fasciatus
E. fuscoguttatus E. fuscoguttatus
E. maculatus E. maculatus
E. malabaricus E. malabaricus
E. polyphekadion E. polyphekadion
G. albomarginata G. albomarginata
P. areolatus P. areolatus
P. laevis P. laevis
P. leopardus P. leopardus
P. lori P. lori
P. luzonensis P. luzonensis
P. pascalus P. pascalus
P. sp P. sp
P. sp1 P. sp1
P. sp2 P. sp2
P. sp3 P. sp3
P. sp4 P. sp4
P. sp5 P. sp5
Data sets transformed to fourth root values.
P. sp6 P. sp6
s. sp2 s. sp2
V. albimarginata V. albimarginata
V. louti V. louti
20
A. rogaa A. rogaa
C. argus C. argus
C. boenak C. boenak
SF
SN
C. formosa C. formosa
C. leopardus C. leopardus
C. miniata C. miniata
C. sp C. sp
C. sp1 C. sp1
C. urodeta C. urodeta
E. fasciatus E. fasciatus
Serranidae - Fishing Effort by Depth
E. fuscoguttatus E. fuscoguttatus
E. maculatus E. maculatus
E. malabaricus E. malabaricus
E. polyphekadion E. polyphekadion
G. albomarginata G. albomarginata
of mean abundances for each species of serranid are shown in Fig. 16.
P. areolatus P. areolatus
species tended to be more abundant in shallow, fished habitats (Fig. 17).
P. laevis P. laevis
P. leopardus P. leopardus
P. lori P. lori
P. luzonensis P. luzonensis
P. pascalus P. pascalus
P. sp P. sp
P. sp1 P. sp1
P. sp2 P. sp2
P. sp3 P. sp3
P. sp4 P. sp4
P. sp5 P. sp5
P. sp6 P. sp6
9.232 SE vs 1.25 ± 0.254 SE). Richness did not vary with fishing effort, with 19 species
s. sp2 s. sp2
V. albimarginata V. albimarginata
recorded on fished reefs and 22 on unfished reefs. Overall, the serranids were the most
V. louti V. louti
with depth, rather than with fishing history of reefs. Changes in the abundances of Lutjanus
PCA found that variation in abundance of the 6 species that were recorded as the principal
targets of Indonesian fishing in the late 1980’s by Russell and Vail (1988) occurred primarily
diverse family of reef fishes encountered in the study, with a total of 31 species recorded in
deployments, while both lutjanids and lethrinids were each represented by 17 species. Plots
FIGURE 16. Mean number of each species of serranid recorded by BRUVS. Abbreviations as for Fig. 14.
bohar, L. gibbus and L. decussatus contributed most to variation in the data set and these three
MEEKAN, CAPPO, CARLETON & MARRIOTTSHARK & FIN-FISH SURVEYS MEEKAN, CAPPO, CARLETON & MARRIOTT
DF
DN
SF
SN
L. bohar
L. decussatus
C. argus
DFDN
SN
T. lymma
SF
L. obsoletus
Dim 2 19.9 %
L. gibbus
Dim 1 61.71 %
FIGURE 17. PCA on abundance (# hr-1) of 6 species that were the primary targets of Indonesian
fishermen. Data sets transformed to 4th root values. Abbreviations as for Fig. 12.
21SHARK & FIN-FISH SURVEYS MEEKAN, CAPPO, CARLETON & MARRIOTT
DISCUSSION
Baited remote underwater video stations (BRUVS) provide a simple and non-destructive
means to assess the abundance of sharks and selected reef fishes on the oceanic coral reefs
and shoals of northern Australia. Our results confirm those of Meekan and Cappo (2004),
who showed that shark assemblages tended to increase in diversity with depth, so that the
greatest number of species was recorded in water depths (>30m) typically beyond the range
of other non-destructive techniques such as underwater visual counts (UVC) by SCUBA
divers. BRUVS could also be deployed easily in habitats such as open blue water beyond the
edges of reef drop-offs that present significant safety challenges to divers. Additionally,
because BRUVS use baits to attract sharks and reef fishes, they sample those species likely to
be most affected by fishing activity, providing a visual record that can be archived for later
analysis. As there is no need for an observer to be present, BRUVS avoid the issue of diver-
avoidance behaviour, particularly by serranids, large labrids and lutjanids that are hunted by
Indonesian spearfishermen (Russell and Vail 1988). The extent to which such behaviours
introduce bias and confound abundance estimates by UVC techniques is unknown, but are
likely to be significant in areas that are heavily fished.
Despite these advantages, it is important to recognise that, like all sampling techniques,
BRUVS also have limitations (see Willis et al. 2000, Cappo et al. 2003 for reviews). One
principal issue is that BRUVS can provide only a relative estimate of abundance, since the area
from which sharks and fishes are attracted to the bait bag and camera is unknown (but could
be modelled). However, our results, in combination with those of Meekan and Cappo (2004)
show that BRUVS provide a cost-effective and rapid means to estimate the relative abundance
patterns of sharks and reef fishes (including rare species) over a wide range of reef and open
water habitats, most of which are beyond the range of SCUBA divers.
Comparison of Shark Assemblages on Reefs within
the MOU74 Box with the Rowley Shoals
Our results confirm the conclusion of Meekan and Cappo (2004) that there is a major
difference in abundance of sharks between reefs in the MOU74 Box and the Rowley Shoals.
Although numbers of reef sharks (principally the whitetip reef shark Triaenodon obesus and the
grey reef shark Carcharhinus amblyrhynchos) were similar in the shallow habitats of fished
(Scott), historically fished (Ashmore, Cartier) and unfished (Rowley Shoals) reefs, there was a
highly significant decline in abundance in deep reef habitats on all fished reefs. The average
numbers of sharks recorded in deep reef habitats at the unfished Rowley Shoals were
consistent, averaging approximately 1.3 hr-1 at all sites, a value that was more than 4 times that
of the same habitat at Scott Reef and more than twice that of Ashmore Reef. Counts at
Cartier Reef approached those of the Rowley Shoals, but these were strongly influenced by
22SHARK & FIN-FISH SURVEYS MEEKAN, CAPPO, CARLETON & MARRIOTT
the low number of BRUVS deployments on this small reef combined with the presence of a
single school of juvenile grey reef sharks in one site.
Differences in abundance of sharks between fished and unfished reefs were most striking in
the 2003 pilot surveys when BRUVS were hung from the surface in open water off reef drop
offs. In this mode, Meekan and Cappo (2004) recorded 5.3 sharks hr-1 (nearly all Carcharhinus
amblyrhynchos and C. albimarginatus) at Mermaid Reef, while no sharks were recorded in open
water deployments at Scott Reef or in the 2004 survey of Ashmore Reef. Replication of open
water deployments at the remaining reefs of the Rowley Shoals were prevented by the
reassignment of the Customs vessel to fisheries patrol functions midway during the field trip in
2004.
Over-fishing is the most plausible explanation of differences in the composition and abundance
of shark assemblages between the MOU74 Box and the Rowley Shoals. Indonesian shark
fishermen typically deploy longlines in deep reef areas and in open water adjacent to reefs
(Russell and Vail 1988), and these were the habitats where the differences in abundance of
sharks between the MOU74 Box and the Rowley Shoals were most apparent. Notably,
preferred target species such as the silvertip whaler shark Carcharhinus albimarginatus were not
sighted at any reef in the MOU74 Box, while hammerhead sharks occurred only in low
numbers. Our results confirm the suggestion of other studies that overfishing has seriously
depleted the abundance of shark populations in the MOU74 Box (Dennis et al. 2005, Fox and
Sen 2002, Russell and Vail 1988, Wallner and McLoughlin 1996). For the silvertip whaler shark,
Indonesian fishing may have led to extirpation of this species in parts of its former range.
Legislation to protect Ashmore Reef from fishing by Indonesians was enacted in 1988.
Permanent deployment of a Customs vessel at the reef in the last decade has meant that there
has been regular enforcement of the protected status of the reef. Similarly, Cartier Reef has
been protected from all forms of fishing since 2000 and patrols visit the reef to ensure that
illegal poaching does not take place. Despite the potential for some illegal fishing to still occur,
our study suggests that there has been some recovery of shark populations at these reefs. In
deep water, where the effects of fishing are pronounced, total numbers of sharks at both
Ashmore and Cartier Reefs are now significantly greater than those of Scott Reef, where
fishing continues. At Cartier Reef, shark numbers in deep water approached those of the reefs
of the Rowley Shoals, although this conclusion must be treated with caution due to the very
limited numbers of BRUVS deployed at this site. The recovery of these deep habitats may
have been aided by the presence of sharks in shallow water, where populations may have been
less affected by fishing. It is important to note however, that this recovery does not include
some components of the assemblage, such as Carcharhinus albimarginatus, the tiger shark
Galeocerdo cuvier, or two species of hammerhead shark (Sphyrna lewini and S. mokarran).
Furthermore, in open water just beyond the reef, there has been no apparent recovery of
populations whatsoever, despite almost 20 years of legislative protection.
23SHARK & FIN-FISH SURVEYS MEEKAN, CAPPO, CARLETON & MARRIOTT
The extirpation of sharks from tropical waters by fishing is an increasingly common event that
has been occurring over enormous areas of the tropical Pacific and Western Atlantic in recent
years (Baum et al. 2003, Myers and Worm 2003). Our results show that the effects and
recovery from fishing do not occur in a uniform fashion across all components of shark
assemblages. Whitetip sharks (Triaenodon obesus) were common on both fished and unfished
reefs, reflecting that they tend to be a territorial, widely dispersed species that principally
occurs in shallow habitats, and as a result are less vulnerable to Indonesian fishing. In contrast,
grey reef sharks and silvertip whalers are curious and aggressive species that tend to aggregate
in response to underwater noises (Meekan and Cappo 2004) and are most abundant in deeper
habitats near reef drop offs. These attributes are likely to increase their susceptibility to
capture, with the consequence that they occur in very low numbers on both historically fished
and fished reefs. In deep habitats, populations of grey reef sharks showed some evidence of
recovery while silvertip sharks did not. Recovery of silvertip whalers may occur from
deepwater habitats around the reef bases, because the species has been recorded in depths of
800m (Last and Stevens 1994). There may be territorial behaviour of these two species with
the consequence that there is less replenishment of new individuals from populations
unaffected by fishing. Confirmation of this suggestion awaits tagging and genetic studies that
can monitor the migratory pathways and diurnal movements of these sharks.
Our findings have some important implications for management strategies of these reefs. They
suggest that protection of reefs under the Marine Protected Areas system (combined with
enforcement of that status) can have a significant impact on the recovery of shark populations,
although recovery will differ among species. Unfortunately, we lack critical information on the
movement and migratory patterns of reef sharks that would allow us to speculate on the
spatial scale at which protection should occur. Evidence from Ashmore and Cartier Reefs
suggests that protection of single reefs may be sufficient to allow populations of grey reef and
whitetip sharks to recover. This appears not to be the case for silvertip and hammerhead
sharks. For these species, protection may need to encompass a number of different reefs that
cover the expanse of the home ranges of these species. Due to the total lack of migration and
movement pattern data for these sharks, we have no idea how large these areas might be.
Despite the lack of movement data, our results clearly show that the MPA status of these
reefs is extremely useful, since they offer a baseline against which the effects of shark fishing
on northern reefs and the efficacy of management regimes such as MPAs in returning reefs to
original populations can be assessed (Meekan and Cappo 2004).However, in the last 5 yrs
there has been a rapid increase in the incidence of illegal shark fishing by Indonesians
throughout northern Australian waters. Other surveys by AIMS (Heyward and Cappo et al.
unpubl. data) on the nearby Karmt and Sahul shoals suggest that this fishing has had the same
effects on silvertip whaler shark populations as those seen within the MOU74 Box. Recently,
sighting of Indonesian vessels by Customs and tourist operators indicate that shark fishermen
are now beginning to target sharks on Rowley Shoals reefs. Given the importance of these
reefs as baselines, and the degraded state of shark populations in equivalent habitats
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