Surveys of Shark and Fin-fish abundance on reefs within the MOU74 Box and Rowley Shoals using Baited Remote Underwater Video Systems

Surveys of Shark and Fin-fish abundance on reefs within the MOU74 Box and Rowley Shoals using Baited Remote Underwater Video Systems

Surveys of Shark and Fin-fish abundance on reefs within the MOU74 Box and Rowley Shoals using Baited Remote Underwater Video Systems

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 2006

SHARK & 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 i 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

SHARK & FIN-FISH SURVEYS MEEKAN, CAPPO, CARLETON & MARRIOTT ii LIST OF FIGURES ANDTABLES 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

SHARK & FIN-FISH SURVEYS MEEKAN, CAPPO, CARLETON & MARRIOTT iii 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.

SHARK & FIN-FISH SURVEYS MEEKAN, CAPPO, CARLETON & MARRIOTT iv 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

SHARK & FIN-FISH SURVEYS MEEKAN, CAPPO, CARLETON & MARRIOTT v 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.

SHARK & FIN-FISH SURVEYS MEEKAN, CAPPO, CARLETON & MARRIOTT 1 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

SHARK & FIN-FISH SURVEYS MEEKAN, CAPPO, CARLETON & MARRIOTT 2 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 3 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.

SHARK & FIN-FISH SURVEYS MEEKAN, CAPPO, CARLETON & MARRIOTT 4 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.

SHARK & FIN-FISH SURVEYS MEEKAN, CAPPO, CARLETON & MARRIOTT 5 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.

6 TABLE 1. Summary of benthic BRUVS deployment and number of sharks sighted by reef and species. Location N sets Effort (hrs of tape) Min Depth (m) Max Depth (m) Alopias pelagicus Carcharhinus albimarginatus C. amblyrhynchos Galeocerdo cuvier Hemipristis elongata Hemitriakis falcata Nebrius ferrugineus Stegostoma fasciatum Sphyrna lewini Sphyrna mokarran Triaenodon obesus 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

SHARK & FIN-FISH SURVEYS MEEKAN, CAPPO, CARLETON & MARRIOTT 7 FIGURE 2. Location of deployment sites at Ashmore, Cartier, Scott Reefs (MOU74 Box) and Mermaid, Clerke and Imperieuse Reefs (Rowley Shoals).

SHARK & FIN-FISH SURVEYS MEEKAN, CAPPO, CARLETON & MARRIOTT 8 A 0 10 20 30 40 50 60 70 Depth ( m ) 2 4 6 8 10 12 No of obs B 7.11 8.34 9.57 10.80 12.03 13.27 14.50 15.73 16.96 18.19 19.42 Time 4 8 12 16 20 24 28 32 No of obs FIGURE 3. A. Number of deployments of BRUVS by depth at all sites. B. Deployments of BRUVS by time of day at all sites.

SHARK & FIN-FISH SURVEYS MEEKAN, CAPPO, CARLETON & MARRIOTT 9 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 Abundance ( # hr -1 + SE ) A. pelagicus C. albimarginatus C. amblyrhynchos G. cuvier H. elongata H. falcata N. ferrugineus S. fasciatum S. lewini S. mokarran T. obesus Shallow 0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 A. pelagicus C. albimarginatus C. amblyrhynchos G. cuvier H. elongata H. falcata N. ferrugineus S. fasciatum S. lewini S. mokarran T. obesus 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

SHARK & FIN-FISH SURVEYS MEEKAN, CAPPO, CARLETON & MARRIOTT 10 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) FAshDAM12 FAshDPM9 NImpDAM26 NImpDPM23 NMerSAM3 FAshDPM8 NCleDPM21 NImpDAM22 NImpDAM28 NImpDPM24 NMerDPM10 FAshDAM6 FCarDAM15 FAshSAM13 FAshSAM3 NImpSAM30 FAshDPM10 NMerSPM7 FCarSPM11 NCleDAM19 NMerSAM11 FAshDPM16 FSouDPM46 NCleDAM18 NCleDPM20 NMerDAM9 NImpDAM25 FAshSPM1 FAshDAM7 FAshSPM2 FAshSPM4 FAshSPM5 FAshSPM8 FSouDAM47 FSouDAM45 0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 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.

SHARK & FIN-FISH SURVEYS MEEKAN, CAPPO, CARLETON & MARRIOTT 11 A Dim 1 45.57 % Dim 2 24.1 % Deep Shallow C. albimarginatus C. amblyrhynchos G. cuvier H. falcata S. lewini S. mokarran T. obesus B Dim 1 45.57 % Dim 2 24.1 % Fished Not Fished C. albimarginatus C. amblyrhynchos H. falcata S. lewini S. mokarran T. obesus C Dim 1 45.57 % Dim 2 24.1 % DF DN SF SN C. albimarginatus C. amblyrhynchos H. falcata S. lewini S. mokarran T. obesus 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.

SHARK & FIN-FISH SURVEYS MEEKAN, CAPPO, CARLETON & MARRIOTT 12 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 Abundance ( # hr -1 + SE ) A. pelagicus C.

albimarginatus C. amblyrhynchos G. cuvier H. elongata H. falcata N. ferrugineus S.

fasciatum S. lewini S. mokarran T. obesus Fished 0.0 0.1 0.2 0.3 0.4 0.5 A. pelagicus C. albimarginatus C. amblyrhynchos G. cuvier H. elongata H. falcata N. ferrugineus S. fasciatum S. lewini S. mokarran T. obesus 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.

SHARK & FIN-FISH SURVEYS MEEKAN, CAPPO, CARLETON & MARRIOTT 13 Location Abundance ( # hr -1 + SE ) A. pelagicus C. albimarginatus C.

amblyrhynchos G. cuvier H. elongata H. falcata N. ferrugineus S. fasciatum S. lewini S. mokarran T. obesus 0.0 0.4 0.8 1.2 1.6 2.0 Ashmore Reef A. pelagicus C. albimarginatus C. amblyrhynchos G. cuvier H. elongata H. falcata N. ferrugineus S. fasciatum S. lewini S. mokarran T. obesus Cartier Reef A. pelagicus C. albimarginatus C. amblyrhynchos G. cuvier H. elongata H. falcata N. ferrugineus S. fasciatum S. lewini S. mokarran T. obesus South Scott Reef A.

pelagicus C. albimarginatus C. amblyrhynchos G. cuvier H. elongata H. falcata N. ferrugineus S. fasciatum S. lewini S. mokarran T. obesus 0.0 0.4 0.8 1.2 1.6 2.0 Clerke Reef A. pelagicus C. albimarginatus C. amblyrhynchos G. cuvier H. elongata H. falcata N. ferrugineus S. fasciatum S. lewini S. mokarran T. obesus Imperieuse Reef A. pelagicus C. albimarginatus C. amblyrhynchos G. cuvier H. elongata H. falcata N. ferrugineus S. fasciatum S. lewini S. mokarran T. obesus Mermaid Reef 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

SHARK & FIN-FISH SURVEYS MEEKAN, CAPPO, CARLETON & MARRIOTT 14 Deep Abundance ( # hr -1 + SE ) Ashmore Cartier Scott Clerke Imperieuse Mermaid 0.0 0.4 0.8 1.2 1.6 2.0 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).

SHARK & FIN-FISH SURVEYS MEEKAN, CAPPO, CARLETON & MARRIOTT 15 Demersals (#hr-1 ) by Sites within Reefs - Fourth Root transformed, Bray-Curtis, UPGMA (beta=-0.1) FAshDAM12 FAshDPM16 FCarDAM15 NCleDAM19 NImpDAM26 NImpDPM24 FAshDAM6 FAshDPM10 FAshSPM1 NCleDAM18 NImpDAM22 NCleDPM21 NImpDAM28 NImpDPM23 NCleDPM20 FAshDAM7 FAshDPM9 FAshSPM8 NImpDAM25 FAshDPM8 FAshSAM13 FAshSAM3 FAshSPM2 FAshSPM5 FAshSPM4 FCarSPM11 NImpSAM26 NImpSAM30 NImpSPM27 0.0 0.2 0.4 0.6 0.8 1.0 1.2 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. Dim 1 21.68 % Dim 2 12.9 % Deep Shallow A. furca C. leopardus L. amboinensis L. bohar L. decussatus L. erythropterus L. gibbus L. olivaceus L. ravus L. rubrioperculatus L. semicinctus M. grandoculis P. leopardus P. lori P. pascalus 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.

SHARK & FIN-FISH SURVEYS MEEKAN, CAPPO, CARLETON & MARRIOTT 16 A. Only Lutjanidae plotted A. furca A. rutilans A. virescens L. bohar L. gibbus L. kasmira L. monostigma L. quinquelineatus L. semicinctus L. vitta M. macularis M. niger S. nematophorus -0.9 -0.6 -0.3 0.0 0.3 0.6 Axis 1 -0.8 -0.4 0.0 0.4 0.8 Axis 2 Fished Not Fished L. fulviflamma L. decussatus B. Only Lethrinidae plotted G. euanus L. amboinensis L. atkinsoni L. erythracanthus L. erythropterus L. nebulosus L. obsoletus L. olivaceus L. ravus L. rubrioperculatus L. semicinctus L. xanthochilus M. grandoculis -0.8 -0.4 0.0 0.4 0.8 Axis 1 -0.8 -0.4 0.0 0.4 0.8 Axis 2 Fished Not Fished G.

grandoculis

SHARK & FIN-FISH SURVEYS MEEKAN, CAPPO, CARLETON & MARRIOTT 17 C. Only Serranidae plotted C. argus C. boenak C. formosa C. leopardus C. miniata C. sp C. sp1 C. urodeta E. fasciatus E. fuscoguttatus E. maculatus E. polyphekadion P. leopardus P. lori P. pascalus P. sp P. sp1 P. sp6 V. albimarginata V. louti -0.9 -0.6 -0.3 0.0 0.3 0.6 Axis 1 -0.8 -0.4 0.0 0.4 0.8 Axis 2 DF DN SF SN Fished Not Fished G. albomarginata E. malabaricus P. areolatus 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).

SHARK & FIN-FISH SURVEYS MEEKAN, CAPPO, CARLETON & MARRIOTT 18 Sampling Effort by Depth Abundance ( # hr -1 + SE ) Lutjanidae Lethrinidae Serranidae DF 2 4 6 8 10 12 14 16 18 20 Lutjanidae Lethrinidae Serranidae SF Lutjanidae Lethrinidae Serranidae DN 2 4 6 8 10 12 14 16 18 20 Lutjanidae Lethrinidae Serranidae SN FIGURE 13. Mean number per hour of lutjanids, lethrinids and serranids recorded by BRUVS. DF – deep fished, SF – shallow fished, DN – deep unfished, SN – shallow unfished. Lutjanidae - Fishing Effort by Depth Abundance (DblSqr( # hr -1 ) + SE ) A. furca A.

rutilans A. virescens L.

bohar L. decussatus L. fulviflamma L. gibbus L. kasmira L. monostigma L. quinquelineatus L. rivulatus L. semicinctus L. vitta M. macularis M. niger M. sp S. nematophorus 0.0 0.3 0.6 0.9 1.2 1.5 DF A. furca A. rutilans A. virescens L. bohar L. decussatus L. fulviflamma L. gibbus L. kasmira L. monostigma L. quinquelineatus L. rivulatus L. semicinctus L. vitta M. macularis M. niger M. sp S. nematophorus SF A. furca A. rutilans A. virescens L. bohar L. decussatus L. fulviflamma L. gibbus L. kasmira L. monostigma L. quinquelineatus L. rivulatus L. semicinctus L. vitta M. macularis M.

niger M. sp S. nematophorus 0.0 0.3 0.6 0.9 1.2 1.5 DN A. furca A. rutilans A. virescens L. bohar L. decussatus L. fulviflamma L. gibbus L. kasmira L. monostigma L. quinquelineatus L. rivulatus L. semicinctus L. vitta M. macularis M. niger M. sp S. nematophorus SN 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 values.

SHARK & FIN-FISH SURVEYS MEEKAN, CAPPO, CARLETON & MARRIOTT 19 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 Abundance (DblSqr( # hr -1 ) + SE ) G. euanus G. grandoculis L. amboinensis L. atkinsoni L. erythracanthus L. erythropterus L. nebulosus L. obsoletus L. olivaceus L. ravus L. rubrioperculatus L. semicinctus L. xanthochilus M. grandoculis sp1 sp2 sp3 0.0 0.4 0.8 1.2 1.6 DF G. euanus G. grandoculis L. amboinensis L. atkinsoni L. erythracanthus L. erythropterus L. nebulosus L. obsoletus L. olivaceus L. ravus L. rubrioperculatus L. semicinctus L. xanthochilus M. grandoculis sp1 sp2 sp3 SF G. euanus G. grandoculis L. amboinensis L. atkinsoni L. erythracanthus L.

erythropterus L. nebulosus L. obsoletus L. olivaceus L. ravus L. rubrioperculatus L. semicinctus L. xanthochilus M. grandoculis sp1 sp2 sp3 0.0 0.4 0.8 1.2 1.6 DN G. euanus G. grandoculis L. amboinensis L. atkinsoni L. erythracanthus L. erythropterus L. nebulosus L. obsoletus L. olivaceus L. ravus L. rubrioperculatus L. semicinctus L. xanthochilus M. grandoculis sp1 sp2 sp3 SN 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 ±

SHARK & FIN-FISH SURVEYS MEEKAN, CAPPO, CARLETON & MARRIOTT 20 9.232 SE vs 1.25 ± 0.254 SE). Richness did not vary with fishing effort, with 19 species recorded on fished reefs and 22 on unfished reefs. Overall, the serranids were the most 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 of mean abundances for each species of serranid are shown in Fig. 16. Serranidae - Fishing Effort by Depth Abundance (DblSqr( # hr -1 ) + SE ) A. rogaa C.

argus C.

boenak C. formosa C. leopardus C. miniata C. sp C. sp1 C. urodeta E. fasciatus E. fuscoguttatus E. maculatus E. malabaricus E. polyphekadion G. albomarginata P. areolatus P. laevis P. leopardus P. lori P. luzonensis P. pascalus P. sp P. sp1 P. sp2 P. sp3 P. sp4 P. sp5 P. sp6 s. sp2 V. albimarginata V. louti 0.0 0.5 1.0 1.5 2.0 DF A. rogaa C. argus C. boenak C. formosa C. leopardus C. miniata C. sp C. sp1 C. urodeta E. fasciatus E. fuscoguttatus E. maculatus E. malabaricus E. polyphekadion G. albomarginata P. areolatus P. laevis P. leopardus P. lori P. luzonensis P. pascalus P. sp P. sp1 P.

sp2 P. sp3 P.

sp4 P. sp5 P. sp6 s. sp2 V. albimarginata V. louti SF A. rogaa C. argus C. boenak C. formosa C. leopardus C. miniata C. sp C. sp1 C. urodeta E. fasciatus E. fuscoguttatus E. maculatus E. malabaricus E. polyphekadion G. albomarginata P. areolatus P. laevis P. leopardus P. lori P. luzonensis P. pascalus P. sp P. sp1 P. sp2 P. sp3 P. sp4 P. sp5 P. sp6 s. sp2 V. albimarginata V. louti 0.0 0.5 1.0 1.5 2.0 DN A. rogaa C. argus C. boenak C. formosa C. leopardus C. miniata C. sp C. sp1 C. urodeta E. fasciatus E. fuscoguttatus E. maculatus E. malabaricus E. polyphekadion G. albomarginata P. areolatus P.

laevis P. leopardus P.

lori P. luzonensis P. pascalus P. sp P. sp1 P. sp2 P. sp3 P. sp4 P. sp5 P. sp6 s. sp2 V. albimarginata V. louti SN FIGURE 16. Mean number of each species of serranid recorded by BRUVS. Abbreviations as for Fig. 14. Data sets transformed to fourth root values. 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 with depth, rather than with fishing history of reefs. Changes in the abundances of Lutjanus bohar, L. gibbus and L. decussatus contributed most to variation in the data set and these three species tended to be more abundant in shallow, fished habitats (Fig.

17).

SHARK & FIN-FISH SURVEYS MEEKAN, CAPPO, CARLETON & MARRIOTT 21 Dim 1 61.71 % Dim 2 19.9 % DFDN SF SN C. argus L. bohar L. decussatus L. gibbus L. obsoletus T. lymma DF DN SF SN 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.

SHARK & FIN-FISH SURVEYS MEEKAN, CAPPO, CARLETON & MARRIOTT 22 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

SHARK & FIN-FISH SURVEYS MEEKAN, CAPPO, CARLETON & MARRIOTT 23 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.

SHARK & FIN-FISH SURVEYS MEEKAN, CAPPO, CARLETON & MARRIOTT 24 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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