Anakao fringing reef system: biodiversity and anthroprogenic impacts
Item Type Report
Citation Walker, R. & Fanning, E. (Eds.) Frontier Madagascar Environmental Research. Report 1. 32p.
Publisher Society for Environmental Exploration, UK and Institute of Marine Sciences, Toliara, Madagascar.
Download date 02/10/2021 11:04:39
Link to Item http://hdl.handle.net/1834/897 Frontier Madagascar Environmental Research
REPORT 1
Anakao fringing reef system: biodiversity and anthropogenic impacts
Frontier Madagascar 2003
Frontier Madagascar Environmental Research
REPORT 1
Anakao fringing reef system: biodiversity and anthroprogenic impacts
Walker, R., & Fanning E. (eds)
Frontier-Madagascar
University of Toliara Society for Environmental The Marine Sciences Institute Exploration Madagascar UK
Toliara 2003 Suggested Technical Paper citation:
Frontier Madagascar (2003) Walker, R. & Fanning, E (eds) Anakao fringing reef system: biodiversity and anthropogenic impacts. Frontier Madagascar Environmental Research report 1.. ISSN 1479-120X Society for Environmental Exploration, UK and Institute of Marine Sciences, Toliara, Madagascar.
© Frontier Madagascar
ISSN 1479-120X Anakao Fringing Reef System
Frontier-Madagascar Madagascar, the fourth largest Island on the planet is renowned for its high biological and ecological diversity, characterised by its high abundance of endemic species. Madagascar is one of the poorest nations in the world and very dependent on the resources the natural environment provides. As a result conservation and development work is of paramount importance as efforts are made to preserve an environment under pressure from non-sustainable exploitation. Frontier Madagascar is in the process of carrying out baseline survey work in the southwest coastal region of Madagascar in an effort to provide biological and resource utilisation data for the preparation of sustainable management initiatives for the region.
Institute of Marine Sciences (IH.SM) The Institute Halieautique et des Sciences Marines (IHSM) is part of the University of Toliara, in Madagascar. IHSM is a university centre of learning in the field of marine sciences and runs courses for both undergraduate and postgraduate students. IHSM also provides consultations to government institutions, NGOs and individuals.
The Society for Environmental Exploration (SEE) The Society is a non-profit making company limited by guarantee and was formed in 1989. The Society’s objectives are to advance field research into environmental issues and implement practical projects contributing to the conservation of natural resources. Projects organised by The Society are joint initiatives developed in collaboration with national research agencies in co-operating countries.
Frontier-Madagascar Marine Research Programme (FM MRP) The Society for Environmental Exploration and The Institute Halieautique et des Sciences Marines (IHSM), part of the University of Toliara have been conducting collaborative research into environmental issues since 2000 under the title of Frontier-Madagascar. Frontier-Madagascar conduct research into biological diversity and resource utilisation of both marine and coastal terrestrial environments, of which one component is the Frontier-Madagascar/Darwin Initiative, Madagascar Marine Biodiversity Training Programme. Since October 2001 the initiative has been working with local stakeholders within the marine environment to promotes sustainable resource use through training and education.
FOR MORE INFORMATION Frontier Madagascar BP 413 Toliara Madagascar [email protected]
L’ Institute Halieutique et des Sciences Marines Society for Environmental Exploration (IHSM) 50-52 Rivington Street, London, EC2A 3QP. U.K. Universite de Toliara Tel: +44 20 76 13 24 22 Toliara Fax: +44 20 76 13 29 92 Madagascar E-mail: [email protected] [email protected] Internet: www.frontier.ac.uk Tel: +261 (0) 20 94 43552
Frontier Madagascar Environmental Research Report No. 1 ii Anakao Fringing Reef System
TABLE OF CONTENTS TABLE OF CONTENTS...... iii LIST OF TABLES: ...... i LIST OF FIGURES...... ii EXECUTIVE SUMMARY...... iii ACKNOWLEDGEMENTS...... iv 1. INTRODUCTION ...... 1 1.1 AIMS...... 2 1.2 OBJECTIVES ...... 2 2. METHODOLOGY ...... 3 2.1 STUDY SITE ...... 3 2.2 SURVEY METHODS...... 4 2.2 UVC SURVEY TECHNIQUES...... 4 2.2.1 REEF FISH ...... 4 2.2.1 COMMERCIAL FISH ...... 6 2.2.2 URCHINS ...... 6 2.4 ANALYSIS OF RESULTS...... 7 2.4.1 REEF FISH ...... 7 2.4.2 COMMERCIAL FISH ...... 7 2.4.3 URCHINS ...... 7 3. RESULTS ...... 8 3.1 REEF FISH...... 8 3.1.1 ABUNDANCE...... 8 3.1. 2 SPECIES RICHNESS ...... 9 3.1.3 SHANNON’S DIVERSITY INDICES ...... 10 3.2 COMMERCIAL FISH ...... 10 3.2.1 ABUNDANCE...... 10 2.4.2 DIVERSITY...... 12 3.2.3 BIOMASS ...... 12 3.3 URCHINS ...... 14 3.3.1 ABUNDANCE...... 14 4. DISCUSSION...... 15 4.1 REEF FISH...... 15 4.2 COMMERCIAL FISH ...... 15 4.3 URCHINS ...... 16 5. Conclusion ...... 16 6. References...... 18 7. APPENDICES...... 20 7.1 APPENDIX 1 ...... 20 37.858(S) 38.744(E)...... 20 37.739(S) 38.741(E)...... 20 37.623(S) 38.755(E)...... 20 37.475(S) 38.761(E0 ...... 20 37.212(S) 38.868(E)...... 20 37.071(S) 38.871(E)...... 20 36.975(S) 38.873(E)...... 20 36.855(S) 38.924(E)...... 20 36.744(S) 38.867(E)...... 20 36.644(S) 38.949(E)...... 20 36.519(S) 39.040(E)...... 20 36.418(S) 39.085(E)...... 20
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LIST OF TABLES:
Table 1 List of the 70 reef fish species included in the UVC. 5 Table 2 List of the 12 commercially important fish families included in the UVC. 6 Table 3 List of the 4 urchin species or genera included in UVC 6 Table 4 Abundance, diversity and Shannon Diversity Index of reef fish for the 12 transect 9 (North and South). Table 5 Diversity and abundance of commercial fish families surveyed within the study 11 site.
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LIST OF FIGURES
Figure 1 Study site, northern section of the Anakao fringing reef system, SW Madagascar 3 Figure 2 Abundance and diversity of reef fish across the 12 transects (north and south) 8 Figure 3 Shannon diversity indices of reef fish across the 12 transects (north and south). 10 Figure 4 Total abundance and diversity of commercial fish families across the 12 transects 11 (north and south) Figure 5 Tropic biomass of the 12 families of commercial fish across the 12 transects (north 12 and south). Figure 6 Size class of herbivorous fish across the 12 transects (north and south). 12 Figure 7 Size class of piscivorus fish across the 12 transects (north and south). 13 Figure 8 Size class of invertivorus fish across the 12 transects (north and south). 13 Figure 9 Size class of planktivorus fish across the 12 transects (north and south). 13 Figure 10 Abundance of the 4 urchin species/ genera across the 12 transects (north and south) 14 Figure 11 Biomass of the 4 urchin species/ genera across the 12 transects (north and south) 14
Frontier Madagascar Environmental Research Report No. 1 ii Anakao Fringing Reef System
EXECUTIVE SUMMARY
Twenty four transects were marked by GPS, equally spaced apart across 2.4km of fringing reef, 5km north of the subsistence fishing community of Anakao, south west Madagascar. The study site has suffered impact, a result sedimentation from the draining of the Onilahy River to the north of the site, and extraction by the local traditional reef fishery.
The outer reef slope of the 24 transects were surveyed using underwater visual census (UVC), through SCUBA diving to include 70 species of reef fish, 12 families of fish, exploited by the local fishery (commercial fish) and 4 species or genera of sea urchins. Each group (reef fish, commercial fish and urchins) was repeatedly surveyed 3 times for each transect.
Abundance of reef fish across the 24 transects was non significant the closer one surveyed to the point source pollution (river mouth). Surgeonfish (Acanthuridae) were by far the most dominant family to occur across the 24 transects, with species richness and diversity of reef fish showing little pattern across the whole study area.
Fusiliers (Caesonidae), were the most abundant family of commercial fish across the study site. Commercial fish biomass was generally low across the whole site, herbivores were recorded as the dominant taxa. Most commercial fish were of very small size (10-20cm), with 57.3% of planktivores <10cm in size.
Urchin biomass was considered high for the whole site, with 3 transects recording a biomass of >500kg/ha. Echinostrephus sp comprised 91% of the urchin abundance. Comparisons with previous studies undertaken on other western Indian Ocean coral reefs suggests that the high urchin biomass, plus the dominance of one urchin species (Echinostrephus sp.), the low fish biomass and the high occurrence of smaller fish suggests an imbalance at the ecosystem level brought about by non sustainable fishing pressure.
Anecdotal evidence suggests that high sediment deposition on to the reef is causing added stress to the reef environment. Domination by herbivorus Surgeonfish (Acanthuridae) support the idea, that the reef in places has become dominated by turf algae. The results of this study suggest that this portion of fringing reef is suffering form medium to high anthropogenic impact.
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ACKNOWLEDGEMENTS This report is the culmination of the advice, co-operation, hard work and expertise of many people. In particular acknowledgements are due to the following:
L’INSTITUT HALIEUTIQUE ET DES SCIENCES MARINE (IHSM)
F-M Co-ordinators: Dr. Man Wai Rabenevanana Dr. Mara Edouard Remanevy
SOCIETY FOR ENVIRONMENTAL EXPLORATION (SEE) Managing Director: Ms. Eibleis Fanning Development Programme Manager: Ms. Elizabeth Humphreys Research Programme Manager: Dr. Damon Stanwell-Smith Operations Manager: Mr. Matthew Willson Programme Manager: Ms. Nicola Beharrell
FRONTIER-MADAGASCAR Country Coordinator: Ms. Jemima Stancombe Project Co-ordinator: Ms. Chloe Webster Research Co-ordinator: Mr. Ryan Walker Assistant Research Co-ordinator: Mr Angus McVean Assistant Research Co-ordinator: Mr Gareth Clubb Assistant Research Co-ordinator: Ms. Lucy Verma Assistant Research Co-ordinator: Mr. Craig McCoy Assistant Research Co-ordinator: Ms. Charlotte Rix Assistant Research Co-ordinator: Ms. Gwen Hemary Logistics Manager: Ms. Emily Roberts Logistics Manager: Ms. Jo Morton Dive Officer: Mr. Stuart Cheeseman Dive Officer: Mr. Duncan Ayling Boatman Mr. Patrick Randria Research Assistants Ms. Eluise Andriex, Mr. John Bennett, Mr. Matthew Brennand Roper, Ms. Yvonne Charras, Ms. Louisa Coleman, Mr. Thomas Cuthbert, Ms. Charlene Davies, Ms. Jessica Fedak, Mr. Rory Fraser, Ms. Juliet Gush, Ms. Elizabeth Jackson, Mr. Christopher Jones, Mr. Will Noel, Ms. Elinor Ames, Ms. Rosalind Buckley, Ms. Katherine Burton, Ms. Julie Bygraves, Ms. Jillyan Drummond, Mr. Edward Eastwood, Ms. Elizabeth Gutteridge, Ms. Sally Hanning, Ms. Francesca Hinman, Mr. Luke McMillan, Piyawan Miller, Ms. Sophie Miller, Mr. Thomas Monk, Ms. Jennifer Pope, Ms. Hannah Prior, Ms. Samantha Rex, Ms. Gemma Rowley, Ms. Jennifer Watts, Ms. Rebecca Weeks, Ms. Stephanie Whybrow, Mr. Samuel Yates, Ms. Yvonne Appleyard, Mr. Michael Bloom, Ms. Rhiannon Cottrell, Mr. John Da Mina, Ms. Aisha Dasgupta, Ms. Marie Day, Ms. Rebecca Eastman, Mr. Stefan
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Hatvany, Ms Sandor Hatvany, Mr. Thomas Jeffcoate, Mr. Richard Lee, Ms. Georgina Oliver, Ms. Catherine Prentice, Ms. Roxanne Smee, Ms. Katie Tuite-Dalton, Mr. Tavis Walker, Ms. Catorina Watts, Mr. Liver Wyatt , Ms. Joanna Baldwin, Ms. Holly Barclay, Ms. Clement Bradley, Ms. Tracey Cambridge, Ms. Karen Clarkson, Ms. Jennifer Gray, Ms. Sophie Hoult, Ms. Helen Jackson, Mr. Mark Koojiman, Ms. Eleanor La Trobe-Bateman, Mr. James Lorigan, Mr. Paul Matthews, Ms. Sandra McCord, Ms. Frances Pearson, Ms. Clare Sheppard, Ms. Belinda Shufflebotham, Ms. Anjali Singh, Mr. Richard Willing, Ms. Soo Foo Wong.
We are also grateful to The Institute of Marine Sciences, Toliara.
Editorial Comments: Ms. Nicola Beharrell (SEE)
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1. INTRODUCTION
Madagascar is one of the largest islands in the world, with a coastline of approximately 6000km. Coral reefs cover an area of 2000km2, more than 20% of the coastline (Gabrie et al, 2000), with 1130km of fringing reef (Cooke, et al 2000). The total species diversity of the Malagasy marine fauna and flora is still poorly known (Garie, et al, 2000). The older taxonomic publications based on studies done in the southwest describe more than 6000 species, including approximately 53 genera of corals and 1500 species of fish. Although no systematic work on the total marine biological diversity has been completed a rich diversity would seem reasonable according to Gabrie, et al (2000). Research on Malagasy coral reefs is very fragmented, and at the national level most coral reefs are still poorly known, with the reefs of the southwest coming under the greatest scrutiny from the scientific community. The reef under study faces two major environmental threats, over exploitation by the local traditional fishery, and the influence of sedimentation from the draining of Onilahy River.
This report presents the findings of the assessment of marine fin fish and urchin biodiversity of 2.4km of fringing reef (Fig 1), part of the Anakao fringing reef system. The data was collected between March 2001 and August 2002. In the western Indian ocean the most wide spread and manageable threat to a reef system is over fishing (McClanahan et al, 2000). The traditional coastal fisheries of Madagascar mainly focus on reef formations. These fisheries involve approximately 50,000 fishers operating 22,000 traditional sailing pirogues form 1250 villages. Reef associated species account for 43% of total marine production (DRH/FAO, 1992, Randriamiaana, 1996), with the main ethnic fishing group in the south west region being the Vezo. In 1992 the number of fishers in the Toliara region was estimated at 15,269 operating 8357 traditional sailing vessels (DRH/FAO, 1992). Fin fish production in 1994 amounted to 50,200 tones nationally (Randriamiaana, 1996), with Laroche & Ramanarivo, (1995) stating that annual fin fish yield of the reefs surrounding Toliara to be 12.13 tonnes per km per year, with noticeable signs of over exploitation.
The impacts of traditional fisheries on western Indian Ocean coral reefs has been recently investigated by Watson and Ormond (1994), McClanahan (1997a, 1997b, 1997c, 1998), McClanahan et al (1997) Jennings et al (1995), McClanahan & Kaunda Arara (1996) and McClanahan and Shafir, (1990). Kenyan coral reefs suffering from over fishing typically support high sea urchin abundance with low urchin diversity (McClanahan & Shafir, 1990, McClanahan, 1997a, McClanahan & Muthiga, 1989) due to the removal of top level invertebrate eating predators. This shift in diversity appears to have cascading effects on the entire coral reef ecosystem (McClanahan & Shafir, 1990). Non-sustainable fishing pressure also leads to reduced fin fish diversity and biomass as reported by McClanahan (1997b). Watson & Ormond (1994) also state that fished reefs in Kenya, have a reduced number of larger fish than unfished control sites.
The single greatest human impact on Madagascar’s coastal ecosystems is sedimentation, a result of poor agricultural practices in land (Cooke et al, 2000). Recent acceleration of sedimentation in the Onilahy river basin has been linked to forest cutting and clearing for the development of rice paddies (Tavy) in mountainous northern and eastern regions of the island. Sedimentation studies between 1964 and 1996, conducted on the Grand Racif north of the study site (Fig 1), indicated areas of eroded reef and seagrass bed, and reduced areas of active reef construction (Randriamanatsoa, 1997). A boat based study in by Cockroft & Young (1997), made mainly at inshore sites close to estuaries reports extreme levels of sedimentation
Frontier Madagascar Environmental Research Report No. XX 1 Anakao Fringing Reef System for much of the west coast of Madagascar, noting only 10 out of the 24 transects where coral reefs appeared in good condition. 1.1 AIMS The aim of the survey was to assess the heath of the reef by carrying out an assessment of biodiversity and biomass of fin fish and urchins, with respect to the possible environmental impact of sedimentation, as a result of discharge from the Onilahy River and impact associated with the local traditional fishery. 1.2 OBJECTIVES
To assess:
• Diversity of 70 species of reef fish across 24 transects. • Abundance and biomass of 12 families of commercially important fin fish families across 24 transects. • Abundance and biomass of 4 families/genera of sea urchin across 24 transects.
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2. METHODOLOGY
2.1 STUDY SITE
The study area situated in southwest Madagascar is approximately 20km south of the major port of Toliara (Fig. 1), and 5km north of the fishing village of Anakao. A high proportion of the population of this region live close to the shore, due to the poor agricultural potential of the region, with a heavy dependence the upon natural marine resources the coastal environment provides (Cook et al, 2000). The lagoon area behind the reef is utilised extensively by inter- tidal gleaners from the villages of Soalara and Anakao, and the reef itself is the focus of an active artisanal fishery operated out of Anakao.
Figure 1. Study site, northern section of the Anakao fringing reef system, SW Madagascar.
The Anakao fringing reef forms approximately 1km south of the mouth of the Onilahy River, and runs South for 120km. The study area itself is approximately 2.4km in length, the reef crest is situated approximately 420 to 480m adjacent to the low water mark. The reef crest itself is often exposed at low spring tides and typically slopes off gently to a sandy shelf between 12- 16m in depth, which intern gently slopes out into the Mozambique Channel. The reef includes several gullies and canyons creating an interesting morphology.
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2.2 SURVEY METHODS
The underwater visual census (UVC) consisted of a total of 216 survey dives carried out by marine biologists and trained, non-specialist volunteers over a period of 13 months. The study area was split into 12 transects (Appendix 1), 200m apart and accurately marked using GPS, the whole study area included 2.4km of fringing reef.
The UVC was based on the survey of two life forms; fish and echinoids, with fish being split into two separate groups, reef fish and commercial fish.
2.2 UVC SURVEY TECHNIQUES
The surveys were conducted during periods of high tide during daylight, and completed by buddy teams of SCUBA divers. Each buddy team was dropped into the water on the seaward side of the reef, at the 12 predetermined transects, each of which was divided into a north and a south zone. The surveyors descended until the reef floor was encountered (12-16m depth), then by swimming up the outer reef slope an estimation was made as to the mid-point between the reef crest and reef floor (10-6m depth). The survey then began at this depth, in a predetermined north or south direction. The surveys lasted 30 minutes, and each organism in one of the three categories (Table 1, 2 &3) was surveyed, the surveyor following the profile of the reef, running parallel to the reef crest, swimming slow enough to inspect for each species or families being surveyed. Each surveyor surveyed one of the three categories along a 5m corridor, 2.5m either side of the surveyor, noting down the abundance and size, where necessary, of each of the target species, genera or families. Three repeat surveys were undertaken for each category over the 24 transects (12 North and 12 South).
Due to the fact that accuracy is not 100% when using UVC techniques the method recommended by Sale & Douglas (1981) was adopted for these surveys. Under this method the results of three separate replicate surveys of organisms are combined to produce a “best estimate” of the fauna present. In addition to this, the largest, rather than the mean, number of individuals seen is recorded for each species, genera or family present when analysing the data of the three surveys for each of the 24 transects. From this a best estimate of abundance and diversity can be calculated for each station. The method adopted has been reported to yield an 82% accuracy for recording species and 75% for individuals known to be present (Sale & Douglas, 1981).
2.2.1 REEF FISH
High biological diversity amongst Indian Ocean coral reefs makes it impossible to census every species encountered; therefore it was important to identify certain target species, and families to be included in the UVC. The reef fish survey includes 70 species covering 8 families and occupying 6 tropic levels in the coral reef ecosystem (Table 1). This included piscivores (Serranidae), coralivores (Chaetodonitae), invertivores (Pomacanthidae, Labridae, Balistidae, Mullidae), herbivores and planktivores (Acanthuridae) and spongivores (Zaniclidae). Species included in the census are visually and numerically dominant, without cryptic behaviour, easily identified underwater by non-specialists (Mumby et al, 1995), and are associated with reef slope habitats (English et al, 1994).
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Table 1. List of the 70 reef fish species included in the UVC.
Angelfishes (Pomacanthidae) Ear-spot Pomacanthus chrysurus Emperor Pomacanthus imperator Regal Pygoplites diacanthus Semicircle Pomacanthus semicirculatus Three-spot Apolemichthys trimaculatus Two-spined Centropyge bispinosus Yellowbar Pomacanthus maculosus Butterflyfishes (Chaetodonitae) Bennett’s Chaetodon bennetti Black-backed Chaetodon melannotus Blackburns Chaetodon blackburni Black pyramid Hemitaurichthys zoster Chevroned Chaetodon trifascialis Klein’s Chaetodon kleinii Lined Chaetodon lineolatus Longfin bannerfish Heniochus acuminatus Longnosed Forcipiger longirostris Madagascar Chaetodon madagascariensis Masked bannerfish Heniochus monoceros Meyer’s Chaetodon meyeri Raccoon Chaetodon lunula Redfin Chaetodon trifasciatus Saddleback Chaetodon falcula Somali Chaetodon leucopleura Spotted Chaetodon guttatissimus Teardrop Chaetodon unimaculatus Threadfin Chaetodon auriga Vagabond Chaetodon vagabundus Yellowhead Chaetodon xanthocephalus Zanzibar Chaetodon zanzibarensis Goatfish (Mullidae) Dash-and-dot Parupeneus barberinus Two-barred Parupeneus bifasciatus Yellowsaddle Parupeneus cyclostomus Groupers (Epinephelini) Coral hind Cephalopholis miniata
Lyretail Variola louti Peacock Cephalopholis argus Potato Epinephelus tukula Saddleback Plectropomus laevis Squaretail Plectropomus areatus White spotted Epinephelus punctatus Surgeonfishes (Acanthuridae) Blackstreak Acanthurus nigricauda Blue-lined Acanthurus lineolatus Brushtail tang Zebrasoma scopes Convict Acanthurus triostegus Eyestripe Acanthurus dussumeiri Goldring Ctenochaetus strigosus Lieutenant Acanthurus tennenti Palette Paracanthurus hepatus Bluespine Naso unicornis Powder-blue Acanthurus leucosternon Sailfin tang Zebrasoma desjardinii Striped bristletooth Ctenochaetus striatus Thompson’s Acanthurus thompsoni Orangespine Naso lituratus Spotted Naso brevirostris Triggerfishes (Balistidae) Black Melichthys niger Bluethroat Sufflamen chrysopterus Clown Balistoides conspicillum Mousetache Balistoides viridescens Orange-striped Balistapus undulates Picasso Rhinecanthus aculeatus Redtooth Odonus niger Rippled Pseudobalistes fuscus Scythe Sufflamen bursa Wedge Rhinecanthus rectangulus Yellow margin Psudobalistes flavimarginatus Wrasses (Labridae) Bird Gomphosus caerulus Checkerboard Halichoeres hortulanus Goldbar Thalasomma hebraicum Humphead Cheilinus undulates Moorish idol (Zaniclidae) Zanclus cornutus
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2.2.1 COMMERCIAL FISH
The commercial fish survey focused on 12 different families (Table 2) targeted by the local traditional reef fishery. The families under consideration are those specifically cited by Océan Consultant (2000) as families that are regularly targeted and caught by the local traditional fishers operating in the Toliara region. Several families cited by Ocean Consultant (2000) were disregarded, for example morays were discounted due to their cryptic nature, species that inhabit the reef matrix are under counted by 91% during UVC (Willis, 2001). Demersal species such as flatheads and pelagic species such as needlefish, barracuda and jacks were also disregarded due to nature of the survey methods meaning that most individuals will be missed and therefore under represented by surveyors. The surveyors were required to identify the fish to family level and estimate their size to within a range of 10cm, this was the only information recorded.
Table 2. List of the 12 commercially important fish families included in the UVC.
Common name Family Emperors (Lethrinidae) Fusiliers (Caesonidae) Goatfishes (Mullidae) Groupers (Epinephenili) Parrotfishes (Scaridae) Rabbitfishes (Siganidae). Snappers (Lutjanidae) Soldier/squirrelfishes (Holocentridae) Surgeonfishes (Acanthuridae) Sweetlips (Haemulidae) Triggerfishes (Balistidae) Wrasses (Labridae)
2.2.2 URCHINS
The urchin survey included 4 species or genera (Table 3), indicators to varying environmental conditions within the reef environment (McClanahan, 1994). The four urchins included in the survey were chosen due to their high abundance, and ease of recognition. Abundance of each of the four urchins was recorded by the surveyors.
Species / Genera: Echinostrephus sp, Echinometra mathaei, Tripneustes gratilla, Diadema sp.
Table 3. List of the 4 urchin species or genera included in UVC.
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2.4 ANALYSIS OF RESULTS 2.4.1 REEF FISH
Shannon’s diversity indices were calculated for each transect in order to make some comparison of biodiversity as well as species richness. It was decided that this was a valid statistic to use despite the fact that not all species within the community were sampled, as due to the use of a restricted species list for the surveys, this effectively represented an artificial complete community to be surveyed for each transect.
Subsequent tests for statistical significance were performed on this data using the statistics package SPSS.
Reef fish abundance and species richness was also calculated when analysing the results.
2.4.2 COMMERCIAL FISH
Commercial fish abundance and diversity was calculated when analysing the results. By recording the size class of individuals when surveying commercial fish, biomass was able to be calculated. Size classes were divided into 10cm increments an described above. Lengths to weight calculations were used to give a biomass (wet weight) calculation for each family of commercial fish surveyed for each transect (L-W calculations taken from fishbase). Length calculations were standardised as each fish recorded fell into a 10cm category and was not measured to within 1cm, therefore a standard mean length of 15cm was given to all the records in the 10-20cm category for example, and so on. Due to the methods of the commercial fish survey only recording fish to family level, the length to weight calculations for families has to be standardised. This was done by taking a mean for every species known to occur in the region for each family surveyed
2.4.3 URCHINS
Urchin abundance was calculated when analysing the results. It has been suggested by McClanahan, (1998) that high abundance of sea urchins suggests an imbalance in the reef ecosystem caused by removal of predator species due to over fishing. To test this 100 of each of the 4 species of urchin surveyed were weighed to gain the mean wet weight of each species, therefore allowing the biomass (wet weight) per hectare (ha) of each group of urchin surveyed to be calculated for each station. Each survey area for each station covered approximately 5 x 200m of reef slope, for each station the number of observed urchins of each of the 4 species was multiplied by 20. The weight of this total of urchins, for each species was calculated to give a figure of wet weight urchin biomas per ha.
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3. RESULTS
3.1 REEF FISH 3.1.1 ABUNDANCE The abundance values obtained (calculated as the sum of the maximum totals of each species observed from the 3 surveys) for the 24 transects (1S to 12N) are listed in Table 4 and represented graphically in Figure 2. As can be seen from Figure 2, there is no trend in the overall abundance of reef fish along the survey area running from South to North. This is highlighted by the fact that reef fish abundance was lowest at transect 6S (61 counts) and 7N (59 counts) but was highest at transect 6N (254 counts). Addition of a trend line indicates a slight decline in abundance along the reef area surveyed from South to North, but a regression analysis of this data indicated no statistical significance in reduced abundance of reef fish from a north to south direction.
Abundence and diversity of reef fish Abundence
Sp Diversity
45 Abundence 300 Linear (Abundence) 40 Linear (Sp Diversity) 250 35
30 200
25 150 20
Diversity 15 100
10 50 5
0 0 1S 1N 2S 2N 3S 3N 4S 4N 5S 5N 6S 6N 7S 7N 8S 8N 9S 9N 10S 10N 11S 11N 12S 12N Transect
Figure 2. Abundance and diversity of reef fish across the 12 transects (north and south)
Overall, the surgeonfish (Acanthuridae) were by far the most dominant family with 152 counts made, with the butterflyfish (Chaetodontidae) the next most abundant, with a consistently high abundance (between 27 and 68 counts) throughout the study area. Groupers (Epinephelini) and goatfish (Mullidae), however, were consistently the families with the lowest abundance.
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Table 4. Abundance, diversity and Shannon Diversity Index of reef fish for the 12 transects (North and South).
Reef Fish Station Diversity Abundance SDI
1S 29 118 2.993684 1N 34 134 2.96681 2S 29 106 3.097018 2N 27 104 3.045365 3S 39 149 2.867547 3N 25 108 3.274003 4S 27 157 2.568601 4N 29 201 2.846941 5S 23 106 2.928318 5N 25 107 2.410211 6S 26 61 2.549548 6N 33 254 3.021741 7S 30 119 2.613174 7N 18 59 2.95992 8S 26 91 2.574784 8N 18 75 2.837164 9S 25 95 3.024741 9N 29 134 2.823404 10S 27 187 3.051513 10N 30 176 2.747212 11S 22 140 2.724705 11N 22 69 2.376221 12S 27 87 2.646469 12N 21 117 2.975718
3.1. 2 SPECIES RICHNESS
Species richness (taken as the total number of species observed during the 3 surveys) peaks at transects 3S, with 39 species recorded (Fig. 2) and is at its lowest at station 7N and 8N with 18 species recorded.
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Shannon diversity indices for the 12 transects
0.6
0.5
0.4
0.3
0.2 Shannon diversity indices Shannon diversity 0.1
0 1N 2S 2N 3S 3N 4S 4N 5S 5N 6S 6N 7S 7N 8S 8N 9S 9N 10S 10N 11S 11N 12S 12N Transect
Figure 3. Shannon diversity indices of reef fish across the 12 transects (north and south).
3.1.3 SHANNON’S DIVERSITY INDICES
A list of the Shannon diversity indices obtained for each transect is given in Table 4, these results are also represented graphically in Figure 3. The highest value obtained for Shannon’s diversity statistic was for transect 2N (H’= 3.097 (3 d.p.)) and the lowest value for transect 11S (H’= 2.376 (3 d.p.)). However, as can be seen from Figure 3, the results obtained for this analysis were quite erratic, with no distinct pattern in diversity being apparent except for a slight general decline, as indicated by the trend line on Figure 3. A subsequent regression analysis carried out using SPSS in order to determine whether this trend was significant yielded a p-value of 0.104, indicating that this trend is not statistically significant at the 5% accuracy level.
3.2 COMMERCIAL FISH 3.2.1 ABUNDANCE
Abundance of 12 different families targeted by the local traditional reef fishery shows some variation over the study site. Abundance peaks at transect 11N with 697 counts, lowest abundance occurs at transect 7S with 124 counts. Fusiliers (Caesonidae), were the most abundant family on transect 2S with 519 counts. The trend line (Fig. 4) suggests very little variation in abundance across the 24 transects spanning the study site.
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Total abundence and diversity of commercial fish families
14 800
12 700
600 Abundenc 10
500 8 400 6 Diversity 300
4 200
2 100
0 0 1S 1N 2S 2N 3S 3N 4S 4N 5S 5N 6S 6N 7S 7N 8S 8N 9S 9N 10S 10N 11S 11N 12S 12N Transect
Figure 4. Total abundance and diversity of commercial fish families across the 12 transects (north and south).
Table 5. Diversity and abundance of commercial fish families surveyed in with the study site.
Commercial fish families Station Diversity Abundance 1S 11 179 1N 12 147 2S 12 431 2N 10 569 3S 10 301 3N 10 460 4S 8 420 4N 8 330 5S 9 400 5N 7 399 6S 8 626 6N 12 434 7S 11 124 7N 10 461 8S 10 403 8N 11 498 9S 11 596 9N 9 211 10S 9 414 10N 12 116 11S 10 279 11N 9 697 12S 12 157 12N 11 483
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2.4.2 DIVERSITY
All 12 families were recorded at least once, occurring on 1 or more occasions during the study. transect 1N, 2S, 6N, 10N and 12S showed the highest diversity with all 12 families of commercial fish recorded (Table 5, Fig. 4). The lowest diversity was displayed at transect 5N, the trend line (Fig. 4) shows little variation in diversity from south to north.
3.2.3 BIOMASS
Tropic Biomass of Commercial Fish
400
350 Herbivores Piscivores 300 Invertivores 250 Planktivores 200
150 Biomass (kg) 100
50
0 1S 1N 2S 2N 3S 3N 4S 4N 5S 5N 6S 6N 7S 7N 8S 8N 9S 9N 10S 10N 11S 11N 12S 12N Transect
Figure 5. Tropic biomass of the 12 families of commercial fish across the 12 transects (north and south).
The 12 families surveyed included fish form four tropic levels, herbivores, pescivores, invertivores and planktivores. Biomass is generally low across the survey transects of the reef (Fig. 5), with herbivores the dominant taxa. Biomass peaks at transect 11N with herbivorous commercial fish biomass at 343.2 kg. The mean biomass for herbivorous fish was 46.2 kg (s.d. 74.2) per transect, much greater than 5.9 kg (s.d 6.4), 7.5 kg (s.d. 6.6) and 13.4 kg (s.d. 13.8) for pescivores, invertivores and plantivores respectively. Site 11N was the only transect to not have all fish from 4 tropic levels represented, with a complete absence of pescivores. Many transects recorded a biomass of less than 3kg of pescivores, invertivores and plantivores.
Herbivorus Fish
800 700 600 500 400 300
Abundence 200 100 0 0-10 10-20 20-30 30-40 40-50 50-60 60 70 70 80 80 90 100 Size class (cm)
Figure 6. Size class of herbivorous fish across the 12 transects (north and south).
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Piscivorus Fish
350 300 250 200 150 100 Abundence 50 0 0-10 10-20 20-30 30-40 40-50 50-60 60 70 70 80 80 90 100 Size class (cm)
Figure 7. Size class of piscivorus fish across the 12 transects (north and south).
Invertivorus Fish
250
200
150
100 Abundence 50
0 0-10 10-20 20-30 30-40 40-50 50-60 60 70 70 80 80 90 100 Size class (cm)
Figure 8. Size class of invertivorus fish across the 12 transects (north and south).
Planktivorus Fish
4000 3500 3000 2500 2000 1500
Abundence 1000 500 0 0-10 10-20 20-30 30-40 40-50 50-60 60 70 70 80 80 90 100 Size class (cm)
Figure 9. Size class of planktivorus fish across the 12 transects (north and south).
The size classes recorded for commercial fish occupying the 4 tropic levels across the study area was low. With 35.2% (n=753) of herbivorous fish and 37.1% (n=208) of invertivores recorded in the 10-20cm size class (Fig. 6 & Fig. 8). Well over half the number of plantivores were in the size class of 0-10cm, (57.3%, n=3479) (Fig. 9). Piscivores were consistently the largest tropic class of commercial fish, 45.3% (n=287) were recorded in the 20-30cm size class (Fig. 7).
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3.3 URCHINS 3.3.1 ABUNDANCE
Throughout the study area urchins (echinoderm) were the dominant class of invertebrate, with a maximum count of 160 and 168 at transect 6N and 7S respectively, for the four species included in the census. Figure 10 shows that the rock boring urchin Echinostrephus sp to be consistently the most dominant species, making up 91% (n=1380) of the abundance across the whole site. The variation between the abundance of the 4 species was tested using a one way ANOVA (p= <0.001), suggesting a very strong variation in urchin population sizes amongst the 4 species. Three transects had an urchin biomass of greater than 500 kg per hectare (Fig. 11). Biomass peeked at N11 (799.5 kg/ha.), transect N1 and S12 have high levels of urchin biomass, 607.1 kg/ha. and 588.1 kg/ha. Respectively (Fig. 11). The mean urchin biomass for the whole study was recorded as 308.9 kg/ha (s.d. 200.6), interestingly even though Echinostrephus sp. accounts for 91% of abundance, total biomass was only 22%, due to the size of the species.
Urchins Abundance
Diadema sp 180 Echinometra mathei 160 Tripneustes gratilla 140 Echinostrephus sp 120 100
80
Abundance 60 40
20 0 S1 N1 S2 N2 S3 N3 S4 N4 S5 N5 S6 N6 S7 N7 S8 N8 S9 N9 S10 N10 S11 N11 S12 N12 Transect
Figure 10. Abundance of the 4 urchins species/genera across the 12 transects (north and south).
Urchin Biomass
Diadema sp 900 Echinometra mathei 800 Tripneustes gratilla 700 Echinostrephus sp 600 Total biomass 500 400 300
Biomass (kg/ha.) Biomass 200 100 0 S1 N1 S2 N2 S3 N3 S4 N4 S5 N5 S6 N6 S7 N7 S8 N8 S9 N9 S10 N10 S11 N11 S12 N12 Transect
Figure 11. Biomass of the 4 urchins species/genera across the 12 transects (north and south).
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4. DISCUSSION
4.1 REEF FISH It is possible that the abundance and diversity of reef fish present on the stretch of reef under study maybe negatively influenced by the deposition of sediment discharged into the local marine environment from the Onilahy River mouth. If this sediment was having noticeable micro effect within the study site the diversity and abundance of reef fish would be negatively influenced the closer the one samples to the point source (transect 12N). Visibility on the reef was as low as < 1m on occasions during the study, placing stress on the environment due to low light levels. Visible signs of pressure were evident, for example a high abundance of macro algae (pers. obs), resulting in an abundance of algivourous (herbivores) surgeonfish (Fig 5). A result of nutrient over enrichment and coral degradation due to macro algae out competing coral colonies in a sediment, and therefore nutrient rich environment.
McClanahan and Obura (1997) stated that during their study of a sediment impacted reef in southern Kenya visibility when surveying was only at a minimum of 7m, comparisons between this and our study were difficult to make as the level of sedimentation was deemed low by the authors of the Kenyan study. The added pressure of fish extraction was not an issue for the Kenyan study, as the research was carried out within a marine protected area. At present no data regarding physical sedimentation is available for the Anakao reef, making comparisons of sediment deposition with previous studies difficult.
Estimations can be drawn regarding the potential impacts of sedimentation, from anecdotal observations and environmental indicators such as fish and urchin abundance. Statistically there is no reduction in diversity and abundance of reef fish as you move closer to the point source of sedimentation. This result maybe due to the study site being too small to show any spatial negative impacts, if the study was increased, and surveys carried out stretching further south along the fringing reef an increase in fish diversity and abundance may be evident, the further one samples from the river mouth.
Fishing effort can also have negative implications for the abundance and diversity of non target reef fish. Watson and Ormond (1994) claimed that butterflyfish abundance and diversity was significantly greater in non fished sites compared to fished sites on Kenyan coral reefs. This suggests that fishing pressure creates deficiencies within the reef ecosystem, therefore effecting abundance and diversity amongst target and non target species alike. Watson and Ormonds (1994) study could suggest that fishing pressure alone on the Anakao fringing reef has negative affects amongst diversity and abundance of non target reef fish such as butterflyfish and angelfish. 4.2 COMMERCIAL FISH Biomass of the 12 families of commercial fish within the study site was low. Most fish are very small, in the 10-30 cm size range, at some transects the biomass of the 12 families of commercially exploited fish by the local traditional fishery was as low as < 10kg per transect (approximately 1 km2). Watson and Ormond (1994) reported that on reefs in Kenya exposed to exploitation by traditional fishing communities the mean abundance of all Lethrinus spp (emperors) was over 500 times less than on protected reefs. Lutjanidae (snapper) and 2 species of rabbitfish Variola louti and Cephalopholis argus were significantly more abundant in the protected area. Most commercial fish present in our study were very small in size (Figs. 6-9), as one would expect by the low biomass. Similar work carried out on Kenyan reefs by Watson &
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Ormond (1994) and McClanahan et al, (2000), concluded that over fished reefs are characterised by an absence of larger fish.
One needs to presume that extraction through fishing, or influences due to sedimentation are negatively influencing populations of species targets by fishers on the Anakao fringing reef system. This hypothesis has to be presumed due to supporting evidence from work carried out on similar exploited western Indian Ocean reefs (Watson and Ormond 1994, McClanahan, 1995 & 1998), but is unable to be tested due to the exploitation of every available, exploitable fringing reef system in south west Madagascar, therefore making it impossible to locate a control site. Our results show that herbivorous fin fish taxa, such as Scaridae, Siganidae and Acanthuridae are the most abundant of the four tropic levels of commercial fish surveyed. This could be a possible result of a high algal turf coverage (pers obs) dominating the substrate. 4.3 URCHINS
Sea urchin population variation in the Indian Ocean is thought to be correlated with predation by triggerfish (Balistidae), where the highest levels of predation produce the highest population variation (McClanahan, 1998). McClanahan (1998), states a dominant abundance by one species of urchin reduces overall sea urchin population variation. This can be seen by the domination of Echinostrephus sp throughout the study site. McClanahan, (1989), states that 90% of Echinometra mathaei predation is caused by finfish. Triggerfish (Balistidae) are the most important predators of sea urchins (McClanahan, 1995), with sea urchin abundance and dominance increasing, with an increase in predator free space. Transect 11N recorded an urchin biomass of over 799.5 kg per ha. High urchin biomass can be used as an indicator to chronically over fished reef environments (McClanahan,1998). The general domination of Echinostrphus sp on most transects suggests the absence of any controlling predator species, possibly a result of over fishing.
5. CONCLUSION
Urchin abundance, biomass and diversity, coupled with a low biomass and absence of any larger individuals of commercially targeted fish by the local fishers suggests that fishing pressure within the study site was non sustainable, having a negative effect on the biodiversity, and trophic structure of the Anakao fringing reef.
The abundance and dominance of Echinostephus sp. and the possible decreased abundance of certain reef fish suggests the reef is suffering from second order effects of fishing. Comparisons between fished and unfished sites in Kenya revealed that finfish populations were four times denser in protected than unprotected reefs. Sea urchin populations were >100 times denser and predation rates on sea urchins (E. mathaei), were four times lower in unprotected than protected reefs.
Heavily fished reefs in Kenya display a lower diversity of substrate complexity with sites dominated by algal turf (McClanahan and Shafir, 1990), further evidence that fishing has detrimental top down effects on the ecosystem. The impact of sedimentation is more difficult to quantify without collection of data concerning sedimentation rates. From anecdotal evidence it can be assumed that sediment input form the Onilahy river is having negative effects on diversity and abundance of reef species. Large areas of the reef are reduced to dominance by algae turf with very little visible Scleractinian coral growth. During surveying light penetration
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Clearly it can be concluded that the area of fringing reef under investigation is suffering levels of medium to high anthropogenic impact. It appears due to the low fish biomass and that fishing effort appears to be concentrated on the reefs of the near by island of Nosy Ve, that the reefs have suffered from acute impact over a number of years and fishers now concentrate there efforts else where.
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6. REFERENCES
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McClanahan, TR (1997c) effects of fishing and reef structure on east Africa coral reefs. Proceedings of the 8th International Coral Reef Symposium, 2: 1533-1538.
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7. APPENDICES
7.1 APPENDIX 1
Transect no. GPS point
1 37.858(S) 38.744(E) 2 37.739(S) 38.741(E) 3 37.623(S) 38.755(E) 4 37.475(S) 38.761(E0 5 37.212(S) 38.868(E) 6 37.071(S) 38.871(E) 7 36.975(S) 38.873(E) 8 36.855(S) 38.924(E) 9 36.744(S) 38.867(E) 10 36.644(S) 38.949(E) 11 36.519(S) 39.040(E) 12 36.418(S) 39.085(E)
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