The Status of Invertebrate Resources at Manihiki and Rakahanga Atolls
The Status of Invertebrate Resources at
Manihiki and Rakahanga Atolls, Cook Islands.
By Ngereteina George & Richard Story
September 2014 – Miscellaneous Report: 03/14 Ref No. NG-3/0814 | Inshore & Aquaculture Division Table of Contents
Acknowledgments ...... 2 Summary ...... 3 1.0 Introduction ...... 4 1.1 Pearl oyster (pārau) assessment ...... 4 1.2 Trochus (torōkati) assessment ...... 4 1.3 Rugose Clam (pa’ua) assessment ...... 5 1.4 Sea cucumber (rori) assessment ...... 5 1.5 Management and existing harvest regimes ...... 5 1.6 Objective of the assessment surveys ...... 5 2.0 Methods...... 6 2.1 Survey design ...... 6 2.2 Survey methodologies ...... 6 2.3 Data analysis and reporting ...... 7 3.0 Results and discussion ...... 8 3.1 Survey coverage ...... 8 3.2 Density (all species) ...... 8 3.3 Sea cucumber ...... 9 3.3.1 Density ...... 9 3.3.2 Size distribution ...... 10 3.3.3 Stock estimate for selected sea cucumber species ...... 12 3.4 Blacklip pearl oyster (pārau) ...... 13 3.5 Trochus ...... 13 3.6 Manihiki rugose giant clam (pa’ua) ...... 13 3.6.1 Density and size structure comparison with previous assessments ...... 14 3.6.2 Mortality rates ...... 16 3.7 Crown of thorns ...... 17 4.0 Recommendations ...... 18 5.0 References ...... 20 6.0 Appendices ...... 21 Appendix 1. Locations of survey stations...... 21 Appendix 2. GPS coordinates for stations at each site...... 23 Appendix 3. Summary statistics for the two sites...... 25 Appendix 4. Invertebrate species recorded at the two sites ...... 27 Appendix 5. Summary of MMR unpublished paua data 2002 ...... 31
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Acknowledgments
The Ministry of Marine Resources (MMR) acknowledges with gratitude the partnering and financial support provided by the EU Funded Global Climate Change Alliance: Pacific Small Island States (GCCA: PSIS) Project; Environmental Monitoring to Enhance Community Livelihoods and Build Resilience to Climate Change in Low Lying Atolls of the Cook Islands, for the assessment of invertebrate resources in Manihiki and Rakahanga.
Many have contributed to the completion of this report from, data collection, analysis, interpretation and writing. Many thanks to MMR Secretary, Ben Ponia, Director of Inshore Fisheries and Aquaculture, Kori Raumea for general guidance, MMR staff, Tangi Napara, Tina Weier, Ravengakore Tuteru, Tuteru Taripo, Wireless Pupuke and part-time MMR staff Maui Tairea, for your efforts in the field and GIS work. Franck Magron, and Ian Bertram of Secretariat of the Pacific Community (SPC), for support in GIS and analytical and guidance in drafting the report. Teina Rongo (National Coordinator for the GCCA: PSIS project) for editorial support.
Lastly meitaki kore reka to the Mayors, members of the Island Councils, Executive officers and community of Manihiki and Rakahanga; our work wouldn’t have been successful without your support.
Ministry of Marine Resources PO Box 85, Avarua, Rarotonga, Cook Islands T: +682 28721 F:+682 29721 E: [email protected]
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Summary
This report presents the results of invertebrate resource assessments completed in May 2014. Transect were used to assess resources over six days in Manihiki and three days in Rakahanga. Sea cucumber assemblages are low in terms of species recorded and size structure. Density of lollyfish was close to regional reference density at Manihiki, but three times higher in Rakahanga, although abundant they were small (average size 122 ±0.9 mm). In Manihiki trochus density was 11.5 (± 6.8) per hectare and absent in Rakahanga. There is no potential for commercial fishing of sea cucumber and trochus at this time.
The assessment determined moderate densities of black lip pearl oyster on fringing reefs and patch reefs for both atolls. This data can be added to information currently collected for deep lagoon strata’s in order to estimate total abundance of wild pearl oyster stocks in both atolls.
Giant clam is abundant in Manihiki but rare in Rakahanga. When clams stocks are reduced to low levels stock recovery periods are uncertain. To safe guard this important food resource against recruitment failure there is a need to be cautious when determining large scale community harvest. The current prohibition on the exports of clams from Manihiki and the protection of clam stock by Ra’ui is excellent and encouraged. Other tools the council could consider are long term closures, (Ra’ui), minimum size limits and very moderate harvest quantities determined after resources surveys for large scale community harvest. Permanent resource monitoring sites should be established to regularly assess recruitment of juvenile clams.
Crown of thorns numbers are very low and do not pose a problem at this time.
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1.0 Introduction
Manihiki and Rakahanga are part of six atolls in the northern group of the Cook Islands. Both atolls are approximately 1,225 kilometres from Rarotonga, the capital of the Cook Islands. Communities on both islands purchase food supplies and necessities in bulk for inter-island cargo ships which can be between two to three months between ship visits. Manihiki is served by inter-island aircraft which visits the island on a fortnightly basis, however occasionally there are chartered flights. Communities commute between atolls by outboard powered aluminium boats. Residents have adapted to the isolation, irregular shipping/aircraft services by primarily living off the sea and land.
Manihiki and Rakahanga communities were once heavily involved in pearl production, however due to increased pearl production cost and decreasing pearl value as a result of global trends in production and marketing this activity has subsided to a few fully active farms. Other sources of income include handcraft, export of modest volumes of seafood, small private business and through government employment. Handcraft is primarily produced by women and these valued products are sought after in the Cook Islands. The atolls have significant lagoons with a maximum depth of 70 meters in Manihiki and 33 meters for Rakahanga. The communities carry out similar fishing activities, collecting invertebrates (primarily for clams and lobster), netting, spearfishing, fixed coral fish traps, and several methods of lagoon and deep water hook and line fishing for fin-fish. Fisheries resources are abundant and in times of excess catch, the catch is dried and shipped to Rarotonga as gifts or for sale.
1.1 Pearl oyster (pārau) assessment Some invertebrate assessments have been conducted on Manihiki in the past. Wild pearl oysters (pārau) stocks have been assessed in the 80’s and 90’s by Sims (1988, 1991) and RDA International (1997). Sims assessed six permanent transects in deep lagoon strata repeatedly using SCUBA in 1987, 1988 and 1991 respectively. Average densities were 56,000, 46,500 and 28,700 pearl oysters per hectare for 1987, 1988 and 1991 respectively. Simms reports the decline in wild pearl oyster density (abundance) was due to collecting young oysters for pearl farm production and 8% rate of natural mortality. RDA International provides a population estimate of 3.1 million wild pearl oysters in 1997, however provides no estimates of density. The Ministry of Marine Resources is currently surveying the wild pearl oyster stocks at both islands.
1.2 Trochus (torōkati) assessment Trochus were surveyed in Manihiki by Ponia (1998). Ponia 1998 states that 400 adult trochus were introduced to Manihiki in 1985. Trochus density was higher on the fringing reefs along the western sector of the lagoon compared to other sectors. Trochus densities from transect survey’s on fringing reefs ranged from 80 to 2300/ha and 30/ha on coral patch reefs. Ponia (1998) concluded that the stock was not commercially viable. In 2005 the Manihiki community collected 3.9mt of trochus (cleaned dry weight) for commercial export.
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1.3 Rugose Clam (pa’ua) assessment Langdon (2012) assessed population and size distributions of rugose clams at Manihiki, to advise the Island Council on a sustainable harvest level for a tere party1 to Rarotonga during the constitution celebrations in 2012. Council recorded 5383.5 kg of edible flesh harvested by the community for the constitution event, this equates to around 107,0002 clams harvested for the tere party.
1.4 Sea cucumber (rori) assessment Sea cucumbers have not been quantitatively assessed in Manihiki or Rakahanga. There is growing foreign interest to source supplies of sea cucumber for the production of beche-de- mer. An assessment of sea cucumber resources is therefore important to explore the potential for commercial exploitation.
1.5 Management and existing harvest regimes In Manihiki giant clams, coconut crabs and lobster are prohibited from exports, there are no harvest size limits for these resources for local consumption. Occasionally the Council may permit large scale giant clam community harvest over a limited period (weeks) for community needs. Recent large scale harvest occurred in 2008 and 2012. The 2008 clam harvest was permitted for a short period with no controls on the quantity (kg) or minimum size (length), however MMR officials estimated the total harvest to be in excess of 7 tons of edible flesh. In 2012 Council requested an assessment of clam resources to help determine if the stock would allow for a harvest quantity of around 3-4mt. A limit of two 20 litre buckets was permitted for harvest in Manihiki per households of Manihiki and Rakahanga (one 20litre bucket equates to 22kgs edible clam flesh). Community were encouraged to harvest clams between the 11-15cm size ranges. This was an effort to protect smaller less reproductively mature clams and to protect larger clams as brood stock to contribute to continuous stock recruitment. In Manihiki, around a fifth of the reef flat, fringing reef and lagoon area, from Porea to Motu Akamaru is ra’ui (traditional protected area), where persons cannot pass through or harvest resources in this Ra’ui. In Rakahanga harvesting of giant clams is prohibited, coconut crabs and lobsters are prohibited under Ra’ui for extended periods.
1.6 Objective of the assessment surveys The objective of this study was to collect data to provide estimates of density and size structure of various invertebrate resources of interest in Manihiki and Rakahanga atolls. The goal was to provide quantitative data on the current status of resources to determine whether these resources have the potential for commercial exploitation, and to explore sustainable harvest strategies, should some species be determined viable for commercial harvest. Specifically, the objectives of the surveys were to:
• Assess the status of sea cucumber and other invertebrate resources on Manihiki and Rakahanga.
1 Travelling group of people (e.g. church, youth, sports group etc.) 2 From a study by staff at MMR Manihiki, edible flesh averages 40g for clams between 10-15cm (total shell length), 60g for clams between 15-20cm and 50g for clams between 10-20cm.
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• Provide population estimates of assessed stocks and fishable stock estimates for those sea cucumber species present in commercially viable quantities for decision-making purposes. • Provide status information on the species of importance to subsistence fishing, and determine whether or not there is a need for improved management.
2.0 Methods
2.1 Survey design For ease of comparing information gathered from invertebrate surveys throughout the country, the methods used by Ministry of Marine Resources in the southern Cooks Islands (Raumea et al 2013) were replicated in Manihiki and Rakahanga. At each atoll the reef and lagoon habitats were divided into four sectors (North, East, South and West). In each sector, survey stations were distributed within the fringing reef, lagoon patch reef, reef flat and reef crest zones (Fig 1). Given the limitations (time, manpower) and logistics, no surveys were carried out in the deep lagoon habitat and beyond the surf zones. The reef habitat areas where certain species of invertebrates are most likely to be found were calculated using satellite image and delineating the areas of species preferred habitats.
Figure 1. Profile of a complex reef system (source Pakoa et al 2014). The habitats assessed in Manihiki and Rakahanga, were fringing reef, lagoon patch reef, reef flat and reef crest
2.2 Survey methodologies Reef benthos transects (RBt) were used to assess the presence, abundance and size structure of invertebrate resources. RBt surveys were conducted on fringing reef, lagoon patch reefs, reef flat and reef crest areas to a depth of less than four meters. The method involves two snorkelers equipped with slates for measuring and recording abundance and size of invertebrates within transects. An RBt station comprises six 40 x 1 metre transects spaced approximately 5 metres apart (Fig 2). Species and habitat data were recorded and a waypoint logged to an accuracy of ≤ 10 m for each station.
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Figure 2. Illustration of reef benthos transect method, six 40 x 1 meter transect constitutes a station (source Pakoa et al 2014).
2.3 Data analysis and reporting The Reef Fisheries Integrated Database (RFID) provided by the Secretariat of the Pacific Community (SPC) was used for data entry, validation and analysis. RFID produced information on: species presence by station to assess the number of species recorded; average density by species and stations (to assess abundance); and length frequency by site and species to assess size distribution.
Analysis of stock status indicators follows standard assessments of invertebrate resources described in Pakoa et al (2014) and the Cook Islands reef resources status report (Pinca et al. 2009). In this report density (the number of individuals per unit area) is expressed as individuals per hectare3 (ind/ha-1).
Population estimates were estimated when density and size structure information for particular species were adequate. To estimate population size, the mean density, i.e overall mean density or habitat specific mean density was extrapolated by multiplying the density by the total area of habitat of interest.
3 A hectare is 10,000 square meters (e.g. 100m X 100m), a rugby field is 100m by 68m or 6800 square meters which is just over half a hectare.
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3.0 Results and discussion
3.1 Survey coverage Sixty five and forty seven stations were surveyed in four habitats in Manihiki and Rakahanga respectively (Table 1). In total an area of 1.53 ha-1 and 1.11 ha-1 was surveyed on Manihiki and Rakahanga respectively. It took four surveyors six working days (28th April - 2nd May 2014) to complete the survey in Manihiki and three days (3rd - 5th May 2014) by the same number of surveyors to complete Rakahanga. Locations of survey stations are shown in appendix 1 and GPS waypoints are tabulated in appendix 2.
Table 1. Coverage of reef benthos transects and estimated areas for reef zones/habitats. Number of Area assessed Total habitat Island Habitat Transects stations (m2) area (ha-1) Reef crest 20 120 4800 183.9 Reef flat 15 90 3600 302.11 Manihiki Lagoon patch reef 4 16 640 37.87 Fringing reef 26 156 6240 253.97 Reef crest 20 120 4800 65.35 Rakahanga Reef flat 9 54 2160 184.75 Fringing reef 18 104 4160 89.31 .* Due to the size of lagoon patch reefs in Manihiki, 4 replicate transect were completed per stations as opposed to 6 transect as intended
3.2 Density (all species) Four species of sea cucumbers were recorded in Manihiki and Rakahanga, the most common species was the low value lollyfish (rori toto) (Table 2). Rakahanga had more than triple the overall mean density for lollyfish than Manihiki. Surf redfish (rori puakatoro) were found on the reef crest which is a narrow strip of approximately 60m and 50m width for Manihiki and Rakahanga respectively which extending right around the atolls. Blotched pearl oyster (pipi) appeared abundant in Rakahanga (15,525 ind/ha ± 15,514), however the species displayed an aggregated pattern of distribution and was recorded in two of the 47 stations assessed. Statistic for transects and stations are found in appendix 3.
Table 2. Overall mean density (ind.ha-1) for invertebrates surveyed by RBt in Manihiki and Rakahanga (2014). Pictures of species are found in appendix 4.
Manihiki Rakahanga Groups Species Local name O_Mean SE O_Mean SE H. atra Rori toto 5351 1260 16734 1780 B. vitiensis n/a 3.8 2.7 39.0 27.8 A. mauritiana Rori puakatoro 24.4 10.5 24.8 22.3 Echinoderms B. argus Rori kuru 2.9 1.7 - - S. maculata Veri-aria - - 0.9 0.9 E. diadema Vana 5.1 5.1 3.5 2.1 H. mammillatus ‘Atuke 4.5 4.5 0.9 0.9
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L. multifora ‘Etu-moana 26.0 14.5 - - A. planci Taramea 0.6 0.6 - - T. maxima Pa‘ua 7586 1653 0.9 0.9 P. maculata Pipi 117 100 15525 15514 Bivalves P. margaritifera Pārau 59.3 25.2 34.6 16.1 A. ventricosa Kuku 4393 1809 128 74 P. muricata Kotate - - 332 206 T. niloticus Torōkati 11.5 6.8 - - D. maximum Ungakoa 210 134 - - Gastropods S. luhuanus n/a 1117 525 - - C. tigris Pōre‘o - - 1.8 1.2 n/a: Local name unknown to author or not available O_mean = overall mean density, this is the density for all stations surveyed SE = standard error on the mean
3.3 Sea cucumber
3.3.1 Density Density for lollyfish at Rakahanga was the highest recorded for the islands assessed the Cook Islands during 2012, 2013 and 2014 and is three times greater than the regional reference for reef benthos transect (Table 3). On both atolls the density for surf redfish was 10 times less than the regional reference. Golden sandfish were recorded in Manihiki and Rakahanga but not in other islands assessed in Cook Islands during 2012-2014. The species distribution is limited and was found in the shallow sandy area of Tukao bay (Manihiki) and the lagoon side from Tua I Omoe to Tua I Moturoa (Rakahanga). This species is high value (2013 @ NZD70/kg4 dried weight) and is of interest for aquaculture.
Table 3. Comparison of sea cucumber overall mean densities (individuals per hectare) for Manihiki and Rakahanga with other islands in the Cook Islands and regional references (ind/ha). Brackets indicate standard errors.
R/R/D5 Species Aitutaki# Mangaia# Palmerston# Rarotonga# Mauke* Mitiaro* Manihiki Rakahanga (RBt) 11,138 6285 1634 8543 850 171 5351 16734 Lollyfish 5,600 (975) (1265) (850) (1780) (535) (213) (1260) (1780) 1598 549 81 579 48 38 24 25 Surf redfish 200 (446) (106) (48) (168) (13) (13) (11) (22) 248 2174 731 Greenfish - - - - - 3,500 (59) (891) (356) Flower sea 2279 3658 2875 29 16 - - - na cucumber (1,111) (1,131) (966) (26) (10) White 1417 371 2108 41 14 - - - na snakefish (541) (169) (469) (35) (8)
4 The value fishers processors should expect for large, well processed product (high grade dried beche-de-mer) in 2013. 5 Regional reference densities are an average of the upper 25% of the highest densities from the Pacific dataset held by SPC. Reference density determined by SPC are based on in-water survey records over the period 2002- 2012 and are from over 2000 survey stations, across 91 sites in 17 countries.
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2.9 Tigerfish ------120 (1.7) Golden 3.8 39 ------na sandfish (2.7) (27.8) R/R/D (RBt) = regional reference density for sea cucumbers assessed by reef benthos transects na = information not available # source; Raumea et al 2013 * source; George 2014
3.3.2 Size distribution Population size structures were determined for lollyfish and surf redfish. Lollyfish is an abundant species at both islands, but a low value species (2013 @ NZD14/kg6 dried weight) and surf redfish (2013 @ NZD46/kg dried weight) is a medium value but less abundant species in Manihiki and Rakahanga. Mean size for lollyfish was 129mm and 122mm at Manihiki and Rakahanga respectively (Fig 3). Mean size for surf redfish was 173mm and 129mm at Manihiki and Rakahanga respectively (Fig 4). Note the low sample size for surf redfish at both sites. The mean sizes for lollyfish and surf red fish are small. This could be due to nutrient poor lagoons substrates. Size structure was not determined for the remainder of sea cucumber species recorded due to very low number of records.
6 Lollyfish and surf red fish values are prices in 2013 for large, well processed product (high grade dried beche- de-mer).
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Figure 3. Size distribution for H. atra, lollyfish (rori toto) at Manihiki (top graph) and Rakahanga (bottom graph).
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Figure 4. Size distribution for A. mauritiana, surf redfish (rori puakatoro) at Manihiki (top graph) and Rakahanga (bottom graph).
3.3.3 Stock estimate for selected sea cucumber species Reef zones (habitat) area estimates (Table 1), species densities (Table 2) and weight information reported in Koroa et al 2013 (Table 4) were used to determine population and biomass (Table 5). On both islands surf redfish were predominantly found on reef crest and lollyfish was found in reef flat, lagoon patch reefs and fringing reefs.
The recommended minimum harvest size for lollyfish and surf redfish for commercial export is 200mm live/unprocessed length. For a commercial harvest of sea cucumbers, it is advised that only a portion (no more than 20-30%) of the population above the recommended minimum size be harvested. This proportion can be adjusted as a fishery develops/matures. The proportion of the population of lollyfish greater than 200mm live length is 7.8% and 4.2% at Manihiki and Rakahanga respectively. For surf redfish the proportion of the population greater than 200mm is 2.6% at Manihiki and 6.7% at Rakahanga. Given the low exploitable biomass (Table 5) these resource would not support a commercial fishery which benefits the wider community.
Table 4. Average live weights of lollyfish and surf redfish greater than 200 mm and proportion retained after gutting.
Species Lollyfish Surf redfish Live weight (g) 355 510 % retained after gutting 31 54
Table 5. Population estimates of lollyfish and surf redfish and proportions of populations (numbers and weight) greater than 200 mm in live length for the Maninhiki and Rakahnga. Fishable stock (live and gutted weights) is estimated as 20% of the populations greater than 200 mm.
Lollyfish
Estimate Wet Harvest level 20% of biomass Proportion Population > Island population biomass (mt) > 200mm 200 mm (numbers) (mt) Live weight Gutted/dried Manihiki 4,162,500 7.8% 322,590 115 23 7/0.7 Rakahanga 4,586,000 4.2% 191,250 68 14 4/0.4
Surf Redfish
Estimate Wet Harvest level 20% of Proportion Population > Island population biomass biomass (mt) > 200mm 200 mm (numbers) (mt) Live weight Gutted/dried Manihiki 1592 2.6% 42 <1 - -/- Rakahanga 4560 6.7% 304 <1 - -/-
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3.4 Blacklip pearl oyster (pārau) Abundance information gathered for pearl oysters from this study supplements information gathered for a wild pearl oyster population study currently underway in Manihiki. The overall mean density for pearl oyster at Manihiki is 59.3 (± 25.2) per hectare for all stations in all habitats (reef flat, reef crest, fringing reef and lagoon patch reefs). However the population in Manihiki is not evenly distributed amongst the four shallow habitats assessed. Pearl oyster were absent in the high wave action zones (reef crest and the reef flat) but present in fringing reefs and lagoon patch reefs. The average number of pearl oysters per hectare on fringing reefs and lagoon patch reefs was 256 (± 89.7) and 516 (± 291.3) respectively. Given the estimate habitat areas (Table 1) the population of wild pearl oysters for fringing reefs and lagoon patch reefs is estimated at 65,000 and 20,000 respectively.
Pearl oysters at Rakahanga has similar pattern of distribution to that of Manihiki, was absent in the high wave impacted zone but found on fringing reefs at density of 166.7 (±70.1) per hectare. The estimate population of wild pearl oysters at Rakahanga for fringing reefs is 15,000. This survey did not cover deep lagoon habitats therefore unable to provide pearl oyster population estimates for this important habitat in Manihiki and Rakahanga.
3.5 Trochus Trochus are found on coral reefs and feed on turf algae. Trochus was found in very low densities along the fringing reef on the west side of Manihiki lagoon and was rare on lagoon patch reefs in Manihiki and absent in Rakahanga. It has been 24 years since trochus was introduced to Manihiki yet density is still low. This relatively low density is also reported for other atolls in the Cook Islands (excluding Aitutaki). Trochus populations are well established at Rarotonga and Aitutaki. It is not well understood why trochus populations have not established on atolls of the Cook Island. Suitable trochus habitats may be limited in atolls of the Cook Island. Agricultural farming (e.g. oranges, bananas) were significant activities in Rarotonga and Aitutaki during the 1970’s/80’s. This activity may have altered reef systems to encourage growth of turf algae, which is a key component of trochus diet. In Penrhyn, Manihiki and Palmerston atolls trochus densities were found to be greater along the western fringing reefs. This could be due south east trade winds driving lagoon circulation, promoting larvae settlement along the western portion of lagoons.
Trochus is commercially exploited in Aitutaki and is an important subsistence species in Rarotonga. At Aitutaki trochus are harvested when market prices are favourable and when densities are greater than 500 trochus per hectare. Given the low density of trochus at Manihiki this resource would not support a commercial fishery in the immediate term.
3.6 Manihiki rugose giant clam (pa’ua) Giant clam is an important invertebrate food resource for atoll communities. The resources can be easily overfished due to its slow growth, non-mobility, it lives in shallow areas and its mantel colour patterns which make them stand out amongst reef habitat. Giant clam begin life as fertilised egg, after hatching it drifts for 7 to 10 days then settles on hard substrate. They first mature as males then later in life they become hermaphrodites, capable of producing
13 | P a g e sperm and eggs. Size at full maturity (male and female) varies across the pacific region, they mature at 80-130mm. Figure 5 illustrate the life cycle of giant clams.
Figure 5: Life cycle of giant clams (source SPC 2011).
3.6.1 Density and size structure comparison with previous assessments Two previous studies on giant clams conducted in Manihiki were found. MMR unpublished data (appendix 5) from a survey conducted in 2002 was exhumed and analysed for density and size structure. The survey was based on counting and recording the size (length) of clams within 81 replicate 1 x 1meter quadrats sampled on three lagoon patch reefs (Amoko, Motu Teru and Turoro) and 162 replicate 1 x 1meter quadrats sampled on fringing reefs (Ngake and Porea). Langdon 2012 reports on a rapid assessment of giant clams in Manihiki. During this assessment clam size information were grouped into <100mm, 100-150mm and >150mm. Langdon 2012 survey was based on 20 replicate 40 x 2meter transects distributed over four lagoon patch reefs and 42 replicate 40 x 2 meter transect sampled on fringing reefs. These two data sets were compared with results from this survey. The three surveys assessed clams in shallow areas (<5m).
Densities in 2002 were within the order of 100,000 per hectare and decreased significantly to 2012 and 2014 levels (Fig 6). The methods of assessment in 2002 can be considered to be more biased than those conducted in 2012 and 2014. Forty meter long transects cover several micro habitats within a reef zone, such as sandy barren areas to dead or live coral with high numbers of clams. However 1m x 1m quadrates are likely to cover less variation in habitats within a reef zone. Coupled with this, is surveyor bias, where surveyors may tend to place quadrates over habitats where clams are present and avoid habitats that do not contain clams. Despite this there was no significant difference in density between fringing reefs and lagoon patch reefs in the 2002 assessment and in the 2014 assessment, however there was a significant difference in density within these two habitats in 2012. There is however a significant difference in density in the 2002 assessment when compared to the assessment in 2012 and 2014. Density on lagoon patch reefs in 2012 was 40,000 (±17,700) and fringing reef was 12,100, (±7600).
Size frequency distribution combines all records from habitats surveyed and are compared between 2002 and 2014 assessments (Fig 7). Juveniles (recruits) from 2002 are moving over time through the size classes, in 2002 average size was 93.3mm (±0.5) and in 2012 average
14 | P a g e size was 113 (±0.9). There is low recruitment (number of juveniles) in 2014 compared to 2012.
Rugose giant clam (pa’ua) was abundant in Manihiki (7586 ind/ha) but rare in Rakahanga (<1 ind/ha). Estimates of clams for various habitats in Manihiki are shown in table 6.
Table 6. Estimates clam abundance for various habitats in Manihiki
Habitat Density Population Type Size (ha) ind/ha-1 SE Absolute SE Reef flat 302.1 6425 3173 1,941,000 958,595 Fringing reef 254.0 15517 3603 3,940,900 915,054 Lagoon patch reef 37.9 14109 1965 534,300 74,415 Total 594.0 6,416,217 1,948,063
Figure 6. Comparison of rugose giant clam (pa’ua) density from in-water assessments in Manihiki.
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Figure 7. Comparison of rugose giant clam (pa’ua) size distribution from in-water assessments conducted in 2002 and 20012 in Manihiki
3.6.2 Mortality rates Total mortality (Z) for various aquatic species has been estimated through length frequency converted to age frequency distribution. The method requires the measuring of fish/invertebrates within a relatively short period (ideally < one-month) and determination or converting lengths to ages. For this study the length frequency distribution for clams surveyed in 2014 (Fig 7) was converted into age frequency distributions using the inverse von-Bertalanffy growth function (VBGF) to determine the age structure of the clam population in Manihiki. The inverse VBGF is of the form;
t(L) = to -1/K(ln(1-L/L)
where:- to: the age at which a clam has 0 length (assumed to be 0), K: the growth rate of clams (0.0683/yr-1) and 7 L: the maximum length clams grow to (24.7cm ). A regression line was fitted to data of the aged based catch curve, excluding, the initial ascending data points, which represent groups of individuals either too small (which may have very different natural mortality rates compared to larger clams) or cryptic, which may not be totally vulnerable to surveyors. Total mortality (Z) was estimated from the right hand slope of the regression. Natural mortality (M) was derived using the general regression equation of Hoeing (1983) for invertebrates:
Ln(M) = 1.44 -0.982*lnt max Where tmax is the maximum known age in years (22). Fishing mortality (F) was determined by Z – M = F Conversions of mortality rate to percentage, was determined by:
7 von-Bertalanffy growth parameters K and L: were from a study in French Polynesia (Gilbert A (2005))
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Z% = 100*(1-exp-Z)
The estimated instantaneous total mortality rate (Z) for T. maxima is 0.298 per year equivalent to 25% mortality per year (Fig 8). This means that each year a quarter of pa’ua stocks in each age class die, either as a result of natural causes (predation) or from fishing activities (harvesting). Natural mortality is estimated at 0.203 (18%) per year, and fishing mortality at 0.095 (9%) per year.
Figure 8. Length converted to age distribution for T. maxima (pa’ua) at Manihiki. Data on the descending slope (dark circles) are included in the regression (sample size 1,384).
3.7 Crown of thorns Crown of thorns were not recorded in Rakahanga and recorded in very low density in Manihiki and at this time they are not a problem. If crown of thorns are present in large numbers – for example, more than 8-10 individuals per hectare or more than 5 crown of thorns are seen in a five minute swim – they begin to cause significant damage to coral reefs.
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4.0 Recommendations
Due to low density of sea cucumber and trochus resources there is no-potential for commercial fishing. Trochus stocks need to be greater than 500 individuals per hectare to warrant commercial exploitation. The most abundant sea cucumber species is lollyfish. Manihiki lollyfish density is close to the regional reference density and lollyfish density was very high in Rakahanga. However, average sizes are small; 122-129 mm for Rakahanga and Manihiki respectively. In the region lollyfish are commonly found at 230 mm, nearly twice the average size recorded in Rakahanga. The low density of these species could be due to limited favourable habitats for these species at these atolls.
Golden sandfish (Bohadschia vitiensis) was recorded at both Manihiki and Rakahanga but not in islands of the southern Cook Islands. This species is high value and is of interest for aquaculture. Consideration could be given to improving the spawning/fertilization potential of this species by corralling them into certain areas or through hatchery technology.
Clams are an important food resource for Manihiki and Rakahanga communities. Clams are sedentary animals and many males and females need to be close together for successful reproduction. Experience in Aitutaki and elsewhere in the region has shown that this resource is fragile. If there are no effective controls on clam harvest/exports, stocks will decline to a point where they can no longer reproduce effectively to replenish exploited stocks. It is important that this scenario of declining population leading to local extinction dose not repeat itself in Manihiki. The need for managing this food resource should be of vital importance to Council and community. Density and size structure of the population in Manihiki is healthy, but at Rakahanga the stock is just about extinct.
Instantaneous total mortality for clams at Manihiki is estimated at 25 %, meaning a quarter of the number of clams in each size/age class die as a result of natural causes (i.e. predation) and from fishing. Manihiki Island Council and community have maintained excellent management measures (ra’ui, prohibition on exports, on-island subsistence consumption permitted) to control exploitation. This should be maintained and could be strengthened with the introduction of a minimum harvest size and to be very cautious when determining total harvest quota for periodic large scale community harvest. Recommended minimum harvest size is 100mm, however a greater minimum harvest size (e.g.120mm) would be better. Large scale community harvest could be organise every 3-5 years as opposed to annually and quota should be set at no greater than natural mortality rate (<18%) of the adult population. An assessment of clam abundance and size distribution prior to determining large scale community harvest is recommended.
Options such as transferring mature clams or collection of juvenile clams through spat collection programs in Manihiki and transferring these to Rakahanga could be explored to rebuild stocks in Rakahanga. However this comes with significant resources (people, time and funds).
It is recommended that permanent sites be selected from a subset of stations assessed from this and previous assessment in Manihiki. This is to enable species specific comparison of
18 | P a g e density over time. To monitor juvenile clam recruitment it is important that’s these permanent stations be monitored on a regular basis (e.g. every two years) and analysed on a station by station basis.
Crown of thorns (taramea) were in very low density at Manihiki and absent in Rakahanga therefore they are not a problem. However for future reference, if crown of thorns are present in large numbers – for example, more than 8-10 individuals adults per hectare or more than 5 crown of thorns are seen in a five minute swim – they begin to cause significant damage to coral reefs.
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5.0 References
George N., Story R., 2014. The status of invertebrates resources at Mauke and Mitiaro: September 2014. Ministry of Marine Resources. Rarotonga, Cook Islands.
Langdon G., 2012. Manihiki Lagoon Paua Assessment Summary Report: May 2012. Ministry of Marine Resources. Rarotonga, Cook Islands.
Pakoa K., Friedman K., Moore B., Tardy E., Bertram I. 2014. Assessing Tropical marine Invertebrates: a Manual for Pacific Island Resource Managers. Coastal Fisheries Science and Management Section, Secretariat of the Pacific Community (SPC): Noumea, New Caledonia.
Pinca S., Boblin P., Friedman K.J., Kronen M., Magron F., Awira R., Pakoa K., Lasi F., Tardy E. and Chapman L.B. 2009. Cook Islands country report: Profiles and results from survey work at Aitutaki, Palmerston, Mangaia and Rarotonga (February and October 2007). Pacific Regional Oceanic and Coastal Fisheries Development Programme (PROCFish/C/CoFish). Secretariat of the Pacific Community (SPC): Noumea, New Caledonia. xxxiv, 339 p.
Ponia B., 2012. Manihiki Trochus Survey Report: July 1998. Ministry of Marine Resources. Rarotonga, Cook Islands.
Raumea K., George N., Pakoa K., Bertram I., Sharp M. 2013. The status of sea cucumber resources at Aitutaki, Mangaia, Palmerston and Rarotonga, Cook Islands: June 2013. Noumea, New Caledonia: Secretariat of the Pacific Community.
RDA International Incorporated. 1997. Lagoon Ecology Monitoring and Management Project, Manihiki Lagoon, Cook Islands. California, United States of America.
Sims N., 1988. Stock Assessment of Pearl Oyster Resources in the Cook Islands: Ministry of Marine Resources. Rarotonga, Cook Islands.
Sims N., 1991. A study on The Cultured Black Pearl Industry: Pacific Regional Team Centre for Pacific Development and Training. Hawaii, USA.
SPC 2011. Guide and information sheets for fishing communities. Secretariat of the Pacific Community (SPC): Noumea, New Caledonia.
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6.0 Appendices
Appendix 1. Locations of survey stations.
Manihiki Invertebrate Survey Map
RBt stations
Reef crest
Fringing reef Reef flat
Lagoon patch reef
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Rakahanga Invertebrate Survey Map
Fringing reef
RBt stations
Reef flat
Reef crest
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Appendix 2. GPS coordinates for stations at each site. Manihiki
Station Station Station Date Latitude Longitude Date Latitude Longitude Date Latitude Longitude # # # 28/05/2014 1 -10.3778 -161.0116 29/05/2014 23 -10.4496 161.014 31/05/2014 45 -10.4347 -160.9558 28/05/2014 2 -10.3775 -161.013 29/05/2014 24 -10.4512 -161.0154 31/05/2014 46 -10.4377 -160.9543 28/05/2014 3 -10.3774 -161.0145 30/05/2014 25 -10.4566 -161.0025 31/05/2014 47 -10.4271 -160.9573 28/05/2014 4 -10.3826 -161.0173 30/05/2014 26 -10.4592 -161.0022 31/05/2014 48 -10.4266 -160.9584 28/05/2014 5 -10.3836 -161.0175 30/05/2014 27 -10.4614 -161.0034 31/05/2014 49 -10.4253 -160.9556 28/05/2014 6 -10.3843 -161.0164 30/05/2014 28 -10.4579 -160.9935 2/06/2014 50 -10.4161 -160.9729 28/05/2014 7 -10.4012 -161.0268 30/05/2014 29 -10.4588 -160.9932 2/06/2014 51 -10.4159 -160.9735 28/05/2014 8 -10.4009 -161.0284 30/05/2014 30 -10.4599 -160.9927 2/06/2014 52 -10.4139 -160.9718 28/05/2014 9 -10.4005 -161.0294 30/05/2014 31 -10.4544 -160.9841 2/06/2014 53 -10.4001 -160.9887 28/05/2014 10 -10.4108 -161.0336 30/05/2014 32 -10.4554 -160.9836 2/06/2014 54 -10.3984 -160.9872 28/05/2014 11 -10.411 -161.0309 30/05/2014 33 -10.4563 -160.9833 2/06/2014 55 -10.3994 -160.9893 28/05/2014 12 -10.4138 -161.0311 30/05/2014 34 -10.4568 -160.9829 2/06/2014 56 -10.384 -160.9996 28/05/2014 13 -10.4248 -161.0355 30/05/2014 35 -10.4509 -160.9758 2/06/2014 57 -10.3809 -161.0018 28/05/2014 14 -10.4238 -161.0331 30/05/2014 36 -10.4522 -160.976 2/06/2014 58 -10.3838 -160.997 28/05/2014 15 -10.4248 -161.0331 30/05/2014 37 -10.4545 -160.9747 2/06/2014 59 -10.3748 -161.0054 29/05/2014 16 -10.4409 -161.0378 31/05/2014 38 -10.4498 -160.9669 2/06/2014 60 -10.3743 -161.0063 29/05/2014 17 -10.4344 -161.0334 31/05/2014 39 -10.4526 -160.9666 2/06/2014 61 -10.3727 -161.0039 29/05/2014 18 -10.4328 -161.034 31/05/2014 40 -10.455 -160.9658 2/06/2014 62 -10.4134 -160.9874 29/05/2014 19 -10.4398 -161.0262 31/05/2014 41 -10.4458 -160.9608 2/06/2014 63 -10.433 -160.9999 29/05/2014 20 -10.4434 -161.0266 31/05/2014 42 -10.4461 -160.959 2/06/2014 64 -10.4221 -161.0178 29/05/2014 21 -10.4442 -161.0269 31/05/2014 43 -10.449 -160.9563 2/06/2014 65 -10.4043 -161.0224 29/05/2014 22 -10.4478 -161.0131 31/05/2014 44 -10.4372 -160.9562
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Rakahanga
Station Station Station Date Latitude Longitude Date Latitude Longitude Date Latitude Longitude # # # 4/06/2014 1 -10.026510 -161.103300 5/06/2014 24 -10.002700 -161.083700 6/06/2014 47 -10.020100 -161.101300 4/06/2014 2 -10.026990 -161.100600 5/06/2014 25 -10.004720 -161.085200 4/06/2014 3 -10.028220 -161.100800 5/06/2014 26 -10.000300 -161.090300 4/06/2014 4 -10.030560 -161.104600 5/06/2014 27 -9.999530 -161.094000 4/06/2014 5 -10.029410 -161.101200 5/06/2014 28 -10.003000 -161.094300 4/06/2014 6 -10.033980 -161.104800 5/06/2014 29 -10.009420 -161.096800 4/06/2014 7 -10.030730 -161.101000 5/06/2014 30 -10.016840 -161.097000 4/06/2014 8 -10.036600 -161.097700 5/06/2014 31 -10.021640 -161.097800 4/06/2014 9 -10.029710 -161.097600 5/06/2014 32 -10.037260 -161.094400 4/06/2014 10 -10.029390 -161.095900 5/06/2014 33 -10.035710 -161.088900 5/06/2014 11 -10.029740 -161.092900 5/06/2014 34 -10.029510 -161.084600 5/06/2014 12 -10.029830 -161.091100 5/06/2014 35 -10.022050 -161.081600 5/06/2014 13 -10.028490 -161.089600 5/06/2014 36 -10.016020 -161.080100 5/06/2014 14 -10.024370 -161.085000 5/06/2014 37 -10.012470 -161.078700 5/06/2014 15 -10.023770 -161.086600 5/06/2014 38 -10.006790 -161.076900 5/06/2014 16 -10.021390 -161.083700 5/06/2014 39 -10.001060 -161.079500 5/06/2014 17 -10.021570 -161.085500 6/06/2014 40 -10.000200 -161.081700 5/06/2014 18 -10.018380 -161.083000 6/06/2014 41 -9.999098 -161.083400 5/06/2014 19 -10.017970 -161.083900 6/06/2014 42 -9.997117 -161.088400 5/06/2014 20 -10.012820 -161.082800 6/06/2014 43 -9.997294 -161.097500 5/06/2014 21 -10.011080 -161.081800 6/06/2014 44 -10.001180 -161.098700 5/06/2014 22 -10.005860 -161.080700 6/06/2014 45 -10.009020 -161.099700 5/06/2014 23 -10.006210 -161.081900 6/06/2014 46 -10.015820 -161.100400
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Appendix 3. Summary statistics for the two sites. ‘All Transects’ represents statistics for all transects (i.e. overall mean); ‘Transects P’ represents statistics for transect where a species was recorded (present); ‘All Stations’ represents results of all stations (i.e. overall mean density for all stations); ‘Stations P’ represents results for stations where a species was recorded (present). Transect = 1 m x 40 m, station = six transects of 1 m x 40 m each.
Manihiki reef benthos transect (RBT) assessment data summary
Species All Transect Transect_P All Station Stations_P Species Group Mean SE N Mean SE N Mean SE n Mean SE n Beche de mer Actinopyga mauritiana 24.9 5.6 382 351.9 44.8 27 24.4 10.5 65 131.9 47.0 12 Beche de mer Bohadschia argus 2.6 1.3 382 250.0 0.0 4 2.9 1.7 65 62.5 12.0 3 Beche de mer Bohadschia vitiensis 3.9 2.8 382 500.0 250.0 3 3.8 2.7 65 83.3 41.7 3 Beche de mer Holothuria atra 5080.5 760.1 382 10722.4 1498.4 181 4979.5 1726.2 65 6605.4 2246.4 49 Beche de mer Holothuria atra (big) 382.9 109.0 382 2215.9 583.8 66 380.4 160.6 65 749.4 304.9 33 Bivalve Arca sp 3380.2 809.6 382 15941.4 3495.7 81 4393.3 1808.8 65 11422.5 4397.6 25 Bivalve Pinctada margaritifera 49.7 12.1 382 593.8 105.8 32 59.3 25.2 65 350.4 117.9 11 Bivalve Pinctada maculate 119.8 68.1 382 5083.3 2491.7 9 117.3 99.8 65 1906.3 1522.8 4 Bivalve Tridacna maxima 7467.9 851.1 382 15588.8 1571.6 183 7604.2 1652.5 65 13007.1 2486.0 38 Gastropod Conus sp. 1140.1 294.9 382 25617.6 2710.9 17 1116.7 525.5 65 10369.0 3370.5 7 Gastropod Dendropoma maximum 214.7 84.9 382 8200.0 2092.6 10 210.3 134.2 65 2733.3 1422.8 5 Gastropod Tectus niloticus 11.8 5.1 382 750.0 129.1 6 11.5 6.8 65 250.0 48.1 3 Star Acanthaster planci 0.7 0.7 382 250.0 1 0.6 0.6 65 41.7 1 Star Leiaster speciosus 26.2 9.2 382 833.3 177.7 12 26.0 14.5 65 281.3 121.7 6 Urchin Echinothrix diadema 5.2 5.2 382 2000.0 1 5.1 5.1 65 333.3 1 Urchin Heterocentrotus mammillatus 4.6 3.0 382 583.3 220.5 3 4.5 4.5 65 291.7 1
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Rakahanga reef benthos transect (RBT) assessment data summary
Species All Transect Transects_P All Station Stations_P Species Group Mean SE N Mean SE N Mean SE n Mean SE n Beche de mer Actinopyga mauritiana 25.2 21.6 278 1400.0 1150.0 5 24.8 22.3 47 583.3 458.3 2 Beche de mer Bohadschia vitiensis 39.6 13.5 278 785.7 177.5 14 39.0 27.8 47 366.7 231.8 5 Beche de mer Holothuria atra 13418.2 2135.9 278 17111.2 2671.1 218 13309.8 4940.3 47 15257.6 5606.4 41 Beche de mer Holothuria atra (big) 3556.7 716.0 278 9416.7 1756.2 105 3590.4 1250.1 47 5625.0 1867.9 30 Beche de mer Synapta maculata. 0.9 0.9 278 250.0 1 0.9 0.9 47 41.7 1 Bivalve Arca sp. 129.5 70.8 278 9000.0 2362.9 4 127.7 74.0 47 2000.0 315.5 3 Bivalve Pinctada margaritifera 35.1 8.9 278 423.9 67.5 23 34.6 16.1 47 162.5 62.9 10 Bivalve Pinctada maculata. 15748.2 6810.4 278 547250.0 149090.8 8 15524.8 15514.0 47 364833.3 364333.3 2 Bivalve Pinna muricata 337.2 109.1 278 6696.4 1327.6 14 332.4 205.9 47 3906.3 1727.0 4 Bivalve Tridacna maxima 0.9 0.9 278 250.0 1 0.9 0.9 47 41.7 1 Gastropod Cypraea tigris 1.8 1.3 278 250.0 0.0 2 1.8 1.2 47 41.7 0.0 2 Urchin Echinothrix diadema 3.6 2.2 278 333.3 83.3 3 3.5 2.1 47 55.6 13.9 3 Urchin Heterocentrotus mammillatus 0.9 0.9 278 250.0 1 0.9 0.9 47 41.7 1
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Appendix 4. Invertebrate species recorded at the two sites Species Photo Species Photo Rori toto Rori kuru
Lollyfish Leopardfish
Holothuria atra Bohadschia argus No local name Veri-aria
Golden sandfish Kingfish
Bohadschia Synapta maculata vitiensis Rori Puakatoro Vana
Long spine sea Surf redfish urchin
Actinopyga Echinothrix mauritiana diadema
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Species Photo Species Photo Atuke Taramea
Brown pencil Crown of thorns urchin
Heterocentrotus Acanthaster planci mammillatus Avake Paua
Short Spine Sea- Rugose giant clam urchin,
Tripnestus Tridacna maxima gratilla Etu moana Pipi
Multicolour star Blotched pearl fish oyster
Linckia Pinctada maculata multifora
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Species Photo Species Photo Parau Torokati
Oyster shell Trochus
Pinctada Tectus niloticus margaritifera Kuku Ungakoa
Ventricose ark Large worm shell shell
Dendropoma Arca ventricosa maximum Kotate No local name
Prickly pen shell Red-lip stromb
Pinna muricata Strombus luhuanus
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Species Photo Species Photo Poreo Rori U
Cowry shell Black teatfish
Cypraea tigris Holothuria nobilis Rori pua Ariri
Flower sea Rough turban shell cucumber
Holothuria Turbo setosus cinerascens Matu rori/Rori tapou/Rori Etu moana ka’a/Rori tapou
White snakefish Blue starfish
Holothuria Linckia laevigata leucospilota Kina
Pink sea urchin
Echinometra mathei
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Appendix 5. Summary of MMR unpublished paua data 2002 Lagoon patch reef Fringing reef Location Kaoa Kaoa Kaoa Ngake Ngake Ngake Ngake Ngake Ngake Porea Porea Porea Porea Station Rep Amoko Motuteru Turoro L.1 L.2 L.3 L.4 L.5 L.6 L.1 L.2 L.3 L.4 1 1 5 22 4 9 12 5 29 17 2 18 0 8 18 1 2 1 5 16 15 5 7 10 7 0 18 2 2 18 1 3 5 4 4 16 5 32 8 11 2 10 0 6 18 1 4 1 25 11 6 13 13 8 21 20 18 0 13 19 1 5 1 5 4 9 2 10 8 24 19 16 0 1 18 1 6 1 48 3 18 9 7 6 4 8 2 6 5 12 1 7 4 27 8 15 10 13 3 7 13 13 0 8 22 1 8 4 28 13 9 28 6 6 3 6 13 0 18 19 1 9 3 18 3 10 8 13 14 2 3 18 0 5 18 2 1 1 38 4 18 3 7 6 25 9 18 2 20 18 2 2 0 15 3 1 7 7 4 23 18 18 5 19 4 2 3 0 13 0 15 18 17 30 11 21 3 3 0 7 2 4 11 8 1 3 5 18 11 35 23 13 5 1 10 2 5 0 8 29 8 2 3 3 25 21 15 16 2 7 2 6 1 20 0 16 6 18 8 5 38 4 5 2 4 2 7 0 33 12 25 5 12 35 1 24 3 0 18 10 2 8 17 15 35 18 21 3 23 1 15 1 0 12 5 2 9 18 29 4 3 15 12 40 1 7 6 12 18 18 3 1 1 36 14 10 17 20 36 8 1 2 6 1 13 3 2 15 2 0 4 7 10 5 18 16 0 0 2 5 3 3 6 18 0 2 14 8 36 16 2 1 0 9 10 3 4 3 5 3 11 11 12 7 7 9 1 4 9 8 3 5 3 3 9 4 16 9 11 34 3 1 5 7 6 3 6 1 5 2 5 14 1 5 3 3 2 0 11 4 3 7 1 64 0 14 20 7 42 1 8 0 0 18 4 3 8 5 24 12 7 32 4 3 0 6 5 2 14 6 3 9 26 0 3 8 25 25 4 2 16 11 0 18 19
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