FINAL REPORT TO QUEENSLAND FISHERIES MANAGEMENT AUTHORITY

Transcription

1 SURVEY OF HOLOTHURIA SCABRA (SANDFISH) ON WARRIOR REEF, TORRES STRAIT FINAL REPORT TO QUEENSLAND FISHERIES MANAGEMENT AUTHORITY Timothy Skewes Charis Burridge Burke Hill PO Box 120 Cleveland, Q 4163 Ph (07) tim.skewes@marine.csiro.au January 1998

2 Survey of Holothuria scabra (sandfish) on Warrior Reef, January ACKNOWLEDGEMENTS The Queensland Fish Management Authority (QFMA) initiated this project. Environment Australia provided a valuable contribution towards funding the project with the QFMA meeting the remaining costs. The project would not have been possible without the assistance of officers from the following agencies: Queensland Boating and Fisheries Patrol (QB&FP), Australian Customs Service (ACS) and the Australian Fisheries Management Authority (AFMA). Personal thanks for assistance beyond the call of duty go to Mr Bruce Kingdom (QB&FP), Mr Mick Bishop (AFMA), Sgnt Robbie Graham (Queensland Water Police), and the captain and crew of the patrol vessel WAURI.

3 Survey of Holothuria scabra (sandfish) on Warrior Reef, January SUMMARY The abundance of Holothuria scabra (sandfish) on Warrior Reef in January 1998 was between 63% and 86% (95% CI, mean 77.5%) smaller than in November The H. scabra stock in November 1995 was considered as over-exploited, therefore this reduction indicates a serious depletion. Both the fishery (2+ years old) and recruiting (one year old) year-classes are depleted, the latter the result of a depleted breeding population, indicating a direct stock-recruitment relationship for H. scabra on Warrior Reef. This indicates that the fishery stock is in a downward spiral, with progressively smaller breeding populations leading to smaller and smaller recruitments. Severe management measures are required to avoid a total collapse of the stock. Only small changes in the reef top community structure were recorded, most notably a small reduction in the percentage cover of live coral. Based on these results, the report makes the following recommendations: 1. The remaining stock of Holothuria scabra (sandfish) on Warrior Reef, and especially the large animals on the western side of the reef, should subject to little, if any, fishing pressure for at least one year. 2. A further assessment of the H. scabra stock should be carried out in one year s time to monitor population trends, especially recruitment. This assessment should include population density and age structure. 3. Future surveys should monitor changes to the reef system in case there are effects on the environment caused by the depletion of the H. scabra on the reef top, both from the loss of an important component of the reef-top community, and from fishing practices such as reef walking.

4 Survey of Holothuria scabra (sandfish) on Warrior Reef, January INTRODUCTION The beche-de-mer fishery provides an opportunity for Torres Strait Islanders to gain an income from the sea. However, beche-de-mer have proven susceptible to over-exploitation because they are large, easily seen and collected and do not require sophisticated fishing equipment or processing. There is little knowledge of the size and sustainability of this resource in Torres Strait and the management agency for this fishery, the Queensland Fisheries Management Authority (QFMA), is concerned that the Holothuria scabra (sandfish) resource in Torres Strait is being over exploited. These concerns are also shared by the Torres Strait Fisheries Scientific Advisory Committee (TSFSAC). Beche-de-mer has historically been a very valuable fishery in Torres Strait and continued to be a prominent fishery up until the Second World War. Recently, the fishery in Torres Strait has seen a resurgence. It started on the PNG side of the border on Warrior Reef for Holothuria scabra (sandfish) in the late 1980 s and lasted until the fishery was closed in 1993 due to concerns about overfishing. The catch during this time ranged between 109 t and 192 t dry weight, mainly consisting of H. scabra, although the proportion of this species in the catch dropped from 100% during the period to only 30% in 1993, the remainder being made up of lower value Actinopyga spp. (Lokani et al., 1996). The closure remained in force until October 1995 and the fishery then operated under a management plan which included a 40 t (dry wt)/year TAC, a closed season and minimum sizes. More recently, the PNG fishery has been closed indefinitely due to more concerns about overexploitation and illegal fishing. The beche-de-mer fishery on the Australian side of Torres Strait experienced a similar boom which began in 1994 with the Australian catch in 1995 estimated to be between 1,200 to 1,400 t wet-weight; all but ~ 50 t of this being, H. scabra (QFMA, pers. comm.). It was mainly centred on Warrior Reef. Levels of exploitation were similar to those prior to the closure of the PNG fishery, and a survey in November 1995 (Long et al., 1996) found that the fishery was probably over exploited. Since that time the fishery has operated on the basis of competitive yearly TACs (wet weights) of 260 t for sandfish (H. scabra), 260 t for teatfish (including black teatfish, H. nobilis and white teatfish, H. fuscogilva), 260 t for prickly redfish (Thelenota ananus) and a collective quota of 80 t for all other species of beche-de-mer (mostly Actinopyga spp.). In addition, minimum size limits are in place for the major species and a limit of 18 cm total length (TL) applies to H. scabra. In late 1997, the QFMA had serious concerns over the quality of logbook data being provided by participants in the commercial fishery. Indirect sources of information indicated that the quota had most likely been reached for the quota period. The inadequacy of the logbook data forestalled action to close the fishery to the taking of H. scabra on quota grounds. Furthermore, anecdotal evidence collected by QFMA suggested that the resource was becoming heavily depleted, and that there was potential habitat damage being done to the fishery. The combination of these events led to the commissioning of this survey to establish an independent assessment of the status of the stock and environmental impacts from fishing activities. This report contains the results of that survey.

5 Survey of Holothuria scabra (sandfish) on Warrior Reef, January METHODS Description of the study area Warrior Reef has an area of 165 km 2 and is the largest reef on the Australian side of the border in the Torres Strait Protected Zone (TSPZ). It accounts for approximately 10% of the reef area in the entire TSPZ. It is part of the Warrior reef complex which is a dominant feature of Torres Strait and extends over a third of the way across Torres Strait as an almost continuous boundary. Warrior Reef is approximately 34 km long north-south and averages about 5 km wide. About one-third of the reef is shallow seagrass habitat (Fig. 1). This habitat is mainly found on the eastern half of the reef. There is a series of shallow sand banks that run up the centre of the reef to about two thirds of the way up Warrior Reef. West of this sand bank is mainly a sand rubble zone with algae replacing seagrass as the dominant biotic component. Australian fishers are mainly based at Tudu Island, at the southern end of Warrior Reef. The northern end of the reef abuts the Australian PNG border. Sample design The limited field time available meant that only 50 sites could be surveyed on Warrior Reef. The results of the previous survey of reef top resources on Warrior Reef in November 1995 showed that most Holothuria scabra were found in the seagrass habitat that covered approximately one third of Warrior Reef (Table 1, Fig. 1). Therefore, all sites in the present survey were located in the seagrass habitat. Table 1. Results of a survey by CSIRO of Holothuria scabra on Warrior Reef in (Abundance estimates are per transect = 40 m 2.) (s.e. = standard error) Area (ha) Sites n H. scabra (per trans) s.e. Seagrass habitat 5, Other habitat 11, Warrior Reef Total 16, The 50 sites were distributed throughout the seagrass habitat on Warrior Reef (Fig. 1). Of these sites, 41 were in the same location as sites sampled in the November 1995 survey. Field sampling Sampling was carried out from 14 to 17 January Conditions were optimal with low winds (0-10 kts) and good water visibility (3-6 m visibility). Sampling was done during the high tide in the middle of the day. Sampling was done from a dinghy with a tethered diver and two observers. Sites were located using GPS. At each site a diver laid out a 20 m transect line and swam along the line collecting all holothurians within 1 m either side of the transect line. The holothurians were returned to the dinghy where they were individually weighed. At each site the percentage cover of seagrass and algae were estimated as were the relative proportions of the dominant species in the seagrass and algae communities. The abundance of dominant sessile megabenthos along the transect such as alcyonarians (soft corals), ascidians and sponges (percent cover); starfish and urchins (qualitative abundance) were recorded. The percentage cover of sand, rubble, consolidated rubble, boulders (> 0.5 m), coralline algal pavement and live coral were also recorded for each site.

6 Survey of Holothuria scabra (sandfish) on Warrior Reef, January Data analysis Survey data was entered into a GIS (ARCView) which was used to produce distribution maps of Holothuria scabra on Warrior Reef. The current distribution map was then compared to the November 1995 distribution data. The relationship between the number of H. scabra seen at sites sampled during both surveys was investigated using regression. The counts of H. scabra per transect were used to calculate an average and variance of the population in the seagrass habitat on Warrior Reef. These abundance figures were then compared to the abundance figures for the same habitat in November The distribution of the site counts were extremely non-normal, even after log transformation. Therefore we used an analysis of parameter estimates with the data modelled to a Poisson distribution by maximum likelihood. The model allowed for a comparison of the two sample data sets with appropriate significance tests (the chi squared statistic) and standard errors computed for the ratio changes. The analysis was done in SAS GENMOD procedure using a Poisson distribution and a log link function. There was one site that had extremely high counts of H. scabra during both surveys (57 and 12 in 1995 and 1998 respectively). As this was the same site both times, it was analysed in the model as a separate treatment to the remaining data. Initial testing with an interaction term showed a non-significant interaction between the single site and the remaining data indicating a consistent change in time for the two treatments. Thereafter the model was run with the two treatments and no interaction term. Wet weights for individual H. scabra were converted to total length using established morphometric relationships (Conand, 1990) and used to construct length frequencies for analysis. Estimates of growth were used to establish the age of H. scabra in the samples. The length frequency of the current survey was then compared to the November 1995 length frequency data. Environmental data (percent cover of dominant biotic and substrate components) from the two surveys were compared using a paired t-test in SAS PROC MEANS. RESULTS Holothuria scabra distribution The distribution of Holothuria scabra seen during the survey in January 1998 was similar to that seen during the survey in November 1995 (Fig. 2) and there was a significant relationship between the number of H. scabra seen at repeated sites during the two surveys (P < , r 2 = 0.901). The highest concentrations of H. scabra were found in the seagrass beds from half to about 2/3 of the way up Warrior Reef (Fig 2). There was no relationship between the number of H. scabra and dominant environmental components that could explain this pattern (Table 2). It may be that the distribution pattern observed is the result of fishing pressure. Australian fishermen were mainly based at Tudu Island at the southern end of Warrior Reef and therefore fishing pressure was greatest at the southern end of Warrior Reef and progressed up the reef as southern areas were fished out. Similarly, the northern end of Warrior Reef was subject to illegal fishing pressure by PNG fishers (Lokani, 1996).

7 Survey of Holothuria scabra (sandfish) on Warrior Reef, January Table 2. Pearsons s correlation matrix (probability of T r = 0 in brackets) between square-root-transformed abundance of Holothuria scabra and arcsine-square-root transformed percentage seagrass cover; live coral cover, soft sediment, rubble, consolidated rubble, pavement and boulders for Warrior Reef tops. SGCOV: % seagrass cover; LCO: % live coral cover; SOF: % soft sediment cover; RUB: % rubble cover; CON: % consolidated rubble cover; PAV: % pavement cover; BOU: % boulder cover. *** P < 0.001; ** P < 0.01; * P<0.05. SGCOV LCO SOF RUB CON PAV BOU H. scabra * (0.398) (0.048) (0.065) (0.254) (0.137) (0.281) (0.675) Holothuria scabra abundance There was a significant reduction in the number of Holothuria scabra per site between the two surveys (chi sq = 36.27; d.f. = 1; P < ) (Table 3). The abundance of H. scabra in the seagrass habitat on Warrior Reef in January 1998 was between 63% and 86% (95% CI) smaller than in November 1995 with a mean reduction of 77.5%. This does not account for the H. scabra in other (non-seagrass) areas of Warrior Reef, however, even if the abundance of H. scabra in the other areas remained the same as the November 1995 survey, the mean reduction in abundance between the two surveys would still be 72.0%. Table 3. Abundance of Holothuria scabra in the seagrass habitat during the two surveys. Also shown are the results of parameter estimation using maximum likelihood and Poisson distribution to compare the two survey results. (s.e. = standard error) Nov-95 (per trans) Jan-98 (per trans) Ln ratio of change (1995:1998) Model reduction (% of 1995) Reduction Upper 95% CI (% of 1995) Reduction Lower 95 % CI (% of 1995) H. scabra s.e Population structure The length frequency of Holothuria scabra collected during the January 1998 survey showed that most were smaller than the legal minimum size of 180 mm total length (TL) (Fig. 3). This was similar to the November 1995 length frequency data and indicates a selective depletion of the fishery-size beche-de-mer (>180 mm TL) caused by high fishing pressure. However, the abundance of the smaller (< 180 mm TL) year-class is also low in comparison to the November 1995 data, and is probably the result of a depleted breeding population the previous summer (1996/97), which indicates a direct stock-recruitment relationship for H. scabra on Warrior Reef. Given the established growth rates of this species (Long et al., 1996), the smaller (<180 mm TL) beche-de-mer are probably a little over one year old, and will pass through the minimum size during 1998.

8 Survey of Holothuria scabra (sandfish) on Warrior Reef, January November 1995 n = Number Number January 1998 n = Total length (mm) Figure 3. Length frequencies of Holothuria scabra collected at approximately 50 sites on Warrior Reef in November 1995 and January Total lengths were derived from wet weight using the relationship: ln(weight(gms))=2.28ln(total length(mm))-6.35 (Conand, 1990). Environmental data During the survey, data was also gathered on the biotic and substrate components of the reef-top environment. Seagrass was the dominant biotic component of the habitats that were sampled in both years. There was no change in the average percent cover of seagrass between the two sample occasions (Table 4). However, there was a significant increase in the amount of Thallassia hemprichii in the seagrass community (from 39% to 48% cover). This increase was accompanied by a decrease in the cover of Cymodocea spp. from 6.6% in 1985 to 0% in Cymodocea spp. and T. hemprichii are morphologically very similar so it may be that the former was missed during the survey. In any case, the changes in seagrass community structure were not great and probably reflect a steady state. Algae cover did not change significantly during the two surveys. The percentage cover of soft sediment did change (from 80.9% to 88.7%) but not significantly. There was a significant reduction in the cover of live coral on the bottom. Even though the absolute amounts of live coral were low (max 0.39 % cover in 1995) there is a possibility that this change is related to fishing practices, such as reef walking. Further surveys would be needed to confirm this trend. Other conspicuous megafauna such as starfish and urchins were in similar numbers during both surveys. Based on this limited information, there does not appear to be large changes in the reef top environment at this time, however, the changes in community structure that may result from removing an important reef component such as holothurians would probably take some time to manifest. Future surveys to monitor changes in the reef system are required.

9 Survey of Holothuria scabra (sandfish) on Warrior Reef, January Table 4. Paired comparison of the percentage cover of dominant biotic and substrate components during the two surveys, the average change of the paired sites, and the results of paired t-test of the variables. (s.e. = standard error) Variable Nov-95 (% cover) Jan-98 (% cover) Mean change (% cover) T Prob> T Seagrass cover s.e Thallasia hempricii s.e Enhallus acoroides s.e Halophila ovalis s.e Halodule uninervis s.e Algae cover s.e Soft sediments s.e Live coral s.e CONCLUSIONS The abundance of H. scabra in the seagrass habitat on Warrior Reef in January 1998 was between 63% and 86% (95% CI, mean 77.5%) smaller than in November The H. scabra stock in November 1995 was considered as over-exploited, therefore this reduction indicates a serious depletion. Both the fishery (2+ years old) and recruiting (one year old) year-classes are depleted, the latter the result of depleted breeding population, indicating a direct stock-recruitment relationship for H. scabra on Warrior Reef. Based on available data on size at sexual maturity (Preston, 1993) and breeding seasonality (Conand, 1990), most of these one year old beche-de-mer will not breed until next summer (1998/99). During 1998, they will pass through the minimum size and would be available to fishing. For the stock to rebuild, these animals need to be protected. It is unlikely that the size of this year-class is anywhere near 240 t wet weight therefore any quota would have to be considerably less. This year-class should be subject to only very limited, if any, fishing effort over the next year. The results of this survey indicate that this fishery is in a downward spiral, with progressively smaller breeding populations leading to smaller and smaller recruitments. Severe management measures are required to avoid a total collapse of the stock. A further assessment of the H. scabra stock should be carried out in one year s time to monitor population trends, especially recruitment. This assessment should include population density and age structure. Interviews with some fishermen indicate that fishing for very large (> 1 kg) beche-de-mer has occurred on the western side of Warrior Reef. This may be a remnant population of older breeding animals and may be an important source of larvae for future recruits to the fishery, providing some protection to overfishing. If this is correct, then consideration should be given to closing fishing to the area west of the sand banks that run down the middle of Warrior Reef for two thirds of the way up the reef.

10 Survey of Holothuria scabra (sandfish) on Warrior Reef, January Only small changes in the reef top community structure were recorded, most notably a small reduction in the percentage cover of live coral. Future surveys should monitor changes to the reef system in case there are effects on the environment caused by the depletion of the H. scabra on the reef top, both from the loss of an important component of the reef-top community, and from fishing practices such as reef walking. As well as changes to the benthic community structure, changes to the structure of the sediment on the reef top should be investigated, as sediment turnover is one of the main ecological roles of holothurians on the reef top. RECOMMENDATIONS 1. The remaining stock of Holothuria scabra (sandfish) on Warrior Reef, and especially the large animals on the western side of the reef, should subject to little, if any, fishing pressure for at least one year. 2. A further assessment of the H. scabra stock should be carried out in one year s time to monitor population trends, especially recruitment. This assessment should include population density and age structure. 3. Future surveys should monitor changes to the reef system in case there are effects on the environment caused by the depletion of the H. scabra on the reef top, both from the loss of an important component of the reef-top community, and from fishing practices such as reef walking.

11 Survey of Holothuria scabra (sandfish) on Warrior Reef, January REFERENCES Conand, C. (1990) Fishery resources of the Pacific islands. Part 2. The Holothurians. FAO Fisheries Technical Paper, 272.2, FAO, Rome, 108 pp. Lokani, P. (1996) Illegal fishing for sea-cucumber (beche-de-mer) by Papua New Guinae artisinal fishermen in the Torres Strait Protected Zone South Pacific Commission Beche-de-mer Information Bulletin, vol. 8, pp Lokani, P., Polon, P., and Lari, R. (1996) Management of beche-de-mer fisheries in the Western Provice of Papua New Guinea South Pacific Commission Beche-de-mer Information Bulletin, vol. 8, pp Long, B.G., Skewes, T.D., Taranto, T. (1996). Distribution and abundance of beche-de-mer on the Warrior reefs, Torres Strait. Report to the Queensland Fisheries Management Authority, Brisbane Queensland, January 1996, 70 pp. Long B.G. and Skewes T.D. (1997). Distribution and abundance of beche-de-mer on Torres Strait Reefs. Beche-de-mer Information Bulletin 9:18-22 Preston, G. (1993) Chapter 11: Bêche-de-mer. pp , In: Wright A. and L. Hill eds. Inshore marine resources of the South Pacific: information for fishery development and management. FFA/USP Press, Fiji.