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A COMPARISON OF THE ABUNDANCE, SIZE COMPOSITION AND HABITAT SELECTION OF GIANT CLAMS IN EXPLOITED AND NON-EXPLOITED AREAS

Introduction

The fisheries stocks of several species in the Wakatobi National Marine Park, South East Sulawesi, Indonesia such as holothurians, lobsters and giant clams have declined in recent years, and have increased the threat to the biodiversity of the hermatypic coral and reef fish in the region due to habitat destruction and fishing pressure (Stazel et al, 1997).  It is believed that with the establishment of a ‘no take’ zone, holothurians, giant clams, lobster and many other reef fish species would be able to recover to levels that have occurred in the past.  The no fishing area is 500 metres in length from the high water mark to a distance of 20 metres seaward from the reef edge.  The width of the no fishing area varies from 100 – 150 metres depending on the tide, which effectively alters the size of the reef flat.  As for most marine parks located throughout Indonesia, the Wakatobi National Marine Park of Southeast Sulawesi supports numerous resident fishing communities.  These communities inevitably provide a dilemma between the dependency of a local fishing community on the resource and the conservation efforts to protect the marine organisms, their habitat and biodiversity of coral reefs. 

Giant clams are one of the most conspicuous of the numerous invertebrates found on the Indo-Pacific coral reefs.  In the region, giant clams have long been a traditional food source and the use of the shell has been important in some locations.  They were once very common inhabitant of Indonesian coral reef ecosystems throughout the archipelago.  Despite there long standing history, they are presently overexploited in most countries.  Their decline and increased exploitation has occurred despite the fact they are a fully protected species by law (Forestry Ministerial Decree No. 12/Ktps/11 1987).  The policing of this law is difficult hence, subsistence and commercial harvesting still contributes considerably to the demise of giant clams with Indonesians continual coastal population expansion.  

Success of a population depends on the density and their reproductive capability.  Giant clams are hermaphrodites, however, they primarily use external fertilisation.  The spawning of a single individual causes a cascading response by its neighbours, thus, increasing the possibility of cross-fertilisation.  Hence, in order for fertilization to take place, populations must have a high number of individuals due to the “allele effect”, if the distance between males and females is too great, fertilization will not take place, which has great implications for the fishery.  As the distance between individuals as a consequence of constant removal, increases the likelihood of local extinction.  Therefore, the main life history traits may explain the fragility of giant clam stocks and in combination with increasing demand has lead to the current state of the giant clam fisheries, where biological overexploitation may occur well before economic exploitation.

Therefore, the aims of this study are: 

·         To compare the abundance and size composition of giant clams in the no fishing zone and adjacent exploited areas.

·         To establish the influence of habitat type on the effectiveness of the no fishing zone.

·         To discover whether a change in the size of harvested giant clams has occurred over the last 10 years.

·         To provide a synopsis of the past and present giant clam fishery in Wakatobi National Marine Park.

 

Methodology

To achieve the proposed aims a series of data was collected throughout July and August 2002 within the Wakatobi national marine park, in particular around Pulau Kaledupa, in which the no fishing zone exists. 

(i)                   Ecological Data: 

In order to determine the effect of the marine protected area on the size, abundance and density of giant clams, a series of data was collected from 7 sites (Samplea, Kaledupa buoy 1, Ridge 1, Sambano, Mantigola, Pak Kasims and Hoga buoy 3) within the Wakatobi national marine park.  Hence, two unit stocks, the fishing zone and no fishing zone, with fringing reefs of varying quality were assessed.

At each sub-unit a horizontal belt transact, 50 metres in length and 5 metres in width was constructed at constant depths of 1, 3 and 10 metres.  Hence, samples were completed along the upper reef slope (10 metres), reef crest (3 metres), and reef flat (1 metre).  Along each belt transact data on the number of species of giant clam, species abundance, the number of species; and size of individuals was recorded.  Each transact was replicated three times at each depth, hence, 63 transacts were completed during July and August.

To estimate the percentage cover of each habitat type a line intercept transact method was employed to determine the coral life forms directly beneath each transect line.  The coral life forms were recorded using the category table developed by English, Wilkinson and Baker (1995).  The length of each coral category was measured, which enables the calculation of the percentage cover of each coral type.  This is computed by dividing the total length of the coral type by the length of the transact line and then multiplying by 100.  This method provides a detailed assessment of the type and diversity of the substrate cover in the defined area of reef surveyed. 

(iv)                Fisheries Data: 

In order to determine whether a change in the size of clams harvested has occurred over the last ten years, five de-shelled clam piles were examined.  The practice of deshelling most species at a time mono-specifically in the same place for several years meant that collections of giant clams were easy to locate.  The top layer of the pile was removed and the bottom layer was unearthed, and both measured (length and width (cm)), so that a comparison between the two layers the bottom being the oldest in theory and the more recent top layer, to investigate the possibly effects on harvesting the populations of these species.  A short interview was also conducted with the house owner to derive the age of the top and bottom layer of the clam pile (Indonesian: “Tumpuk Kima”).

Fifteen semi-structured interviews were completed in Samplea and Furake to assess the current status of the giant clam fishery and establish the opinions of the locals regarding the no fishing zone.   Each interview conducted lasted approximately forty minutes, initially the interview was translated from English to Bohaso Indonesian in Furake and in Samplea the interview was further translated to Bohaso Bajau.  The interviewee’s replies were then translated back from Bohaso Bajau to Bohaso Indonesian (for Samplea only) and then to English. 

 

(v)                 Statistical Analysis: 

With the use of the statistical package SPSS version 10.0 for social science, the following analysis was completed: 

Report

A dissertation entitled A comparison of the abundance, size composition, age class structure and habitat selection of giant clams in the no fishing zone with adjacent exploited areas in the Wakatobi Marine National Park, South East Sulawesi, Indonesia will be produced by Sophie-Marie Green, University of York by February 2003.

 

 

 

Habitat selection and species associations of Ophiothrix sp. 2 in the Wakatobi Marine National Park, S.E. Sulawesi, Indonesia

 

Caroline Harris, University of Leeds

 

Home supervisor             Dr John Grahame

Field supervisors             Nerida Wilson

Dates of study             28th July 2002 to 22nd August 2002

 

Introduction

Ophiothrix sp.2 is one of the 2000 described species of the Class Ophiuroidea which belongs to the Phylum Echinodermata. Ophiuroids, or brittle stars as they are more commonly known, are found in all types of marine habitats. Many species are cryptic and live beneath rocks and coral rubble. Others have been found to inhabit both hard and soft corals and also the water canals of large sponges. As well as forming habitat associations, the echinoderms are well known for their host abilities. Until now Ophiothrix sp.2 has never been studied in the Wakatobi Marine National Park and so nothing was known about any of its associations.

 

Aims

  1. To quantify the abundance and distribution of Ophiothrix sp.2 at different depths, sites and times of day.
  2. To determine whether Ophiothrix sp.2 exhibit any preference in the substrate they inhabit.
  3. To examine parasite/commensal organisms associated with Ophiothrix sp.2.

 

Objectives

  1. To count the number of Ophiothrix sp.2 found within quadrats placed at various depths at each site.
  2. To monitor permanent quadrats over a 36 hour period.
  3. To record the substrate on which each individual is found.
  4. To quantify organisms living on Ophiothrix sp.2

 

Methodology

A 1m² quadrat was held against the reef and all Ophiothrix sp.2 present were recorded. The substrate on which each individual was found and any other observations of interest were also noted. Six replicates were made 5 metres apart. This was conducted at three different depths (reef crest, 5m below reef crest and 10m below reef crest) and at three different sites (Home reef, Pak Kasims and Sampela).

 

Six permanent quadrats of varying depths were also set up at Home reef. They were monitored at 4 hour intervals over a 36 hour time period (7am, 11am, 3pm and 7pm on day 1 and 7am, 11am and 3pm on day 2).

 

From each of the three sites 15 specimens of Ophiothrix sp.2 were collected. Back in the laboratory all parasites/commensal organisms found were described and removed for further identification.

 

Preliminary results

As yet no statistical analysis has been conducted on the data obtained. However some interesting observations have been noted. A greater abundance of Ophiothrix sp.2 was found at Home reef and Pak Kasims compared to Sampela. At these sites, greatest numbers were found at 5m and 10m below the reef crest. The individual permanent quadrats showed no change in abundance over the 36 hour period  which indicates a strong association with the substrate/host animal. Ophiothrix sp.2 was found to inhabit a variety of substrates/host animals although it was found most commonly on soft corals and gorgonians. The diversity of parasites/commensals living on Ophiothrix sp.2 was found to be lower at Sampela compared to Home reef and Pak Kasims.

 

Dissemination of results

A dissertation thesis will be produced in 2003.