2.1 FIELD STOCK SURVEY
The tow method adopted by Munro (1986) in his survey to assess giant clam stocks in Kiribati and the same method used in the FAO-funded giant clam stock survey in Tuvalu (Braley, 1988) was also employed in this survey. Two divers with snorkel gear (mask/snorkel and fins) are towed along the sides of a small boat from a 5m length of galvanised 1.5in. pipe secured perpendicular to the boat just behind the foredeck (Figure 1). Short lengths of rope were required to secure the pole to the boat and to provide loops which could be held by the divers being towed. The keel of the boat provided a natural border so that the two divers would not count some clams twice. The width of the tow track scanned by the divers varied due to clarity of the water and densities of clams. If there were high densities the combined width of two divers would be as little as 4m but in low density situations and clear water it may have been from 10–14m combined width. In rare cases of towing over mostly sand or on the ocean-side tows with very clear water the combined width may have been 16–20m. Beyond these widths (normally no wider than 10–14m) it was nearly impossible to spot clams and also cover the area in front of the diver carefully at the speed which we towed. Our speed of towing ranged from 60–80m/min. but the speed and time of the tow were not used to determine distance of the tow as in Munro (1986).
To determine the distance of each tow a large red or orange buoy with rope and anchor was placed at the starting point. A straight line distance tow was made to approximate distances of 250–400m. When the boat engine was stopped the information of clam numbers, depth of tow, width of tow track scanned, percentage of coral cover, and percentage of live coral was relayed to one of the survey team members in the boat while the other team member in the boat was taking readings of the buoy with an optical Rangefinder [Rangefinder 1200m]. A clear image of the buoy is seen through the viewing lens but when the buoy is not focused for the second parallel lens a yellowish image of the buoy is superimposed on the clear image through the viewing lens. With careful adjustment for distance the two images merge and the distance is simply read off the dial. To check for different viewer's error the second team member in the boat would also check the Rangefinder for the distance. When a replicate tow was made the boatdriver was instructed to move about 50–100m perpendicular to the tow track and return at an angle to the first tow track back toward the buoy. In situations where the reef edge was very narrow or was clearly the only area where any coral was present a tow without replication would be made. Two tows were made at each of Fakaofo, Nukunonu, and Atafu atolls on the ocean side of the atoll reef. Tows were all on the leeward side of these atolls and the aim was to have a brief look at the presence/absence of clams and condition of the coral. There was not enough time to cover large areas outside of the lagoon. Therefore, the results of the survey will represent an estimate of the stocks available in the lagoon in depths less than 12–15m.
FIGURE 1:
Method of towing to assess clam stocks [after Braley, 1988]
The Rangefinder was calibrated in Australia before commencing this trip but was also recalibrated in Fakaofo, and on Nukunonu once during the survey and once just before leaving for Atafu.
In addition to the tow survey, we ran 3 reef flat transects at Fakaofo and Nukunonu, and 2 transects at Atafu. The main purpose of these transects was to check the densities of clams on the reef flat areas so that a rough estimate could be made of the portion of the population inhabiting these shallow, significantly large areas of the atolls. In addition to the giant clams we counted holothurians present (to be presented in a separate short report). The transect tape was laid out perpendicular to the reef front edge starting from the intertidal area on the lagoon side of the flat. Stakes or coral stones were used to indicate 10m intervals out to 200m, and thereafter intervals were to be measured every 50m to the reef crest. Two men would walk, one each side of the transect line and count numbers of clams (and holothurians) present within 3m each side of the transect line. The only other transect work done was subtidally at Fakaofo to show Mose how to use this technique of quantifying organisms. Since these transects were made to separately estimate clams on the reef flat from the tows made in the lagoon proper the analysis of results is done differently than the estimates made in the Tuvalu survey (Braley, 1988). In Tuvalu it was necessary to estimate whether 25%, 50%, or 75% of the adjacent reef area to a tow (including both lagoon area and reef flat area) was suitable for clams to live on. In the present survey, I can consider that the suitable lagoon bottom area (with the same depth range as the tow) halfway between a tow and its two closest tows is 100% as suitable as the tow (sampled area). The estimate for the reef flat is then added on separately in the total estimates.
The only two species of giant clam found live on these survey were Tridacna maxima (fahua nao) and Tridacna squamosa (fahua taka).
2.2 FIELD MEASUREMENT OF CLAMS
Measurement of the shell length of T. maxima was made from different sites in the lagoons of each atoll. Hand-held vernier calipers were used and SCUBA gear when required to measure 50–100 individual clams from each site. At least one of the sites chosen at each atoll was a pristine area with numerous coral bommies and high densities of clams far from the village islet. At Nukunonu clams were also measured with snorkel on a reef jutting into the lagoon just off the village. The size of these clams located off the village will be compared with the pristine areas far from the village.
2.3 PEARL SHELL OYSTER FIELD WORK
Patch reefs in the middle of the lagoon or a steep slope with coral cover on the lagoon reef were chosen as sites to make several dives to 100–120ft. (30.5–36.5m) to check cursorily for the presence of pearl oysters. A dive was made down to 100ft. quickly as possible, then to 120ft. where coral crevices, overhangs, etc. were checked for presence of pearl oysters. We also made count estimates in 1m2 areas of the number of spondylid oysters present at this depth, then again at 100ft., 80ft. and shallower. We tried to keep the dives as short as possible because we were extremely limited with the number of bottles of compressed air (2 for pearl oyster deep dives, 1 for giant clam measurements per atoll). Mose and I took turns on these dives until we got to Atafu where we had more tanks but less time available.
2.4 VILLAGE QUESTIONAIRE SURVEY
The questionaire form used was modified only slightly from the one used in the Tuvalu survey (Braley, 1988). Appendix 2 lists the questions in English and in Tokelauan. The procedure was to go to houses in early-mid evening in two groups (1 or 2 persons/group) and ask to speak with a man or woman in the household. Questions were asked in Tokelauan and written down (or interpreted to myself first) in English. The names and ages of men and women were noted so that we could try to keep about 5 men and women in each of the three age groups (teen-30; 30–50; 50+). The aim of the questionaire was to record the views of a wide spectrum of people in the community with regards to giant clams. Some of the questions were constructed to see how people view the reef and clams in an ecological sense. When a person gave unusual answers there was an attempt to probe more into why they took their view.
Identification sheets showing all of the species of giant clams were shown to persons being interviewed and the people were told that T. squamosa and T. maxima are separate species to correct a commonly held belief that the fahua taka grows up to a large size from the smaller fahua nao.
In Atafu we were very short of time for the survey and had to rely on the Agriculture Extension Officer and a team of assistants to conduct the questionaire survey whilst the field team carried out the towing survey. Unfortunately only 16 persons were questioned rather than the desired 30 but this number should still give an idea of the trend of thought in Atafu.
PEARL SHELL:
The interviewees were asked if they had ever seen a pearl oyster shell taken from their atoll lagoon. If so, when and where did they find it or see one which someone else had found.
HOLOTHURIANS:
Some interviewees were asked to identify from pictures in Cannon and Silver (1986) which ‘sea cucumbers’ they have seen on their atoll reefs. These will be used in a separate short report.
