Pasua (Tridacna maxima) size and abundance in Tongareva Lagoon, Cook Islands

Study Extent

Tongareva Lagoon, Cook Islands

Sampling

The survey took place over 14 days from 3–17 May 2006 with a follow-up survey at sites Te Vo, Tepetepe and between Ahuamiria and Atutahi on 24 July 2006. Fieldwork was conducted by the author, Mataora Marsters and Taimana Manata from the Tongareva Marine Research Centre (TMRC) with additional assistance from Tomas Samuela Jr., Tuku Marsters, Martin Chambers and Baar Tapu. Site selection was designed in an attempt to survey a wide cross section of the lagoon environment, for example toka (distinct coral heads), tuarai (small scattered coral heads close to shore) and kauniho (reef edges). Sites were stratified according to the section of the lagoon (e.g. far southern section) and were selected by travelling in a boat within these sections in a relatively straight line (e.g. parallel to the shore) for a set period of time (e.g. 10 minutes) until suitable habitat was encountered. From preliminary observations and based on existing biological information concerning pasua (Lewis 1987), suitable habitat was defined as toka and tuarai down to a depth of 10 metres, and kauniho, which weren’t predominately sandy.

Method steps

1. The survey area was delineated by four 50-m ropes, which were knotted to form a square. This was then placed randomly over the survey site although in the case of kauniho, the lines were placed at the edge of the kauniho extending towards the shoreline. Five transect lines were then placed within the square at 10-m intervals. All pasua found within 5 m on each side of the transect line were counted and a proportion of the total population measured. In cases where low numbers of pasua were present, every second pasua was measured but in situations where large numbers were found, measurements varied from every tenth to every one-hundreth pasua.
2. The size distribution of pasua was evaluated by recording the length, measured in centimetres, along the shell opening (lip) using callipers. The census of pasua abundance in the lagoon was calculated by counting the number of pasua within each survey area; density was calculated as the total.

Pasua giant clams

1. Tridacna maxima
   common name: Pasua rank: species

Tongareva Lagoon, Cook Islands