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Part One: Manta Tow Technique
DATA RECORDING Ambient variables The ambient variables recorded include, information about the survey (reef name, time, date, data collectors) and the weather conditions (Appendix I). The weather conditions are recorded as wind strength, cloud cover, sea state and tide, and are described as: Wind Wind strength is recorded as a category from 1 to 5, described in Table 1.
Table 1. Wind strength categories.
Cloud Cloud cover is quantified in terms of eighths of the sky area covered by cloud. The unit of measure is the okta. From a position where the entire sky can be seen, estimate the amount of cloud as a fraction of eight. Thus, a cloudless sky is recorded as 0 eighths or oktas and an overcast sky is recorded as 8 oktas. Sea state Sea state is described by a modified Beaufort scale (Table 2).
Table 2. Sea state description
Tide The tide state is defined as either low, high, falling or rising and is determined from a Tide table. These states are described in Table 3.
Manta tow variables For each two minute manta tow, the number and size of COTS, percentage cover of live coral, dead coral and soft coral, presence of feeding scars, visibility and any observations of note are recorded (Appendix I). COTS numbers The number of COTS observed during a manta tow is recorded. COTS size The size of COTS is recorded as: juvenile (J), 5 cm or less; sub-adult (A); or adult (B), greater than 15 cm (Table 4). If no COTS are seen, then this column is left blank. When several size categories are seen, only the most numerous category is recorded in this column, and a note is made of the other size categories in the 'other' column. Note. Juvenile COTS are not usually seen while manta towing.
Table 4. Size of COTS categories.
Live coral cover Percentage cover estimates of live coral are made from the total tow path area observed during each 2 minute manta tow. Live coral refers to the living reef-building Scleractinian corals. This does not include the non-Scleractinian corals such as Millepora, Heliopora and Tubipora. Live coral is coloured by the presence of living tissue and can be easily recognised by its colour and the detailed structure of the corallites. The percentage cover estimates of live coral are recorded as one of 6 categories (Table 5). Dead coral cover Dead coral is defined as coral that is not covered by living tissue but still has distinguishable corallum structure. A 'newly' dead coral is easily recognised by its brilliant white colour. The white skeleton is then colonised by a succession of algal types, initially with turf algae giving the dead coral a dull, greenish/brown colour. Eventually, coralline algae take over giving the coral skeleton a smooth pink appearance. By this time, the detailed structure of the coral skeleton is lost, due to the algal colonisers and weathering, and the colony is considered part of the substrate, or if fragmented, becomes rubble. The percentage cover estimate of dead coral is recorded as one of 6 categories (Table 5). Note. Once a coral skeleton has become encrusted by coralline algae it is considered to be substrate and not dead coral.
Table 5. Percentage cover estimates. For each category (except 0) a plus (+) or minus (-) is added to denote whether the cover estimate falls into the upper or lower half of each category.
Scars Feeding scars are patches of recently dead coral easily recognised by their white colour (Figure 4). Such scarring is often indicative of feeding activity by COTS and is useful in locating cryptic populations. However, COTS are not the only organisms that cause coral scarring. The gastropod Drupella, can produce feeding scars similar to those of COTS, however, COTS scars are distinguished by their large size and stark white colour. Feeding scars of COTS are recorded as absent (A), present (P) or common (C) (Table 6).
Figure 4. Acropora coral with distinctive feeding scar. Soft coral cover Soft corals, as their name implies, lack the hard limestone skeleton typical of scleractinian corals. There are many forms of soft coral, but in a manta tow survey we only look for the 'fleshy' soft corals, such as those from the families Alcyoniidae, Neptheidae, Xeniidae and Briareidae. Soft coral cover is also recorded as a percent cover category (Table 5). Visibility Water visibility estimates are recorded during the first manta tow and subsequently whenever the visibility changes. An estimate of visibility is made by diving below the surface and looking at the two floats attached along the tow rope at six metre intervals. Depending on how far the observer can see along the tow rope, the visibility is recorded as one of four categories (Table 7). For example, if the observer can see the boat motor and beyond, then the visibility is scored as category 4 and if the nearest float cannot be seen, then the visibility is scored as category 1 (Figure 5).
Table 7. Categories for estimating visibility.
The other column is for any observations such as the reef structure, slope, diversity, fish abundance, number of giant clams, coral mortality or any information required for the reef aesthetics sheet (Appendix IIa). Giant clams are counted for each tow. However if the observer feels clam counts may interfere with counting COTS, then a note should be made in the other column to say 'no data' for clams. Reef aesthetics surveys The reef aesthetics data sheet is designed to provide descriptive information about the reef slope. The data are obtained from the collective manta towers' observations. The result is a qualitative impression of the reef's topography and value (Appendix II). Procedure When recording the manta tow data the observer writes descriptive notes in the 'other' column on the manta tow data sheet that describe the reef slope and depict areas of change. At the end of the day's work, these notes are used in conjunction with a discussion by the observers to form an overall impression of the reef. To help form this impression, a series of attributes have been devised to describe the reef. A category is assigned for each attribute from the description sheet (Appendix IIb) and recorded on the reef aesthetics data sheet. Data recording A reef outline is drawn on the data sheet (Appendix IIa) and is divided into four zones. For each zone, ten descriptive attributes are assigned to describe the area. Below this are four rows of boxes, each row corresponding to a reef zone. In each box a number or letter representing a category is recorded. See Appendix IIb for categories. Zone The zones, marked on the reef outline are labelled in a clockwise direction starting from the back reef (leeward side).
The reef slope is defined as the average angle of the slope in the zone and has been categorised as follows:
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Figure 5. Visibility category estimates for manta tow surveys.