March 30, 2011

What is the goal and practical application of transect deployment in a Reef Check survey?


By Reef Check’s Executive Director Dr. Gregor Hodgson

Reef Check Tropical – What is the goal and practical application of transect deployment in a Reef Check survey?

The Reef Check coral reef monitoring protocol is based on the use of a transect line to measure distance and area and a plumbline to determine a point sample location along the line. A fundamental consideration when using a transect line to sample reefs is how the transect line is deployed. On one extreme, the transect could be pulled tight above the reef while at the other extreme the transect could be deployed with more slack to follow the vertical relief of the reef. In fact, in 1972, James W. Porter1 published a paper on a survey protocol using a chain link transect line that closely followed the surface of the reef.

What is the goal and practical application of transect deployment in a Reef Check survey? The fundamental question is: are we trying to sample a horizontal representation of a 3-D structure – the surface of the reef? The answer is yes because our plumbline method and instructions are based on the plumbline aligned perpendicular to the transect. So how does the position of the transect line with respect to the angle of the reef at a given point affect our estimates of cover? My conclusion is, “not much.”

In some respects, this is actually a scale question along the lines of, “how long is the coast of Sweden?” If you followed every vertical cm of relief, you might be able to “use up” the 100 m tape in 20 m of horizontal distance. There are several practical issues involved as well. Given that we are using fiberglass survey tapes these days as transect lines, the transect line can also float if it is not stretched taut. Slack lines can swing widely in current and waves making plumbline use difficult.

For RC, the primary objective is that we want to let the transect line stay within each depth zone and follow the depth contour. Then we want the line to follow the vertical relief sufficiently so that we are not creating 1+ m vertical gaps between the line and the reef. In practice, in a wave washed environment, the line is typically given a quick wrap around a rock every 10 to 20 m to prevent swinging so the result is that some of the transect is in direct contact with the reef while in other areas it may end up getting pulled relatively taut – hanging above the reef.

Since we are point sampling – a slack line deployment could create a bias by bringing sample points slightly closer together by following the relief more closely, and to shorten the horizontal distance covered by the entire transect — probably not a serious bias in most situations.

Even on a perfectly calm day, most transects will probably end up including a mixture of some relatively taut and some more slack sections such that we reduce sections of swinging transect or sections hanging more than 1 m above the substrate. In terms of what to aim for and how to teach, I would suggest pulling the line a bit tighter on flat sections and letting it drape when going over large coral heads/rocks that create 1+ plus vertical gaps. Remember that the fiberglass tapes will break if pulled too tight.

Thank you to Dr. Jonathan Shrives, a long-time Reef Check Trainer now based in Jersey, for requesting clarification on this issue.

1Patterns of Species Diversity in Caribbean Reef Corals
James W. Porter, Ecology Vol. 53, No. 4 (Jul., 1972), pp. 745-748