Rapid assessment of coral reef status using the photoquadrat method.

I learned to SCUBA dive not for the sheer pleasure of seeing the beautiful coral reefs but primarily to get to understand the coral ecosystem’s status. The coral reefs are constantly under threat due to excessive fishing as well as illegal fishing in this highly productive marine ecosystem. There is a need to find out the current status of coral reefs such that proper resource management measures can be implemented. One of the ways to assess the condition of the reefs is with the aid of still pictures taken from the coral reef. These are analyzed using a software designed for the purpose. The method is called the photoquadrat method.

What is the Photoquadrat Method?

The photoquadrat method uses still pictures taken in a series with  the aid of a  fiberglass measuring tape, usually 50 to 100 meters long. For a 50 meter measuring tape, 50 pictures have to be taken. This means a picture of the coral reef is taken every meter. A picture of the coral cover is taken at a distance of about 30 centimeters from the bottom. Some coral reef researchers use a pod to standardize on distance, but a rapid assessment can be done through just mere estimate of distance. A sample quadrat is shown below.

A sample photoquadrat.

Factors Affecting the Photoquadrat Method

In principle, the idea of the photoquadrat method is simple – take a series of pictures along a transect line. In reality, however, taking pictures underwater is replete with factors that may affect the quality of data obtained using the photoquadrat method. Among the difficulties I encountered in the field are the following:

• Poor visibility of reefs due to high water turbidity. High water turbidity can render the pictures unclear and unfit for coral reef identification and analysis.
• Strong water current due to wave action. Some of the coral reef assessment times fall on bad weather days. It is quite difficult to focus the camera while you sway due to strong wave action on the surface. This is particularly true in shallow (ca. 15 to 20 feet depth or less) areas.
• Irregular coral reef configuration. Coral reefs are not always in reef flats so there may be a discrepancy in the depth of follow-up studies.

• Underwater camera out of power. You can never predict if the batteries you use will be reliable. Sometimes, the camera’s batteries are exhausted while underwater forcing us divers to surface and reload. Reloading sometimes is unsuccessful as underwater cameras can be foggy due to the heat generated by the underwater camera. A foggy underwater camera cannot take good pictures.
• Fear of the unknown. Even though I have SCUBA dived many times, there’s some kind of a feeling of fear that can affect the quality of the pictures taken. My buddy usually swims ahead to lay the measuring scale as transect and I am left alone focusing my attention on the corals oblivious of what’s going on around.  I might accidentally bump on something hostile. Occasionally, I encounter sea snakes and manta ray along the transect. I’m apprehensive of either the shark or the crocodiles. I am thankful many of the areas we assessed are overfished. If fishes are few, most likely these predators are absent due to the food chain principle. They have nothing to eat so they’re somewhere else.

How are the Pictures Analyzed

The pictures taken using the photoquadrat method are analyzed with the aid of a coral point count software dedicated for the purpose. I am using CPCe, an image analysis software by Kohler and Gill (2006). The software automatically provides random points for coral reef identification and classification according to the level of resolution the researcher wants, be it the coral’s genera or just plain general classification such as live coral, dead coral, soft coral, among others. The analysis can be exported to MS Excel for interpretation.

A better understanding of the coral reef’s status through the photoquadrat method can then aid the coral reef resource managers to act where it’s needed and what coral reef-related issues should be addressed. Among those issues are crown-of-thorns starfish outbreaks.

Literature Cited

Kohler K.E., Gill S.M. (2006) Coral Point Count with Excel extensions (CPCe): a visual basic program for the determination of coral and substrate coverage using random point count methodology. Computers & Geosciences, 32, 1259–1269.