February 25, 2010

Technical Question of the Month: Video Transects


Each month, Reef Check will answer a technical question regarding the monitoring protocol of our coral reef or rocky reef programs. If you have a question you would like answered, please email rcinfo@reefcheck.org.

Reef Check California — Why do we use visual (slate) instead of video fish transects?

The best methods for collecting data in the marine environment depend on many factors including environmental conditions, behavior of the organisms of interest, cost and relative safety of collecting techniques. What you are measuring (metric), whether it be the number of blue rockfish or the size of abalone on a reef, can often require completely different data collection techniques. The parameters that we are interested in measuring are abundance (number of selected indicator species), diversity (number of different indicator species), and size. Many of our volunteers have often asked us whether or not video transects might be a more suitable method than the Reef Check Protocol for measuring these parameters. Studies from tropical reefs have shown that visual (or slate) censuses are the more accurate technique to determine the number of fish species and the abundance of lower-density species, which are either cryptic (hidden), evasive, or difficult to distinguish from the background1. Video transects were found to be adequate for measuring species with high abundances that tend to school.

A large proportion of our target fish species in California are cryptic and found deep within crevices, such as rockfishes. We also have the additional difficulty of kelp filling the entire water column, thus reducing chances for observation. Therefore, the visual diver census is the method that best fits our conditions and organisms of interest. Our maximum depth of 60ft for all surveys means that we sample relatively shallow, nearshore rocky reefs. Rocky reef surveys at deeper depths in California are generally collected using submarines and remotely operated vehicles (ROVs) that employ video transects. Although visual diver transect data would likely collect more accurate data on the cryptic species at these depths, diver surveys below 60ft are much less safe and therefore less suitable than video. Another often overlooked task associated with video surveys is the post processing of the video. An hour spent recording video means at least an hour spent watching video, recording the data, and entering it into a database. This can greatly increase the amount of time spent per transect and therefore can increase costs significantly. If the video operator does not record in cracks and crevices, some animals may be missed. While there are factors that can contribute to a reduction in precision of slate transect data, such as attraction of fishes to divers, time spent looking down recording data, and slight differences in technique between divers2, this is most accurate and least expensive way to gather data in our given conditions.

1Tessier, E. et al. 2005. Visual censuses of tropical fish aggregations on artificial reef: slate versus video recording techniques. Journal of Experimental Biology and Ecology 315:17-30.
2Harvey, E. et al. 2002. Estimation of reef fish length by divers and by stereo-video A first comparison of the accuracy of and precision in the field on living fish under operational conditions. Fisheries Research 57:255-265.