Underwater video analysis

Posted on December 28, 2013

Although electrofishing is probably the most commonly used method of surveying stream fishes, it can lead to serious short and long-term damage to the animals concerned. For example, in one study of rainbow trout, X-ray examination showed that the majority of electrofished individuals incurred spinal injuries. However, technical advances in the field of underwater video analysis now allow alternative, non-destructive methods of fish sampling. South African researchers used East Cape redfin (Pseudobarbus afer) and Cape kurper (Sandelia capensis) as test species to explore the potential of underwater video analysis for assessing the diversity and abundance of fishes. Working in headwater tributaries of the Swartkops River, they sampled ten pools with a mean water depth of 30-70 cm. They used a colour camera on a stand 20 cm above the stream bed at the tail of each pool. Over a three-day period they filmed for a total time of 60 minutes (60 subsamples x 30 seconds x two deployments) and then after a one-day break they sampled all pools using three-pass electrofishing. For both study species, relative abundance was estimated in two ways: from the video data (as the maximum number of individuals present in the field of view at the same time, averaged over all subsamples) and from the electrofishing data (as the number of fish caught per cubic metre swept). When the two measures of redfin abundance were plotted against one another, there was a direct, highly significant correlation. However, similar plots of the Cape kurper data showed a different pattern, with the electrofishing index remaining static and low even when the visual index reached high levels. This result was explained by the fact that Cape kurper are cryptic fish that are likely to retreat into complex habitats when disturbed by electrofishing activities. Taken together, the findings for the two species suggest that video-based measures of fish abundance can be as reliable, or more reliable, than those based on electrofishing, while at the same time being preferable in terms of damage minimisation. For these reasons they could be particularly valuable when rare or endangered fish species are involved. The main constraint on the use of underwater video techniques is the degree of water clarity.

Reference: Ellender, B.R. et al. 2012. Underwater video analysis as a non-destructive alternative to electrofishing for sampling imperilled headwater stream fishes. Aquatic Conservation: Marine and Freshwater Ecosystems 22, 58–65.