The assessment of fish and elasmobranch (sharks and rays) communities plays an important role in monitoring the health of the ocean. Non-invasive, non-destructive methods that can detect fishes are preferable, particularly when dealing with endangered species or when working in protected areas. Baited remote underwater video surveillance (BRUVs) is used all over the globe in management and conservation programs that target fish and elasmobranchs. The stereo-video method employed with BRUVs allows for not only the identity of fish species but can also be used to assess abundance and biomass, which are essential for effective fisheries management. However, there are caveats with using BRUVs, as with all methods, and small cryptic species, night predators, and fishes not attracted to the bait bag may not be captured on video. Alternatively, there has been a recent surge of interest in the capability of using environmental DNA (eDNA) to characterise biodiversity. The technique works on the basis that all living things leave traces of their DNA in the environment; for example, in blood, hair, scales, and poo. Remarkably, recent advances in DNA sequencing technologies now allow us to assess these ‘genetic breadcrumbs’, and by targeting specific areas of an organism’s genome, determine the species that are present.

Our study compared these two methods (BRUVs and eDNA) in their capacity to characterise fish communities. We sampled seawater for eDNA and deployed BRUVs at the same locations across reef and seagrass habitats. What our study concludes is that no single approach is better – you get a more holistic picture of the fish diversity present when you combine data from the two approaches. Furthermore, we show that both methods are sensitive enough to resolve fish communities that are specific to either seagrass or reef habitats – an interesting find considering that we initially thought that DNA in the ocean would be one homogenous soup. Rather, what we suggest occurs in the ocean is that the DNA degrades very quickly and becomes diluted as you move further away from the source, which collectively provides the structure that we observed in fish eDNA. Combined, the ability to both capture molecules and movies provides a powerful tool that can now be used to more effectively monitor and conserve the biodiversity in our oceans.

https://onlinelibrary.wiley.com/doi/abs/10.1111/cobi.13183

Dr Michael Stat is a lecturer at Macquarie University in Sydney, Australia.

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