Data & Results Newsletter
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In the Right Place at the Right Time: A Comparison of Sampling Methods for Larval Fish
The Wells National Estuarine Research Reserve (NERR) has been surveying larval fish populations in the Webhannet Estuary since 2009 using a traditional plankton net. This long-term plankton net survey tells us a lot about the fish larvae entering our estuary and how their composition and timing changes from year-to-year; but what other fish species are in the area? eDNA methods can help fill in some of the blanks! The question is: What’s the best method for collecting water samples to get the most eDNA? Does it matter how I collect my samples? In this newsletter, we will explore several methods of collecting eDNA water samples and compare them to traditional methods of collecting and visually identifying larval fish.
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The Venn Diagrams below show the species that were detected each month using each of the four sampling methods:
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These results show us a few things:
eDNA is a great way to supplement traditional sampling. In all months, traditional sampling only yielded 0, 1, or 2 species, while eDNA detected MANY more. For the traditional plankton net to capture a particular species, that individual must be in the larval stage swimming at 1 m below the water surface and carried by a current through the exact location of the plankton net…in the right place at the right time! In contrast, an individual fish swimming offshore could disperse DNA that could be carried by a current into the estuary and into one of the eDNA samples. But eDNA cannot fully replace the larval fish survey. First, eDNA cannot tell you if a fish is young or old, alive or dead, near or far. Second, traditional sampling can sometimes detect species otherwise not detected by eDNA. Just look at September! This brings us to our next point…
Different methods give different results. While there is some overlap between the species detected by the different methods, there are also a lot of differences. For a species to be detected by a particular method, its DNA (or the fish itself in the case of traditional sampling) has to be contained within the water being collected. In other words, it has to be…in the right place at the right time! That’s important to remember when looking at your results: the species you detect are not necessarily all the species present nearby. So then what method should I use to detect the most species???
Some eDNA methods are better than others. In this study, the Net Rinse sampling method detected the most fish species in all months, while the Continuous sampling only detected one (if any) species each month. To be detected via the Continuous method, DNA must pass through the opening of a very narrow tube. On the other hand, the plankton net acts like a massive filter in the water column, 'catching’ DNA that can then be rinsed off into the Net Rinse sample bottle. As we have seen in some other studies, collecting more water will typically help you detect more species. A plankton net can essentially filter hundreds or thousands of liters of water in a one-hour tow, allowing you to “sample” a massive volume of water without having to actually collect it all.
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When designing an eDNA study, it is important to consider not only how much water to collect but also how to collect it. Results from this study show that different eDNA and traditional sampling techniques yield different results in terms of fish species detection and provide some initial insight into a few unique sampling methods. Our results highlight that eDNA methods can complement traditional monitoring methods to provide a more complete picture of fish assemblages.
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Emptying the larval tow net. Our new favorite filter size?
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