Small acoustic targets in Lake Geneva

I mentioned in an earlier post the acoustic detection of some very small targets. We saw them in early April and found it curious, but their numbers were low and we did not think too much about it other than to consider how we might mask them to prevent our measurements of zooplankton back scatter from becoming biased high. Over the course of April their numbers increased dramatically, almost exponentially so. We decided that we needed to make an effort to try and capture these creatures so we could explain the backscatter when we publish our data. Scientists must thing well in advance to defend their work. So we rigged a larval fish trawl (1.5 m x 1.0 m mouth with 1000 micron mesh) so it could fish the pelagic zone (open water) at depths of 5 to 15 m below the surface where the bulk of the small scatterers reside (see video of the trawl being retrieved. Be patient.). So we did that and caught zooplankton and some plant debris, but we also caught chironomid pupae and what we believe to be newly hatched flies (see photos). They are also known as midges or non-biting mosquitoes.  They are about a one centimeter long.  I thought we might be detecting larval fish, but I was proven wrong with this important sampling. Thanks JC and Jean for making it happen, and Phillippe for helping us trawl and his expertise in the pressure probe we used to understand the depths fished.

When we got off the water, I did a quick internet search and found a paper written by my friend Jean Kubecka in 2000 describing the acoustic target strengths of chironomids he had sampled in some European reservoirs. He reported they had target strengths of -75 to -64 decibels, just a twitch bigger than the targets we were seeing (-80 to -68). Apparently we can detect them because they carry air in their exoskeletons or because the exoskeleton has a different density than the water. Whatever the reason, I'm convinced that is what we've been detecting in Lake Geneva. Jan's description of their position in the water column is exactly what we observed. He described them as sedentary in the water and that is what we saw. Apparently they are in Loch Ness and Jan concluded his paper by saying that the Lake's monster as seen on acoustic echograms may in fact be midges.

Thanks Ann for helping me take the microscope photos of the chironomids.