Tracking fish using acoustic telemetry in the Missouri River is never easy. Even with good conditions on the Big Muddy we are working at the edge of what technology allows. The acoustic telemetry tags that we implant inside pallid sturgeon are controlled by electronic micro-circuitry, and powered by a small battery. The circuitry in the tag creates a unique electronic pulse and a transducer in the tag converts that pulse into an acoustic signal, or pulse of sound. Acoustic receivers on our tracking boats detect that pulse of sound from the tag using specialized, directional, underwater microphones, or hydrophones. The acoustic signal is processed by the receivers telling the tracking crew which direction to go to find the fish, and which fish it is.
Let’s use a human analogy to describe the difficulty of detecting acoustic tags implanted in sturgeon in the Missouri River. Imagine you saw your friend across a field and wanted to attract their attention. Your friend is far away. Of course you would call to your friend by name as loudly as possible (an acoustic signal). Even if your friend has very sensitive ears (a good acoustic receiver) the distance your friend can hear you (detection range) is limited by how loudly you can yell (signal strength). But if there are trees between you and your friend the sound is blocked or if the wind is blowing loudly your voice is drowned out and you will need to be much closer to your friend for them to hear you.
Moving sand and turbulent water make the Missouri River a very noisy river. Under the best of conditions, acoustic tags implanted in pallid sturgeon can be detected only 300 to 500 meters away, or about the width of the Lower Missouri River. As water levels rise, the river gets wider and noisier. Increased sediment transport can quickly dampen acoustic signals and reduce detection range. Sandbars, dikes, woody debris and dunes of sand along the bottom create many places that sturgeon can hide to avoid detection. Under extreme flow conditions our detection ranges may fall to fewer than 100 meters. At very high river flows, the analogy is more similar to you trying to attract your friend’s attention from somewhere across a crowded football stadium during the Superbowl. Fortunately, we have some tools in our pocket to deal with those situations.