By Katie Sinclair
A short DIDSON clip of a cownose ray swimming near the SERC dock
One of the biggest challenges of studying life in the Chesapeake Bay is poor visibility. In the water, you literally can’t see your hand in front of your face. So how do you see what’s going on under the water? Sampling with nets can give us a pretty good picture, but if you want to observe what’s happening at a specific moment you need to use another approach.
The Fish and Invertebrate Ecology Lab is using Dual-frequency Identification Sonar (DIDSON) to see what their eyes can’t. The DIDSON uses sound to sketch a picture of the environment much like a fish-finder or depth-finder on a boat. When sound hits an object in the water or the bottom, it bounces back. The result is video footage that can be downloaded and later analyzed. The resolution of the image is pretty impressive, considering it’s the product of sound waves. Many species can be identified by their distinct shapes and movement patterns, and their sizes can be estimated. DIDSON was first developed for the military to locate enemy swimmers , but is now being used as a handy tool by anyone who needs to see in murky waters.
The Fish and Invertebrate Lab started using DIDSON with the goal of better understanding the size of river herring populations in Chesapeake Bay. Funded jointly by the National Fish and Wildlife Foundation and the Smithsonian Institution, the project aims to generate accurate population counts of river herring in the Choptank and Nanticoke Rivers. Once a species that supported a major fishery on the Atlantic coast, river herring (actually two species known as alewife and blueback herring) have declined dramatically over the past 10 years. In order to figure out the best way to conserve and manage these populations, the first step is to figure out how many herring are out there. The murky waters and large run sizes make it nearly impossible to get an accurate visual count. Other methods, such as electrofishing and looking at icthyoplankton (fish eggs and larvae) can potentially work as counting methods, but first it must be established how accurate they really are. By deploying the DIDSON during the spring herring runs, footage of the runs can be recorded and the number of fish counted. DIDSON also allows fish to be recorded for several months straight without requiring someone to be out in the field sampling all day and night.
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