By John Page Williams, CBF Senior Naturalist and avid boater and fisherman. In this two-part series, John Page gives anglers some insight into the images on your sonar fish finders—what they mean and how you can use that information to catch more fish.
Ever see an image like this on your fish finder?
Here is what is going on:
This image shows dense schools of bait fish (mostly menhaden) and some larger fish (probably rockfish) suspended high in the water column over 40 feet of water in the Severn River, about a mile above the Route 50 bridge in Annapolis, Maryland, on August 1, 2010. The fish are swimming at depths between 10 feet and 18 feet, where they have water as cool as possible (around 81 degrees instead of 83 degrees at the surface) and enough dissolved oxygen to “breathe.” The dissolved oxygen level in the zone where the fish are holding is 2 to 5 mg/l (milligrams per liter), low enough to cause stress to the menhaden and rockfish but not enough to kill them. Below 18 feet, the dissolved oxygen tails off to lethal levels of just 1.7 mg/l at 20 feet and 0.9 at 35 feet.
In the Chesapeake at this time of year, the water at the surface is generally warmer than the bottom, because it is closer to the sun, and fresher, because it has flowed into the Bay or a river from rainfall. The bottom water, on the other hand, is denser because it is cooler, and saltier because some of it has flowed up the Bay from the Atlantic Ocean. If the weather is stable, without heavy rainfall and strong winds, the lighter surface water sits like a lid on the bottom water.
Meanwhile, trillions of algae cells at the surface die every day and sink into that sealed-off bottom water, where aerobic (oxygen-consuming) bacteria decay them. This process literally sucks the life out of the deep water, causing what we now call “Bad Water” where fish and crabs get stressed (2-5 mg/l) or “dead zones” where they can’t live at all (less than 2 mg/l). The result of this process is a situation like the one you see in the image above.
Sometimes, the boundary line between warmer, fresher, well-oxygenated water and cooler, saltier, poorer water is very sharp:
See the lines going across the screen at 16 feet and 24 feet? Most sonar manufacturers call them thermoclines, a term that refers to sharp temperature changes, but in an estuary like the Chesapeake, those breaks can also be caused by abrupt changes in salinity, or a combination of both factors. In this image, taken in the open Bay off the mouth of the Chester River in May, each of the two breaks is caused by changes in both salinity and temperature. Those density changes are sharp enough to bounce sonar echoes! Notice how fish tend to suspend on or just above them.
Part 2On Thursday, John Page will be back with a look at how you can use this information to catch more fish—in both the short run and long run.