Friday, May 11, 2012

Shallow water chirping

You hear the claims that chirp technology lets you see deep, and here is some real proof. A Garmin GSD26 with an Airmar 599LH transducer painting the bottom at an impressive 11,706 feet. That's over two miles deep. I think this is the current record for finding bottom with a marine chirp fish finder, or at least this is by far the deepest screen shot I have seen to date, and I have seen a lot of them. Look at the depth range scale. The bottom of it is at 13,200 feet, and I would bet given time we will see even deeper screen shots if anyone can find water deep enough. I'm sure the unit was using every bit of its 3000 watts on low chirp to do this. But this time, we are going to the opposite extreme to look at how well this technology operates in very shallow water.

Our test boat is a GSD26 equipped Pursuit with a 1000 watt Airmar B265LH inhull transducer. The deepest water we were in during our shallow water expedition was less than 13'. All of the screen shots were taken using high chirp, in high auto gain, with auto ranging. The GSD 26 varies its power based on depth, and all of these screen shots were taken with about 30 watts being used.

Our first stop was under the the new Ringling bridge just one piling to the east of the main channel. This is a somewhat notorious location. After the new bridge construction, there were old bridge pilings, and debris left just a little to close to the surface, as some surprised corner cutting boaters discovered. This required to contractor to do some additional clean up. Remnants of old pilings, abutments, and concrete rubble are all over the place providing good fish habitat. This is also not a good place to drop an anchor unless you want to leave it there.

As we idled over the area you could clearly recognize old concrete pilings still standing, and flat chunks of bridge decking. Reinforcing steel is still sticking out of the concrete.

This screen shot is a continuation of the previous one. You can see part of an old piling, and several fish targets on the bottom. What's notable about these targets is that they are just a few inches off of the bottom, and you see them as a separate objects. Non-chirp fish finders would likely show these fish targets as part of the bottom. 

The amount of detail you can see in shallow waters is is incredible. Very sharp bottom detail, and resolution. It's almost like having a microscope.

I know minnows abound, because they show up as targets especially in shallow water, whether they are alone, or in a school. If you were in 75' of water this image could be of Amberjacks under the boat. In this case it's a small school of minnows. When you pass through bait fish like surface schooling blue runners, you actually see them as hundreds of individual fish. The image below is oyster bed bottom, and it looks like a bed of nails. I have learned how to distinguish the local bottom types.

Seabed bottoms have very clear signatures with the chirp system. I can now discern the different local bottoms such as land runoff silt, sand, sea grass, exposed limestone, and others. I think that with a little practice you could very effectively use chirp technology to survey bottom types and measure their areas for environmental studies. I'm going to play with the idea a bit by laying down tracks and importing the data into Garmin's Homeport software along with marker waypoints indicating bottom type.

All of the typical chirp fish finder features work just as well in shallow water as they do in deeper water. The image below is using the shift function to get a better view of the bottom. The point of all of this is to say this technology works great, whether you're fishing at an artificial reef in 20', or your daytime sword fishing in 2000', and it is a quantum jump over traditional non-chirp systems.

I'm leaving this little vignette with a little bit of chirp op art. We were caught near the bridge by a passing wake that undulated the bottom, and, and also the fish targets. Cool I thought, and suitable for framing.

Many thanks to Dr. Doug Greene for indulging me in getting these screen shots. We were testing out, and playing with the final system configuration, including the new Yamaha NMEA 2000 engine interfaces. A minor last point, in the screen shots you can see differences in the speed that is being reported. The upper speed number is coming from the Yamaha interface via a paddle wheel, along with the temperature. I would trust the GPS calculated speed first. %#&^!! HTML editor, sometimes I just can't get rid of linefeeds (<br />), I mean I can, but the editor just puts it back. You won this time Google, but I'll be back.


  1. Informative post! I never tried to screenshot the data from the transducer but now that I've read your blog, I think I'm gonna do it some other time, to know the quality of my fish finder.

  2. detailed post and easy to understand.