Lowrance Downscan image

[Dan Carney]

Nelly and other Lowrance guys, I just had my new unit out in the Sound for the first time and am still getting used to it. When I first started seeing images like the one below, I thought they were bait balls, now I am wondering if they were schools of bigger fish (maybe even salmon)? Any thoughts? This was captured at Thatcher pass on Friday, and all I caught were dogs.
Thx.
Dan

DSI_1.jpg (134.8 KiB) Viewed 6321 times

[Nelly]

Definitely a tightly packed bait ball indicating that it was being harrassed by a predator species.

In other words, herring usually travel in schools that are readily apparent and easy to mark on your sounder. However, when "somebody" is feeding on a number of herring, they tend to display classic schooling behavior (well defined tight ball) which is how I interpret your screen shot.

Salmon, as all bony fishes (Class Osteichthyes) have a swim bladder containing air which marks very well as bright arches.

Sharks on the other hand are a cartilaginous fish (Class Chondrichthyes) and remain neutrally bouyant in the water column due to a large, oil filled three-lobed liver) The oil in the liver has a slightly lower density than water and therefore sharks do not always light up your screen the way salmon and herring do!

Hope that answers your question and good luck with your Lowrance!

[Dan Carney]

Nelly,
I initially thought those were bait balls too, but I never really saw any fish around them. Your comments about dogfish not showing up very well makes sense to me. What I wasn't sure of is; I have seen fish arches on 50 and 200 kHz units, but my new unit runs on 800/455 kHz like the structurescan units. So far I have mostly used it in shallow fresh water on 800 kHz downscan mode. There you get the picture like images and fish look like little spots in the structure. I haven't found any literature or reference videos that show fish arches on 800 or 455 kHz. So what I am wondering is if you get fish arches at 455 kHz on your unit.
Thanks and happy 4th. Happy Birthday America.
Dan
USN(Ret)

[Nelly]

Hey Dan, you really made me put my thinking cap on for that question but I think I can help.
You are correct in using 800 kHz in shallow (less than 80 feet) water. Once I get into 90 or 100 foot depths - which is most of the time for me- my Structure Scan is pinging at 455 kHz. Also, remember to crank the sensitivity until you get visual "noise", then turn it down a bit. Think of it like your "Squelch" on your VHF radio and you're on the right track.

Last month in Sitka we found chinook feeding on a candlefish flat ranging from 60 to 100 feet deep. At 455 kHz, I was indeed seeing faint arches in the down scan image and "streaks" in the side view indicating salmon under our boat. We were actually able to mark individual fish, turn the boat to the area indicated by the side scan and catch that particular fish!

I did play around with 800kHz but I'm getting the impression that salmon show up better in the 455kHz frequency. It could be that salmonids just resonate better at that frequency.

Hope that helps and good luck, T

[Dan Carney]

I was out today at tide point, I did see some faint arches at 455kHz a couple of times.
Had my dad hooked up twice, but we didn't get them in the boat.
Thanks for the help.
L8R
Dan