I have no direct experience with the use of fish finders in rivers, but have fished many of the various "meat holes" on several rivers. My impression is that both the commotion of fishing pressure (boat motors, oars, all those lines) and fighting fish in the water are the primary determinants of how active or put off the fish are going to be.

In most terminal areas, the bite tends to be in flurries with dead time in between, rather than a steady bite all day. I think this is due to the fish being spooked into not biting or moving to a different area. The fish, however, have short memories and after a quiet period will get active again or move back into the hole because of its good holding water qualities (after all, we anglers don't fish bad water!?!).

I also think this short memory contributes to an acclimation affect, in which the fish just gets used to all the commotion. Thats why the fish come back rather than vacate permanently.

I've also noticed that a struggling fish on a line can sometimes antagonize other fish into hitting. How many times have you observed two or three fish hooked one right after the other. But this aggressive behavior eventually switches to fear and the remaining fish either temporarily vacate or or just quit hitting for a time.

Related to this is the 'moving fish is always most aggressive' theory. When fish arrive in a new area, its most aggressive and will pick up. This goes for fish migrating in from downstream or fish that have been spooked from one part of the hole to another.

Add to this such intangibles as barometric pressure, water clarity, sunshine, etc., affecting the bite and you're bound to get this on and off activity at any high pressure spot.

Specifically regarding sonar detection, I think like any other disturbance, if its not severe enough to make the fish beat fins to the next hole, they'll get acclimated to it and eventually engage in suicidal behavior again.

Or I could be 150% wrong.

[This message has been edited by obsessed (edited 05-17-2000).]