Bam..the mercury is in the silt/water/muck etc from various pollution sources, not from rotting fish..the cats have higher levels of mercury not due to eating the dead but like carp for instance they are extremely hardy, and will live through pollution that would kill lesser fish. So where other fish would reach a certain level of contaminates and die, catfish would just keep on going like nothing was wrong. This tolerance allows them to live in places and eat things that many other fish would not and sadly it also build the levels of poisons in em
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Channel cats are a blast and sadly I do not know any real hot spots here for them. I grew up in PA and hunted them all the time down there. Like you said they are big predators and one of my favorite spots was a deep hole that shallowed and narrowed to a stream that eventually opened up again. Shad used to collect at this shallow narrow section in big schools. The shad were 5-8 inches easy and fat. What I would do was toss a net and catch about a dozen of them when I arrived early in the morning. I'd take half of them and cut em up and toss em in the cooler, the other half I would keep alive in a large bucket. I'd take one rod, hook a live shad with a large 4/0 or so so hook, right behind the dorsal fin, then make a slit just before the tail on the belly. Not a big cut but enough to make the shad bleed and move frantically. I'd then free line that one and place a 2 oz weight with a chunk of shad on the bottom on a second rod.
I'll tell you watching a shad dance, then suddenly put a bend in the rod as a large cat would tear off was great fun...the payout could be huge, but like mentioned it could also be slow in between. Not like bass fishing. I might have only gotten 1-3 fish in a day..but those fish were typically 10-30 pounds...
Another great thing I used was chicken liver, fresh. If you have trouble casting it you can use cheese cloth to wrap around it before you hook it on. It is hard for a cat to pass up chicken liver though...looks great in the water and just oozes "come eat me"!
As mentioned circle hooks are great to use as cats want to pick up and move with their food while they eat. I've had a big cat pick up an 8 inch shad and slowly move away, and after a 30 second or so pause when I set the hook it simply came right out of it's mouth because he hadn't taken the hook in yet. It is hard not to nail that hook, but patience is key and giving them a few moments to move the hook into their mouths is crucial. Knowing this circle hooks present a great system for not gut hook the cat, especially for catch and release. The hardest part is learning to use pressure instead of a huge hook slam with circles.
Lemme know if you ever get around to doing any serious cat fishing. It is another of my passions that I haven't done in some time. I've mainly focused on carp here in MA, and while I know big cats exist in rivers like the M'mack and CT, I haven't really ever sat down and looked for em here. If I come across any ideas I'll let ya know.