Almost anything can be chopped up for chum but best are oily fish —mackerel, sardines, anchovies, etc. When I am using fish for bait I almost always cut up the heads into small pieces (along with the guts) and toss them out for chum.
If you're chumming for small smelt to use for live bait pieces of bread work well. Some even suggest using pizza dough.
I also just received a tip from someone I consider an expert: "My secret weapon for chumming smelt (both toppies and jackies) is attached. Has to be patè so it dissolves. A couple scoops (say each can has 6 scoops) and I can throw the cast net with confidence. Always get bait."
I agree with the canned cat food, i usually stuck a can or two into a mesh bait bag and let it soak in the water, each can lasts fairly long, usually around an hour. Sure beats constant chopping of fish/squid chum.
The cheaper the canned cat food, the better for the most part.