I've been catching a decent number of Wolf Eel this week, in Foundry Cove (Willow Creek) - the best spot in that neighbourhood is the sparkly fishing place near the magic tree, but I caught some at the other two fishing places too. As far as I can tell they'll only bite for frogs, and since it says more valuable bait is better I had my household breed up a big stack of whirlyflower frogs. Once you've got two of the same frog to breed more, whirlyflowers are no more effort to breed than leaf frogs, and if you end up with more than you need, the leftover frogs sell for a lot more!
Using a club vibe of focused does seem to help too. Focused sims are supposed to do better at fishing, and the club vibe doesn't wear off after a few hours like anything else you can use to make them focused.