Mackerel lure and rod
Leave the Fishing Rod's lure slot empty so it fishes with French Fry; Mackerel and Gar each take 40% of bites and Pike 20%.
Any island works for this free pool, but the Forest is where most players first see it.
Drip Mackerel
Drip Mackerel exists with the usual 10% drip chance per hooked fish, up to 25% with the Wishbone.
Plan on about 25 bites per drip Mackerel (10 with the Wishbone), with no bait cost at all.
Mackerel sell price
Mackerel sells for $6, a dollar more than Gar from the same free pool.