Raw mackerel is a fish that can be caught with a big net at any big net fishing spot. It requires a fishing level of 16 to catch and can be cooked into mackerel with level 10 cooking. The most common spot to fish mackerel is Catherby. There's a possibility that 10 raw Mackerel will be required for a part in the quest In Aid of the Myreque.
There are only seven fishing locations at which mackerel can be caught:
Catherby beach: A popular spot that's close to a bank.
Fishing Guild: The closest spot to a bank, with a fishing shop in the area as well as a spawn of a harpoon and a big net. It is usually recommended because it is close to the bank and there are a lot of fishing spots.
Burgh de Rott: Requires In Aid of the Myreque. There is a bank in Burgh de Rott not too far from these spots. This area is notable for its especially high density of Net/Harpoon spots—there are a lot of them very close together, so not a lot of running around is needed to get from one spot to another.
Isafdar: Requires Regicide. As with the Net/Bait spots here, this area is too far from a bank to be practical.
Jatizso: Requires The Fremennik Isles. Just like the Cage/Harpoon spots, the Net/Harpoon spots here are very close to a bank and excellent for players who don't like a lot of company when they fish.
Feldip Hills: There are aggressive enemies nearby, but fishing here is not as inconvenient as it used to be.
Ape Atoll: Requires Monkey Madness I. There is a fishing spot located near the agility course and another on the south-west side of the island. There's no bank on the whole island, and players can only fish when they are in human form, which means the monsters on the island will become aggressive. Because of this, it's not an effective place to fish.