Mackerel brining time before smoking
I smoke mackerel and eel just with fine salt and ground black pepper in the open belly (kept open with a couple of toothpicks). Vertical smoking about 6h at 35-40ºC smoke.

If you marinate them, I don't know if the meat wouldn't fall apart. I marinate (just the fillets and just marinated) covered with wine vinegar and lemon juice (80% to 20%), garlic, a little salt or no salt and black pepper. After 4-6h it's ready to eat over toast, as a snack with cold dry white wine. Best next day. Holds for a week in fridge (covered with marinade). Sardines are the best like this.