So I pasted mackerel pie into my search box and read a recipe. It's basically the same as Shepherd's Pie, with canned mackerel instead of meat.
I’ve seen the pie with whole mackerel (about 6) with the heads sticking out of the pie crust like spokes on a wheel. Mackerel head soup is also hearty seacoast fare.
In France (off the subject) they have worked up dishes from fish most of us use as bait. Baked mullet... etc...