Dave:

herrings: not very oily
smoked herrings: bit oilier
mackerel: quite oily
smoked mackerel: very oily

For me, at some point in the range of fat/oil content, my delicate gall baldder and normally hearty intestines scream 'Noooooooooo!'. I love smoked mackerel, but have to limit consumption or my innards will protest.

But that goes for all concentrated high fat foods, in my case. And my mother's, but she ended up having the troublesome organ (gall bladder) removed. I'm trying to keep mine.