This is going back many, many moons but as a boy my father used to take me salmon 'tickling' at various locations around scotland, and in the evening at camp clean the fish, press the fillets together flesh to flesh, wrap it in sheets of newpsaper one sheet at a time to make one thick 'parcel'... wet it in the river and then burry the parcel in the coals of a died down fire for half an hour or so. if it was a big salmon, maybe an hour.
an alternative if the newspaper had been used for other things would be to get a roaring fire going, surrounded by flat rocks and as it died down, small fillets or 'darnes' where placed skin side down on the hot rocks.
when the flesh flakes with the tip of a kinfe its done, and tasted superb
a drop of spit on a rock somewhere will show you if its hot enough to cook on.
maybe a bit old skool, but i can still taste it now thinking back. watch out for the bones in pike, not tried it myself but been warned.
TBL.