I copied this from Clayton Diskerud on the GLA site. I'm sure he won't mind posting it here. It's excellent & an easy way to prepare lake trout.
"I've done fish boils for groups from 4 to 40 and must say that I can't give you specific quantities. It seems to come out fine each time, so I doubt that it is important to be precise.
Last month when we had four couples over for fish boil I used two lake trout. I steaked them in sizes varying from about an inch (toward head of fish) to 2+ inches (toward the tail). The red potatoes I counted out at 1.5 per person, so there were 15 in the pot. For medium sized onions it's your call. I used about 10, but did cut some in half.
I use regular table salt. I do think it is important to use a full cup in the first boil (with potatoes) and another cup when the fish and onion are added for the 2nd boil. I don't know the capacity of my fish-boil kettle, but it must be close to two gallons. I use enough water to cover the fish in the insert basket.
Also, if you don't want to pour off the oil and residue that floats to the top, simply use a large serving spoon and skim it off. You certainly don't need to use the flashy burn-off show the restaurateurs employ.
Clayton Diskerud"