TOPIC:

Is it illegal to put fish scrap back into the lake?

You should not discard fish guts directly back into Lake Michigan in Indiana. According to  Indiana's Lake Michigan Clean Marina Guidebook , fish waste (offal) should be disposed of in a sanitary manner—such as in designated cleaning station grinders or in trash receptacles—to avoid pollution and public health issues, rather than being returned directly to the water. 

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Lickety's answer is correct if you are in Indiana and on Lake Michigan. Elsewhere in Indiana, there appears to be no restriction on disposing of fish offal. There is no restriction about cleaning fish in Indiana while on a boat, just don't throw the remains in the lake - put it in a bucket or garbage bag and dispose of on land. 

In Illinois, (on Lake Michigan) there's a restriction about cleaning fish on a boat if there's a size limit on the fish. That pretty-well leaves out tossing the remains overboard - except for perch. 

In Michigan, fish can be cleaned on board a boat, but the "offal" can't be tossed overboard. 

When talking to a Wisconsin officer, he said their DNR views disposing of fish guts in the lake is no different than leaving a gut pile in the woods when deer hunting. (As long as the boat is more than 1/2 mile offshore.) However, elsewhere in their regs a fish with a size limit (say 15" for cohos) can't be cut to make any part of the fish you are taking home less than 15 inches. So filleting a 22 inch coho is probably legal, cutting a 16-incher is illegal. 

New York, Ohio, PA and Ontario all have their own versions of fish cleaning restrictions.