Hi Bill,
The test results that I mentioned was on growing out juvenile carnivores. There wasn't substantial growth difference between a 35% protein food and a 55% protein food. Note these were growing fish which are using as much protein as they ever will to build muscle and tissue. The growth difference occurred in varying levels of starch and fat. Marine fat like crude menhaden oil works best in my opinion. This was the case for a carnivore and omnivore used in testing. Honestly there is no difference in digestion between a LMB, Bluegill or carnivorous catfish.
I can post a link if you would like. I will warn you that its quite lengthy.
Whole Menhaden, Herring or Anchovy meal are great (92% - 93% digestibility). I have found that Menhaden is very highly palatable for carnivores. Parts meals even of marine sources are generally very poor quality. Fish really aren't designed to eat warm blooded protein. Extruding the food at high temperatures effects the protein very little. However it does cook away some fat and destroys naturally occurring vitamins.
I have studied fish nutrition going on seven years now on my own. Read boring studies, done my own testing and messed around with different formulas just to see how it worked out.
