Fish intolerance?

I'm trying to narrow down the source of Simba's IBD. So far the common denominator in each of the food he's thrown up is fish in the forms of "fish" (friskies canned), "fish meal" (nature's recipe dry), and "Menhaden fish meal" (nature's variety instinct dry). However there is Salmon and Whitefish in the canned foods he's eating now (4health regular and grain free), but he hasn't thrown this food up so far. I'm having trouble understanding this.

The "fish" and "fish meal" could be any random mix of fish the manufactures can get to process. "Menhaden" is a specific fish (AKA mossbunker or bunker) not commonly used for human consumption but as a lower cost additive in animal food and dietary supplements as a omega-3 source (and raw material in things like lipstick). Technically it is a filter fish and low on the food chain. The salmon and whitefish are higher on the food chain than Menhaden so my guess would be that it could be something in the processing for those fish meals and menhaden fish meal. The salmon and whitefish in his canned food probably get processed in a different manner and are of higher quality to begin with. I have one cat who has problem with just shellfish in his food. I personally stay away from food with generic "fish" because that could be anything. I usually stick with named fish that I recognize and avoid any shellfish.

Makes sense to me probably because my Dad and I both have food allergies that the processing of the food makes a difference. I can eat raw tomato (well nightshade family foods in general) but can't have anything that was canned or jarred at any point without problems. My Dad can eat organic farm poultry with no antibiotics or additive but nothing from a commercial farm. Makes food in our family seem really health conscious since it has to be fresh and as natural as possible, lol.