Well, you have not been looking very carefully in the field or reading the journals. Direct consumption of dead salmon and their by products is well documented and important.

Do a search. Direct consumption is perhaps the most important pathway for marine nutrients to get to fish. Either by consumption of flesh or eggs. Then fry as they become available. Followed by indirect pathways through invertebrates eating flesh. Followed by sorption onto substrate of organic material as it decays and dissolves. Choose your pathway. It all gets back to the fish.