Without seeing what the fillet looked like before smoking, I'll take a guess that there were a number of small white nodules scattered throughout the fish. A friend had a coho caught at LaPush last year that had them. We contacted Dr. Martin Chen, WDFW. He provides the following information in a pamphlet he is working on concerning two parasites that might be the problem:

Henneguya spp.

This myxozoan parasite forms small white or yellow cysts about the size of a grain of rice in the flesh of marine fishes like salmon, rockfish, and flatfish. These usually do not affect the flavor, texture, or safety of the meat, but one species H. salmincola may produce visible cysts filled with milky fluid in coho and sockeye salmon, reducing the attractiveness of the flesh. However, this is not a common problem.

Kudoa spp.

Myxozoan parasites of the genus Kudoa form white cysts in the muscle of salmon and many other local marine fishes. The frequency of this problem is very low in wild-caught salmon. The cysts may be large enough to form noticeable white patches or streaks in the meat. After the death of the fish, the parasites release an enzyme that breaks down muscle tissue and causes the flesh to become soft and less desireable as food. The enzyme is fairly heat-stable and may continue to breakdown tissue even if the meat is smoked.