What type of fish was it? I've seen this a few times on the bbq but forgot what type of fish it was, either chum, or pinks? just smoke longer or cook longer, all the fat comes to the surface, ya looks nasty but cook a little longer and slower and flip the filet if possable to let the fat drip off and dry out a bit you'll be amazed at the taste, unless this is some kind of morfadite fish , then crab bait done it a few times on the bbq fliped it let the grease hit the coals and flame up a bit to dry it out turne out excellant, never done it in a smoker befor so would be trial and error