Assuming you bleed them out before cleaning I'd not worry about the lateral line. It sounds like you're already doing it right if there is such a thing. Anything done in cooking preparation will only serve to mask the unwanted effects of decomposing fish flesh.

If fish caught and cleaned in the morning can be kept a day in the canoe with no ill effects, I'd suspect the flaw is coming during frozen storage. As I recall frozen always had more of a fishy flavor than fresh anyway.

FWIW - My current freezer is a horrible self defrosting model. Ice cream may be rock hard or way too soft depending on timing. Needless to say we don't keep things in our freezer very long. That and length of storage may be something to consider. Is there ever frost inside the bag?