I recently made a Dunkelweizen with the Platinum strain Weizen Yeast from White labs. When served from the keg and a bit cold the aroma was fine but when decanted to a growler and allowed to warm up a bit the aroma was of freshly cleaned trout. Not quite pleasant. I have seen this once before at a competition and was wondering if anyone has a clue to what may cause the issue. The beer was fermented in a corny keg at 70 deg with a 4 week primary and direct transfer to another keg. I used Atmos 300 as a foam control agent (similar to Fermcap S). The beer as force carbonated at 36 degrees at 12 PSI for 2 weeks. Thanks all.
The only thing I can think of is the water. Sometimes in the spring here in Mpls we get very fishy smelling water for a week or so. I believe it has to do with switching over the aquifers.
Maybe serve the beer with some tartar sauce? 
Trout? Yuck. Walleye? Mmmm.
Maybe it was something in the growler. Have you tried pouring from the keg into a clean glass and letting it warm up, then giving it a sniff? If it’s still fishy drink it extra cold. 
Under pressure or just with an airlock/equivalent?
If the water source was surface water (river, stream, reservoir), then it could be the water flavoring the beer. Compounds such as geosmin and methylisoborneal give the water a fishy or muddy taste. Proper filtering (read slooow) through an activated carbon filter will remove their taste and aroma from the water.
I think it is the yeast and the fermenting temperature.
Last night, our club had a wheat beer tasting. 11 wheat beers in all. One of the beers had a distinct “fish guts” aroma that several of us commented on (I actually couldn’t place it, but someone said “fish guts” and sure enough, that’s what it smelled like). Turned out the beer was brewed with WLP300 and was fermented at 70°F.
The beer did not have any aroma of that in the taste. It seemed a little one dimesional, grassy and a little clove.
I like fermenting wheat beers at around 62°F.