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.