Well lets see. Most fish have a strong fish smell anyways. Most fish spawn this time of year, be it smelt,crapie.shad,walleye,perch,carp,suckers etc.including frogs and turtles. I could almost bet it's because of the spawing cycle and that fact that there is no clorine in the water to give it a fresher smell. The water tempiture is raising every day thus making the fish increase the natural slim over their bodies to help fight infections as well,and if you have every grabbed a hold of a fish in the winter you'll notice the slim is not as prevailent as it is in the heat of summer. So my bestest guestest is, the natural excretion of the fishes slim for defenses to the raising water tempeture and water born diseases. Shad have a very strong smell to their slim too, really oily stinch.

I second what Beekeeper said. I've always smelled that same odor when fishing a bream bed, even down in the river swamp lakes where there's not much stuff that has a sweet smell.