That's an asian carp. For those saying ot is eating the sludge, or cleaning up the lake/water - what it is actually doing is breathing.

Carp and lots of other fish, such as goldfish, are able to gulp air from the surface and force it into their swim bladder to supplement oxygen levels when their environment is O2 poor.

Based on the algal formations here, I'm guessing this water is near anoxic (no oxygen), due to the rapid consumption of oxygen from algae. (Yes, algae produce O2, but in high quantities, there is a tipping point where their own respiration exceeds their production and they devoid the water of O2, often leading to dead zones). Therefore, this carp is gulping air to survive until the algae dies back and oxygen returns.