A much more powerful version of the Water Release: Five Feeding Sharks, the user summons a thousand sharks that enter into a wall formation and rain down to attack the opponent.
In the manga, these sharks are formed from the water. In the anime, they appear to be summoned sharks. (correction: the manga like the anime, showed real sharks, which is why Gai couldn't tell which shark had the scroll)
In the Shōnen Jump publication of chapter 506, the name of this technique was pronounced Senjikizame. It was corrected to Senshokukō in the tankōbon release.