The obvious bit is the chunk of flesh missing....but when you look longer, there is a bit of tail gone as well, so he was probably attacked from the rear.
Most predators will try and swallow whole, front first.....so my guess is a large tailor
Gives a bit of a lie to those who were saying in another post that fish feel pain....he was obviously still thinking about food...not sure I'd be hungry with a chunk missing out of me that proportion of my body.