It all depends. Did you use a digital camera? If so, the highest quality pic is on the flash memory card. If you use a program to transfer pics from the memory card, it sometimes resizes them and does other things you don't want. So hopefully you have the raw pic from your cam. I would try this:
1) Copy the pic directly to your computer.
2) Crop it down a little. You should try and cut out most of the surrounding area because the fish are such a small part of the pic.
3) You'll have to experiemnt with the zooms and resizing. So make several copies of your picture files, and pic the best.
4) The edit your pic, you can use paint, photoshop, and mircrosoft has a picture editor that comes with office sometimes. So, pick one of those and try your luck.
Its really hard to give exact steps to fix your pic. Obviosuly it needs to be cropped some, but the rest is experiemnting. The pic from a digital camera is usually much large than the one you have posted. Work with the original and you should have something better. Post any further questions. Maybe a few of the edited pics if you need help.