just looks like a shark with a tractor beam...

And you would suggest fixing it how, exactly?
Criticism is fine, but just making a snarky comment with nothing positive to add, nor any suggestion on how to fix it really doesn't contribute much, James.

Try duplicating the fish a number of times and motion blur each one a little then change the transperancy. You could also change it's size and/or shape a tiny bit ... does that help?

Great idea ... fish acutally looks smug about the whole thing!

For realism I would definatly think about the fact that this is all happening in water, the water would have a cone of distortion from begining to end of the fish`s movement also I dont think the line would be so straight, fish use their tails in a side to side motion and although at speed seem to move straight its not really the case.

try taking a picture of a moving fish... (or find a picture that hasn't been taken with a quick shutter speed (as the goal is to make it realistic - not what 'we' think it would look like); Analyse the shot, and then transfer your findings into your image (not literally of course)

Perhaps a gradient transparency mask, so it looks like the blur fades more towards the shark. Good luck!