You could make the fish have weights/sizes and worth more or less depending on how big or small the fish you catch is.

Like one shark could be worth $20 and another, bigger shark could be $50.
(idk existing fish prices lol)

Yeah, neither do I.

Isn't that similar to N-Pulses fishing system when they were still around?
Although they didn't have the fish as "items" you could drop or see in your inventory, it was all calculated behind the scenes and you would just go "sell fish" to sell them for the weight.

http://i.imgur.com/nzRPxXZ.png

The server Atrius does it by weight.