Nice job! I really love the concept - I had a similar idea in which you started as a tiny fish and ate bigger fish, working your way up the food chain. The art you chose worked well for the game, but it would be nice if it was original. I understand that it's hard to do that within the 48-hour jam time, though. The music/sound effects fit well and contributed to the game atmosphere.
Part of what makes a game engaging and keeps players hooked is difficulty. It's crucial to balance making the game playable and not so hard that the player just quits immediately (which I often have a hard time with), but also hard enough that the player wants to keep playing, and feels like there is a challenge. In this game, I felt like it was far too easy. After the third fish, I couldn't find any fish that were bigger than me, let alone even the same size. Make sure to challenge your players.
Besides this, this was a really fun game! I enjoyed playing :D