Fishes might die when they consume the plastic pellets (or nurdles).
Literally, fishes do not eat the pellets, but they swallow them, which they become full or have their digestive systems obstructed and that cause their death.
1.When the fish eats plastic pellets, it dies.
2.When the fish eats plastic pellets, it becomes dead.
NOTE: 'dead' is an adjective, so you can't say 'it dead'. You should say 'it is dead' or 'it becomes dead'