There are four different examples of food chains in the movie "Finding Nemo".
1) Whales Eat Krill
2) Sharks eat smaller fish
3) Gulls (birds) eat anything they can
4) Humans (fishermen) eat fish (implied through the harvesting or attempted harvesting of fish scene)
5) Crabs scavenging for food from/along the pipes on the ocean floor
6) Baracuda (scary fish) eats fish eggs at beginning of movie and Nemo's mom.
7) The french shrimp cleans the tank by eating the algae and other build up.