Irrawaddy Dolphin

"I prevent fish from overpopulating"

If you see a lot of dolphins in an area, then it generally means that the local ecosystem is healthy enough to support them. They are apex predators that make sure that the populations of their prey remain healthy and do not grow too big, which can disrupt the food chain. They help maintain their ecosystem by having a diet of fish, mollusks, and aquatic crustaceans such as crab and shrimp. Irrawaddy dolphins provide income for coastal communities through ecotourism. They are well recognized for their “smiling” faces and are known for their ability to spit water which is thought to be used as a way to herd fish.

Learn more on Wikipedia