FWS is responsible for the recovery of terrestrial, freshwater, and catadromous species. These are known as catadromous fish. The European eel ("Anguilla anguilla") is a species of eel, a snake-like, catadromous fish. This elusive fish is catadromous in nature; it migrates to the ocean to breed. A catadromous species, it is tolerant of high and low salinities. The fish are the only catadromous species in the Hudson's estuary. This catadromous breeding system also ensures randomised mating, creating a panmictic population. The American eel, a catadromous fish was also noted to traverse Philadelphia's waters. It is a catadromous species which is native to the Indo-Pacific and northern Australia. Others, exemplified by the eel, are catadromous, doing the reverse.