The stream animals of Hanalei are rich in uniqueness but low in diversity. The large stream animals, such as fish, crustacea and molluscs, evolved from marine ancestors and part of the lifecycle requires a return to the ocean. They live their adult lives and lay their eggs in the river, but upon hatching, the larvae drift out to sea where they live as plankton for several months. Although the lifecycle is similar to salmon, Hawaiian stream organisms do not return to the stream in which they were born.

The native stream fish in Hanalei are collectively called o`opu. Three species of gobies are endemic (unique) to the Hawaiian islands. The o'opu alamo'o (Lentipes concolor) and o'opu nopili (Sicyopterus stimpsoni) live in the fast flowing waters of the river. They have sucker-like fins that allow them to climb waterfalls and thus can be found at the very headwaters of the Hanalei river. Another endemic goby, the o'opu naniha (Stenogobius hawaiiensis) lives in the lower slower sections of the river. The largest goby in the Hanalei river is the indigenous (native but not unique) o'opu nakea (Awaous guamensis). This species provides an important fisheries to the Hanalei community, when after floods large numbers of fish come down to the lower sections to breed. Also in the lower section of the river is the endemic, predator "sleeper" or o'opu akupa (Eleotris sandwicensis).

Other native stream animals such as insects and snails do not return to the ocean but spend their entire lives in the river. Many of these, such as damselflies are even more unique than the fish because they use special habitats such as waterfalls and are found only on the island of Kauai. One such species is the tiny endangered Newcomb’s snail (Erinna newcombi), which lives in the high springs that feed the Hanalei river and a few other streams on the north shore of Kauai.

At the opposite end, where the river meets the ocean it forms an estuary. This estuary is important for many native fish such as mullet or ama'ama (Mugil cephalus) and flagtails or aholehole (Kuhlia sandwicensis) . These fish breed in the ocean but schools of juveniles move into the Hanalei estuary to feed and develop away from large predators.

For more information on Hawaii's freshwater fish see Freshwater Fishing in Hawaii, a brochure by the Division of Aquatic Resources, June 1993.

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