Today, more than 50 species of alien fishes, invertebrates, reptiles, amphibians and plants are established in our streams and reservoirs. Many of these species were intentionally released with the hope that they would become established, and in some way improve the quality of life here in Hawai‘i. Others were simply dumped in our streams with no thought given to possible consequences.
Some of the earliest aquatic introductions occurred during the 1800’s and accompanied the first Asian immigrants to these islands. The Chinese catfish, rice paddy eel, soft-shell turtle, carp, goldfish and the Japanese weather fish (dojo) were among the first exotic freshwater animals brought to Hawai‘i. Most of these species were brought in for food, but a few, like the goldfish, were also brought in for ornamental purposes.
During the early 1900’s, and through the 1960’s, several species of topminnows (locally known as medaka or tabai) were released in our streams and reservoirs for mosquito control. Various species of Tilapia were brought in to help the sugar plantations control weeds in their irrigation systems and to provide baitfish for the aku (skipjack tuna) fishery. Gamefish, such as the largemouth and smallmouth bass, trout and tucunare were brought in to provide sport and recreation.
The rise of the environmental movement during the 1970’s brought about a change in the way people viewed our native stream animals. Previously unappreciated, Hawai‘i’s fauna and flora were now recognized as being unique and precious in their own right. The focus shifted from ‘improving’ these resources to preserving what was left. Government sponsored introductions of alien species ceased, and requests to import alien species by institutions and private individuals came under increased scrutiny. By this time, however, approximately 70 different species of aquatic animals had been intentionally released in our streams and nearshore waters; more than half of which had become established.
During the 1980’s and 1990’s more alien species began appearing in our streams and reservoirs. Species such as the Convict cichlid, Midas cichlid, Johanni cichlid, Jewel cichlid, Suckermouth catfish, Armored catfish, Stickfish, Apple snail, and Grass shrimp can all trace their origins to the aquarium fish and aquaculture industries. The Asiatic clam, which is widely distributed in streams, reservoirs and taro patches on Kaua‘i, Maui and O‘ahu, is thought to have been smuggled in by Asian immigrants for food purposes. A ship’s ballast tank is believed to be the way an alien goby species entered Hawai‘i.
Some of the impacts these alien species are having on our native stream animals and habitats are readily apparent. Suckermouth catfish and crayfish, for example, dig holes in reservoir and stream banks, causing erosion and increasing the amount of silt in our stream water. Smallmouth bass are voracious predators that feed on native ‘o‘opu (gobies) and ‘ōpae (shrimp). Even seemingly harmless species, like the guppy and swordtail are known to carry parasites that can infect ‘o‘opu.