The only tropical and archipelagic state of the US, Hawaii is a set of volcanic islands that are nestled in the Polynesian region of the Pacific Ocean. The main islands of Hawaii, which include Oahu, Maui, the Big Island, Kauai, Lanai, and Molokai, each have their own unique communities of flora and fauna. Unlike the states of the mainland US, these islands are geographically young.
Due to its location and the nature of how its land masses have expanded and evolved, Hawaii has but a few endemic freshwater species. Obviously, the islands are not connected by streams and rivulets with self-sustaining populations of freshwater fish. Instead, they boast considerably long coastlines that are regularly visited by marine fish and mammals that seasonally migrate across the Pacific.
Regardless, Hawaii’s main islands have their own hotspots for freshwater fishing, largely due to the introduction of species from the mainland US. These have been stocked into natural lakes and streams that continuously receive clean rainwater or groundwater sourced from aquifers. With resources that are wisely managed and protected by natives and state agencies, fish that are found only in the Hawaiian Islands can continue to evolve and populate their waters alongside legally and unintentionally introduced fauna.
Hawaii’s Endemic and Native Freshwater Fish Species
1) O’opu alamo’o (Lentipes concolor)
Also known as the Hawaiian freshwater goby, the o’opu alamo’o is a remarkably strong fish for its minute size. Though it grows to a maximum length of just 7 inches (18 cm), it can swim upstream at speeds of up to 90 meters (98 yards) per hour! Inhabiting streams of all of the state’s main islands, it thrives best in cool and fast-moving waters that are devoid of obstructions and anthropogenic alterations.
The o’opu alamo’o is highly valued by Hawaii’s native communities. Prior to the unification of the state’s islands, separate settlements attempted to cultivate this species in polyculture systems associated with taro paddies. The gobies helped prune overgrown taro leaves and consumed any pests that were in the water.
Capable of ascending waterfalls by latching onto bottom substrates, this small fish is a diadromous omnivore. Adults spawn in freshwater bodies that are located away from main river channels. Their eggs are carried downstream and into coastal areas, where they hatch into larvae and form part of the oceanic plankton community. Tides carry them back into freshwater systems.
2) O’opu nopili (Sicyopterus stimpsoni)
The o’opu nopili is also referred to as the Nopili rockclimbing goby or Stimpson’s goby. It is highly sensitive to ecological disturbances as it lives only in the torrential streams of Hawaii’s mountain regions. Its adults are restricted to the upper reaches of cool streams, where they feed on diatoms and filamentous algae above gravelly substrates. Territorial, they defend their own feeding patches and cultivate their own gardens.
Known for being an attractive fish, the spawning males of this rare species display bright hues of blues and reds above a base of white and black speckles. They can measure up to 7.8 inches (19.8 cm) long in freshwater pools with optimal conditions. Their hatchlings, which are washed into coastal areas, must travel upstream to return to their spawning pools.
3) O’opu naniha (Stenogobius hawaiiensis)
Unlike the rest of Hawaii’s endemic gobies, the Naniha goby or o’opu naniha does not inhabit the upper portions of stream systems because it is a poor climber and swimmer. Instead, it is tolerant of a wide range of water conditions and can be found in inland, coastal, and marine waters. Specimens that are found inland, however, are usually adults that are ready to spawn. Their larvae naturally drift out to sea, where they spend several months feeding on invertebrates and algae.
A handful of Hawaii’s native marine fishes, as well as its native night heron (Nycticorax nycticorax), feed on Naniha goby fry, larvae, and adults. The maximum size of o’opu naniha, which ranges from just 4 to 5 inches (10 – 13 cm), makes them a perfect snack for larger predators. The remaining populations of this fish are increasingly threatened by the introduction of non-native species that, apart from feeding on their eggs, harbor exotic pests and diseases.
4) O’opu akupa (Eleotris sandwicensis)
The Sandwich Island sleeper or o’opu akupa lacks the pelvic fin structure that allows other native gobies to travel upstream and climb waterfalls. As a result, it makes its nests in crevices along the bottom reaches of streams. Its eggs are naturally washed out to sea, where they hatch after just one day of development. Before being recruited into freshwater streams, their larvae become part of the oceanic community of zooplankton.
Today, this species is restricted to the main Hawaiian islands. Its largest populations are found in the streams of Oahu, in regions that are downstream of any man-made obstructions or natural alterations. It usually hides in leaf litter or seeks shelter underneath large rocks. Opportunistic and able to grow to a full length of 13 inches (33 cm), it has shown a higher tolerance for suboptimal conditions (when compared to other endemic gobies).
5) O’opu nakea (Awaous stamineus)
Fairly abundant on the island of Kaua’i, o’opu nakea is Hawaii’s largest and most common endemic goby. In optimal conditions, it can grow to a full length of about 14 inches (36 cm). It is able to feed on suspended organic particles, benthic algae, aquatic insects, worms, crustaceans, and larvae. Although it maintains an omnivorous diet, it is not known for feeding on smaller fishes.
From August to November, o’opu nakea adults form spawning aggregations in riffle zones that are located close to estuaries. This is why this species is primarily found along the lower regions of streams. Though it prefers to spawn in riffles, it spends the majority of its adult life in slow-moving and relatively deep waters above fine substrates. As this olive-colored and speckled fish burrows underneath rocks, its presence is often betrayed by its eyes peeping through gravel.
6) Hawaiian flagtail (Kuhlia xenura)
This ray-finned flagtail naturally occurs in waters that are found close to or along the shoreline. Able to tolerate both freshwater and low salinity conditions, it favors conditions in estuaries, tidal pools, and shallow reefs with either rocky or sandy substrates. Its diet shifts depending on whether it occupies freshwater or marine systems. When inland, it subsists on insects, planktonic crustaceans, and algae.
Generally nocturnal, the Hawaiian flagtail is distinguished by its large eye, a slightly concave bend along the top of its head, its silvery color, and its deeply forked tailfin. The rear margin of its tailfin, which frames a paler submarginal band, is usually black. Its mature specimens, which tend to gather in schools, rarely measure more than 8.7 inches (22.1 cm).
7) Flathead mullet (Mugil cephalus)
Found in subtropical to tropical water systems all around the world, the flathead mullet is a common and commercially important coastal fish. It can be found in various habitats, from freshwater rivers and streams to more saline estuaries and coastal zones. As it is a euryhaline fish, it can quickly adapt to changes in salinity. When found in freshwater bodies, it chiefly subsists on a diet of algae. In marine environments, it is more likely to feed on zooplankton and detritus.
Due to its value as a source of high-quality proteins, the flathead mullet is farmed and fished in many countries. In Hawaii, where it is locally referred to by natives as ‘Ama’ama’, it is readily found in the pools and riffle zones of the lower reaches of streams. Though its smaller specimens have many predators, its mature adults usually become too big for native birds and fish to prey on.
Introduced Freshwater Fish Species in Hawaii
1) Oscar (Astronotus ocellatus)
Widely popularized by the ornamental freshwater fish trade, the remarkably colorful oscar is now found in many parts of Europe and the US. In its native range, where it is normally found in slow-moving waters with high concentrations of suspended sediments, it is frequently sold as a food fish. Due to its importation as an exotic fish into Hawaii, primarily as a species for recreational fishing, it has now formed established populations.
In Hawaii, the record size for oscar is 2 pounds and 6 ounces (1 kg). In its native environments, this fish can easily measure more than 3 pounds (1.4 kg). Easily identified because of its red marking and rings above a darker body, it releases its eggs in circular nests found in shallow and calm waters. The best time to spot this species is during its spawning period, which lasts from March to September.
2) Goldfish (Carassius auratus auratus)
The domesticated goldfish is another freshwater fish species that has become established far outside of its native range. Due to escapees, many of its self-sustaining populations compete with endemic communities of fauna for food and space. Remarkably hardy, it is known for negatively impacting freshwater food webs and for growing to enormous sizes on an omnivorous diet of wild food.
In Hawaii, goldfish escapees have managed to form populations in some freshwater streams. They are now readily found in reservoirs of all the main islands. Introduced into the state before the 1900s, this non-native fish may threaten the survival of endemic species if its populations continue to expand and become more widely distributed.
3) Smallmouth bass (Micropterus dolomieu)
Deliberately introduced into Hawaii’s freshwater systems due to its popularity with sport fishers, the smallmouth bass is one of the most prized game fishes. Owing to its muscular features and its streamlined shape, it is a fantastic swimmer and an apex predator. As sport fishing for this species is not particularly common in the Hawaiian Islands, local anglers are often able to catch large specimens. In Wailua, for example, 3-pound (1.4-kg) smallmouths are practically guaranteed for the seasoned fisher.
The smallmouth bass was reintroduced into the state several times because the initial introductions failed to lead to established populations. Currently, this species is present in the clear waters of Oahu, Big Island, and Kauai. It is generally restricted to gravelly and rocky streams and lakes.
4) Channel catfish (Ictalurus punctatus)
The most common catfish species throughout its native range, the channel catfish is an extremely important food and game fish. Hardy and able to grow to hefty sizes, it has a knack for thriving in both pristine and slightly polluted water systems. Due to local and international demand for its flavorful meat, it is now commercially fished and grown in aquaculture setups. As a result, it has also been introduced into distant islands.
Channel catfish have deliberately been stocked into the Nu-uhanu and Wahiawa reservoirs, both of which are located on the island of Oahu. In these locations, they can legally be fished by local anglers that are willing to abide by fishing rules. Outside of these reservoirs, channel cats may also be found in streams, lakes, and pools, though their population sizes largely vary.
5) Rainbow trout (Oncorhynchus mykiss)
Highly prized for its attractive colors, feistiness as a sport fish, and protein-rich meat, the rainbow trout is considered a valuable species wherever it is found. As it is targeted by many professional anglers in North America, it has been introduced in many states. It is now found in practically all major freshwater systems with cool, clean, and fast-flowing currents in the US. Thus, it comes as no surprise that this fish has also been stocked into some of Hawaii’s waters.
Brought into Hawaii as a sport and recreational fish, the rainbow trout can be found in some high-altitude streams, lakes, and rivulets in Maui, the Big Island, Molokai, Oahu, and Kauai. Established populations can be accessed by registered fishers in several reservoirs and in some natural reserves of Kauai. The stability of their presence there as a non-native species, however, is heavily affected by factors such as food availability and water temperature.
6) Tucunaré (Cichla ocellaris)
The tucunaré, or the peacock bass, is a tropical cichlid that is primarily found in the Amazon Basin. This eye-catching freshwater fish is remarkably colorful, with iridescent scales and vertical bands that come in shades of yellow, blue, and green. It is a predatory fish with a knack for striking aggressively. Due to its energetic fighting ability, it is prized by many sport fishers.
To meet the demands of local sport fishers, the tucunaré was deliberately introduced into Hawaii’s waters as a recreational fish. It is typically found in relatively large reservoirs, where it feeds on a variety of smaller non-native fish like mosquitofish, threadfin shad, bluegill, and tilapia. During its spawning period, which lasts from March to September, females lay their eggs on rocky areas. The survival of the eggs hinges on the protection of at least one parent fish.
7) Bluegill (Lepomis macrochirus)
Although the natural range of bluegills is largely centered around the Rocky Mountains, it is now found in all major freshwater systems of the US. A key component of the food web wherever it is found, its medium size and its tendency to breed rapidly make it an important forage food for larger game fish. Its native habitats include vegetated lakes, rivers, ponds, and wetlands.
Bluegill was first introduced into Hawaii in 1946 as a recreational freshwater species. Now, its established populations are found in many of the state’s prized waterbodies. Anglers can search for it in several areas of Oahu, including Lake Wilson and the ponds of Diamond Head. Its population trends across the islands are largely varied due to differences in fishing pressures and habitat diversity.
8) Largemouth bass (Micropterus salmoides)
Although this species is known by dozens of names, the common name “largemouth bass” is the most memorable because it alludes to its enormous gape. Able to prey on animals that measure up to almost half its full size, this voracious carnivore is an apex predator in its own right. Apart from its large mouth, with upper jaws that extend far back to behind its orbital region, it is set apart by its large adult size and a rough, horizontal stripe along the length of its sides.
To meet the demands of local sport fishers and lovers of aquatic fare, it was introduced into Hawaii in 1896. This black bass favors conditions in the reservoirs of the Big Island, where slow-moving waters have structurally diverse bottoms. To successfully catch its favored prey types, it relies on the camouflage provided by submerged logs, vertical vegetation, and weeds.
9) Threadfin shad (Dorosoma petenense)
A pelagic freshwater species that is commonly found in reservoirs, rivers, and lakes, threadfin shad are frequently stocked into ponds as a forage fish. Its small size and its rapid reproduction rate make it a crucial member of the food chain throughout its native range. Despite its capacity to grow to a maximum length of 8 inches (20 cm), it rarely grows to more than an inch long in highly diverse bodies of water.
The threadfin shad was intentionally introduced into Hawaii as a baitfish. Though the early introductions of this species were unsuccessful in reservoirs and rivers, it is now widely distributed throughout the freshwater systems of the major islands.
10) Mosquitofish (Gambusa affinis)
Known for their capacity to thrive in waters with low dissolved oxygen conditions, high temperatures, and high salinities, mosquitofish are a remarkably hardy freshwater species. They have proven to be highly valuable in nature and in developed areas due to their appetite for mosquitoes and their larvae.
Unfortunately, despite their potential for reducing the spread of mosquito-borne diseases, their introduced populations have proven to be alarmingly harmful to Hawaii’s aquatic ecosystems. Willing to eat just about anything that fits into their mouths, they have developed a penchant for preying on the eggs, fry, and juveniles of some of Hawaii’s endemic gobies. Many of Hawaii’s endemic insects are not safe from this fish’s seemingly boundless appetite either!
11) Swamp eel (Monopterus albus)
Also known as the rice paddy eel, the swamp eel has been introduced into many parts of the world as it is commercially important and consumed as a food fish. It is not a “true eel”, despite its common name, and is more aptly described as an “eel-like fish”. Its anguilliform body measures up to 16 inches (41 cm) long, has a tapered tail, and is devoid of both pectoral and pelvic fins. Its present fins are best described as rudimentary.
The swamp eel is an invasive, air-breathing fish. Its presence in Hawaii poses many challenges because it preys on a variety of endemic amphibians, small fish, and invertebrates. Because it’s able to reproduce rapidly, its populations can become quickly established in new habitats. There are now local efforts to manage existing populations and minimize their spread.
12) Mozambique tilapia (Oreochromis mossambicus)
The Mozambique tilapia is a dull-colored yet remarkably tasty fish. As it is often farmed in aquaculture setups, it has been introduced into many tropical and subtropical regions outside of its native range. Due to this fish’s robust and hardy nature, its populations can rapidly become well-established and may prove to be invasive.
In Hawaii, the Mozambique tilapia was introduced as a food fish in the mid-1900s. This non-native fish is now perceived as a threat to the survival of endemic freshwater fauna. Difficult to manage and control, it can quickly adapt to changes in environmental conditions as well as anthropogenic alterations to wild habitats. It’s crucial that efforts to restrict the spread of this species are supported by locals and tourists alike.
