How a mosquitofish brought in to kill mosquito larvae became major freshwater ecosystem problem

A small fish introduced to Hawaii more than a century ago to control disease-carrying mosquitoes has become a major threat to the islands' native freshwater wildlife.

The eastern mosquitofish (Gambusia affinis) was brought to Hawaii in 1905 from the southeastern United States during a period of agricultural expansion. The intention was to reduce mosquito populations, but the fish soon began feeding on a much wider range of native aquatic species.

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According to assessments and conservation reports from the US Fish and Wildlife Service (USFWS), the invasive fish has contributed to declines in native aquatic insects, including several threatened species of Hawaiian damselflies.

Why was the mosquitofish introduced to Hawaii?

The eastern mosquitofish was introduced to Hawaii as a biological control measure against mosquitoes. It was subsequently released widely into streams, drainage ditches and taro fields.

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However, instead of restricting its diet to mosquito larvae, the fish adapted to eating various small aquatic organisms.

The USFWS Ecological Risk Screening Summary identifies Gambusia affinis as a species with high ecological risk. Its ability to tolerate different water temperatures and low oxygen levels allows it to survive in a wide range of freshwater environments.

Mosquitofish threaten Hawaii's native damselflies

One of the most significant impacts has been on Hawaii's native damselflies, belonging to the Megalagrion genus.

USFWS assessments identify predation by introduced fish as one of the major factors behind declines in these native insects.

Before non-native fish became established, many high-elevation Hawaiian streams had relatively few native freshwater predators. As a result, native aquatic insects evolved without strong defences against fast-moving fish.

When mosquitofish spread into lower- and middle-elevation streams, they began preying on young Hawaiian damselflies, known as naiads. These aquatic juveniles live in calm pools and along stream edges, making them vulnerable to fish predation.

Native Hawaiian damselflies lose parts of their range

According to USFWS assessments, species including the crimson Hawaiian damselfly (Megalagrion xanthe) and flying-earlobe Hawaiian damselfly (Megalagrion nigrohamatum) have disappeared from substantial parts of their former ranges.

The affected areas include parts of Oahu, Molokai, Maui and Hawaii Island where mosquitofish have become established.

The decline highlights how an introduced predator can affect species that evolved in an ecosystem without similar fish predators.

Invasive fish disrupt Hawaii freshwater ecosystems

The ecological impact of mosquitofish extends beyond Hawaiian damselflies.

USFWS information indicates that the fish can compete with native stream species, including Hawaii's endemic freshwater gobies, commonly known as o'opu. Both depend on small aquatic organisms for food.

Mosquitofish also consume native freshwater crustaceans, including endemic shrimp known as o'pae kala'ole.

Their feeding behaviour can have wider effects on stream ecosystems by removing organisms that influence algae and other plant growth on streambeds.

Why mosquitofish are difficult to control

One reason the invasive fish is particularly difficult to manage is its highly adaptable diet.

USFWS biologists note that mosquitofish are not dependent solely on mosquito larvae. When mosquito populations decline, the fish can switch to other small aquatic animals, allowing established populations to survive while continuing to prey on native species.

Their reproductive strategy adds another challenge.

Female mosquitofish give birth to live young rather than laying eggs. This means that even a single pregnant female introduced into an isolated stream pool could potentially establish a new population relatively quickly.

Removing mosquitofish from Hawaii streams is challenging

Eliminating established mosquitofish populations is difficult because Hawaii's freshwater systems are interconnected.

Streams can link with groundwater springs, mountain taro fields and coastal areas, making it harder for wildlife managers to isolate affected habitats.

According to conservation assessments, trapping and chemical control can also pose risks to native species living in the same waterways.

As a result, conservation programmes increasingly focus on preventing invasive fish from reaching vulnerable habitats in the first place.

Conservation efforts focus on protecting high-elevation streams

The USFWS and state wildlife agencies are working to protect high-elevation streams where natural waterfalls can act as barriers against invasive fish moving upstream.

Wildlife managers are also installing artificial barriers to prevent mosquitofish from entering protected habitats.

In some areas, captive-bred Hawaiian damselflies are being released into safer headwater streams in an effort to rebuild declining populations.

Conservation authorities also stress the importance of preventing people from deliberately or accidentally moving mosquitofish into remote streams.


Inputs from TOI