Mālama Mokupuni/Caring for Our Island Environment: Life and Death in the Stream—‘O‘opu, ‘Opae, and Tahitian Prawns

By Rachel Laderman

On the east side of Hawai‘i Island, north of Hilo and along the Hāmākua coast, streams cascade down steep slopes, flow quickly over boulders, leap into wailele (waterfalls), rest briefly in calm ki‘o wai (pools), and riffle on towards the ocean. To live in this tumultuous zone, fish, crustaceans, and other native species have developed unique characteristics. Unfortunately, some unscrupulous prawn harvesters put all stream life at risk.

‘O‘opu and ‘Ōpae

Before looking at the harmful activity in Hawai‘i’s streams, let’s visit some of the amazing inhabitants. The native fish and crustaceans are still connected to their ocean origins—they lay their eggs in the streams, the young are washed down to the ocean, and juveniles then return up the streams. 

Hawai‘i is home to five native fish species with this creative lifecycle, four of which are endemic (only found in Hawai‘i). Some facts on two kinds of these small fish, collectively called ‘o‘opu:

• Before returning to streamlife, ‘o‘opu nōpili (Sicyopterus stimpsoni) undergoes a metamorphosis with its mouth shifting from the front of its head to the underside, becoming a suction disk for climbing. This ‘o‘opu is near threatened.
• ‘O‘opu nōpili can change color to match their environment.
• The male nōpili guards a nest of underwater eggs.
• ‘O‘opu ‘alamo‘o (Lentipes concolor), even rarer than nōpili, uses fused pelvic fins and a lot of muscle to climb vertical waterfalls—including Honomū’s 420-foot high Akaka Falls!

Hawai‘i streams are also home to two native shrimp species—the ‘ōpae kuahiwi or kala‘ole (Atyoida bisulcata), or mountain ‘ōpae, and ‘ōpae ‘oeha‘a (Macrobrachium grandimanus)—found primarily in slower moving, lower stream reaches. A few ‘ōpae facts:

• ‘Ōpae, like most crustaceans, are very sensitive to stream changes.
• Mountain ‘ōpae are excellent climbers, even able to climb completely out of the water.
• The ‘ōpae ‘oeha‘a have one striped pincher larger than the other, giving them their name: ‘oeha‘a means “to walk crookedly.”

The Problem with Prawns

Tahitian prawns (Macrobrachium lar, also known as giant freshwater shrimp) were introduced on Moloka‘i and O‘ahu as a food crop in the early 1950s. Today they are common in streams on the larger five Hawaiian Islands. “Like the native ‘ōpae, the invasive Tahitian prawns drift out to sea as larvae, then migrate back into different streams to grow into adults and live,” explains Troy Sakihara, aquatic biologist with the Division of Aquatic Resources, Hawai‘i Department of Land and Natural Resources (DLNR). “This explains the wide dispersal of these species.”

“One reason for the decline in ‘ōpae kala‘ole are the Tahitian prawns, which compete with and prey upon the native ‘ōpae,” says Troy. In addition to crowding out native species, Tahitian prawns have also brought another problem—people using poisons to harvest them.

In the spring of 2018 and again in the fall of 2020, DLNR Division of Conservation and Resources Enforcement (DOCARE) investigated cases of more than six streams that were poisoned for the purpose of harvesting Tahitian prawns from North Hilo to south Hāmākua. Household pesticides were used to kill the prawns, and also killed much of the other aquatic life in these special streams. This dangerous practice can sicken anyone who eats the prawns. We do not know why this method is used—besides poisoning the streams, it is very hazardous to the harvesters, their families, and the public. Pets that have eaten poisoned prawns have gotten very sick.

“These poisonings not only risk the public’s health when they unknowingly eat a poisoned prawn, but clearly this illegal activity has dramatic and negative impacts on the otherwise pristine streams flowing from the mountains and into the ocean on Hawai‘i island,” said DOCARE Chief Jason Redulla. “It takes months for life to return to the poisoned areas. Anyone convicted of poisoning a Hawai‘i stream could face felony criminal charges and civil penalties, with significant fines and/or jail time.”

For updates in civil action since this article was written, check DLNR/DOCARE for news releases.

What to Do

• Know the source of your prawns; be confident they are safe to eat and were harvested legally and humanely, i.e., with a net that will not harm native stream life.
• If you see dead prawns, or have any information on stream poisoning, notify DOCARE at 808.933.3460, 643.DLNR, or via the free DLNRtip app. Note the time of day, date, vehicle, and license plates. Provide photographs if possible and a call-back number.
• If you observe prawns crawling out of the water, freeze them as soon as possible. The quicker this information is provided the sooner specialists can get water and sediment samples.

Rachel Laderman, Sustainable Pacific Program, Lynker LLC/NOAA Affiliate, Hawai‘i Island