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Tuna in the Stream?

January 9, 2013

Tuna (in Samoan), is also known as the giant mottled eel, or the marbled eel (Anguilla marmorata).

Eel on land

Occasionally, a giant mottled (or marbled) eel will move about on land for a short time in search of food.

Description: This freshwater eel has the widest distribution of the anguillid eels. It can be found in the tropical Indo-Pacific region from East Africa to French Polynesia as well as rivers in Japan and Mozambique. Adults are long and slender and have brown to black marbling on their backs over a greyish yellow base, and a white belly. They can reach lengths of 2 meters weighing a robust 20 kilograms, though they typically measure around 80 cm in American Samoa streams. This species is extremely long-lived with age estimates up to 40 years.

Habitat and Diet: Don’t be alarmed if you see an eel slithering your way. Tuna are capable of limited overland travel, especially during rainy periods which keep their skin moist out of water. They can be found in all areas of the stream from the mouth to the headwaters. They are primarily nocturnal and eat a wide range of prey including crabs, fish, and frogs.

Reproduction: Tuna spend their adult lives in freshwater or estuaries, but migrate to the ocean to reproduce. Their leaf-shaped larvae drift into the ocean and hide among plankton for about 4 months before returning to streams. Young migrate upstream feeding and growing for 8-20 years before returning to the sea to reproduce.

Threats: Eels, like all freshwater fish, are cold-blooded and breathe using gills. Since they cannot regulate body temperature, their bodies are the same temperature as the water. Temperature affects the eels’ metabolism and the amount of oxygen in the stream.  In undeveloped areas with abundant tree cover, the water temperature remains relatively constant; therefore, the animal’s metabolism and amount of oxygen in the water will also be relatively constant.

However, in more developed areas where land has been cleared for housing or agriculture, the water is more exposed to the sun and temperature fluctuations during the day. The warmer water increases the eels’ metabolism making them hungrier. The relative warmth simultaneously  decreases the amount of oxygen in the water, which makes it more difficult for tuna to breathe.

Additionally, pollution such as sewage or agricultural runoff from surrounding development further depletes oxygen levels in the water. Since these animals need to access the ocean as part of their life cycle, they may need to travel through more open areas as part of their journey to or from the ocean. If the amount of dissolved oxygen in the water is low due to lack of shade or elevated pollution, the animals’ progress may be deterred.

The Inventory & Monitoring Program monitors animal populations annually, and water quality parameters quarterly. Monitoring includes vital measurements such as temperature and dissolved oxygen.

Eel in Net

The field crew carefully removes an eel from a shrimp net during a survey

Giant Mottled Eels  in PACN National Parks: 

The giant mottled eel is common in streams on Tutuila and Ta‘u Islands in the National Park of American Samoa.This species is also found in Asan Stream on the island of Guam in War in the Pacific National Historical Park.

—A. Farahi, NPS

Biological Technician

Caught on Video: We captured this tuna on video last time the stream crew was working in American Samoa.

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