Hawai'i's Environment
Surviving saltless streams
By Jan TenBruggencate
Advertiser Staff Writer
Several Hawaiian freshwater stream animals must return to the sea at some point in their lives.
The native gobies, the 'o'opu, return to the sea to lay their eggs, and their young drift on the ocean before returning to the streams.
This ability to survive in water of different salinities helps explain how Hawaiian streams were populated, even though salt water surrounds us.
It turns out science is still learning about the ability of marine life to tolerate fresh water.
The tasty saltwater Pacific white shrimp, a major player in Hawaiian aquaculture, can survive quite well in fresh water.
While the shrimp requires salinity in the water in which it reproduces, it can grow up and apparently thrive in fresh water, according to new research conducted in Hawai'i.
Kathleen McGovern-Hopkins, the Windward O'ahu extension agent with the Sea Grant program, said it was known that the shrimp could survive in low salinity, and that they grew in hard water, which is water with a high mineral content.
McGovern-Hopkins found they grow just fine in fresh water, even when it's soft water.
There are several ways in which the shrimps' ability to thrive in fresh water could be beneficial, she said. One is with diseases that survive only in salt water.
"We have so many disease problems. Perhaps we can avoid them in fresh water," she said.
The Pacific white shrimp, Penaeus vannamei, is a native to the Pacific coasts of Central and South America. It is grown by several aquaculture operations throughout Hawai'i, of which Ceatech on Kaua'i's west side is the largest.
McGovern-Hopkins knew that the Pacific white shrimp could survive low salinity. Heavy rainfall on saltwater shrimp ponds has reduced salinity without killing shrimp. A Moloka'i farm was able to grow them to adult size in salinities as low as 1 to 2 parts per thousand.
She said she took 7-day-old post-larval shrimp and began reducing the salinity from the 31 parts per thousand of seawater.
In just two days, the salinity was cut to 7 parts per thousand and they were doing fine. At the end of a week, the shrimp were in soft, fresh water.
"I really didn't expect them to survive," she said.
But they did.
Now her investigation will move to just how well they grow in fresh water.
Among the issues is whether they grow faster or slower in fresh water, whether their flavor and appearance changes, and whether freshwater growth affects the keeping quality of the shrimp when they go to market.
Jan TenBruggencate is The Advertiser's Kaua'i bureau chief and its science and environment writer. Reach him at jant@honoluluadvertiser.com or (808) 245-3074.
