WASHINGTON – Papa salmon plus mama salmon equals … baby trout?

Japanese researchers put a new spin on surrogate parenting as they engineered one fish species to produce another in a quest to preserve endangered fish.

Idaho scientists begin the next big step next month, trying to produce a type of salmon highly endangered in that state – the sockeye – this time using more plentiful trout as surrogate parents.

The new method is “one of the best things that has happened in a long time in bringing something new into conservation biology,” said University of Idaho zoology professor Joseph Cloud, who is leading the U.S. government-funded sockeye project.

The Tokyo University inventors dubbed their method “surrogate broodstocking.” They injected newly hatched but sterile Asian masu salmon with sperm-growing cells from rainbow trout – and watched the salmon grow up to produce trout.

The striking success, published in Friday’s edition of the journal Science, is capturing the attention of conservation specialists, who say new techniques are badly needed. Captive breeding of endangered fish is difficult, and attempts to freeze fish eggs for posterity so far have failed.

“They showed nicely that … they produced the fish they were shooting for,” said John Waldman, a fisheries biologist at Queens College in New York.

“Future work should look to expand this approach to other fishes in need of conservation, in particular, the sturgeons and paddlefish,” he added. “We have a lot of species of fish around the world that are really in danger of becoming extinct.”

The Japanese researchers’ ultimate goal: Boost the rapidly dwindling population of bluefin tuna, a species prized in a country famed for its tuna appetite.

“We need to rescue them somehow,” said Goro Yoshizaki, a Tokyo University marine scientist who is leading the research.

First, Yoshizaki’s team started with “salmonids,” a family that includes both salmon and trout and one of concern to biologists because several species are endangered or extinct.

Initial attempts to transplant sperm-producing cells into normal masu salmon mostly produced hybrids of the two species that didn’t survive.

This time, Yoshizaki engineered salmon to be sterile. He then injected newly hatched salmon with stem cells destined to grow into sperm that he had culled from male rainbow trout.

Once they were grown, 10 of 29 male salmon who got the injections produced trout sperm, called milt.

Here’s the bigger surprise: Injecting the male cells into female salmon sometimes worked, too, prompting five female salmon to ovulate trout eggs. That’s a scientific first, Yoshizaki said.

The stem cells were still primitive enough to switch gears from sperm-producers to egg-producers when they wound up inside female organs, explained Idaho’s Cloud.

Then Yoshizaki used the salmon-grown trout sperm to fertilize both wild trout eggs and the salmon-grown trout eggs. DNA testing confirmed that all of the dozens of resulting baby fish were pure trout, he reported.

Moreover, those new trout grew up able to reproduce.

Those first experiments, funded by a Japanese research institute, used still fairly plentiful species to develop the technique. Now comes Idaho’s attempt to prove if the method is really useful in trying to produce the endangered sockeye salmon.

Last January, Yoshizaki helped University of Idaho scientists collect and freeze immature sperm tissue from young sockeye salmon being raised at a state-run hatchery. Next month, he’ll be back to help Cloud thaw the tissue and implant it into sterile rainbow trout.

In Japan, Yoshizaki is focused on bluefin tuna, noting that standard “marine ranching” techniques are difficult for tuna that can reach man-size.

He has begun experiments into how to produce baby tuna from mackerel, which are nearly a thousand times smaller than adult tuna. If it works, “we can save space, cost and labor,” he predicted in an e-mail interview.