Salmon spawn trout in a promising test

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 research, published in today's edition of Science, is capturing the attention of conservation specialists, who say new techniques are much needed. Captive breeding of endangered fish is difficult, and efforts to freeze fish eggs long term have failed.

"[The Japanese scientists] 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.

The Japanese researchers' 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 leading the research.

Yoshizaki's team started with "salmonids," a family that includes 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.

Next, Yoshizaki engineered the salmon to be sterile. He 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 that 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 first, Yoshizaki said.

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

Yoshizaki then used the salmon-grown trout sperm to fertilize 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 said.

Moreover, the new trout grew up able to reproduce.

In 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 tough for tuna that reach human size.

He has begun experiments on 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 said by e-mail.