Salmon spawn endangered trout; the story sounds fishy but it's true. By injecting specialized trout sex cells into sterilized but otherwise healthy salmon embryos, Japanese scientists wound up with male salmon that ejected trout milt (semen) and female salmon bearing trout eggs. Further, trout offspring of these altered salmon were totally normal and able to reproduce as trout—some eliding their salmon ancestry even in their appearance—thus raising hopes for spawning endangered fish species within more common ones.

"There are a lot of fish species or populations facing extinction because of environment destruction and overharvesting," says marine bioscientist Goro Yoshizaki, who led the effort. "Obviously environmental protection and [halting] fishing is the best solution…[but] we need some backup system to conserve endangered species."

Yoshizaki and his colleagues at the Tokyo University of Marine Science and Technology have been searching for a method to preserve endangered fish species for years. Simply freezing fertilized fish eggs has proved impossible because of their size and fat content. Preserving spermatogonia, the specialized cells that can grow into sperm or eggs, is possible, but the question is whether it would create the desired fish.

The researchers injected such spermatogonia from the rainbow trout into newly hatched masu salmon embryos that had been sterilized, because salmon spermatogonia would easily outproduce the introduced trout version [see photo below]. Once inside the salmon's body, the cells migrated on their own to the fish's testes or ovaries. "They can walk to their home by themselves," Yoshizaki says.

Once home, the cells proliferated and matured into viable eggs and sperm. When the adult fish mated, one out of every four fry looked like a wild rainbow trout and the other three offspring bore the orange coloring of their salmon parents [see photo above]. All developed normally and produced normal trout offspring. "The female salmon produce trout eggs and male salmon produce trout sperm," Yoshizaki says. "Consequently, simply by mating them, we can regenerate extinct trout."

Yoshizaki and his team obtained similar results when they injected trout spermatogonia into other fish species (brown trout and Japanese char). Given the encouraging results, freezing of endangered fish species's spermatogonia has begun. "We have started cryopreservation of [the spermatogonia of the] endangered sockeye salmon population in Idaho with NOAA [(National Oceanic & Atmospheric Administration)]," Yoshizaki says.

But the real target of the effort is Japan's favorite fish: the bluefin tuna. The fastest fish in the sea (and one of the largest) has been rapidly declining due to overfishing but its size and habits make it difficult to raise in captivity. "If we can produce surrogate mackerel, whose body size is 500 to 1,000 grams—nearly 1,000 times smaller than tuna—we can save space, cost and labor to produce [a] tuna baby," Yoshizaki says. And that means humanity won't have to worry about a big species of fish that got away.

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