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Trout surrogates could support single spawning salmon SPAWNINGImagine you're beside a rocky stream in the mountains and you see a clutch of fish eggs.You think they belong to salmon or trout, but you're not a fish expert and how different could they be, anyway?Turns out, pretty different!If they are chinook salmon eggs, they're probably orphans.Chinook salmon (Oncorhynchus tshawytscha) spawn once in freshwater streams after a substantial migration back from the ocean and then die.If they are rainbow trout eggs, their parents are probably around.Rainbow trout (Oncorhynchus mykiss) spawn in freshwater streams multiple times during their life.Why some salmonids are singlespawners and others are multiplespawners is not well understood.However, Goro Yoshizaki and colleagues at the Tokyo University of Marine Science and Technology and the University of Tsukuba, Japan, have uncovered a potential reason for this difference and it involves reproductive stem cells: the small but important cells that give rise to eggs and sperm.First, the scientists wondered what happens to these stem cells before and after spawning.When they looked at the immature gonads of rainbow trout (multiple-spawners) and chinook salmon (single-spawners) under the microscope, they saw lots of these stem cellsready to turn into sperm or eggs.After the fish had spawned once, they checked again and saw stem cells in the ovaries and testes of rainbow trout, but not in those of chinook salmon.This suggested that singlespawning species lose their ability to produce eggs and sperm after one reproductive cycle, while this is retained in multiple-spawners.Outside JEB reports on the most exciting developments in experimental biology.