Some of the differences between river salmon and beach salmon included:
Body depth of males - male beach salmon are "deeper" from back to belly than their river counterparts. This trait influences mating success, Hendry says. "Deeper-bodied males gain access to more females during mating but, in a river with a strong current, a deep-bodied male would be inefficient hydrodynamically, and would have a more difficult time, so river fish have adapted by becoming slimmer, more streamlined."
Size of females at breeding age - river females are larger than their beach counterparts. Hendry points out that the river female's larger size enables her to bury her eggs deeper in gravel, decreasing the chances that the eggs will be disturbed or destroyed by high water flows.
"Rapid evolution has also been documented for other organisms," Hendry points out. "The unique twist to our study is that we were able to demonstrate these differences resulted in reproductive isolation." Scientists examined tiny ear bones known as otoliths, which have a sort of bar-code identifying each fish as to where it had been born, the river or the beach. "We can look at breeding adults and know who is a river resident, who is a beach resident, and who is a beach immigrant - a fish born in the river but now breeding at the beach," said Hendry. "We found that a large percentage of the adults spawning in the beach came from the river - almost 39 percent each generation," he said. "If those fish were successful in producing offspring, the two populations would homogenize."
Genetic analysis of "beach immigrants" relative to "residents" revealed just the opposite: immigrants are considerably less successful at producing offspring. This difference may have arisen because immigrants have reduced mating success or
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Contact: Elizabeth Luciano
luciano@journ.umass.edu
413-545-2989
University of Massachusetts at Amherst
18-Oct-2000