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Craig Stockwell and Margaret Mulvey, ecologists at the University of Georgia's Savannah River Ecological Laboratory, looked at the issue by examining populations of mosquito fish that had been moved 60 years before to control whining arthropods. According to records, 900 young fish hit the road in that move (far more animals than are involved in the reintroduction of larger species). "We expected that most rare alleles would be retained," Stockwell says. "But a good portion of them were lost in the newly-founded population."
This was all the more surprising because the female fish retain some sperm in their reproductive tracts, meaning some males that didn't make the trip were probably represented among the new population.
What are the implications of this research for species reintroductions?
Stockwell notes that "if you grab a handful of animals, by chance, you
are not going to sample all the genetic variability in the parent population."
He recommends the use of screening and surveys to monitor genetic diversity
when fish are moved to reestablish populations. 
But such monitoring would be much easier with relatively numerous fish than with, say, whooping cranes, whose wild population, after decades of preservation and reintroduction work, totals all of about 150. Stockwell acknowledges the sampling he recommends would be difficult, dangerous and expensive in many cases.
What
can go wrong?
These are some of the key dangers of moving small numbers of organisms
to a new location for purposes of reintroduction:
- Inbreeding
- Hybridization, or interbreeding, with local species
- Failure to survive
Want to know what house flies are good for? Yes, a thousand times, Yes!
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