Biologists Jason Guignard and Brian Deason surveying salmon carcasses. (Photo: John Hannon, ucsc.edu)
Wild salmon populations weaker than previously thought: study
Friday, February 10, 2012, 00:10 (GMT + 9)
Only about 10 per cent of the fall-run Chinook salmon reproducing in California's Mokelumne River are naturally spawned wild salmon, scientists have discovered. The rest of the fish are hatchery-raised.
This phenomenon has been concealing the fact that too few wild fish are returning to the river to maintain a natural population because, as most California hatchery fish are unmarked, they look just like their wild counterparts. Population surveys are thus unable to distinguish between naturally wild and hatchery-raised fish.
|Juvenile chinook salmon. (Photo: news.ucsc.edu)
The results were published in the journal PLoS ONE.
In this study, scientists employed a new isotopic approach to show that a natural spawning population of Chinook salmon (Oncorhynchus tshawytscha) considered relatively healthy actually represents a decrease in population when the hatchery produced specimens are taken into account. The researchers marked hatchery fish using a novel technique that allows for the detection of traces of a hatchery diet in the ear bones (otoliths) of adult fish.
"We expected to find hatchery fish, but the sheer number of hatchery fish returning to spawn in the wild is surprising", said first author Rachel Johnson, a fishery biologist affiliated with the Institute of Marine Sciences at UC Santa Cruz and with the Bay-Delta Office of the US Bureau of Reclamation, Phys.Org reports. "It looked like a healthy population of fish returning to spawn, but the reality is that without the hatchery fish the wild stocks are not sustaining themselves."
The frequency and extent of connectivity between natal sources makes up the demographic and genetic limits of a population. However, the role that the influx of hatchery-produced adults may play in changing population dynamics and the fitness of natural populations remains for the most part unquantified, the scientists noted.
|Chinook salmon spawning adults. (Photo: news.ucsc.edu)
Results demonstrate that only 10.3 per cent of adults spawning in the river had otolith values lower than 8.5 per cent, which is characteristic of wild salmon. When considering the total amount of fish in river and hatchery, findings show that 90.7 to 99.3 per cent of returning adults were produced in a hatchery, with the best estimate standing at 95.9 per cent.
“When population growth rate of the natural population was modeled to account for the contribution of previously unidentified hatchery immigrants, we found that hatchery-produced fish caused the false appearance of positive population growth”, the scientists wrote. “These findings highlight the potential dangers in ignoring source-sink dynamics in recovering natural populations, and question the extent to which declines in natural salmon populations are undetected by monitoring programmes.”
Johnson believes the numbers show that efforts to boost salmon numbers may be doing more harm than good.
"Humans are influencing the wild stocks, but we have not been adequately measuring that in our monitoring of endangered species", she added.
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