Minijacks -- A Major Headache
by Bill Rudolph
Three NOAA Fisheries researchers from the agency's science center in Seattle have taken a fresh look at some Northwest hatcheries and found a troubling tendency that is likely hampering some regional supplementation efforts.
The researchers say the production of large numbers of young Chinook called "minijacks," young males that mature so early they never leave their river of origin before returning to their early rearing grounds, show extremely low reproductive success. Jack Chinook are sexually precocious males that return early as well, but they spend one year at sea.
Published in the April 30 online edition of Transactions of the American Fisheries Society, the paper, "Variation in Minijack Rate among Hatchery Populations of Columbia River Basin Chinook Salmon," found a tenfold variation in minijack rates among several spring and summer Chinook hatcheries in the Columbia Basin. It was authored by Deborah Harstad, Donald Larsen and Brian Beckman.
Since growth rates at hatcheries are influenced by diet, water temperature and feeding rates, young males can mature much earlier than in the wild. In some cases, like the White River captive broodstock program in the Wenatchee Basin, minijacks accounted for more than 71 percent of the return from the 2009 brood year.
The authors said the high minijack rate for the White River program was greater than two times the overall rate for all the spring Chinook release groups they reviewed, and demonstrated that the White River fish "probably experienced high growth at a critical period when growth influences the onset of maturation."
A flagship research hatchery in Cle Elum, Wash., operated by the Yakama Tribe, sported a 48.5-percent minijack rate from the 2010 brood year, while the Carson Hatchery not far above Bonneville Dam, produced 30 percent minijacks from the 2009 brood.
In 2009, the Winthrop Hatchery had a minijack rate of only 14.6 percent, and the Leavenworth Hatchery 21.4 percent. In northeast Oregon, the Imnaha Hatchery had a 57.4-percent minijack rate in 2006, and the Lostine Hatchery had a 51.5-percent minijack rate that year.
In the wild, minijacks are thought to make up less than 5 percent of returning fish.
Another finding in the paper was recognition of the large difference in minijack rates between integrated hatchery programs where natural-origin adults and hatchery returns are used for broodstock, and segregated programs, where only returning hatchery fish are used for broodstock. Minijack rates are much lower for the segregated programs.
The authors report that minijacks are never used for broodstock at production hatcheries in the Columbia Basin and hypothesize that domestication selection over generations has required a higher growth rate and/or body size to achieve the same minijack rate as integrated programs.
They were cautious about the potential for altering minijack rates in integrated programs for supplementing wild populations, especially those listed for protection under the ESA, and said such programs may be violating the requirement that calls for supplementation programs to minimize alterations to the genetic and phenotypic characteristics of these natural populations.
They cited another study by Ford (2012) that found a negative correlation between spring Chinook in a supplementation hatchery and the reproductive success of their progeny in the wild. "The differences in reproductive success between the hatchery and natural environments were largely due to low reproductive success of minijacks in the natural environment," said Harstad et al.
The paper said supplementation programs may be "significantly" altering breeding success and fitness among naturally spawning stocks, but such an analysis was beyond the scope of their paper and warranted more study.
The high minijack rates they found throughout Columbia Basin hatchery programs "represent a direct reduction in the number of male smolts released and may result in a reduction in the number of males returning as full-sized anadromous adults for harvest or spawning."
Researchers Search for Methods to Reduce High Numbers of 'Mini-Jacks' Produced by Hatcheries by Staff, Columbia Basin Bulletin 12/11/9
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