Study Looks At Rate, Impacts Of Hatchery Steelhead ‘Residualism’ On Methow River

A major focus of hatchery reform is the genetic management of broodstocks, but a recent study shows that how hatchery juveniles are raised is equally important.

Some 8 percent of summer steelhead released from the Winthrop National Fish Hatchery on the Methow River remained in the river and didn’t migrate. These “residual” steelhead can impact natural populations through competition, predation and interbreeding with returning anadromous adults, the study says.

Researchers investigated how age at release, size and maturation status influenced the production rate of residual summer steelhead between 2010 and 2015. They found that migration data, using PIT tags, identified 1,783 residual steelhead out of 21,598 released steelhead juveniles. It also found that the residual steelhead were of two types: immature male and female parr, and precociously mature males.

“The residualism rate was similar for steelhead smolts reared on a one or a two year program,” said Chris Tatara, research biologist at NOAA Fisheries’ Northwest Fisheries Science Center, Manchester Station, and Don Larsen, research biologist, NOAA’s Science Center in Seattle. “However, the two smolt age produced different types of residuals.”

The age-1 program, he said, produced predominantly immature male and female parr, while the age-2 program produced precociously mature male residuals. 

“The different residual types have different genetic and ecological consequences for the natural populations.  Residual steelhead rarely returned as anadromous adults,” Tatara and Larsen said.

According to the study, wild steelhead typically spend two or more years in freshwater before undergoing the physiological changes to allow for life in seawater (smolting) and migrating to the ocean. However, for economy of time, space and expense, most hatchery steelhead are reared on an accelerated growth regime and released to produce seaward migrants after a single year. “But, doing so may require intentional artificial selection for advanced spawn timing in hatchery fish.”

Programs that aim to supplement natural populations for recovery purposes are mandated by hatchery reform guidelines to use natural-origin broodstock that exhibit natural spawn timing. Those programs have recently been developed to produce age-2 smolts, the study says.

The rate of residualism for natural steelhead is hard to quantify especially in large rivers because of the small size and cryptic coloration of these fish, the researchers said.

“In steelhead it is even more difficult because it is hard to distinguish between a residual steelhead and a rainbow trout,” they said. “In a hatchery setting it would be best to produce the fewest number of residuals possible in order to maximize anadromous production and to minimize genetic introgression of hatchery fish into natural populations.”

The open access study, “Age at Release, Size, and Maturation Status Influence Residualism in Hatchery Steelhead,” was published online May 22, 2019 in the North American Journal of Fishery Management.

Tatara’s and Larsen’s co-authors are Penny Swanson, division director, and Deborah Harstad, research scientist, Environmental and Fisheries Sciences, NOAA Fisheries Northwest Science Center; Mollie A. Middleton, scientist, and Jon T. Dickey, research scientist, School of Aquatic and Fisheries Sciences, University of Washington; Chris Pasley, hatchery manager, Winthrop National Fish Hatchery, U.S. Fish and Wildlife Service; and Matt Cooper and Michael Humling, fisheries biologists, Mid-Columbia Fish and Wildlife Conservation Office, U.S. Fish and Wildlife Service.

Among the study’s findings:

— Age at release significantly affected the predominant residual phenotype. Age-1 steelhead residuals were dominated by smaller parr of both sexes (fish smaller than 146 millimeters fork length – 5.5 inches), while age-2 residuals were dominated by mature males, although both phenotypes were present in both groups.

— Collections of residual steelhead in the Methow River indicated that the parr residuals grew as well as natural-origin juveniles. That suggests potential competition with natural steelhead for food resources and habitat.

— Steelhead from both the age-1 and age-2 treatments residualized in the vicinity of the hatchery in nearly equal proportions.

— Detections of PIT tags within the Methow River basin indicated that precocious male residuals may overlap in both space and time with spawning anadromous adult steelhead, posing a potential genetic management risk.

— Both residual phenotypes had poor overwinter survival, and only 1 of the 1,783 residual fish eventually returned to the Methow River as an anadromous adult.

“We conclude that the ecological and genetic consequences of residual steelhead far outweigh their potential contribution to anadromous production, and measures should be taken to reduce their production by changing hatchery rearing practices,” the study says.

Tatara and Larsen said the researchers had identified three possible hatchery practices that could reduce release of residual steelhead. 

1. Incorporate a volitional release and stock remaining residuals in a terminal fishery. 

2. Manually sort immature parr and mature males from the population prior to release. 

3. Perhaps the “most ideal would be to manipulate the growth patterns to minimize production of residuals in the first place,” they said.

“Ultimately, the success of an S2 (age-2) regime must be judged by comparing the relative reproductive success of S1 (age-1) and S2 returning adults,” the study concludes. “This research is currently underway and will hopefully provide for a comprehensive understanding of the costs and benefits of this approach for the WNFH and other facilities considering the production of age-2 steelhead smolts.”

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