7631

• UCREFRP Catalog Number: 7631
• Author: Byrne, A. and T. C. Bjornn.
• Title: Predicting the Effect of Supplementation with Hatchery Fish on a Native Steelhead Stock - a Modeling Approach.
• USFW Year: 1991.
• USFW - Doc Type: 91-26,
• Copyright Material: NO

Information bulletin
U.S. DEPARTMENT OF THE INTERIOR NO.
FISH AND Wit nl IFF SFRVIrF DATE Mar. 1991
Predicting the Effect of Supplementation with
Hatchery Fish on a Native Steelhead Stock-a
Modeling Approach
The Northwest Power Planning Councils River
Basin Fish and Wildlife Program has as its goal the
doubling of salmon and steelhead production in the
Columbia River basin. To meet this goal, projects to
enhance and rehabilitate stream habitat, improve
downstream smolt survival, and stock streams with
hatchery fish will be implemented. If preservation of
native stocks is a goal of management, tools are
needed to help assess the consequences of
supplementation programs before they are begun.
A Life-history Model was Used to
Investigate Management Options
Because long-term studies that assess the effects
of hatchery supplementation on native fish are
lacking, we used a model to predict the consequences
of various management strategies on the native
steelhead Oneorhynchus mykiss stock in the Lochsa
River, Idaho. Native fish are the indigenous stock of
the drainage that have no hatchery lineage. We
simplified the steelhead life-cycle by assuming that
all naturally produced fish migrated to the ocean
after spending 2 summers in the river and returned
to spawn after a little more than 2 years at sea. This
allowed us to examine the effects of stocking by
generations, without the complications arising from
variable smolt age and length of time at sea. A
model with one gene and two alleles, to identify
hatchery (genotype HH) and native fish (genotype
Research Information Bulletins (RIBS) are internal Fish and Wildlife Service documents whose purpose is
to provide information on research activities. Because RIBs are not subject to peer review, they may not be
cited. Use of trade names does not imply U.S. Government endorsement of commercial products.
M, was used to assess the effect of supplementing
the native stock with hatchery fish over 30
generations. Hatchery fish were planted in
generations 1 to 15 but not during generations 16 to
30, so that the response of the native fish to a
long-term supplementation program could be judged.
Before stocking began, all steelhead in the river were
assumed to be native.
We focused the study on four variables: the initial
number of native spawners, the fitness of progeny of
hatchery fish that spawn naturally in the river, the
number of planted hatchery fish, and the amount of
mating overlap among the genotypes. The initial
number of native spawners was chosen to represent
low (30% of maximum) and high (80% of maximum)
drainage seeding rate. The fry fitness rate is a
measure of the survival, from fry-to-smolt relative to
native fish, of progeny from hatchery parents that
spawn in the river. The number of matings between
the genotypes was obtained from multiplying a
mating overlap rate with the abundance of the less
common of the two genotypes involved. For example,
if the mating overlap was 200/b, and 2,000 native
spawners and 1,000 hatchery spawners returned, the
number of native x hatchery fish matings was 200. A
deterministic model was used because we were
interested in judging what set of supplementation
conditions might be detrimental to the survival of
native fish, not in assessing the effects of a variable
environment. We examined the effect of planting