Oncorhynchus mykiss:
The Quandary of a Highly Polymorphic Speciesunder the U.S. Endangered Species Act
by:Kathryn Kostow
Oregon Department of Fish and Wildlife
In the late 1990s, steelhead populations in most of the Columbia Basin and California
were listed as threatened or endangered under the U.S. Endangered Species Act.
Resident life histories of Oncorhynchus mykiss were not included in the listings.
This decision was challenged legally.
Components of the ESA quandary:
“Distinct” populations of a taxonomic species can be listed as “species” under ESA; “Distinct” is not defined by the act.
NMFS adopted criteria for defining “Evolutionarily Significant Units” (ESUs) to serve as their “distinct” populations for listing purposes;
Consistent with their criteria, NMFS originally found that sympatric trout and steelhead are in the same ESU;
The USFWS and NMFS share ESA jurisdiction over Oncorhynchus mykiss, with the USFWS responsible for trout and NMFS responsible for steelhead; The agencies disagreed about the need to list this species.
Steelhead in many ESUs were listed but the trout were not.
NMFS was sued for listing only part of their ESUs.
NMFS Policy and Criteria for Defining ESUs
An ESU is “… a population (or group of populations) that
1) is substantially reproductively isolated from other conspecific population units, and
2) represents an important component in the evolutionary legacy of the species.”
Waples 1991
Formally adopted as a federal policy applicable to NMFS
Trout and steelhead could be combined into a single ESU
Or each life history could qualify as their own ESU
depending on whether or not they meet both criteria:
2) represents an important component in the evolutionary legacy of the species.
1) substantially reproductively isolated from other conspecific population units,
Trout and steelhead could be combined into a single ESU
Or each life history could qualify as their own ESU
depending on whether or not they meet both criteria:
2) represents an important component in the evolutionary legacy of the species.
1) substantially reproductively isolated from other conspecific population units,
Frequent interbreeding is not necessary.
NMFS combined life histories in several ESUs:Summer & winter steelhead, spring & fall chinook, etc.
1) substantially reproductively isolated from other conspecific population units?
ESUs may include multiple “demographically independent” populations.
Gene flow may be historic but recent, periodic or rare.
ESUs are large-scale groupings.
To be combined, trout & steelhead only need to interbreed to the same degree as
populations in other ESUs. What has NMFS done before?
Evidence for or against reproductive isolation:
Genetic evidence
Otolith data
Life history data: spawning time and location
Observations of the life histories spawning together
Evidence that the life histories can produce the alternate life history
Geographic distribution and physical barriers
Distribution and Physical Barriers:Three Patterns
3. Trout are above a long-standing natural barrier and are outside of historic steelhead range.
1. The two life histories are currently sympatric;
2. Trout are above an artificial barrier that now blocks steelhead access into an area where the two life histories were historically sympatric;
Trout and Steelheadare Sympatric
Trout PopulationsIsolated above Dams,but within HistoricSteelhead Range
Trout PopulationsIsolated aboveNatural Barriers
Yakima steelheadcombined with othersto form the Mid-Columbia ESU
55
Genetics Survey of Trout and Steelhead(Pearsons et al.)
Unable to differentiatetrout from steelhead; but four Yakima steelhead populations were differentiated.
Genetics Evidence
Teanaway River, Yakima Basin
: Gene Flow
The O. mykiss problem:
Most samples were juvenilesfrom areas of steelhead/troutsympatry.
55
Other Genetics Surveys: Natural and Artificial Barriers
Waples: North Fork Clearwater
Leary: Snake River
Populations above Dams
Currens et al.:White River
Knudsen et al.:Kootenai
Populationsabove Waterfalls
Experimental Studies
Evidence that the life histories can produce the alternate life history
Parentage Assignment Rate
Hood River Steelhead Pedigree (Ardren and Blouin)
0
0.05
0.1
0.15
0.2
0.25
0.3
0.35
0.4
0.45
0.5
Parent Pair Mom Only Dad Only Neither Parent
Frequency
1991 Parents
1995 Parents
1996 Parents
1997 Parents
Two steelheadparents
Steelhead Mom;“Resident Dad”
Steelhead Dad;“Resident Mom”
MaybeNatural Strays
0%
5%
10%
15%
20%
25%
30%
35%
40%
45%
StsFxStsM
StsFxRbM
ResFxStsM
ResFxRbM
RbFxStsM
RbFx RbM
Crosses
0%
10%
15%
20%
25%
30%
35%
40%
45%
StsFxStsM
StsFxRbM
ResFxStsM
ResFxRbM
RbFxStsM
RbFx RbM
Crosses
Per
cen
t o
f re
leas
ed o
ffsp
rin
g
that
wer
e d
etec
ted
Grande Ronde Experimental Crosses (Ruzycki et al.)Detection of smolts at mainstem dams produced by crosses
Sts: Steelhead parentRes: Residual parent
(resident offspring of steelhead)Rb: Wild trout parent
0%
5%
10%
15%
20%
25%
30%
35%
40%
45%
StsFxStsM
StsFxRbM
ResFxStsM
ResFxRbM
RbFxStsM
RbFx RbM
Crosses
0%
10%
15%
20%
25%
30%
35%
40%
45%
StsFxStsM
StsFxRbM
ResFxStsM
ResFxRbM
RbFxStsM
RbFx RbM
Crosses
Per
cen
t o
f re
leas
ed o
ffsp
rin
g
that
wer
e d
etec
ted
Grande Ronde Experimental Crosses (Ruzycki et al.)Detection of smolts at mainstem dams produced by crosses
Sts: Steelhead parentRes: Residual parent
(resident offspring of steelhead)Rb: Wild trout parent
Summary of other evidence:
Extensive overlap of trout and steelhead spawning times and distributions;
Trout and steelhead observed on the same redds, apparently spawning together; most
frequently male trout acting like jacks;
Many steelhead (and fluvial and adfluvial trout) sex ratios are 60% to 80% females suggesting some males in the populations are resident;
Three scenarios of trout and steelhead distribution:
3. Trout are above a long-standing natural barrier and are outside of historic steelhead range.
1. The two life histories are currently sympatric;
2. Trout are above an artificial barrier that now blocks steelhead access into an area where the two life histories were historically sympatric;
Up to NMFS to decide final ESU boundaries.
What is the extinction risk of an ESU that includes both life histories?
Previous NMFS status reviews only “counted” steelheadand assessed the extinction risk of only steelhead.
The status and extinction risk of a steelhead/trout ESU may be quite different than that of just the steelhead:
Distribution
Population productivity
Population structure
Diversity
Abundance
Assuming that trout and steelhead are combined in ESUs
Distribution:
Current distributionof the five listedColumbia Basin ESUs
Distribution of the ESUsif trout above the damsare included.
Abundance:
In areas of sympatry in the inland Columbia Basin, adult trout appear to comprise
90% to 95% of the adult O. mykiss present.
Lower Deschutes River
0
20,000
40,000
60,000
80,000
100,000
120,00019
71
197
3
197
5
197
7
197
9
198
1
198
3
198
5
198
7
198
9
199
1
199
3
199
5
199
7
199
9
200
1
Year
Num
ber
of
fish
Steelhead Basin-wideAdult Trout in Lower Mainstem
0
2,000
4,000
6,000
8,000
10,000
12,000
198
6
198
8
199
0
199
2
199
4
199
6
199
8
200
0
Num
ber
of
fish
Year
Yakima Basin
Steelhead Basin-wideAdult Trout in 25 km Index Area, Upper Mainstem
ESUs may be secured from extinction risk by trout even while steelhead are in danger of extinction.
And therein lies the ESA quandary.
The loss of steelhead would constitute a significantchange in the character of an ESU.
ESA speaks clearly about avoiding extinction.
What does ESA say about avoiding a change in character?
Trout (the ESUs) are still present everywheresteelhead have already become extinct.
If trout and steelhead are in the same ESUs...