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Determination of Smoltification Status in Juvenile
Migratory Rainbow Trout and Chinook Salmon inMinnesotaMary T. Negus
a
a Minnesota Department of Natural Resources, Division of Fisheries, 5351 North Shore Drive,
Duluth, Minnesota, 55804, USA
Version of record first published: 08 Jan 2011.
To cite this article: Mary T. Negus (2003): Determination of Smoltification Status in Juvenile Migratory Rainbow Trout and
Chinook Salmon in Minnesota, North American Journal of Fisheries Management, 23:3, 913-927
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913
North American Journal of Fisheries Management 23:913–927, 2003 Copyright by the American Fisheries Society 2003
Determination of Smoltification Status in Juvenile Migratory
Rainbow Trout and Chinook Salmon in Minnesota
MARY T. NEGUS* Minnesota Department of Natural Resources, Division of Fisheries,
5351 North Shore Drive, Duluth, Minnesota 55804, USA
Abstract.— The progress of smoltification was quantified with an enzyme assay (gill sodium-
potassium-activated adenosine triphosphatase, or ‘‘Na,K-ATPase,’’ activity) in nonnative pop-
ulations of migratory salmonines that are stocked or naturally spawned in Minnesota tributaries
of Lake Superior. This information was needed to develop stocking strategies that would maximize
imprinting at intended locations and to determine the causes and consequences of early emigration
in stream-reared fish. Species and strains tested included (1) chinook salmon Oncorhynchus tshaw-
ytscha during hatchery rearing and after stocking, (2) two strains of O. mykiss—steelhead (natu-
ralized) and a Kamloops (hatchery) strain of migratory rainbow trout—during hatchery rearing
and after stocking, and (3) stream-reared steelhead emigrants captured in smolt traps. The ATPase
activity level that distinguished smolts from nonsmolts was 11 mol Pi · (mg protein)1 · h1 for
chinook salmon and 10 mol Pi · (mg protein)1 · h1 for all groups of rainbow trout tested. Plots
of ATPase activity against fork length, weight, and body depth indicated that 91% of chinook
salmon and 100% of rainbow trout were stocked at sizes that exceeded a minimum size for smolting.
Temperature appeared to influence smoltification of chinook salmon. Stocking into a river signif-
icantly increased ATPase activity levels of hatchery chinook salmon and Kamloops rainbow trout,
but data were inconclusive for steelhead. Most stocked steelhead fry and naturally spawned steel-
head that emigrated before age 2 were not smolts and may have emigrated as a result of natural
exploratory movement. My results suggest that stocking all fish in May or June, controlling rearing
and stocking temperatures of chinook salmon, and controlling the sizes attained by migratory
rainbow trout in the hatchery may maximize in-stream smolting and future homing to desired
locations.
Chinook salmon Oncorhynchus tshawytscha
(naturalized) and two strains of O. mykiss—natu-
ralized steelhead and hatchery-reproduced Kam-
loops rainbow trout—are anadromous fish native
to the Pacific coast that have been stocked into
Minnesota tributaries of Lake Superior (Close et
al. 1984; Krueger et al. 1994; Peck et al. 1994).
Broodstock of fall-spawning chinook salmon
(Washington State origin, smolting at age 0) and
spring-spawning rainbow trout of both strains are
captured annually in French River for propagation
at the French River State Fish Hatchery. Age-0chinook salmon, yearling steelhead, and yearling
Kamloops rainbow trout (hereafter Kamloops) are
stocked into Minnesota tributary streams in spring
at ‘‘smolt size,’’ a loosely applied term based on
size and color change in some fish. These fish live
entirely in freshwater and are therefore called mi-
gratory rather than anadromous, but their original
life history patterns, including smoltification, per-
sist. Smoltification is the primary time for olfac-
tory imprinting, although fish may also remember
* E-mail: mary.negus@dnr.state.mn.us
Received September 25, 2001; accepted January 8, 2003
odors experienced before that stage (Hasler and
Scholz 1983; Morin and Døving 1992; Quinn
1993; Pascual et al. 1995; McCormick et al. 1998).
To effectively enhance stocks that will home to
specific locations where angling opportunities are
desired, hatchery fish must be stocked before im-
printing is completed.
Increased salinity tolerance is another charac-
teristic of smoltification, and most studies of smol-
tification have involved populations that migrate
to saline environments (Wedemeyer et al. 1980;
Folmar and Dickhoff 1981). Although salinity tol-erance is obviously irrelevant for smolts in Lake
Superior, the physiological changes still occur and
provide a means of pinpointing smoltification (and
indirectly, the period of imprinting). Gill sodium-
potassium-activated adenosine triphosphatase
(Na,K-ATPase) activity (hereafter called ‘‘ATPase
activity’’), which is strongly correlated with salin-
ity tolerance, can be used to quantify progress of
smoltification (Zaugg 1982; Boeuf and Prunet
1985; McCormick et al. 1987; Johnson et al. 1991;
Schrock et al. 1994). Superficial characteristicscommonly recognized as indicators of smoltifi-
cation (e.g., coloration, body size, condition factor,
and behavior) are imprecise measures of the peak
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914 NEGUS
of this life stage and may vary with species, pop-
ulation, or locale (Zaugg and McLain 1972; Vir-
tanen et al. 1991; Beeman et al. 1995; Haner et al.
1995).
Rainbow trout and chinook salmon stocks inMinnesota waters of Lake Superior have been de-
clining in recent years, and factors contributing to
this decline are under investigation (Schreiner
1995). Despite potential negative effects of pro-
longed hatchery rearing on imprinting and smol-
tification (Wagner et al. 1963; Wedemeyer et al.
1980; Quinn 1993), there is public pressure in
Minnesota to rear all hatchery stocks to the largest
possible sizes before stocking. Adult chinook
salmon, steelhead, and Kamloops routinely stray
to other rivers along the Minnesota shoreline and
beyond, suggesting that they partially imprint inthe hatchery or fail to imprint to their stocking
sites (Jones and Schreiner 1997; Peck et al. 1999;
D. R. Schreiner, Minnesota Department of Natural
Resources, personal communication). Even when
steelhead are stocked as fry (as in French River)
or are naturally produced (as in Knife River), the
young fish often display counterproductive behav-
ior by emigrating at small sizes (before age 2) that
experience low survival (Olsen et al. 2000; Spur-
rier et al. 2000a). Determining the smolt status of
these small emigrants may help to explain thecause of early emigration and the reasons for their
high mortality.
The objectives of this study were to (1) evaluate
smoltification readiness in hatchery chinook salm-
on, steelhead, and Kamloops, (2) determine wheth-
er smoltification is stimulated when fish are
stocked into a stream, (3) identify the smoltifica-
tion status of emigrating steelhead (naturally
spawned or stocked as fry) captured in smolt traps,
and (4) determine whether ATPase activity was
related to size, coloration, condition, water tem-
perature, and date of capture to identify indicators
of smoltification applicable to local stocks.
Methods
Study area and fish populations.—I tested chi-
nook salmon, steelhead, and the Kamloops strain
of rainbow trout being reared at the French River
State Fish Hatchery. The hatchery outflow water
enters the French River (Figure 1) about 100 m
upstream from the mouth. The hatchery-reared
stocks were destined for various Lake Superior
tributaries, where they were expected to imprintand supplement fishing opportunities during
spawning runs (Figure 1). I also tested stream-
reared steelhead that had been stocked as fry in
the French River or naturally spawned (wild) in
the Knife River and that were later captured in
smolt traps located in these rivers.
Minnesota’s shoreline of Lake Superior contai ns
54 tributary rivers or streams with habitat for mi-gratory species (MNDNR 1992). These tributaries
are generally much smaller and less productive
than West Coast rivers where chinook salmon and
rainbow trout originated. The Knife River system
is the only tributary that is accessible to migratory
fish for its entire 113-km length. The remaining
53 streams total about 62 km of habitat, but 25 of
these streams have marginal habitat due to their
small size. The steep landscape in Minnesota’s
North Shore region of Lake Superior forms a bar-
rier to upstream migration of fish in most streams,
and waterfalls often occur only a few meters up-stream from the lake. In general, the streams have
little groundwater input and are subject to high
spring runoff and widely fluctuating flows. Flows
in French and Knife rivers, for example, often
range from 0.1 to 1 m3 /s in summer but can spike
to 6–28 m3 /s or more duri ng storms. Cold winter
temperatures can cause anchor ice, ice dams, and
dry streambeds in some locations in some years.
Warm, dry weather can severely reduce flows and
heat the water, and heavy rainfall or snowmelt can
rapidly increase flows to flood stage. Thesestreams have low productivity and a limited car-
rying capacity for fish. Brook trout Salvelinus fon-
tinalis inhabit the headwaters of most streams, and
lower reaches contain a few brown trout Salmo
trutta, sculpins Cottus spp. and minnows. These
lower reaches are stocked with steelhead fry,
whereas age-0 chinook salmon and yearling rain-
bow trout are generally stocked a short distance
above or below the barriers. Since 1992, Kamloops
stocking has been restricted to three tributaries
near the city of Duluth (MNDNR 1992; Figure 1)
to reduce the probability of interbreeding with nat-
uralized steelhead.
Fish sampling.—To determine whether smolti-
fication was occurring in hatchery stocks (objec-
tive 1), ATPase activity was measured in chinook
salmon, steelhead, and Kamloops during hatchery
rearing to smolt size (Table 1). Hatchery-reared
chinook salmon achieved smolt size at age 0
(reared from the January hatch to May or June),
and the earliest samples were taken in March.
Steelhead were sampled periodically from Novem-
ber at age 0 to May at age 1; Kamloops weresampled from November at age 0 through Septem-
ber at age 1. Ordinarily, 36 samples (1 sample/
fish) were taken from each species or strain on
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915DETERMINATION OF SMOLTIFICATION STATUS
FIGURE 1.—Minnesota’s Lake Superior shoreline, known as the North Shore, including tributaries into which
migratory species are stocked. Letters in circles preceding river names indicate species or strains stocked into those
rivers: C chinook salmon, S steelhead, and K Kamloops strain of rainbow trout. All migratory fish stocked
into Minnesota waters are reared at the French River State Fish Hatchery.
each sampling date. Hatchery year-classes were
1997, 1998, and 1999 for chinook salmon; 1996
and 1997 for steelhead; and 1996, 1997, and 1998
for Kamloops. The 1997 year-class of chinook
salmon was reared with two different thermal re-
gimes: normal rearing temperatures of 5–7C and
accelerated growth temperatures of 10–12C. Chi-
nook salmon rearing temperatures in 1998 were
also elevated (9–12C) to speed growth, and rear-
ing temperatures in 1999 resembled the normal
temperatures of 1997. The ATPase activities of all
fish were measured to determine whether smolting
(indicated by an elevation in ATPase activity
above threshold values) was occurring duringhatchery residency.
To determine whether stocking into flowing wa-
ter stimulated smolting (objective 2), samples were
taken from hatchery-reared chinook salmon, steel-
head, and Kamloops before and after stocking (Ta-
ble 1). No individuals were sampled more than
once. A random sample of hatchery fish was
stocked about 1 km above the French River smolt
trap so they could be recaptured, and fish remain-
ing in the hatchery were sampled on the same day
to represent prestocked fish. Mean ATPase activ-
ities before and after stocking were compared us-
ing pooled-variance t -tests ( 0.05).
To determine the smoltification status of stream-
reared steelhead emigrants (objective 3), steelhead
that had been stocked as fry in French River or
naturally reproduced in Knife River were sampledfrom smolt traps in these rivers. Samples were
taken when at least 3–5 fish/d were being trapped.
Up to 36 fish were sampled per day; fish less than
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916 NEGUS
TABLE 1.—Salmonid populations sampled for gill sodium-
potassium-activated adenosine triphosphate activity. There
was one sample per fish and one assay per sample.
Species or strainNumber
of samples
Hatchery-reared fish
(sampled periodically up to day of stocking)
Chinook salmon
Steelhead
Kamloops
242
343
459
Hatchery fish stocked into French River
(sampled at recapture)
Chinook salmon
Steelhead
Kamloops
98
38
36
Stream-reared fish
(sampled when captured in the river’s trap)
Fry-stocked steelhead (French River)Wild steelhead (Knife River)
368122
80 mm fork length (FL), which were difficult to
sample nonlethally and were always parr, were
omitted when large numbers were captured. The
ATPase activities of these two groups of stream-
reared fish were measured to determine the pres-
ence of smolting.
To determine whether ATPase activity in local
stocks could be related to a more obvious, easilymeasured characteristic (objective 4), ATPase ac-
tivities were plotted against FL (a more consistent
measure than total length because hatchery fish
had obvious fin erosion), weight, body depth, con-
dition factor (105 · weight/fork length3), and sam-
pling date; ATPase activities were also compared
with body color (distinguished by different sym-
bols). The threshold level in ATPase activity re-
quired for smoltification was defined as the upper
end of the range of ATPase activities detected in
the smallest fish tested that had dark bands and
swam strongly against the current (chinook salmon
sampled in March and April and rainbow trout
sampled in November and January). Recommend-
ed guidelines were 0 mol to about 10–12 mol
Pi · (mg protein)1 · h1 for nonsmolting juveniles
and 12–15 mol Pi · (mg protein)1 · h1 or more
for smolting juveniles (R. Schrock, USGS, West-
ern Fisheries Research Center, personal commu-
nication), although these levels vary by species,
procedure, and laboratory. Ewing et al. (1980) dis-
cussed the concept of size thresholds for smolti-
fication; in my study, I quantified minimum sizesfor smoltification in chinook salmon, steelhead,
and Kamloops by visually inspecting the plots of
ATPase activity as a function of fork length and
then by estimating the minimum size at which el-
evated ATPase activities (indicating the onset of
smoltification) were first evident. The ATPase ac-
tivities of three color categories for each species
or strain were analyzed using analysis of variance(ANOVA; 0.05) and posthoc Bonferroni
pairwise comparisons. Water temperatures, FL,
and ATPase activities were plotted against date to
reveal the temporal sequence of changes in all spe-
cies and strains.
ATPase activity assay methods.—At sampling,
each fish was anesthetized and a portion of gill
filament was excised using techniques described
by McCormick (1993) and Schrock et al. (1994).
Each tissue sample (one sample per fish) was
placed in SEI (sucrose, disodium EDTA, imidaz-
ole) buffer solution, numbered, and frozen in liq-uid nitrogen within 30 min. The samples were later
transferred to a 80C freezer until analysis. Each
fish was weighed (g); measured (mm) for fork
length, total length, and body depth; and catego-
rized by body coloration (banded, intermediate, or
silver). The ‘‘banded’’ designation meant that all
parr marks were visible. The ‘‘intermediate’’ des-
ignation meant that parr marks closest to the head
were gone, but marks were still visible near the
caudal fin. The ‘‘silver’’ designation meant that all
parr marks were gone, or those remaining near thecaudal fin were extremely pale. Categorization was
done indoors or out of direct sunlight to reduce
reflection that could affect perceived body color.
Water temperatures in the river and in the hatchery
were continuously monitored. All live fish were
released after sampling.
Gill filament samples were prepared following
the protocol of Schrock et al. (1994), with a few
modifications described here. The samples were
extracted in SEI only, eliminating the solution con-
taining deoxycholate, a detergent additive that was
found to be unnecessary. Samples were ground
with a Tissue Tearor and then centrifuged for 8
min at 2,000 relative centrifugal force (RCF). Su-
pernatant was pipetted off, SEI was added to the
pellet, and each vial was vortexed on the high set-
ting for 10 s. Samples were sonicated for 4–5 con-
tinuous seconds using a Sonics and Materials, Inc.,
Vibra Cell 50-W Ultrasonic Processor set at Out-
put 20 and centrifuged for 10 min at 12,000
RCF; 150–200 L of the enzyme preparation su-
pernatant was transferred to a culture tube. Rep-
licates of each sample, references, and standardswere included in each microassay plate to reduce
the probability of incorrect values due to pipetting
errors. Values of the replicates were averaged.
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917DETERMINATION OF SMOLTIFICATION STATUS
TABLE 2.—Sodium-potassium-activated adenosine triphosphate (ATPase) activities that defined the threshold of smolt-
ing for chinook salmon and migratory rainbow trout. Minimum sizes were the approximate lengths, body depths, or
weights at which elevated ATPase activities were first measured.
Species orstrain
Rearinglocation
ATPaseactivitya
Minimum sizes for smolting
Fork length(mm)
Weight(g)
Bodydepth(mm)
Approximatenumber
per pound (kg)b
Chinook salmon
Steelhead
Kamloops rainbow trout
Steelhead (fry-stocked)
Wild steelhead
Hatchery
Hatchery
Hatchery
French River
Knife River
11
10
10
10
10
71
150
150
140
140
4
40
40
28
28
15
30
30
25
25
100 (220)
10 (22)
10 (22)
a That is, mol Pi (mg protein)1·h1; the first whole integer above the range of values measured in the smallest groups of juveniles
tested.b Number per pound is a common hatchery measurement. Because these fish cannot be blotted effectively when held in a net and because
the minimum weight is a conservative estimate for the low end of smolting, the number of fish has been reduced slightly to provide a
rough guideline for hatchery managers.
The evaluation procedure consisted of two as-
says: one to measure the amount of inorganic phos-
phate (Pi) liberated by the action of ATPase and
one to measure the amount of protein. The phos-
phate assay followed the procedure of Schrock et
al. (1994). The Bio-Rad protein assay was substi-
tuted for the more complicated protein assay used
by Schrock et al. (1994). All phosphate and protein
samples were read at 590 nm on a Biolog Mi-
crostation 96-well plate reader. The ATPase activ-
ity was reported as mol Pi
· (mg protein)1 · h1.
Results
Smoltification Readiness in Hatchery Fish
Elevated ATPase activity levels indicative of
smolting were found in many hatchery fish, but
smoltification was obviously not an all-or-nothing
event; nonsmolts were sampled at nearly every
size and date that smolts were sampled. The thresh-
old ATPase activity for smolting in chinook salm-
on was determined to be 11 mol Pi · (mg pro-
tein)1 · h1, because 10.6 was the highest activity
found in the smallest chinook salmon sampled (Ta-
ble 2; Figures 2, 3). The threshold ATPase activity
for smolting in rainbow trout was determined to
be 10 mol Pi · (mg protein)1 · h1 because 9.8
was the highest activity found in the smallest rain-
bow trout sampled (Table 2; Figures 4, 5, 6).
Stimulation of smoltification between stocking
and recapture
Mean ATPase activity between the time of
stocking and recapture increased significantly in
chinook salmon (t 3.704, df 146, P 0.001)and Kamloops rainbow trout (t 3.003, df
101, P 0.003) but decreased significantly in
steelhead (t 4.396, df 66, P 0.001; Table
3). Only one stocking trial was run with steelhead,
and that group was the only one with a mean
ATPase activity level that exceeded the threshold
for smoltification before stocking (Table 3). Most
stocked chinook salmon emigrated immediately,
and the highest and lowest ATPase activities were
measured in those fish recaptured first. Most of the
later recaptures remained in the stream until smolt-
ing (Negus 2000). The largest numbers of chinook
salmon with low ATPase activities were recaptured
1 d after stocking at low temperatures (below 9 C).
Emigration by stocked steelhead and Kamloops
was more gradual, and about 50% left within 1
week of stocking.
Smoltificationt Status of Fry-Stocked and Wild
Steelhead Emigrants
Elevated ATPase levels indicative of smolting
were measured in some steelhead emigrants cap-
tured on every sampling date in spring, but none
were found in fall samples (Figures 7, 8). Tem-
peratures during all spring and fall periods ex-
ceeded normal hatchery rearing temperatures forsteelhead and Kamloops. Most smolting emigrants
were age-2, or very large age-1 fish. Only one
emigrant in the French River exceeded 244 mm
FL, and the largest emigrant in Knife River was
214 mm FL.
Relationship between ATPase Activity and Other
Measurements
The smallest sizes at which smoltification was
detected were ascertained for each of the strains.
The minimum size of smoltification in chinook salmon (ATPase activity 11 mol Pi · [mg pro-
tein]1 · h1) was 71 mm FL, as seen in plots of
all year-classes combined (Figure 3; Table 2). A
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918 NEGUS
FIGURE 2.—Temporal changes in daily water temperature, sodium-potassium-activated adenosine triphosphatase
(ATPase) activity, and fork length of four groups (three year-classes) of chinook salmon sampled during rearing
in Minnesota’s French River State Hatchery. Error bars have inner tick marks denoting means SEs and outer
‘‘T’’ marks denoting means SDs. In 1997, temperatures were not recorded after early June because most chinook
salmon were stocked by that time. The last groups sampled that spring had been held for 2 weeks at approximately
10–12C. Triangles identify dates when fish were sampled just before stocking trials.
second, even greater increase in ATPase activities
occurred above 15 mol Pi · (mg protein)1 · h1 in
the largest fish from the 1997 accelerated growth
group that was reared at higher temperatures (Fig-
ure 3). The minimum size at which smoltification
occurred for rainbow trout varied depending on
rearing location. Kamloops and steelhead in the
hatchery exhibited elevated ATPase activities at
about 150 mm FL (Figure 6; Table 2). Stream-
reared steelhead appeared to begin smolting at
smaller sizes than fish in the hatchery, roughly
around 140 mm FL. At the time of stocking, 91%
of the chinook salmon and 100% of steelhead and
Kamloops exceeded minimum smolting sizes (Fig-
ures 2, 4, 5). The ATPase activity was not signif-
icantly related to condition factor of any hatcheryor stream-reared stocks tested (Negus 2000).
Elevated ATPase levels were rare in hatchery
Kamloops larger than 260 mm FL, 200 g, and 60
mm body depth (Figure 6; Negus 2000), but these
large nonsmolts were found primarily in summer,
after the normal smolting season. Less than 1% of
the hatchery steelhead exceeded these large sizes
at the time of stocking, but between 21% and 36%
of the Kamloops exceeded at least one of these
measurements when stocked.
Color determination of fish was somewhat sub-
jective, and visi bility of parr marks depended upon
light intensity and viewing angle. Chinook salmon
were especially difficult to classify, so color cat-
egorizations of this species were dismissed. The
ATPase activities differed significantly (P 0.05)
between color categories of hatchery steelhead
(F 2,377 80.940), hatchery Kamloops (F 2,491
21.824), fry-stocked steelhead in French River(F 2,363 68.642), and naturally reproduced steel-
head in Knife River (F 2,118 50.977; Table 4),
although posthoc Bonferroni pairwise compari-
y
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919DETERMINATION OF SMOLTIFICATION STATUS
FIGURE 3.—Individual ATPase activity levels versus
fork length of all three year-classes of age-0 chinook
salmon measured during their rearing periods in Min-
nesota’s French River State Hatchery. The vertical line
delineates the approximate minimum size at which el-
evated ATPase activities were measured in some fish.
sons demonstrated no difference between the in-
termediate and silver color categories of Kam-
loops. In all other cases, dark-banded rainbow
trout had the lowest and silver-colored rainbow
trout had the highest ATPase activities.
Evidence of smolting was seen primarily in chi-
nook salmon reared at or above 9 C and occurred
as early as April (Figures 2, 3). Elevated ATPase
activities were seen in hatchery steelhead and
Kamloops by late February or early March (Fig-
ures 4, 5). The ATPase activities of Kamloops in
the hatchery were depressed after the temperature
reached 12C, but these fish were the largest rain-
bow trout tested (late summer; Figure 5). Stream-
reared steelhead could not be sampled before April
because of ice cover; most fish with elevatedATPase levels emigrated in May or June, and
ATPase activities in French River were depressed
from June and through the fall (Figures 7, 8).
Discussion
Smoltification Readiness in Hatchery Fish
Steelhead, Kamloops rainbow trout, and most
chinook salmon are currently held in the French
River State Fish Hatchery beyond the size when
smoltification can begin. Some Kamloops may be
held past the period of smoltification and regres-sion. Holding fish beyond the start of smoltifica-
tion is not necessarily harmful to the fish, but
hatchery imprinting or sequential imprinting may
explain some of the straying back to French River
by stocked chinook salmon, steelhead, and Kam-
loops. This straying reduces fishing opportunities
at the stocked streams and causes an inflated es-
timate of survival based on counts taken at FrenchRiver. Pascual et al. (1995) suggested that down-
stream emigration during the appropriate season
and physiological state is important to smolt trans-
formation and imprinting, but fish may also re-
member odors experienced before this time. Ma-
turing salmon tend to reverse the sequence of their
outward emigration as juveniles, returning first to
the odors of their release site and continuing on
to the rearing site (hatchery) if its odors can be
detected (Quinn et al. 1989). If there is no stimulus
for further migration once they reach the release
site, the fish may remain there (Dittman and Quinn1996). Seelbach et al. (1994) found that although
smaller steelhead smolts and those stocked farther
upstream in a Lake Michigan tributary suffered
higher mortality than larger smolts and those
stocked into river mouth, those stocked into the
river mouth strayed more. He surmised that min-
imizing straying by stocking upstream outweighs
the costs of higher mortality, when the goal is to
create fisheries in specific locales. The issue of
straying is also important where there is a desire
to maintain spatial segregation between natural-ized and hatchery stocks. Hatchery Kamloops will
spawn with steelhead in a stream setting, but off-
spring having one or more Kamloops parents have
reduced fitness (Close 1999; Negus 1999). Al-
though stocking of Kamloops has been restricted
to reduce the spatial overlap with the naturalized
steelhead during spawning runs, this strategy may
be less effective than desired.
Stimulation of Smoltification between Stocking
and Recapture
Some smoltification was stimulated by place-
ment of chinook salmon and Kamloops in a flow-
ing stream. Although the stocked steelhead did not
respond in a similar way, data were insufficient to
prove that steelhead cannot also be stimulated to
smolt by stocking. Most (93%) of the chinook
salmon were recaptured immediately after stock-
ing. These immediate recaptures included both
smolts and presmolts, whereas those recaptured
later were primarily smolts. The initial mass em-
igration, possibly a schooling behavior, was es-
pecially noticeable after the May 1997 stockingwhen water temperature was about 7C, a tem-
perature that apparently discouraged chinook
salmon smoltification in this study. About 50% of
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920 NEGUS
FIGURE 4.—Temporal changes in daily water temperature, ATPase activity, and fork length of two year-classes
of steelhead sampled during rearing in Minnesota’s French River State Hatchery. Error bars have inner tick marks
denoting means SEs and outer ‘‘T’’ marks denoting means SDs. The triangle identifies the date when fish
were sampled just before the stocking trial.
the steelhead and Kamloops yearlings that were
stocked and recaptured emigrated within 1 week.
Seelbach and Miller (1993) found that hatchery
steelhead smolts with a mean total length of 195
mm (about 187 mm FL; Negus 2000) stocked in
a Michigan tributary to Lake Superior emigrated
at a similar rate and strayed widely.
Smoltification Status of Stream-Reared Steelhead
Emigrants
Early emigration by steelhead has sometimes
been attributed to early smolting, displacement due
to high water, intraspecific competition, and self-
thinning (Grant 1993; Marschall and Crowder
1995), as well as natural exploratory movement.
Early smolting was not found in most small
stream-reared steelhead in this study, because most
of the emigrants under age 2 were not actuallysmolting. High water was also rejected as an ex-
planation for most premature emigration because
capture in the Knife River trap has been signifi-
cantly correlated with lower flows and captures are
rare during spate flows (Morse and Olsen 1999).
Self-thinning due to intraspecific competition im-
plies that the streams are being stocked or naturally
recruited above their carrying capacity for age-1
parr. Estimating the amount of intraspecific com-
petition is beyond the scope of this study, but data
from the Little Knife River (a branch of Knife
River), show that the number of age-1 emigrants
is directly correlated with the number of age-2
emigrants from the same year-class (Spurrier et al.
2000b; Figure 9). Differing carrying capacities for
ages 1 and 2 would be unlikely to produce such a
direct relationship between the two age-classes.
However, large year-classes or better survival to
age 1 in some years may result in more survivors
and more emigrants throughout the period of
stream residency. Leider et al. (1986) found thatsurvival of steelhead appeared to depend on avail-
ability of suitable rearing environments in down-
river areas because many presmolt steelhead in a
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921DETERMINATION OF SMOLTIFICATION STATUS
FIGURE 5.—Temporal changes in daily water temperature, ATPase activity, and fork length of three year-classes
of a Kamloops strain of rainbow trout sampled during rearing in French River State Hatchery. Error bars haveinner tick marks denoting means SEs and outer ‘‘T’’ marks denoting means SDs. Triangles identify dates
when fish were sampled just before stocking trials.
Washington tributary stream moved down to the
main stem, some later moved back upstream, but
many stayed in the main stem until smolting the
following year. Minnesota’s high-gradient tribu-
taries preclude upstream movement by parr, and
these streams are generally much shorter and
smaller than the rivers in their native range. I sug-
gest that natural exploratory movement results in
the premature emigration of parr that quickly reach
the cold, predator-inhabited waters of Lake Su-
perior at a very vulnerable size, and prospects for
survival are poor.
Relationship between ATPase Activity and Other
Measure ments
External features failed to verify smoltification
with certainty, but minimum size, coloration, and
time of year can bracket when smoltification is
imminent. Minimum size represents a size atwhich smoltification may begin under the right
conditions. Minimum sizes similar to those found
in this study (71 mm FL for chinook salmon, 140–
150 mm FL for rainbow trout) have been reported
for age-0 chinook salmon (80 mm FL; Ewing et
al. 1979) and for steelhead (140–160 mm total
length; Chrisp and Bjornn 1978). Once hatchery
fish exceed this size, stocking should be consid-
ered if homing to locations away from the hatchery
is desired, especially if the hatchery outflow enters
a stream that will be accessible to the stocked fish
when they mature. Stream-reared age 2 steelhead
displayed elevated ATPase activity levels at small-
er sizes than hatchery yearling steelhead and Kam-
loops, which may be related to the age differential
or exposure to flowing water and other natural
stimuli in the stream.
Nearly all hatchery Kamloops larger than 260
mm FL, 200 g, and 60 mm body depth and held
into late summer regressed to a presmolt condition,
revealing the transient nature of smoltification in
fish that are retained in a hatchery for some weeksafter the usual time of migration (Fessler and Wag-
ner 1969; Zaugg and McLain 1972; Hoar 1976;
Pascual et al. 1995). Not surprisingly, all but one
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922 NEGUS
FIGURE 6.—ATPase activity levels versus fork length of individual migratory rainbow trout (two strains reared
in the hatchery and two groups reared in streams), including samples collected in all years but separated by three
body color characteristics (banded, intermediate, and silver). The hatchery-reared fish were tested during the period
of hatchery residency up until the time of stocking; the stream-reared fish were tested at the time of capture in
smolt traps. Vertical lines indicate minimum sizes at which elevated ATPase activities were measured in some fish.
stream-reared fish emigrated at smaller sizes. The
percentage of Kamloops larger than 260 mm FL
dropped from 21% before stocking to 14% at re-
capture in the French River, suggesting differential
survival or a reduced tendency toward emigration.
Partridge (1986) found that steelhead larger than
260 mm FL have a tendency to residualize (remain
in the river).
Color change was not necessarily a precise in-
dicator of smolting for individual fish, a finding
similar to those of Zaugg and McLain (1972) and
Virtanen et al. (1991). Condition factor was not agood predictor of smolting in any of the Minnesota
stocks tested, a finding supported by Beckman et
al. (1999), who stated that changes in condition
factor occur for a variety of reasons unrelated to
smolting. In contrast, condition factor has been
directly correlated with smoltification and emigra-
tion for some steelhead in Washington and Oregon
(Fessler and Wagner 1969; Zaugg and McLain
1972; Beeman et al. 1995; Tipping and Byrne
1996).
Temperatures of 13C and above have been
found to inhibit smoltification in steelhead (Wed-
emeyer et al. 1980), and rearing temperatures of
steelhead and Kamloops at the French River State
Hatchery were kept below 12C at all times, exceptfor Kamloops during summer 1997. In contrast,
stream temperatures frequently exceeded 12C
during the spring emigration period. Elevated
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923DETERMINATION OF SMOLTIFICATION STATUS
TABLE 3.—Samples of chinook salmon, Kamloops,
and steelhead taken after stocking and recapture in the
French River smolt trap. Recapture of emigrating fish
stocked in spring 1999 was curtailed when a severe
flood interrupted trap operation on 4 July. Sodium-po-
tassium-activated adenosine triphosphate (ATPase) ac-tivity is reported in mol Pi·(mg protein)
1·h1 .
Numberstocked
Recaptured
Number PercentStocking
date
Mean ATPase activity
Beforestocking
Afterrecapture
Chinook salmon, age 0
100
50
50
50
98
27
16
28
98
54
32
56
14 May 1997
8 Jun 1998
17 May 1999
26 May 1999
6.0
7.4
6.5
9.1
10.0
13.0
13.2
Steelhead yearlings
50 38 76 1 Jun 1998 12.4 8.3
Kamloops rainbow trout yearlings
50
50
41
7
82
14
17 May 1999
28 Jun 1999
9.2
8.0
10.6
10.1
FIGURE 7.—Temporal changes in daily water temperature, ATPase activity, and fork length of steelhead that had
been stocked as fry in the French River, reared in the stream, and captured at the smolt trap as they emigrated
downriver in spring and fall. Error bars have inner tick marks denoting means SEs and outer ‘‘T’’ marks denotingmeans SDs. Each fish was sampled only once at the time of capture in the trap, then replaced in the river below
the trap. Note that a maximum of 36 fish were sampled per day and that we eliminated small fish (80 mm FL)
when larger fish were available.
ATPase activities were not found in hatcher y Kam-
loops after the temperature exceeded 12C, and
ATPase activities of stream-reared steelhead were
depressed in June and in the fall following periods
of elevated temperatures. The elevated tempera-tures used for rearing some chinook salmon in this
study ranged from 10C to 12C, which is reported
to be favorable for growth and survival of this
species (Beckman et al. 1999).
Smoltification in age-0 chinook salmon appears
to be related to spring rearing temperature, al-
though other studies have implicated spring
growth rate as a controlling factor for yearling
chinook salmon in Washington and Oregon (Beck-
man et al. 1998, 1999). Although the Minnesota
chinook salmon reared at warmer temperatures
tended to grow faster, even the largest (fast grow-
ing) fish in cooler water had low ATPase activities,
so spring growth rate had less effect than rearing
temperature. Pascual et al. (1995) found that chi-
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924 NEGUS
FIGURE 8.—Temporal changes in daily water temperature, ATPase activity, and fork length of steelhead that had
been naturally spawned in the Knife River, reared in the stream, and captured at the smolt trap as they emigrateddownriver in spring. Error bars have inner tick marks denoting means SEs and outer ‘‘T’’ marks denoting means
SDs. Each fish was sampled only once at the time of capture in the trap, then replaced in the river below the
trap. Note that a maximum of 36 fish were sampled per day and that we eliminated small fish ( 80 mm FL) when
larger fish were available.
TABLE 4.—Mean ATPase activities ( N ; SE) in different color phases of each strain of migratory rainbow trout tested.
Chinook salmon have not been included here because color differentiation was more ambiguous.
StrainRearinglocation
Color phase
Banded Intermediate Silver
SteelheadKamloops
Steelhead (fry-stocked)
Wild steelhead
HatcheryHatchery
French River
Knife River
5.6 (148; 0.197)6.2 (123; 0.261)
5.8 (239; 0.201)
6.1 (53; 0.343)
9.4 (129; 0.317)8.4 (116; 0.260)
8.2 (89; 0.445)
10.6 (41; 0.687)
10.5 (103; 0.384)8.3 (255; 0.207)
12.9 (38; 0.746)
14.7 (27; 0.847)
nook salmon released into the Columbia River dur-
ing low flows and high temperature produced the
lowest straying rates, possibly because fish stayed
in the rivers longer, imprinting was facilitated at
high temperatures, or the odors for olfactory im-
printing were more concentrated. Results of my
study support the possibility of enhanced chinook
salmon smoltification at 10–12C.
Stream-reared steelhead emigrated primarily
from early May through June, so this may be the
most appropriate time for stocking. Emigration
timing by naturally spawned chinook salmon in
Minnesota waters is unknown (and spawning hab-
itat is limited), but chinook salmon in the Brule
River, a Lake Superior tributary in Wisconsin,
smoltify primarily in May and June of their first
year (DuBois and Pratt 1994).
Management Implications
Hatchery chinook salmon and migratory rain-
bow trout that exceed minimum sizes and have
intermediate or silver coloration in May or June
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925DETERMINATION OF SMOLTIFICATION STATUS
FIGURE 9.—Stream-reared (naturally spawned) steel-
head emigrants captured in the Little Knife River smolt
trap. Each point represents the total number of fish from
each year-class (1987–1999) that emigrated at ages 1 or
2 (Duluth Area Fisheries file data).
probably will smolt within a short time. Fish that
fit these criteria should be stocked as soon as pos-
sible in the location to which imprinting is desired,
especially if returns to remote locations (away
from the hatchery outflow) are desired. Stockingsmaller fish at remote locations and larger fish at
the hatchery site may promote appropriate im-
printing. These recommendations may be less im-
portant in regions where the hatchery does not
discharge into the stocked watershed. Fish stocked
at remote locations should be placed some distance
upstream to enhance imprinting.
Imprinting to locations away from the hatchery
may be compromised in larger fish that have al-
ready partially imprinted and may have regressed
to a presmolt condition, especially if the season of
normal smolting is past. Growth of migratory rain-
bow trout should be controlled in the hatchery to
avoid production of very large (260 mm FL)
individuals. Upstream stocking of very large mi-
gratory rainbow trout should be avoided because
these fish may tend to residualize.
Chinook salmon smoltification appears to be in-
fluenced to some extent by temperature. Increasing
temperature early in the rearing process could help
to bring 100% of the age-0 fish to the minimum
71 mm FL by May. As the season of smoltification
approaches, chinook salmon that will be stockedin remote locations could then be maintained at 5–
7C to reduce imprinting in the hatchery and
stocked into water about 10–12C to enhance
smoltification and imprinting at that time. Those
destined for stocking at a hatchery location could
be held at 10–12C to enhance smoltification and
imprinting before stocking. Stocking at several lo-
cations or on several days from late May throughearly June may reduce immediate mass emigra-
tions of nonsmolts and smolts.
The ATPase activity assays were a useful tool
for defining the approximate size and time of
smolting in chinook salmon and migratory rain-
bow trout in this study. This knowledge will assist
with the effective use of hatchery stocks, enable
managers to make informed decisions regarding
hatchery supplementation in the presence of nat-
uralized stocks, and provide some explanations re-
garding the fate of juveniles that emigrate from
streams before smolting. This type of informationis needed not only to assist in making biologically
sound management decisions, but also to provide
supporting evidence that helps to explain these de-
cisions to the concerned public.
Acknowledgments
I thank Fred Tureson, manager of the French
River State Fish Hatchery, for suggesting this pro-
ject. Randall Hicks was extremely accommodating
in arranging for laboratory space and equipment
at the University of Minnesota in Duluth and,along with Arun Goyal, provided helpful advice
in developing the laboratory protocols. Thanks are
extended to Robin Schrock for advice and refer-
ence samples that were invaluable for setting up
the ATPase assay procedures. Donald Schliep and
Kenneth Olsen (Duluth Area Fisheries) were very
helpful in providing stocking information, age
data, and stream temperature data. Fred Tureson
and Mark Gottwald at the French River State Fish
Hatchery provided the hatchery fish for samples,
Kenneth Olsen provided fish from the French and
Knife river smolt traps, and Tracy Close (Duluth
Fisheries Research) assisted with sample collec-
tion. Charles Anderson, Darryl Bathel, Tracy
Close, Donald Schreiner, and Paul Wingate re-
viewed earlier drafts of this manuscript. This pro-
ject was funded in part by the Federal Aid in Sport
Fish Restoration (Dingell–Johnson) Program.
Completion Report, Study 654, D-J Project F-26-
R Minnesota. Reference to trade names does not
imply endorsement by the Minnesota Department
of Natural Resources.
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Recommended