Montana's Fish Species of Special Concern
Bull Trout
By
Gary Carnefix, January 2002
Classification
and distribution
Bull trout (Salvelinus
confluentus) are in the family Salmonidae, genus Salvelinus (chars),
which also includes introduced brook trout (S. fontinalis ) and lake
trout (S. namaycush). Bull
trout were first described as a separate species from Dolly Varden (S. malma)
by Cavender (1978), who suggested the species originated in the Columbia River
basin and dispersed following deglaciation since the Pleistocene.
Montana populations are limited to the Columbia and Saskatchewan River
basins. Major bull trout drainages
are the Kootenai and Clark Fork (including Bitterroot, Flathead/Swan and
Blackfoot systems). Metaline Falls (Tom Weaver, MFWP, personal communication) and
Bonnington Falls have isolated Clark Fork and Kootenai River populations from
downstream Columbia basin populations for approximately 10,000+ years (Montana
Bull Trout Restoration Team (MBTRT) 2000).
The St. Mary’s River in the Saskatchewan basin, draining north into
Canada, contains the only bull trout population east of the continental divide
in the United States.
Life
history and habitat requirements
[The following account is taken directly from the Restoration Plan for Bull
Trout in the Clark Fork River Basin and Kootenai River Basin, Montana (MBTRT
2000). Text that is italicized and
bracketed has been added by the author.]
Bull trout are native to streams, rivers, and lakes in northwestern
Montana. They are long-lived fish
that do not reach breeding age until at least five years of age [but rare
reports of maturity as young as age two exist, e.g. Shepard and Graham 1983].
Sub-adult and adult bull trout feed primarily on other fish,
resulting in their being dubbed the "cannibal of Montana's streams"
(Anonymous 1929). Bull trout spawn
in the fall, and their eggs remain up to six inches deep in spawning gravels
until spring, when the fry emerge. Young
bull trout remain in the stream for one to four years, huddled among bottom
rocks and other cover. Bull trout
grow up to lengths of 37 inches and weights as heavy as 20+ pounds.
Sub-adult and adult fluvial bull trout reside in larger streams and
rivers and spawn in smaller tributary streams, whereas adfluvial bull trout
reside in lakes and spawn in tributaries.
Bull trout may have either a resident or migratory life history.
Resident fish usually spend their entire lives in smaller tributaries and
headwater streams. Migratory fish spawn and their progeny rear for one to
several rears in tributary streams before migrating downstream to larger rivers
or lakes where they mature and spend most of their adult life.
Adults migrate back to their natal tributaries to spawn, apparently with
a high degree of fidelity (Swanberg 1996, Kanda et al. 1997; unpublished data).
Bull trout also may migrate during the summer to seek colder water and
during the winter to seek relatively ice free habitats (Jakober 1995)
[free from anchor and frazil ice, but selecting surface-ice-covered sites,
Jakober et al. 1998]. Resident
and migratory bull trout can live together and one life history form can
probably give rise to the other.
This variety of life history strategies is important to the stability and
persistence of populations, but also complicates restoration and conservation
because a diversity of high quality habitats are needed.
When individual habitat components are altered, by human or natural
events, bull trout populations may be negatively impacted.
The following summary
accounts of life history and bull trout habitat requirements were derived from
the report The Relationship Between Land Management Activities and Habitat
Requirements of Bull Trout prepared by the Montana Bull Trout Scientific
Group (MBTSG 1998). More specific
details and references are contained in that report.
Spawning
The majority of migratory bull trout spawning in Montana occurs in a
small percentage of the total stream habitat available.
Spawning takes place between late August and early November, principally
in third and fourth order streams. Spawning
adults use low gradient areas (less than 2%) with gravel/cobble substrate and
water depths between 0.1 and 0.6 meters (4 to 24 inches) and velocities from
0.09 to 0.61 m/sec (0.3 to 2.0 ft./sec). Proximity
of cover for adult fish before and during spawning is an important habitat
component. Spawning tends to be
concentrated in reaches influenced by groundwater, where temperature and flow
conditions may be more stable. The
relationship between groundwater exchange and migratory bull trout spawning, and
the spawning habitat requirements of resident bull trout, require further
investigation.
Incubation
Existing studies suggest that successful incubation of bull trout embryos
requires cold water temperatures below 8°C
(46°F),
gravel/cobble substrate with high permeability to allow water to flow over
incubating eggs, and low levels of fine sediment (particles smaller than 6.35 mm
(0.25 inches) in diameter) that smother eggs and fry.
Eggs are deposited as deep as 25 cm (10 inches) below the streambed
surface, and fry do not emerge until 7 to 8 months later, depending on water
temperature. Spawning adults alter
streambed characteristics during redd construction to improve survival of
embryos, but conditions in redds often degrade during the incubation period.
Mortality of eggs or fry can be caused by scouring during high flows,
freezing during low flows, superimposition of redds, or deposition of fine
sediments or organic materials that smother the eggs or fry.
A significant inverse relationship exists between the percentage of fine
sediment in the incubation environment and bull trout survival to emergence.
Entombment appeared to the largest mortality factor in incubation studies
in the Flathead drainage. Groundwater
influence plays a large role in embryo development and survival by mitigating
mortality factors.
Juvenile Rearing
in Tributary Streams
Basic rearing habitat requirements for juvenile bull trout include cold
summer water temperatures (<15°C
(59°F))
with sufficient surface and groundwater flows.
Warmer temperatures are associated with lower bull trout densities, and
can increase the risk of invasion by other species that could displace, compete
with, or prey on juvenile bull trout. Juvenile
bull trout are generally bottom foragers and rarely stray from cover.
They prefer complex forms of cover that include deep pools, large woody
debris, rocky stream beds, and undercut banks.
High sediment levels and embeddedness can result in decreased rearing
densities. Unembedded cobble/rubble
substrate is preferred for cover and feeding, and also provides invertebrate
production. Highly variable
streamflow, reduction in large woody debris, bedload movement, and other forms
of channel instability can limit the distribution and abundance of juvenile bull
trout.
Subadults and
Adults in Tributary Streams
Habitat characteristics that are important for juvenile bull trout of
migratory populations (low water temperatures, clean cobble-boulder substrates,
and abundant cover) are also important for stream-resident subadults and adults.
However, stream resident adults are more strongly associated with deep
pool habitats than are migratory juveniles.
Movement and
Migration in Tributary Streams
Both migratory and stream-resident bull trout move in response to
developmental and seasonal habitat requirements.
Migratory individuals can move great distances (up to 156 miles [250 km])
among lakes, rivers, and tributary streams in response to spawning, rearing, and
adult habitat needs (Swanberg 1996). Stream-resident
bull trout migrate within tributary stream networks for spawning purposes, as
well as in response to changes in seasonal habitat requirements and conditions.
Open migratory corridors, both within and among tributary streams, larger
rivers and lake systems are critical for maintaining bull trout populations.
Subadults and
Adults in Large Rivers
Most migratory bull trout remain in tributaries for one year or more before
moving into large rivers downstream. After
they reach large river habitats, bull trout can remain there for brief periods,
or for as long as several years, before either moving into lakes or returning to
tributary streams to spawn. During
their river residency, bull trout commonly make long-distance annual or seasonal
movements among various riverine habitats, apparently in search of foraging
opportunities and refuge from warm, low-water conditions in mid-summer and [frazil/anchor]
ice in winter. [Muhlfeld et al.
(2003) found that sub-adult bull trout made diel shifts in winter habitat use in
a partially regulated section of the Flathead River in northwestern Montana;
sub-adult bull trout moved from deep, mid-channel areas during the day, to
shallow low-velocity areas along the channel margins without overhead cover at
night.] Little is known about
these movement patterns among basins, but it is likely that river residency and
migratory behavior in each bull trout stock largely reflects local adaptation to
the specific array of suitable habitats historically available in the basin.
The degree of genetic control of migratory behavior in bull trout is
unknown.
Subadults and
Adults in Lakes
Lakes and reservoirs are critically important to adfluvial bull trout
populations. In six of the 12 bull
trout restoration/conservation areas (Flathead, Swan, South Fork Flathead, Upper
Kootenai, Lower Kootenai, and Lower Clark Fork), large bodies of standing water
form the primary habitat for rearing of subadult migratory bull trout and
provide food and cover for fish to achieve rapid growth and maturation.
Growth rates of juvenile bull trout increase substantially as they enter
large river and lake environments and shift their diet from insects to fish.
Despite the importance of lakes and reservoirs, very limited information
is available range-wide on habitat use by bull trout in these waters.
In general, bull trout appear to be bottom oriented in lakes, but use
relatively shallow zones (less than 40 m; 130 ft), provided water temperatures
there are less than 15°C
(59°F).
During summer, bull trout appear to primarily occupy the upper
hypolimnion of deep lakes, but forage opportunistically in shallower waters.
River/lake transition zones appear to be particularly important habitats.
Introduced species, especially lake trout (S. namaycush) and Mysis
shrimp (Mysis relicta) in combination, have been implicated in
drastically altering the food web where they occur, which has led to declines or
extinction of bull trout in many lakes (McIntyre 1998).
Although poorly understood at this time, habitat conditions in lakes and
reservoirs are potentially critical to persistence of migratory bull trout
populations and require additional investigation.
[End MBTRT 2000 excerpt]
Threats
Factors contributing to declines of bull trout populations include habitat
degradation and loss due to land and water management practices; isolation and
fragmentation of populations by both structural (e.g. dams) and environmental
(e.g. thermal or pollution) barriers; introduction of non-native fishes
resulting in competition, predation and hybridization threats; historical
eradication efforts; poisoning to remove non-game species; historical
overharvest; and ongoing poaching and accidental harvest due to
misidentification (Fraley and Shepard 1989; Meehan and Bjornn 1991; Bond 1992;
Ratliff 1992; Leary et al. 1993; MBTSG 1995; Long 1997; Riehle et al. 1997;
Donald and Stelfox 1997; Schmetterling and Long 1999).
Historically, bull trout were often viewed as a nuisance species due to
their predaceous habits, including being targeted for eradication with bounties,
and were fished commercially with nets on Flathead Lake in the early 1900's
(Brown 1971; Leary et al. 1993). Concerns over
their decline, along with accompanying efforts at protection, began to be raised
in the 1950's (Brown 1971; Fraley 1994). Bull
trout are now viewed as an important indicator species for environmental
disturbance, due to their specific requirements for spawning and rearing habitat
and general sensitivity of each life history stage (Fraley and Shepard 1989).
Loss of the migratory component of bull trout life history diversity, where
populations have become isolated in headwater lakes (Wade Fredenberg, USFWS,
personal communication) or stream reaches further threatens the species’
recovery. Small, isolated
populations face increased extirpation risks as a result of direct impacts of
habitat change, random demographic and environmental variation, and genetic
processes (Goetz 1989; Rieman and McIntyre 1993).
Migratory fish are typically much larger than resident individuals, with
greater fecundity and reproductive potential as a result (Marnell 1985, Goetz
1989). Ratliff and Howell (1992)
speculated that large migratory bull trout may be less likely to hybridize with
smaller brook trout (but see Kitano et al. 1994 for contrary evidence).
Management
Several studies report bull trout local population genetic divergence down to
the geographic scale of adjacent tributaries (Leary et al. 1993; Kanda et al. 1997; Spruell et al. 1999; Taylor et
al. 1999). Based on similar patterns of population genetic structure in
steelhead, Parkinson (1984) suggested that populations in geographically
adjacent streams be managed as separate stocks.
Because of their opportunistic feeding habits and late maturity, bull trout are
vulnerable to overharvest and poaching/accidental harvest, especially during
migrations in tributaries (Leathe and Enk 1985, Long 1997, Schmetterling and
Long 1999, Carnefix 2002). Some
Montana bull trout populations (e.g. Swan, South Fork Flathead, Kootenai,
Flathead and Blackfoot) have responded well to more restrictive angling
regulations or closures (Tom Weaver, MFWP, personal communication), and initial
conservation efforts in Montana focused on such measures.
The first minimum length limit was imposed in 1951 (Long 1997).
By 1981, eight of 33 major North and Middle Fork Flathead River spawning
tributaries were closed to fishing and an 18-inch minimum size limit to protect
pre-spawners in the rivers and Flathead Lake was in effect (Fraley et al. 1981;
Tom Weaver, MFWP, personal communication). Regulations closing all state waters
except Swan Lake and Hungry Horse Reservoir to intentionally fishing for and/or
harvesting bull trout became effective in 1993 (Tom Weaver, MFWP, personal
communication). Harvest is
currently permitted only in Swan Lake. Some
level of poaching (Swanberg 1996; Long 1997) and accidental harvest due to
misidentification (Schmetterling and Long 1999) probably continues to impact
bull trout populations, but is difficult to detect, quantify, prosecute or
prevent. Recent efforts to reduce misidentification include a Bull
Trout Identification and Education webpage at the MFWP website (http://fwp.state.mt.us/bulltroutid/default.htm).
The state of
Montana began development of a bull trout restoration plan in 1993.
The final plan, published in June 2000, identifies 115 bull trout core
areas and connecting “nodal habitat” within twelve Restoration/Conservation
Areas (RCAs); sets goals, objectives and criteria for restoration; outlines
actions to meet those criteria; and establishes a structure to monitor
implementation and evaluate effectiveness of the plan.
The stated goal of the plan is “to ensure the long-term persistence of
complex (all life histories represented), interacting groups of bull trout
distributed across the species’ range and manage for sufficient abundance
within restored RCAs to allow for recreational utilization” (MBTRT 2000).
Bull trout conservation is also a stated goal of the Plum Creek Timber
Company’s Native Fish Habitat Conservation Plan (HCP) (http://www.plumcreek.com/environment/HCP-fish.cfm),
for which agreement was reached with the USFWS in October 2000.
Though approved, this HCP/take permit is currently in litigation.
With “threatened” listing (USFWS 1998), USFWS has
separate responsibility under the Endangered Species Act for development of a
federal recovery plan and designation of critical habitat. A draft Recovery Plan built on the foundation of state
restoration plans (USFWS 2002a, Internet-accessible at http://pacific.fws.gov/bulltrout/recovery/Default.htm)
and proposed critical habitat (USFWS 2002b, http://pacific.fws.gov/bulltrout/criticalhab.htm),
were released in November. Initial
public comment periods for both documents have been extended, to March 29, 2003
(Recovery Plan) and May 12, 2003 (proposed critical habitat).
Although all bull trout within the U.S. are now listed as threatened,
this draft Recovery Plan and proposed critical habitat are organized
hierarchically by “local populations” within “core areas” within
“recovery subunits” within 24 “recovery units” within three (of five)
designated “distinct population segments” (DPSs).
The draft Recovery Plan covers the Klamath basin, Columbia River and St.
Mary-Belly River DPSs. Critical
habitat designation is currently proposed only for the Klamath and Columbia
River DPSs. Proposed critical
habitat is limited to bankfull stream channel width within designated stream
segments and excludes habitat within existing approved HCPs with bull trout
“incidental take” permits. In
Montana, 5,341 stream km (3,319 mi) and 88,051 lake/reservoir ha (217,577 ac)
are proposed as critical habitat, of which 60% is in federal ownership, 1%
tribal, 5% state/local and 34% private. Ten
local populations within four core areas have been identified within the
Kootenai River Recovery Unit in Montana. About
119 local populations distributed among 36 core areas within three Recovery
Subunits (Flathead, Upper and Lower Clark Fork) are identified within Montana in
the Clark Fork Recovery Unit. Nine
local populations within six core areas are identified within Montana in the St.
Mary-Belly River Recovery Unit.
Status and trends
[From Restoration Plan for Bull Trout in the Clark Fork River Basin and Kootenai
River Basin, Montana (MBTRT 2000)]. Bull
trout are still widely distributed, although declines in abundance, the loss of
important life history forms, local extinctions, fragmentation, and isolation of
high-quality habitats are apparent throughout the Columbia River basin (Lee et
al. 1997, Rieman et al. 1997). Although
still widespread, strong or protected populations are less common (Rieman et al.
1997). According to the assessment
of aquatic species and resources prepared for the Interior Columbia river Basin
Ecosystem Management Plan, areas supporting strong populations of bull trout
occur in only six percent of available watersheds (Lee et al. 1997).
Many formerly complex, diverse and connected river systems have been
transformed into a patchwork of fragmented habitats with isolated populations.
This isolation may place the remaining populations at risk of extinction
(Rieman and McIntyre 1993; Lee et al. 1997).
Continued loss of habitat associated with detrimental land use practices
further threatens remaining bull trout populations (Rieman et al. 1997).
In Montana, bull trout are still widely distributed throughout their
historic range, although numbers and distribution have declined during the past
century (Everman 1892; Thomas 1992; MBTSG 1995a-e; MBTSG 1996a-f; Peters 1990;
Weaver 1997). [End MBTRT 2000 excerpt]
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