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OCR for page 164
7
Habitat Loss
DIMENSIONS OF THE PROBLEM
Salmon habitat in freshwater is defined by physical and chemical character-
istics of the environment during the portion of the life cycle spent in streams,
lakes, or estuaries. It is generally taken to include
· Water quality: temperature, dissolved oxygen, turbidity, nutrients, and
environmental contaminants.
· Properties of flow: velocity, turbulence, and discharge.
· Geological and topographic features of the stream and its valley: width
and depth, streambed roughness, particle size composition, riffle and pool fre-
quency, and floodplain characteristics.
· Cover: shading, interstitial hiding spaces, undercut banks and ledges,
woody debris and aquatic vegetation.
For many of those features, streamside vegetation plays an important direct
and indirect role in affecting local habitat characteristics. Some biotic compo-
nents of the environment are influenced by physical habitat conditions, including
prey, predators, competitors, and pathogens.
In this report, altered habitat is habitat that has been changed by human
activity but is still accessible to salmon; lost habitat is habitat that used to be
accessible but is no longer. Habitat alteration and loss that lead to reduced
salmon production can occur when either of two conditions exists: anthropo-
genic perturbations transform freshwater spawning or rearing habitat to an un
164
OCR for page 165
HABITAT LOSS
165
natural or otherwise unproductive state or human intervention prevents natural
disturbances from creating or maintaining habitat that is important for salmon
production. Although most anthropogenic changes in habitat result in impair-
ment of the productivity of aquatic ecosystems (Reice et al. l990J, some lead to
increased production by improving survival or growth of one or more life-history
phases. And some types of alterations do not directly affect salmon habitat but
cause changes in the species composition of the aquatic community that might or
might not be favorable to salmon (Reeves et al. 1987) or shift conditions from
those favorable to one salmon species to those favorable to another (Lichatowich
19893. The important point is that habitat can be altered by the direct effects of
human perturbations and by human prevention of natural disturbances (Sousa
1984, Wissmar and Swanson 1990~. Either can impair salmon production, espe-
cially when its spatial or temporal scale differs fundamentally from that of the
natural disturbance regime of an area.
Habitat alterations can have positive or negative outcomes that are often
difficult to predict. For example, removal of streamside vegetation, a frequent
consequence of human alteration of riparian zones, results in increases in solar
radiation and water temperature. Higher light levels and warmer water can
promote algal growth (Gregory et al. 1987), which leads to increased invertebrate
production and more food for rearing salmon (Hawkins et al. 1983~. Higher light
levels also tend to enhance foraging efficiency of stream-dwelling salmon
(Wilzbach 19853. The resulting increased growth rates might confer improved
overwinter survival and increased smell size (Holtby and Scrivener 19893. Large
smelts, in turn, might be better able to escape predation in nearshore environ-
ments and have higher survival rates at sea (Pearcy 19921. All those processes
potentially improve productivity. However, increased temperatures have also
been shown to reduce growth efficiency when food is scarce (Brett et al. 1969)
and to favor competitive dominance of cyprinid fishes, such as redside shiners
(Richardsonius balteatus), over salmon (Reeves et al. 19871. And outcomes can
be complicated by the presence of exotic species or pathogens, which also tend to
be favored by higher temperatures (Li et al. 1987~. Those processes potentially
limit salmon production.
Because so many physical and biological factors, of which temperature is
only one, are influenced by the removal of streamside vegetation and because
interactions between these factors are still poorly understood, predictions of the
specific consequences for salmon of altering stream temperatures to salmon are
often prone to error. Models of the impact of habitat change on salmon generally
suffer from an inability to predict the consequences of interacting ecological
processes (Mathur et al. 1985, Fausch et al. 19883; this is especially true when
models are extrapolated to geographical regions beyond those in which their
quantitative relationships were developed (Shirvell 19891.
Because habitat loss is widely acknowledged to have contributed to the
decline of virtually every species of Pacific salmon in western North America
OCR for page 166
66
UPSTREAM: SALMON AND SOCIETY IN THE PACIFIC NORTHWEST
(Nehlsen et al. 1991), the lack of precise knowledge of relationships between
various types of habitat change and salmon populations need not be a barrier to
improved environmental management. Different land uses (e.g., forestry, agri-
culture, grazing, mining, and urban and industrial development' are practiced at
different locations in a river basin, but they share some effects with respect to
habitat alteration and loss. This chapter identifies some important types of habi-
tat alteration and loss, discusses how these changes influence the functioning of
aquatic ecosystems, and identifies specific consequences for salmon.
NATURAL VERSUS ANTHROPOGENIC DISTURBANCES
AND WATERSHED PRODUCTIVITY
Natural disturbances play a crucial role in the various life-history phases of
salmon. Pacific salmon evolved in freshwater environments that included a
variety of natural disturbances, including seasonal high flows and floods, gla-
ciers, droughts, wildfires, volcanism, landslides and debris flows, and seasonally
extreme temperatures. Their adaptations to life in frequently disturbed freshwa-
ter ecosystems reflect, in part, a need to cope with unusual events. Such adapta-
tions include relatively high fecundity and large eggs, which permit extended
intragravel residence by alevins during periods of unfavorable stream conditions;
excellent swimming abilities of both juveniles and adults; occasional straying
from natal streams by adults; and differentiation into locally adapted populations.
Salmon with prolonged freshwater life cycles appear to be somewhat more flex-
ible in their habitat requirements than those with abbreviated or lacustrine fresh-
water life cycles (Miller and Brannon 1982~. For example, Reimers (1973)
identified five distinct life-history strategies involving different periods of river-
ine and estuarine residence in a single population of fall chinook salmon in
southern Oregon (Table 7-1~. Multiple life-history strategies within populations
might be an effective means of hedging against unusual events.
Not all disturbances result in diminished salmon production. Some cause
short-term population declines but ultimately lead to increased productivity con-
currently with habitat and trophic recovery (Gregory et al. 1987, Schlosser 1991~.
Natural disturbances can alter habitat in ways that stimulate salmon production
and maintain environmental heterogeneity (Neiman et al. 19921. Wildfires and
some types of soil disturbances increase nutrient availability and so enhance
primary production (Walstad et al. 1990~. Floods entrain particulate organic
matter and both large and small woody debris from riparian zones. High flows
cleanse spawning gravel of fine sediment and scour new pools. Wildfires open
forest canopies, provide large woody debris, and create opportunities for early
successional plant communities in riparian zones (Agee 19931. Windstorms and
windthrow provide recruitment of large woody debris and increase the complex-
ity of local habitats. Disturbances of many types increase the transport of nutri-
ents. organic matter, and large woody debris to estuaries (Sibert 1979, Simenstad
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HABITAT LOSS
TABLE 7-1 Major Variations in Fall Chinook Salmon Life Histories in Sixes
Rivera
167
Life-History
Variation
Description
1
2
3
4
Emerge from gravel? move directly downstream through main river and
estuary and into ocean within few weeks.
Emerge from gravel, move into main river (or possibly stay in tributaries)
for rearing until early summer, move into estuary for short period, and
finally move into ocean before period of high productivity in estuary during
late summer and autumn.
Emerge from gravel, move into main river (or possibly stay in tributaries)
for rearing until early summer, move into estuary for extended rearing, and
finally enter ocean after experiencing improved growth in estuary during
late summer and autumn.
Emerge from gravel, stay in tributary streams (or, rarely, in main river)
until autumn rains, and then move directly to ocean.
Emerge from ravel; stay in tributary streams (or, rarely, in main river)
through summer, autumn, and winter, and then enter ocean during next
. .
spring as yearlings.
aA coastal Oregon stream.
Source: Reimers 1973.
et al. 19821. All of those processes are important to maintaining fish production
in aquatic ecosystems (Gregory et al. 1991) and are necessary for normal ecosys-
tem functions and diverse aquatic communities (Poff and Ward 19901. Although
a large natural disturbance can cause a temporary decline in salmon populations,
productivity might rebound to exceed predisturbance levels for extended periods
(Bisson et al. 1988~. Some types of natural disturbances that have beneficial
long-term effects on salmon habitat, such as floods and wildfires, damage prop-
erty and threaten lives and so are aggressively controlled and suppressed. It is
therefore important to view human activity not only as a cause of habitat change,
but also as potentially hindering natural disturbance patterns and recovery pro-
cesses from creating and maintaining productive and diverse habitat.
Productivity declines when habitat alteration and loss impair the successful
completion of life-history stages in the context of a watershed's landscape, its
natural disturbance regime, and its anthropogenic changes. If a salmon popula-
tion exists close to the environmental tolerance limits of its species for ex-
ample, at the edge of its range either geographically or with respect to riverine
environmental gradients relatively minor changes in key habitat characteristics
resulting from natural climatic events or from human activity can influence popu
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68
UPSTREAM: SALMON AND SOCIETY IN THE PACIFIC NORTHWEST
ration viability or expression of full evolutionary potential (Table 7-2 and 7-3~.
Many of the known extinctions of salmon populations over the last century have
occurred near the edges of geographical ranges (Nehlsen et al. 1991, The Wilder-
ness Society 1993), and many of these have apparently been caused or accentu-
ated by human-related habitat losses. Understanding the effects of habitat alter-
ation must include considering changes in the context of an area's natural
disturbance regime (Table 7-2J.
Habitat disturbances can be "cumulative" in the sense that different factors
acting sequentially or concurrently can limit population size or growth during
different phases of freshwater and estuarine rearing periods (Elliott 1985~. To
some extent, populations can adjust to alteration in or loss of habitat in a compen-
satory fashion; after a period of decreased survival, reduction in competition can
lead to increased survival or growth (Chapman 19663. However, not all factors
can be compensated in this manner and interactions between different types of
habitat change may exacerbate the damage each would do independently (Niemi
et al. 1990, Hicks et al. 1991~. Furthermore, anthropogenic changes to habitat
may occur so fast that natural selection processes are unable to adjust and com-
pensate.
Human activities change the frequency or magnitude of disturbances (Table
7-2), and result not only in loss of or alteration in habitat but in substantial
changes in natural recovery processes (see Figure 7-la). Natural disturbances
large enough to have an important impact require recovery intervals that might
include periods of high production followed by re-establishment of density-de-
pendent regulating mechanisms and biological controls that cause a return to
predisturbance levels. There have been relatively few long-term studies of stream-
dwelling salmon after large natural disturbances (Hanson and Waters 1974, Wa-
ters 1983, Elliott 1985, Bisson et al. 1988), but available evidence suggests that
10 years or more might pass after a large disturbance before salmon populations
return to the normal range of predisturbance abundance.
Frequent anthropogenic perturbations of various intensity superimposed on
natural regimes of less-frequent disturbances (Table 7-2) can hinder recovery
processes and prevent populations from returning to their former abundance (Fig-
ure 7-11. Such perturbations gradually "ratchet" populations downward, a pat-
tern typical of salmon populations in areas of progressive encroachment on ripar-
ian zones or areas with chronic input of sediment (Cederholm et al. 1981~.
Frequent, relatively small perturbations tend to increase the year-to-year variabil-
ity of salmon populations. Hartman and Scrivener (1990) concluded that popula-
tion instability was one of the most serious long-term consequences of logging
for coho and chum salmon in Carnation Creek, British Columbia. Characteristic
declines occur because populations do not have time to recover fully before the
next large disturbance.
Very large anthropogenic impacts can cause so much damage to salmon
populations or their habitat that abundance declines precipitously and does not
OCR for page 169
HABITAT LOSS
169
recover (Figure 7-11. Such changes are characteristic of extensive habitat losses
that might occur, for example, if a large portion of a river system were blocked.
Sockeye salmon populations in the Fraser River underwent a major crash in
1913-1914 when rockslides caused by railroad construction in the canyon at
Hell's Gate blocked much of the upper river, including most of the spawning
grounds. Sockeye and other salmon that used the upper Fraser River remained at
critically low densities until construction of fishways in the 1930s (Ricker 19875.
Damage to or loss of habitat was so great that natural recovery was precluded
until the fishways were completed. Many of the stock extinctions noted by
Nehlsen et al. (1991) resulted from similar very large anthropogenic perturba-
tions.
The spatial and temporal scales of anthropogenic habitat alterations that are
imposed on salmon populations often differ in both frequency and magnitude
from natural disturbance regimes. It is the natural disturbance regimes to which
local populations are adapted and that have historically powered the creation of
new, productive habitat: These characteristics must be retained or replicated if
freshwater salmon habitat is to be sustained (Hill et al. 19913.
SEDIMENTATION
Sediment can enter watercourses by various mechanisms, and inputs can be
chronic or episodic. Mobilization of soil particles through surface and gully
erosion delivers small particles (fine sediment) to the stream network. Surface
erosion is normally associated with precipitation but can occur chronically if
human activities generate continuous runoff of sediment-rich water to streams.
The erosion of large volumes of hillslope material, a process termed mass ero-
sion, occurs when large upper soil movements (often rapid), such as landslides,
and deeply seated slope failures, such as earth slumps, deliver coarse and fine
sediment, large woody debris, and fine organic matter to streams.
.
Both surface erosion and mass erosion are normal processes (Leopold et al.
19641; their frequency depends mostly on the geology and erosiveness of soils
and underlying rock and on the intensity and duration of rainfall and snow melt
(Swanson et al. 19875. Some areas have naturally high erosion rates; examples
include sandstone-dominated coastal river basins in northern California and west-
ern Oregon, granitic sediments in northern and central Idaho, and glacial-lacus-
trine deposits in northwestern Washington. That kind of area is often among the
most sensitive to erosion caused by anthropogenic perturbations, such as logging
and road building (Figure 7-21.
Improvements in road-construction and logging methods can reduce erosion
rates. Rice (1992) documented an 80% reduction in mass erosion from forest
roads and about a 40% reduction in mass erosion from logged areas in northern
California due to improvements in forest practices beginning in the middle 1970s.
However, the potential for continued alteration in and loss of salmon habitat
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70
UPSTREAM: SALMON AND SOCIETY IN THE PACIFIC NORTHWEST
TABLE 7-2 Approximate Occurrence Rates of Different Types of Natural and
Approximate Recurrence Interval (years)
Type of Disturbance
Natural Anthropogenic
Daily to weekly precipitation and
discharge patterns
Seasonal precipitation and discharge;
moderate storms; ice formation
0.01 - 0.1 0.001 - 0.
0.1 - 1.0 0.01 - 1.0
Major floods; storms; rain-on-snow events 10 - 100 1 - 50
Debris avalanches and debris torrents
1 Go - 1 coo 20 - 200
OCR for page 171
HABITAT LOSS
Anthropogenic Disturbances
171
Physical and Chemical Factors Influenced by
the Disturbance
Habitat Effects
Stream discharge; channel width and depth;
storage and transport of fine particulate
organic matter; fine sediment transport and
deposition; nutrient concentrations; water
current velocity
Bank-full flows; moderate channel erosion;
high base-flow erosion; increased mobility
of sediment and woody debris; local
damming and flooding; sediment transport
by anchor ice; gouging of stream bed by
ice movement; reduced winter flows with
extensive freezing; seasonal nutrient
concentrations
Inputs of sediment, organic matter and
woody debris from hillslopes, riparian
zones and streambanks; localized scour
and fill of streambeds; lateral channel
movement; streambed mobilization resulting
in redistribution of coarse sediment ant!
flushing of fine sediment; redistribution
of large woody debris; inundation of
floodplains; transport of organic matter
and large woody debris t`' estuaries
Large, short-term increases in sediment and
large woody debris inputs; extensive
channel scour; large-scale movement and
redistribution of substrate, fine particulate
organic matter and large woody debris;
damming and obstruction of channels at
the terminus of the torrent track;
accelerated streambank erosion, resulting
in channel widening; destruction of riparian
vegetation; very large short-term increase
in suspended sediment; subsequent summer
temperature increases from vegetative
canopy removal
Minor alteration of particle sizes in spawning
~ravels; minor variations in rearing habitat;
minor temperature change; altered
turbidity; altered primary productivity
Changes in frequencies of riffles and pools;
changes in particle sizes in spawning
gravels; increased channel width; flooding
of side channels; removal (or sometimes
addition of) cover; relocation of holding
areas. In areas affected by ice: decreased
water temperatures; lower primary and
secondary productivity; egg dewatering
or scour during anchor ice formation and
breakup
Changes in the frequencies of riffles and
pools; formation of large log jams; burial
of some spawning sites but creation of new
areas suitable for spawning; increased
amounts of fine particular organic matter
for processing by the benthic community,
resulting in increased secondary
production; destruction or creation of
side-channels along the floodplain;
increased secondary production and cover
habitat in estuaries
Extensive loss of pool habitat in the torrent
track; loss of spawning gravels; loss of
habitat complexity along edge of stream;
destruction of side-channels and other
overwintering areas; creation of new cover
in the terminal debris dam; creation of
new spawning areas in the sediment terrace
upstream from the debris dam; short-term
loss of aquatic invertebrates; possible
damage to gills from heavy suspended
sediment load; increased primary
production
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172
TABLE 7-2 Continued
UPSTREAM: SALMON AND SOCIETY IN THE PACIFIC NORTHWEST
Approximate Recurrence Interval (years)
Natural Anthropogenic
Type of Disturbance
Beaver activity
Major disturbances to vegetation
Windthrow
Wildfire
Insects and disease
Slumps and earthflows
5- 100 0
(removal of beavers)
100 - 500 50 - 150
(buffer strip blow-down)
100 - 750 50 - 150
(timber harvest rotation)
100 - 500 50 - 150
(timber harvest rotation)
1 00 - 1 ?0OO 50 _00
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HABITAT LOSS
173
Physical and Chemical Factors Influenced by
the Disturbance
Habitat effects
Channel damming; obstruction and redirection
of channel flow; flooding of streambanks
and side-channels; entrainment of trees from
riparian zone; creation of large depositional
areas for fine sediment; conditions that
pr~'m<.~:e anaerobic decomposition and
denitrification, resulting in nutrient
enrichment downstream from the pond
Increased sediment delivery to channels;
decreased litterfall; increased inputs of
large woody debris; decreased riparian
canopy; increased retention of sediment
and fine organic matter; reduced litterfall
Increased sediment delivery to channels;
inputs of large woody debris; loss of
riparian canopy and vegetative cover;
short-term increase in fine particulate
organic matter and nutrients; decreased
litterfall; increased peak discharge;
short-term increase in summer flows from
reduced evapotranspiration; short-term
increase in biochemical oxygen demand in
stream substrate
Inputs of large woody debris; loss of
. . .
Spartan canopy and vegetative cover;
decreased litterfall; short-term increase in
summer flows from reduced evapo-
transpiration
Low-level, long-term contributions of
sediment and large woody debris to streams;
partial blockage of channel; local baselevel
constriction below point of entry; shifts in
channel configuration; long-term source of
nutrients
Enhanced rearing and overwintering habitat;
increased water volumes during low :llows;
refugia during floods; possible blockage to
upstream migration by adults and
juveniles; elevated summer temperatures
and lower winter temperatures; local
reductions in dissolved oxygen, including
areas under ice in winter; increased
production of lentic invertebrates in pond;
increased primary and secondary
production downstream from pond
Increased pool habitat; localized
sedimentation; increased in-channel cover;
increased summer temperatures and
decreased winter temperatures; creation of
eddies and alcoves along channel mat gins;
increased secondary production
Increased sedimentation of spawning and
rearing habitat; increased pool habitat and
in-channel cover' increased water volume
in summer; increased summer
temperatures and decreased winter
temperatures; increased secondary
production; reduced dissolved oxygen in
spawning gravels; scour of egos and
alevins in spawning gravels
Increased pool habitat and in-channel cover:
increased summer temperatures and
decreased winter temperatures; increased
water volume in summer; increased
primary and secondary production
Sedimentation of spawning gravels; scour of
channels below point of entry;
accumulation of gravels behind
obstructions; possible blockage of fish
migrations; increased pool habitat in
coarse sediment and large woody debris
depositional areas; destruction of side
channels in some areas, creation of new
side channels in others; long-term
maintenance of aquatic productivity
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174
TABLE 7-2 Continued
UPSTREAM: SALMON AND SOCIETY IN THE PACIFIC NORTHWEST
Approximate Recurrence Interval (years)
Type of Disturbance
Natural Anthropogenic
Volcanism
Climate change
100- 1,000
1 ,000 - 1 00,000 1 0 - 1 00
(thermal discharges,
riparian canopy removal,
channelization)
Source: Modified from Swanston 1991.
resulting from forestry activities continues. The FEMAT report (1993) noted that
federally owned forest lands within the range of the northern spotted owl contain
about 180,000 km of roads, a substantial portion of which constitutes potential
threats to riparian and aquatic habitats, mostly through sedimentation. An esti-
mated 250,000 stream crossings are associated with the road system, and most of
the crossing structures might be unable to withstand storms with a recurrence
interval of less than 25 years (FEMAT 19933. Road failures often result in debris
torrents in small streams and can be particularly damaging to coho, steelhead, and
sea-run cutthroat habitat.
Increased erosion from land use is not limited to the relatively steep forested
terrain of the Pacific Northwest. For example, glacial sediment deposits in east-
ern Oregon and Washington are widely farmed for the production of dryland
crops. However, because extensive areas are left fallow (i.e., barren of vegetative
cover) each winter, winter rainfall, particularly on frozen soils, causes much
surface erosion and sediment movement to streams. Similarly, urbanization,
mining, excessive grazing, and other land uses can increase sediment production
well beyond background levels.
Increased erosion can impair the reproductive success of salmon in several
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HABITAT LOSS
TABLE 7-7 Current Requirements for Riparian Protection on State-Owned
and Privately Owned Forests in California, Oregon, and Washington
193
Stream Classification
Minimum Riparian Protection
Zone Width Each Side `~f
Stream (m)
Californian
Class I (fish-bearing)
Class II (non-fish-bearing)
Classes III & IV (no aquatic life)
OregonC
Type F (fish use or fish and domestic use together
Type D (domestic use only)
Type N (no fish or domestic use)
Washingtonf
Types I-III (fish-bearing)
Type IV (non-fish-bearing)
Type V (intermittent or ephemeral)
23-46b
15 30b
None
15 30d
6-2ld
o-2le
7-30g
Noneh
None
amp to 50% of overstory and 50% of understory may be removed; exceptions for greater removal
are given.
bDetermined by slope steepness.
CThe Oregon Rules permit timber harvest within npanan management zones as long as conifer
basal requirements are met.
dDifferent ripanan management area widths are specified for small, medium, and large Type F
and Type D streams.
eFor some small Type N streams, understory vegetation and nonmerchantable conifers must be
left within 3 m of the stream.
fWidths of npar~an zones along Washington's streams are determined by process called watershed
analysis; some timber harvest is allowed within r~par~an management areas.
"Designed to recruit, on average, 70% of historical levels of large woody debris.
hVegetative buffers may be required along lower reaches of Type IV waters for temperature
protection and to buffer streams from applications of forest fertilizers.
Source: FEMAT 1993.
ranges (e.g., Burns 19711. Unless food is extremely plentiful (a situation that
rarely occurs in nutrient-poor coastal river systems', higher temperatures de-
crease lower salmon growth efficiencies (Brett et al. 1969, Bisson and Davis
1976, Wurtsbaugh and Davis 1977), which must be offset by increased food
production for temperature increases to be beneficial.
Increased temperatures can influence the migratory behavior of spawning
adults. Bjornn and Reiser (1991) cited several studies that show delays in the
upstream movement of sockeye, chinook, and steelhead adults because natal
streams became too warm. Berman and Quinn (1991) found that adult chinook
salmon behaviorally regulated their body temperature by pausing in areas of cool
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94
UPSTREAM: SALMON AND SOCIETY IN THE PACIFIC NORTHWEST
water during upstream migrations; they concluded that reduced availability of
cool areas could potentially reduce spawning success.
Altered thermal regimes might have an even greater effect on the production
of salmon through effects on interspecific interactions between salmon and non-
salmon fishes, principally competition and predation. Reeves et al. (1987) dem-
onstrated that juvenile steelhead in laboratory streams were aggressively dis-
placed from preferred foraging sites by juvenile redside shiners, a species of
minnow, when experimentally increased temperatures became physiologically
less favorable for steelhead. In a forested river system in southern Oregon, they
found that the outcome of steelhead-redside shiner competition followed a ther-
mal gradient in which stream warming resulted in the retreat of steelhead to
higher-elevation, cooler tributaries.
Many river systems in the Pacific Northwest contain a variety of introduced
species that originated in warmer waters in eastern North America (Li et al.
1987), which were planted to provide a greater diversity of recreational angling
opportunities. Their effects on native salmon populations are often poorly under-
stood, but many species are known to prey on young salmon or compete with
them for food or rearing sites at some stage in their life history. Temperature
increases whether caused by riparian canopy removal, water impoundment,
agricultural and urban runoff, or heated industrial discharges-create conditions
favorable to many warm-water game species and might enable them to gain a
competitive advantage or facilitate their predation on juvenile salmon.
Altered thermal regimes can change other characteristics of habitat in
streams, rivers, and estuaries by altering the structure of plant and invertebrate
communities (Bisson and Davis 1976~. Aquatic plants and macroinvertebrates
have specific temperature preferences. Changes in thermal regime can alter
species composition in ways that might or might not be favorable to salmon
production.
DECREASED LARGE WOODY DEBRIS
Perhaps no other structural component of the environment is as important to
salmon habitat as is large woody debris, particularly in coastal watersheds. Nu-
merous reviews of the biological role of large woody debris in streams in the
Pacific Northwest (e.g., Harmon et al. 1986, Bisson et al. 1987, Gregory et al.
l991J have concluded that woody debris plays a key role in physical habitat
formation, in sediment and organic-matter storage, and in maintaining a high
degree of spatial heterogeneity ("habitat complexity") in stream channels. Loss
of large woody debris from streams usually diminishes habitat quality and re-
duces carrying capacity for rearing salmon during all or part of the year (Hicks et
al. 1991~. As with temperature, the exact manifestation of the effects of woody-
debris loss on salmon is often difficult to predict.
Two general trends with respect to loss of woody debris are clear. First, the
OCR for page 195
HABITAT LOSS
195
distribution and abundance of large woody debris have been extensively altered
in most river systems. Headwater streams have lost woody debris through sev-
eral processes, most related to logging activity. Years of splash damming, a
method of floating logs from upland logging sites to downstream mills, have
scoured channels and removed much of the instream woody debris that was
present. Sedell and Luchessa (1982) found that some coastal river systems in
Oregon and Washington contained hundreds of active splash dams from the early
to middle 1900s. In some areas without splash dams, increased frequency of
landslides caused large debris torrents (Figure 7-2), scoured stream channels, and
created conditions similar to those resulting from splash dams and log drives.
When it became apparent that accelerated hillslope erosion had caused numerous
debris torrents culminating in large, impassable logjams, fishery management
agencies in the middle 1900s undertook aggressive programs of debris removal to
facilitate adult salmon spawning migrations. In addition to removing the log-
jams, stream-cleaning crews often removed large woody debris after logging
even when there was little evidence that the debris actually constituted a migra-
tion barrier (Narver 1971 J.
Harvest of timber from riparian zones in coastal and western Cascade water-
sheds created ideal conditions for early-successional tree species, such as red
alder, which replaced late-successional conifers as the dominant form of riparian
vegetation over large areas (Kauffman 1988~. Recruitment of new debris to
streams from alder-dominated riparian zones was more rapid than from conifer-
dominated stands, but the hardwood debris was smaller, was more prone to
breakage, and decomposed faster than conifer debris (Bilby 1988), so streams in
second-growth forests became progressively debris-impoverished after removal
of the old-growth stand (Grette 1985, Veldhuisen 19901. Rotational harvest ages
of forests on many industrial forest lands (40-60 years) have been short enough to
preclude re-establishment of dominant conifers in rip arian zones (Andrus et al.
19883. The combined effects of those anthropogenic perturbations have led to a
large-scale reduction in the quantity and quality of large woody debris in many
forested headwater tributaries and to a substantial decline in woody debris floated
downstream to lowland streams (Sedell et al. 1988~.
Mainstem rivers and estuaries historically also contained great amounts of
large woody debris (Gonor et al. 1988J. Some of the debris was produced in
uplands and fluvially transported to depositional sites along rivers and their flood-
plains, but woody debris was also recruited from riparian zones adjacent to rivers
and estuaries and entered channels through natural processes of floods, wind-
storms, fires, and beaver activity. Often, the lower reaches of rivers contained
massive accumulations of debris that formed huge drift dams. Much of the wood
in Pacific Northwest river systems was removed for navigational improvements
and flood control in the late 1800s and early 1900s; for example, 368 km of the
Sacramento River, 88 km of the Willamette River, and 24 km of the Chehalis
River (Washington) were cleaned for river navigation from 1867 to 1912 (Sedell
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96
UPSTREAM: SALMON AND SOCIETY IN THE PACIFIC NORTHWEST
et al., 19901. Rivers and estuaries containing large volumes of woody debris
were characterized by spatially complex and diverse channel systems and highly
productive salmon habitat, but many of these areas were lost in the early twenti-
eth century.
The second trend related to alteration of large woody debris abundance has
been simplification of stream channels and loss of pool habitat (Figure 7-3~.
Simplification occurs when loss of small-scale spatial heterogeneity leads to
channel conditions characterized by uniform substrate, depth, and velocity; loss
of sediment and organic-matter retention capacity; elimination of backwaters,
eddies, and side channels; and loss of instream cover. In other areas of North
America where habitat simplification has taken place, streams support fewer
species and are less resistant to community disruption from natural disturbances
(Kerr et al. 1985, Schlosser 19911. Pacific Northwest streams have fewer species
than most other regions (Moyle and Herbold 1987) but have a great diversity of
locally adapted populations. Streams with simplified channels usually contain
fewer species than streams with structurally complex channels (Bisson et al.
1992), or they might be suitable for one age group but not multiple age groups-
an important factor for salmon rearing two years or more in streams. Reeves et
al. (1993) demonstrated that Oregon coastal watersheds with histories of forest
management had lower salmon diversity than unmanaged watersheds. Typically,
simplified channels with scarce woody debris support abundant populations of
underyearling salmon but contain few yearling and older fish (Bisson and Sedell
1984, Hartman and Scrivener 1990, Reeves et al.1993~. Reduction in numbers of
older salmon in simplified streams is often related to loss of pool habitat and
winter cover resulting from elimination of large woody debris (Bisson et al.
1987~. Of the 43 intermediate-size tributaries with histories of forest manage-
ment examined in the FEMAT report (1993) for western Oregon and Washing-
ton, two-thirds had modal pool frequencies below the range of frequencies be-
lieved to have existed historically (generally 25-60%~. In about half the
watersheds, pools comprised less than 20% of channel areas.
MIGRATION BARRIERS
Dams are an important class of migration barriers and are discussed in Chap-
ter 9. Many smaller barriers to migration are probably unknown. Nehlsen et al.
(1991) noted that a substantial fraction of 106 stock extinctions might have
resulted from migration blockages. They quoted from a story told by a Twana
Indian who was born about 1865 but referred to the extinction of a sockeye run in
southern Puget Sound in 1852:
There were some sockeye in Mason Lake, south of Hood Canal Puget Sound
area. These ran up Sherwood Creek from Allyn on Case Inlet. They'd hang
around the lake till ripe, then run up the creeks from there. The Squaxon got
them with a weir in Sherwood Creek. Finally a pioneer named Sherwood built
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HABITAT LOSS
a little dam in the creek and stopped the fish, and they named the creek after
him.
197
Many small populations might have been extirpated by similar activities in the
1800s and early 1900s. Although historical records are infrequent and usually
rely on anecdotal information from people with little ability to identify salmon
species, small dams probably contributed to the extinction of many local breed-
ing populations, as in the case of Atlantic salmon in eastern North America.
Likewise, landslides that blocked migratory routes eliminated some runs before
the middle 1900s, but records were often poorly kept or lost. In many instances,
nonnative stocks of salmon have been introduced into formerly blocked river
systems (Johnson et al. 1991~.
Rearing habitat of juvenile salmon can also be lost to blockages. Some of the
most productive rearing sites in streams are located in backwaters along the edge
of the channel and in side-channel areas (Sedell et al. 1984, Sedell and Beschta
19913. Highways built next to streams and rivers often disrupt access to these
off-channel sites by physically isolating them from the main channel or by in-
cluding culverts that are impassable for juvenile salmon. Culverts that are de-
signed to pass adult salmon might create speeds that exceed the sustained swim-
ming abilities of juveniles. Furniss et al. (1991) give the sustained swimming
speeds of juveniles of several salmon species; they range from about 20 cm/s for
juvenile coho 5 cm long to 70 crn/s for juvenile sockeye 13 cm long. Water
speeds in many culverts are too great to allow juvenile passage at any time except
during periods of low streamflow; in others the outfall of the culvert might be
suspended too high above the water for juveniles to enter.
Unscreened water diversions constitute a potential migration blockage if
downstream-migrating juvenile salmon are entrained in diverted water. Nichols
(1990) identified over 3,000 unscreened water diversions in Oregon, including
1,300 on coastal rivers, that potentially affected salmon-rearing streams. In
addition to blocking migrations, water withdrawals potentially influence avail-
able rearing habitat. Kaczynski and Palmisano (1993) reported that about 60~o of
the water diversions in Oregon were for irrigation and 20% for urban uses.
Palmisano et al. (1993), citing several studies by the National Marine Fisheries
Service, stated that about 70% of Washington's water diversions lacked proper
screening in the late 1970s and that 30% continued to be improperly screened or
designed even after efforts to improve screening.
WATER POLLUTION
Before enactment of the federal Water Pollution Control Amendments to the
Clean Water Act in the 1970s, fish kills in the United States occurred with some
regularity. Dissolved-oxygen concentrations in Oregon's Willamette River in
the 1940s and 1950s often dropped to anaerobic levels because of sewage and
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l9S
UPSTREAM: SALMON AND SOCIETY IN THE PACIFIC NORTHWEST
industrial discharges, creating an uninhabitable zone along a substantial reach of
the river until nutrient discharges were controlled (Warren 19713. Anaerobic
conditions often occurred in upper Grays Harbor, the estuary of Washington's
Chehalis River system, during the 1920s and 1930s in response to effluents from
two sulfite pulp mills, three municipal sewage-treatment plants, and agricultural
runoff (Eriksen and Townsend 19401. One pulp mill, built in 1928 near the
mouth of the Hoquiam River in Grays Harbor, exerted a biochemical oxygen
demand of 115,000 kg/d, a load equivalent to the raw sewage produced by 1.4
million people (Seller 1989~. Water quality was degraded during low river dis-
charges from May to October in Grays Harbor and was severely damaging to
chinook, coho, and steelhead; but it apparently did not substantially affect chum
salmon, which emigrated earlier than the other species and did not rear in the
upper estuary. Pollution-abatement efforts have reduced sewage and industrial
discharges over the last two decades and the upper estuary is no longer anaerobic
in summer, but experimental releases of smells from hatcheries upstream have
shown that a pollution block still exists in Grays Harbor and that exposure of
smolts to water of poor quality has reduced seawater adaptation, increased infes-
tation by a trematode parasite, lowered disease immunity, and possibly increased
vulnerability to predation by birds and squawfish. Smolts in the Chehalis River
system survive at roughly half the rate of smelts from a nearby, relatively unpol-
luted river (Seller 19891. The case study of Grays Harbor has been well docu-
mented and might be representative of the effects of water-quality degradation on
salmon in lower rivers and estuaries with heavy urban and industrial develop-
ment. Although the concentration of pollutants in wastewaters is now regulated
more strictly than before, the volume of pollutants in water could be equal to or
greater than volumes existing before water-quality laws were enacted. Servisi
(1989) estimated that the volume of wastewater discharged into the Fraser River
had tripled since 1965.
Mining is another source of water pollution in Pacific Northwest rivers.
Nelson et al. (1991), citing a study by the Environmental Protection Agency,
reported that in 1961-1975 at least 10 million fish were killed nationally by
mining-related water pollution, although the number of salmon included was not
given. Nelson et al. (1991) provided a thorough discussion of the different types
of water pollution resulting from mining activity: in western North America,
metals and radionuclides from mining wastes can be highly toxic to salmon;
some highly toxic metals, such as copper and zinc, are also highly synergistic-
their combined effects greatly increase lethality. Furthermore, metals can "bio-
accumulate" in fish tissues, causing long-term stress and posing potential health
threats to people consuming the fish. Sediment can be an important byproduct of
mining activity. In an Idaho tributary of the Salmon River, Konopacky et al.
(1985) found that dredging for rare earths generated 500,000 m3 of sediment,
which smothered important downstream spawning and rearing areas of chinook
salmon and steelhead. Spaulding and Ogden (1968) estimated that hydraulic
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HABITAT LOSS
199
mining for gold in the Boise River, Idaho, generated 116.5 million kilograms of
sediment in 18 months.
LOSS OF REFUGES
Because natural disturbances were an important part of the freshwater envi-
ronment of Pacific salmon, many populations, particularly those at the edges of
the range, underwent periodic expansion and contraction in response to local
extinctions and periods of recolonization. That process fostered genetic diversity
and versatility and enabled salmon to be resilient and locally variable (Scudder
1989~. Over the last century, many small populations have become extinct as a
result of human activity (Nehlsen et al. 1991, Frissell 1993), and the geographical
distribution of some species has become highly fragmented (The Wilderness
Society 19931. Within the confines of a river basin, the ability of salmon to
recolonize areas of local population loss, such as a third- to fourth-order tributary
system, might depend on the presence of refugia (survival areas) containing high
quality habitat and relatively stable populations (Sedell et al. 19901. At present,
watersheds without substantial anthropogenic perturbations are limited almost
solely to national parks and designated wilderness areas, and in the Pacific North-
west states these are usually at elevations above the occurrence of anadromous
salmon. High-quality habitat might exist in small patches within river basins that
are subject to land and water management, but often habitat refugia widely dis-
tributed throughout the system there are insufficient for colonization of nearby
disturbed sites (Sedell et al. 19901.
Concern for the continued viability of Pacific salmon on federally owned
forest lands has led to the establishment of "key watersheds" in which high
priority is given to protecting stream habitat (Figure 7-4) (Reeves and Sedell
1992, FEMAT 19933. Protection of habitat watersheds will be achieved by
controlling erosion and by leaving large riparian buffers adjacent to both fish-
bearing and non-fish-bearing streams. However, the distribution of key water-
shed reserves is limited primarily to headwater drainages where national forests
are located (FEMAT 19933; few are in lower river valleys and coastal lowlands.
In the Pacific Northwest landscape, the latter kinds of environment lack refugia
with high-quality habitat for salmon (Frissell 1993), and there seems to be little
hope of future establishment of such areas without considerable public resolve
and financial commitment.
SUMMARY
Habitat for salmon in Pacific Northwest river basins has been lost or exten-
sively altered over the past 150 years. As a result of alteration and degradation of
biophysical conditions, many salmon populations have been extirpated or de
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200
Key Watersheds
Tier ~
Tier 2
UPSTREAM: SALMON AND SOCIETY IN THE PACIFIC NORTHWEST
?
\
FIGURE 7-4 Key watersheds in the Pacific Northwest identified in the president's
forest plan. The shaded area indicates the known range of the northern spotted owl
(Strix occidentalis caurina). Source: FEMAT 1993.
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HABITAT LOSS
201
pleted. Although the full extent of the modifications of salmon habitat will never
be known, some generalizations are possible.
As a result of human development, the current condition of most river basins
in California, Idaho, Oregon, and Washington is significantly different from the
conditions in which salmon evolved. Stream-habitat alterations and losses asso-
ciated with many types of land uses have included increased sediment loading,
higher and more variable water temperatures, reduced amounts of large woody
debris, reduced and simplified riparian plant communities, new barriers to migra-
tions, lower streamflows during some periods and higher peak flows at other
times, loss of stream-bank integrity, simplified channel structure, and reduced
small-scale habitat heterogeneity. These changes have generally reduced the
productivity of river basins for salmon, although some changes have occasionally
increased production.
Anthropogenic habitat disturbances have often resulted in simultaneous
changes in a wide variety of functions, processes, and habitat characteristics. It
has usually been impossible to identify which habitat changes have had the
greatest effect on salmon. Human-caused disturbances have interacted with natu-
ral ones: for example, effects of widespread removal of riparian vegetation, loss
of ground cover, or decreased stability of hillslopes have sometimes not been
manifested until after a heavy rainfall or snowfall. The simultaneous alteration of
many factors had important implications for habitat rehabilitation. First, it is
important to avoid attempting only to improve individual aspects of habitat (e.g.,
pools in streams) without addressing other aspects that might be equally degraded
and are critical to salmon production. It is not enough to improve a pool in a
stream, for example, if the water is too warm to support salmon. In many cases,
these single-factor approaches are not effective (NRC 1992a). Second, rehabili-
tating watershed processes to the extent possible given human development,
including the re-establishment of riparian functions such as providing shading,
organic matter, and large woody debris-is probably more effective in improving
salmon habitat over the long-term than substituting artificial structures for eco-
logical functions.
Habitat changes caused by human activities have occurred at far different
spatial and temporal scales than natural disturbances in the Pacific Northwest.
These differences between anthropogenic and natural disturbance patterns have
interfered with the abilities of salmon to survive and recover from changes in
their habitat. Anthropogenic perturbations have been significant causes of direct
mortality for juvenile and adult salmon. Examples include excessive sediment
inputs, scouring of reads, creation of migration blockages, high water tempera-
tures, and toxic discharges. Catastrophic habitat loss can also occur as a result of
natural disturbances, but the frequency and spatial scale of natural disturbances-
unlike those of many human-caused disturbances are such that salmon's behav-
ioral and physiological characteristics allow their populations to be resilient.
Human activities have prevented natural disturbance regimes from creating
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202
UPSTREAM: SALMON AND SOCIETYIN THE PACIFIC NORTHWEST
or maintaining productive salmon habitat. In addition to imposing new distur-
bance regimes, the land- and water-management actions of an increasing human
population have systematically prevented natural disturbances from providing
crucial raw materials for productive habitat. Examples include flood control and
wildfire suppression, both of which have interfered with processes that provide
woody debris and nutrients to river systems. In addition, stream processes have
been altered to the extent that they cannot respond to natural disturbances in a
normal manner. Rehabilitating ecologically productive watersheds will require
allowing natural disturbances to occur to the greatest extent possible.
Habitat alteration has changed the outcome of interactions between salmon
and other species. Increased water temperatures, for example, have favored
warm-water species at the expense of cold-water species such as salmon. In
addition, many Pacific Northwest river basins contain nonnative fishes intro-
duced from eastern North America and elsewhere, which are often better adapted
to warm temperatures than salmon and which can prey on or outcompete young
salmon. Some habitat alterations unfavorable for salmon have resulted from
exotic animals or plants.
Habitat protection-especially with respect to riparian zones has been very
uneven across different types of land uses and ownerships. Overall, streams on
public lands receive greater protection than those on private lands; those on
forested lands often receive greater protection than those on agricultural and
range lands; and streams and sloughs in urban and industrial areas have generally
received the least protection. Water-quality requirements also differ according to
the predominant use or according to various federal, state, and local discharge
regulations. There often are large variations in the degree of protection afforded
in different places within a river basin. Habitats on private and public lands are
important to salmon, and rehabilitation programs that focus only on public lands
will be less effective than those involving private lands as well (NRC 1995b).
Therefore, cooperation between private and public landowners is important to
protecting and rehabilitating habitat at a watershed scale. Development of coop-
erative habitat-conservation agreements between public and private landowners
and resource managers will help identify critical habitat areas in need of rehabili-
tation while providing site-specific flexibility for landowners to provide different
types of protection measures (see also Chapter 13 and NRC l995b).
So much habitat has been lost or altered that relatively few areas of high-
quality habitat remain, especially in large river valleys and in coastal lowlands-
typically home to large numbers of people. Therefore, protecting and rehabilitat-
ing enough habitat to provide refuges for salmon and sources for recolonization
of other areas that might be improved in the future will entail operating in a
context of human development in other words, ways must be developed for
people and salmon to live together. Although many of the best remaining sites
are in forested headwaters, nodes of good habitat are also needed in large river
valleys and coastal lowlands. These sites can be identified and locally based
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HABITAT LOSS
203
protection measures can be developed. In some cases, landowner incentives can
be used as a financial carrot in place of the more traditional regulatory stick. An
example of such a program is King County's Waterways 2000, an aquatic conser-
vation program funded from county real-estate taxes. It provides with tax reduc-
tions to landowners for dedicating part of their property to riparian protection,
outright purchases of important greenways and conservation easements, and pub-
lic education emphasizing good stewardship of the county's streams. The initial
phase of the program has already helped to protect good habitats in six important
salmon watersheds in this urban area, which includes Seattle (King County Sur-
face Water Management Division 19959.
Representative terms from entire chapter:
natural disturbances
