|
Items in cart [0] |
Questions? Call 888-624-8373 |
| Copyright © 2009. National Academy of Sciences. All rights reserved. Terms of Use and Privacy Statement |
Below are the first 10 and last 10 pages of uncorrected machine-read text (when available) of this chapter, followed by the top 30 algorithmically extracted key phrases from the chapter as a whole.
Intended to provide our own search engines and external engines with highly rich, chapter-representative searchable text on the opening pages of each chapter.
Because it is UNCORRECTED material, please consider the following text as a useful but insufficient proxy for the authoritative book pages.
Do not use for reproduction, copying, pasting, or reading; exclusively for search engines.
OCR for page 28
Salmon Geography and Ecology
~d
INTRODUCTION
Salmon live and travel over a large geographic area. In North America, they
begin their lives in streams as far south as California, as far north as the Arctic
Circle, as far west as the Pacific shore, and as far east as the Continental Divide.
Particular populations migrate from central Idaho more than 1,500 km to the sea;
they might travel many times that distance during the ocean phase of their lives
and return to spawn and die in the same freshwater areas where they hatched.
Thus, during their lives, salmon cross many geographic and human boundaries.
Salmon are parts of ecosystems that cover areas much larger than marked proper-
ties and government jurisdictions. Salmon generation times are usually 2-6 years,
and interdecadal changes in climate influence salmon abundance and production.
Human decisions can be made quickly, but they can have consequences for
irrigation, land-clearing. urbanization, and dams that last for much longer peri-
ods. Conflicts between human wants and the needs of fish occur in a context of
mismatches between time and space scales. To make the mismatches explicit,
this chapter describes the geographic and ecological scales of salmon lives.
The chapter also concerns salmon ecology, the interaction of salmon with
their physical and biological environment. It addresses the state of knowledge on
salmon ecology and raises cautionary notes relevant to plans and expectations for
human interventions. Two aspects are emphasized: ecological interactions that
occur in the river basins and those that occur in the ocean. Salmon are most
visible in river basins, where changes to their environment human-caused and
other are most apparent. But they spend most of their life in the ocean. The
28
OCR for page 29
SALMON GEOGRAPHY AND ECOLOGY
29
ocean can seem distant and constant, but processes going on in the ocean are
probably neither constant nor unimportant with respect to the salmon problem.
Attempts to restore, rehabilitate, or otherwise manage salmon populations will
affect and be affected by abiotic and biotic interactions in both river and ocean
ecosystems.
SALMON LIFE HISTORY AND DISTRIBUTION
The life histories of Pacific anadromous salmon set the geographic stage for
solutions to the problem of their decline. Their life histories have three character-
istics anadromy, homing, and semelparity that are key to considering trends
in abundance and factors responsible for salmon declines; these characteristics
differ among species and individual runs. (See Figure 1-2.)
Anadromy
Anadromous fishes begin their lives in freshwater, migrate to the sea after
some variable period of freshwater residence, feed and attain most of their size at
sea, and then migrate back to freshwater to spawn. Anadromy makes salmon
dependent on a complex of freshwater and marine environments. The large
marine pasture environment supports a much greater biomass of salmon than the
small freshwater environments could possibly support. Freshwater spawning
sites provide a relatively safe haven for egg and larval development.
Homing
Individuals that survive their marine residence and return to spawn almost
invariably spawn in the stream where they began their lives years earlier (Quinn
and Dittman 1992~. This "homing" reduces genetic exchange among salmon
populations. Combined with the action of natural selection in diverse habitats,
homing results in differentiation of populations into distinct units that are repro-
ductively isolated.
Semelparity
Most anadromous salmon die after their first spawning season and thus
spawn only once in their lifetime they are semelparous. Exceptions are cut-
throat and steelhead, some of which migrate back to sea after spawning, and then
return to spawn again after one or more seasons. Semelparity is thought to evolve
in species with a high growth rate and high mortality during long and stressful
migrations; i.e., these species reach a large body size by time of first spawning,
and their probability of surviving more than one migration to the spawning area is
low. A consequence of this life cycle of growing at sea and dying in freshwater
OCR for page 30
30
UPSTREAM: SALMON AND SOCIETY IN THE PACIFIC NORTHWEST
is that the carcasses transport nutrients into comparatively impoverished freshwa-
ter environments, and these nutrients are used by various terrestrial animals
(Cederholm et al. 1989) or cycled through aquatic pathways (Kline et al. 1990,
1993).
Generalized Life Cycle
The salmon life cycle integrates anadromy, homing, and semelparity (Figure
1-21. Generally, in fall or spring, females select sites for spawning, dig crater-
shaped depressions in the stream gravel, and there deposit from a few hundred to
several thousand pea-sized eggs. Males compete to fertilize the eggs, which
females then bury and guard. After about 2 weeks, adults of both sexes die near
the spawning site. The eggs develop in the gravel and hatch several months later.
The larvae, termed alevins, remain buried until their yolk sac is absorbed; they
then emerge as free-swimming fry that either begin feeding in the stream or
migrate from it. Pinks migrate directly to sea, chum do so after a few days or
weeks, chinook after a few months to a year, and coho generally after a year.
Most sockeye populations migrate to a lake for a year before continuing to the
ocean, but some rear in rivers and others migrate to the ocean shortly after
emergence. But some sockeye populations, called kokanee, never migrate to the
ocean but remain in the lakes. Young steelhead and cutthroat usually migrate to
sea after about 2 years in freshwater; some never migrate, but form freshwater
populations. Young salmon (Parr), which are adapted to freshwater, become
prepared to migrate seaward and live in saltwater by a complex developmental
transformation (known as smoltification' that involves physiological, biochemi-
cal, morphological, and behavioral changes.
Anadromous salmon display four patterns of marine distribution that differ
in how far the salmon move from the river of origin. First, juvenile pink, chum,
and sockeye (smelts) enter the ocean and migrate north along the continental
shelf, reaching the northern Gulf of Alaska in late fall (Harts and Dell 19861; they
then migrate south into the open ocean, where they remain until they mature and
return to coastal waters, usually making landfall north of their river of origin.
Second, coho and chinook usually rear in coastal waters, although some migrate
to the open ocean as well. Third, steelhead migrate to the open ocean, but-
unlike pink, chum, and sockeye appear to migrate directly to open ocean as
smelts and return directly to the vicinity of their river of origin, rather than
migrating along the coast (Burgner et al. 1992~. Fourth, cutthroat make relatively
little use of the ocean for feeding, generally spending only the summer at sea near
. . ~ . .
t nelr rivers of ongln.
The duration of marine residence differs among populations even popula-
tions of the same species and among years. Some populations tend to mature at
a later age than others, but ocean conditions that are poor for growth for any
population can delay maturation and return to freshwater. Pink, chum, and coho
OCR for page 31
SALMON GEOGRAPHY AND ECOLOGY
31
generally return to freshwater in the fall shortly before their population-specific
spawning time. Sockeye might return in fall or several months in advance of
spawning. Time differences between return and spawning are highly varied for
chinook and steelhead.
The differences in life history among chinook and steelhead populations are
reflected, in part, by designations that indicate the timing of adult or juvenile
migrations (Healey 1991, Taylor 19901. Chinook are classified by the season
when adults return to spawn. They also are distinguished by their juvenile resi-
dence in freshwater: stream-type chinook spend a year or two in freshwater and
migrate to the sea in the spring of their second or third year of life, whereas
ocean-type chinook migrate to the sea during their first year of life, often after a
few months in the river. Stream-type chinook tend to migrate to open-ocean
waters, whereas ocean-type chinook have a more coastal marine distribution.
Ocean-type chinook also spend much more time in estuaries than do stream-type
chinook. In the Pacific Northwest, stream-type juveniles are often spring-run
adults, and ocean-type juveniles are most often fall-run adults. Fall-run chinook
predominate in the lower reaches of large rivers and in smaller coastal rivers,
spring/summer chinook tend to occur farther inland. Stream-type chinook are
more prevalent in the northern part of the range. Common designations for
steelhead are "summer" and "winter." Summer steelhead return to freshwater
during late summer and fall, spend the winter in rivers, and spawn the following
spring; they typically occur in upper reaches of rivers. Winter steelhead, which
are more prevalent in the lower reaches of rivers, enter freshwater in winter and
spawn soon thereafter.
Individual Species Distributions
Pink Salmon
Pink salmon, the most abundant anadromous salmon in North America, are
relatively scarce in the Pacific Northwest (Heard 19911. Although they spawn in
Puget Sound streams, they are apparently absent in coastal Washington, Oregon,
and California. They generally are fished in nearshore areas as they approach
maturity, which is always reached in their second year. In the southern part of
their range, runs are much larger during odd-numbered years; in the northern part
of their North American range, run sizes tend to be greater in even-numbered
years. Virtually all pink salmon that return to Puget Sound and the Fraser River
do so in odd years.
Sockeye Salmon
Sockeye are the second most abundant anadromous salmon and predominate
in Alaska and British Columbia (Burgner 1991~. They are abundant in the Fraser
OCR for page 32
32
UPSTREAM: SALMON AND SOCIETY IN THE PACIFIC NORTHWEST
River, and significant populations existed in the Columbia River before dam
construction (Chapman 19861. The Columbia River has self-sustaining sockeye
populations in Lake Wenatchee and Lake Osoyoos. One population, which has
been listed as endangered, migrates nearly 1,600 km upstream to Redfish Lake,
Idaho. Populations exist in Puget Sound (in Baker Lake and Lake Washington)
and on the Washington coast in (Lake Quinault, Ozette Lake, and Lake Pleasant).
No anadromous populations seem to exist south of the Columbia River. Sockeye
are fished on their homeward migrations in coastal waters.
Chum Salmon
Chum are distributed more broadly both to the north arid to the south than
pink or sockeye. Although today they are relatively scarce south of the Columbia
River (Salo 1991), they were abundant in the Columbia River and in Oregon
coastal streams south to Tillamook Bay and also in California as far south as
Monterey Bay. Chum tend to make greater use of estuaries than pink or sockeye
(Healey 1982), but like those species, they are distributed in offshore waters and
are taken almost exclusively by commercial, rather than recreational, fisheries.
Although chum can grow to be relatively large, about 5 kg in Washington (Salo
1991), they are not highly valued, because they are often in an advanced state of
maturity and are considered unpalatable by some when caught in coastal waters
(some American Indians do consider chum caught in coastal water palatable and
their roe is highly valued).
Coho Salmon
Coho are native to coastal and interior rivers from Alaska to Monterey Bay,
California. They tend to spawn in small streams. Most coho in the Pacific
Northwest mature at the age of 3 years. Some populations migrate to offshore
waters, but many remain relatively near the coast during their marine residence
(Sandercock 19913. Coho are caught in commercial and recreational fisheries in
waters off the coast and in Puget Sound and the Strait of Georgia.
Chinook Salmon
Chinook, like coho, occur from Alaska to the Sacramento River system but
are generally less abundant than coho. They attain the largest size of any Pacific
salmon (over 40 kg, although 10 kg is typical) and are relatively long-lived,
commonly maturing at the age of 4-5 years. Chinook tend to spawn in large
rivers near the coast and in the interior. As mentioned above, different popula-
tions have markedly different life-history patterns. Stream-type chinook are
fished primarily on their return to freshwater from their wide distribution in open-
ocean waters. Ocean-type chinook, however, are vulnerable to recreational and
commercial fisheries much of their lives because they tend to stay near the coast.
OCR for page 33
SALMON GEOGRAPHY AND ECOLOGY
Steelhead Trout
33
Steelhead occur from Alaska to central California. They migrate directly to
open-ocean waters with little estuarine residence time and usually spend 1 or 2
years at sea (Burgner et al. 19921. Juvenile steelhead commonly co-occur with
coho in streams but are usually less abundant than coho. They are caught gener-
ally in commercial fisheries (mostly by American Indians, although bycatch in
other fisheries can be significant) and recreational fisheries in rivers.
Cutthroat Trout
Cutthroat have similar habitat requirements to steelhead (Bisson et al. 19881.
They are common as anadromous and resident populations in coastal and interior
streams and are native to drainage systems of the continental interior. Cutthroat
are smaller and often less numerous than steelhead. They have a relatively
complex life cycle (Trotter, 1989; Behnke, 1992) generally spending only a sum-
mer at sea. They are caught by recreational fishers but generally are under-
emphasized in research and management, as evidenced by the inability of scien-
tists to define cutthroat populations in a meaningful way (Nehlsen et al. 1991) or
even to consider them in stock-status reports (WDF et al. 19933.
PACIFIC NORTHWEST SALMON AREAS
The region's political divisions do not correlate closely with the life history
and distribution of anadromous Pacific salmon (see Chapter 4 for details of
populations). As a political region, the Pacific Northwest is usually considered to
encompass Idaho, Oregon, and Washington (Figure 1-11. The Columbia River
Basin, however, extends into Canada and includes portions of Montana, Wyo-
ming, Utah, and Nevada. The U.S.-Canada border cuts not only across the
Columbia River Basin, but also across the migratory paths of salmon moving
north and south along the Pacific coast. Oregon and Washington share 522 km of
the Columbia River as a common boundary. The Snake River serves as a portion
of the boundary of Idaho and Oregon. Klamath River headwaters are east of the
Cascades in Oregon and empty into the Pacific Ocean in northern California.
Forest planners consider the area to the west of the Cascades one unit (FEMAT
1993) and the area to the east another (Bormann et al. 1993~.
The committee focused on five biogeographical areas of the Pacific North-
west and on relevant parts of the Pacific Ocean and adjacent Canadian coastal
areas and rivers that differentiate important aspects of the salmon problem (Fig-
ure l-l). Three of the biogeographic areas Puget Sound, the Columbia River,
and the Sacramento River are drainage basins. The two others divide the por-
tion of the U.S. Pacific Coast used by salmon into a north and south region at
Humbug Mountain, 20 km south of Cape Blanco, Oregon. Appendix D discusses
the major landforms and their rivers.
OCR for page 34
34
UPSTREAM: SALMON AND SOCIETYIN THE PACIFIC NORTHWEST
The economies in the five areas are diverse. The Sacramento River, Colum-
bia River, and Puget Sound drainage basins have heavily urbanized regions.
Sacramento and San Francisco are the major urban centers in the Sacramento
drainage. The Columbia River Basin differs from the Sacramento River Basin in
that the largest urban area, Portland, is farther upstream, more than 185 km from
the river mouth. Two smaller urban areas are in the upper reaches of the Colum-
bia River Basin, on tributaries of the Columbia-Spokane and Boise. The Se-
attle-Tacoma urban area covers much of the east side of Puget Sound. In con-
trast, the northern and southern coastal areas do not have major urban centers,
and their economies depend primarily on forestry, fishing, and tourism. Here,
salmon are an important part of the total economy. Although Seattle is a major
fishing port, the Seattle-Tacoma area has a diverse economy in which fishing
plays only a small part. Fishing is least important to the economies of Portland,
Sacramento, Spokane, and Boise. The decline of salmon has produced signifi-
cant impact to what was an important commercial fishing industry, not only in
San Francisco, but also along the entire northern California coast; hundreds of
fishing boats have been idled and a whole way of life has been changed. The
cultural effects have been and continue to be important. As a tourist attraction,
fishing still is important to the San Francisco waterfront economy. In the interior
of the Sacramento River, Columbia River, and Puget Sound basins, agriculture,
ranching, hydropower generation, forestry, and tourism are the dominant eco
nomic activities.
River Basins
The fundamental freshwater geographic unit for salmon is the river basin. A
river basin is the drainage network of a river and its tributaries. It ranges in size
from small, unnamed coastal streams that drain less than a few square kilometers
to large rivers, such as the Columbia and Sacramento, with drainage areas of
thousands of square kilometers. River basins can have subbasins of major tribu-
taries, such as the Salmon River of Idaho, the Yakima River of Washington, and
the Willamette River of Oregon, which are all part of the Columbia River basin.
The term watershed is applied to small and large river systems but usually is
taken to mean intermediate-size drainages of 20-500 km2 (FEMAT 1993, Wash-
ington Forest Practices Board 1993J.
River basins originate in numerous steep, headwater streams that often are
fed by glaciers, snowmelt, or groundwater springs. These small, fishless streams
make up 70% or more of the total length of the drainage network in the Pacific
Northwest (Neiman et al. 1992~. Although not inhabited by salmon, they are
important for salmon because they carry cool water, nutrients, and organic matter
downstream to areas used for spawning and as nurseries. The tiny headwater
streams come together to form slightly larger streams with gradients low enough
for some fishes, but not usually salmon. Farther downstream, gradients lessen
OCR for page 35
SALMON GEOGRAPHY AND ECOLOGY
35
and channels widen; one or two salmon species reach these streams to spawn.
Confluences of small and medium streams eventually form large streams and
rivers that include lake systems and well-developed flood plains that support
salmon. Before flowing into the ocean, a river enters an estuary, where freshwa-
ter and salt water mingle. Estuaries vary considerably in size and in the extent to
which freshwater and salt water intermingle.
Ocean
The North Pacific Ocean forms the second major geographic unit for salmon.
The continental shelf along the Pacific Northwest underlies coastal waters to a
depth of about 200 m and ranges in width from about 7 km off Cape 13lanco,
Oregon, to about 30 km off Grays Harbor, Washington (Good 1993J. At the edge
of the continental shelf, the slope drops 2,000 m to the sea floor.
Off the Pacific Northwest coast during summer, the south-flowing California
Current along the outer shelf and open ocean is underlain by the north-flowing
California Undercurrent, which, when combined with offshore winds, results in a
movement of nearshore surface water offshore and upwelling of cold, nutrient-
rich waters from the deep ocean. In some years, warm surface waters from the
tropics move northward along the coast and inhibit upwelling; this results in
increased sea-surface temperatures and depletion of nutrients (E1 Nino condi-
tions). During winter, southerly winds result in the north-flowing Davidson
Current along the continental shelf, an onshore flow of surface waters, and
downwelling.
As discussed previously, some salmon do not remain in nearshore waters but
travel well out to sea; some tagged salmon of U.S. origin are recovered in the
western Pacific Ocean off the Russian coast. Most salmon migrate along the
North American coast, either north or south, to reach feeding areas, where they
then follow major ocean currents while foraging (Pearcy 1992~.
SALMON ECOLOGY IN RIVER BASINS
To understand the ecology of anadromous Pacific salmon in freshwater, one
must consider species interactions, complex adaptations of juveniles to stream
habitat, and the influence of returning spawners on stream ecosystems.
Species Interactions
Salmon interact with other species in complex ways. The dynamics of
ecological communities are shaped by many direct and indirect interactions
among species. Traditionally, the study of direct pairwise interactions, such as
the interaction between a predator and a prey species, has dominated approaches
to ecological systems, but recent world has demonstrated the importance of indi
OCR for page 36
36
UPSTREAM: SALMON AND SOCIETY IN THE PA CIFIC NORTHWEST
rect effects that cascade through the web of other species and the physicochemi-
cal components of the system (Carpenter 1988, Carpenter and Kitchell 19931.
Competitors can have positive effects on each other in more complex ecological
systems; removing a predator can negatively affect its prey through interactions
that its prey will then have with other species. The physicochemical environment
not only affects but is affected by biology.
More than 100 freshwater and andaromous fishes inhabit the Pacific North-
west, of which about one-third are introduced (Scott and Crossman 1973, Moyle
1976, Wydoski and Whitney 1979, McPhail and Lindsey 19861. The Columbia
River contains 52 native species, including 13 endemic species (species found in
the basin and not elsewhere). The Chehalis River contains 34 native species,
most of which also occur in the Columbia River. However, except in such large
river basins as the Columbia, Chehalis, and Sacramento, the region's freshwater
communities tend to contain few fish species. Typical Puget Sound and Pacific
Coast streams might have two or three salmons (family Salmonidae), two sculpins
(family Cottidae), one or two minnows (family Cyprinidae), and a lamprey (fam-
ily Petromyzontidae).
Large rivers-such as the Columbia, Chehalis, and Sacramento-have more-
diverse native fish communities (25-50 fishes versus 1-6), more-complex food
webs, and less-variable streamflow and temperature regimes than small streams.
Large rivers also contain more introduced fishes (Li et al. 19871. Introduced
fishes in this region such as sunfish (Lepomis spp.), black bass (Micropterus
spp.), perch (Perca flavescens), walleye (Stizostedion vitreum), striped bass
(Morone saxatilis), and catfish (Ictalurus spp.) prefer or tolerate higher tem-
peratures and use more standing-water habitats than do juvenile salmons; thus,
they are often suited to live in reservoirs behind dams. Introduced fishes often
are large and piscivorous, inasmuch as usually they have been introduced to
provide recreational fishing opportunities. Their potential interactions with
salmon (competition or predation), have seldom been evaluated.
Perhaps the most intensively studied ecological interaction between salmon
and nonsalmonid fishes in a large river has been the predation on salmon smelts
in the Columbia River. Nonnative species, such as walleye and smallmouth bass,
eat smelts, but the most important predator is the native northern squawfish,
Pt~chocheilus oregonensis (Rieman et al. 19911. Squawfish removals have been
initiated as a management measure, but experience with other aquatic ecosystems
suggests that any benefits to salmon could be confounded by other species inter-
actions (e.g., squawfish might control other salmon predators) or interactions
between life-history stages (e.g., reduction in predation by large squawfish on
smaller squawfish might result in rapid population rebound and more intense
predation on salmon).
OCR for page 37
SALMON GEOGRAPHY AND ECOLOGY
37
Juvenile Adaptability
Juvenile salmon in streams and rivers tend to consume mostly aquatic and
terrestrial invertebrates carried along by the flowing water (Muncie 1969), but
they also eat small fish, salmon eggs, and occasionally the carcasses of adult
salmon (Kline et al. 1990~. Diet varies among species and seasons. In small
watersheds with dense forest canopies, much of the organic matter in streams
originates in the surrounding forest, and the invertebrate communities are domi-
nated by organisms specialized for processing wood and leaves (Gregory 19831.
In larger watersheds, where streams are too wide to be completely shaded, or in
small streams where the canopy has been removed, the food base of the aquatic
community shifts to algae produced photosynthetically in the stream itself (Bilby
and Bisson 1992~. This shift in primary production from mostly terrestrially
produced to aquatically produced organic matter as streams become larger is of
fundamental importance for the composition of the invertebrate community
(Vannote et al. 1980~. In spite of the shift in the food web, juvenile salmon
generally are able to capitalize on whatever is abundant (Chapman and Bjornn
19691.
Juvenile salmon move within watersheds to take advantage of seasonal envi-
ronmental changes (Bustard and Narver 1975), but the importance of winter
feeding areas has become recognized only recently. Despite temperatures well
below the optimum for growth, some species show impressive weight gains in
late winter and early spring. Juvenile salmon in Pacific Coast streams often
overwinter in areas sheltered from high flows, including off-channel ponds
(Peterson 1982), sloughs, wetlands, and even estuaries (Tschaplinski and Hartman
1983~. In the interior, salmon overwinter in areas free of ice accumulation. Thus,
salmon display remarkable adaptations to the availability of food and refuge in
different parts of their range. However, human activities can alter the physical
characteristics of the habitat and the types of food available for salmon (e.g.,
Hawkins et al. 19831.
Spawners' Effects on Streams
One way in which salmon are connected to their ecosystems that has at-
tracted some attention is what happens to them after they die. Carcasses of
postspawning adults are seldom washed out to sea. Usually, they are removed
from the stream and eaten by a wide variety of terrestrial animals (Cederholm et
al. 19891; are deposited by floods in the riparian zone, where they are eaten or
decompose; or decompose within the stream. Although terrestrial animals must
persist through the year on other sources of food, the salmon can be seasonally
important, particularly for animals fattening for the onset of winter. Migratory
animals also can take advantage of salmon carcasses, as exemplified by the large
population of bald eagles that winter on the lower Skagit River, Washington. The
OCR for page 38
38
UPSTREAM: SALMON AND SOCIETY IN THE PACIFIC NORTHWEST
absence of salmon carcasses can reduce wildlife abundance (Spencer et al. 1991 J.
Decomposition in the riparian zone also can contribute a large amount of nutri-
ents to vegetation (Bilby and Bisson, unpublished data).
Marine-derived nutrients (carbon, nitrogen, and phosphorus) imported into
freshwater systems by salmon are important to the trophic support system for
salmon and other components of the aquatic and riparian communities (Donaldson
1967, Kline et al. 1990, Kline et al. 1993), as evidenced in concentrations of
dissolved organic nutrients and stable-isotope ratios. When not eaten by wildlife,
carcasses can be used through four pathways: autotrophic fixation by aquatic
plants and subsequent transfer through periphyton-based food webs, heterotrophic
fixation by bacteria and fungi and transfer through decomposer-based food webs,
abiotic uptake by sorption within the stream substrate, and direct consumption of
carcass tissues and remaining eggs by fish.
Before they die, however, adult salmon can exert another important influ-
ence on the stream ecosystem by their digging activities. Digging by females
dramatically (but only temporarily) decreases the density of benthic invertebrates
(Field-Dodgson 1987~. In some instances, nest-digging can affect the dimen-
sions and stability of the stream. At moderate to high fish densities such as
often occur in healthy pink, chum, and sockeye populations salmon can widen
a stream by digging along its margins, filling in its pools, and coarsening the
streambed's substrate. Thus, contrary to the general belief that high densities of
spawners are undesirable (Hunter 1959, McNeil 1964), high salmon density can
be associated with high survival. If a flood, as might result when warm rain melts
snow accumulated on a deforested slope, coincided with a poor run of salmon to
a stream, the resulting increase in streambed scour could reduce recruitment.
This could force the population into a cycle of smaller and smaller runs that could
affect the physical structure of the stream and indirectly affect other inhabitants,
such as fish, amphibians, and invertebrates.
Cautions
The complexity of salmon life cycles and the communities in which they
exist should engender caution in proposing simplistic solutions to the salmon
problem in the Pacific Northwest, especially in light of our limited understanding
of salmon ecosystems. A weakness of natural-resource management has been the
tendency to emphasize single-species management by manipulating either the
"valued" species or the physical factors thought to influence its abundance or, at
most, to consider some of the obvious interactions between pairs of species.
Salmon restoration and rehabilitation must be approached in an ecosystem con-
text, bearing in mind that habitat changes might not bring equivalent improve-
ments for all species and could even be detrimental to some. Research on Pacific
salmon has not kept pace with the theoretical and empirical literature on complex
multispecies interactions in other systems. Caution must be practiced in the
OCR for page 39
SALMON GEOGRAPHY AND ECOLOGY
39
application of relatively simplistic salmon-management approaches, such as al-
tering habitat, increasing hatchery production of juveniles, controlling predators,
and introducing or eliminating nonnative species, because we have little ability to
predict their ecological consequences.
SALMON ECOLOGY IN TEIE OCEAN
What happens at sea is important to the conservation and management of
Pacific salmon (Pearcy 1992~. Pacific salmon spend most of their lives in the
ocean, where they grow to maturity after leaving freshwater streams as smelts.
Two aspects are critical to understand the salmon problem and select appropriate
management systems: interdecadal changes in the ocean environment and the
effects of density on growth and survival. Interdecadal changes in the ocean
environment especially in water temperature and currents and associated bio-
logical communities influence growth and survival rates and thus the return of
adults (Francis and Sibley 1991, Beamish and Bouillon 1993, Francis and Hare
19941. Density-dependent effects-how salmon at sea are affected by the num-
ber of young salmon entering the ocean from their natal streams or hatcheries-
might be experienced during the ocean phase and could thwart attempts to supple-
ment populations by stocking during periods of poor ocean conditions (R. Francis
and R. Brodeur, Production and management of coho salmon: A simulation
model incorporating environmental variability, pers. comm., 1991; Beamish and
Bouillon 19931.
Ocean factors are important not only because they directly influence the
number of adults that return to their spawning streams but also because a poor
understanding of their effects can result in misinterpreting both population status
and the potential for an acceptable level of fishing. Management can overesti-
mate or underestimate the abundance of returning adults if ocean effects are not
realistically acknowledged. For example, Lawson (1993) has pointed out that the
success of freshwater salmon-habitat improvements can be masked by changes in
the ocean condition. Likewise, favorable ocean conditions could mask the effects
of habitat degradation. Ocean effects are logistically difficult to observe because
they occur over such large spatial and temporal scales that they are not easy to
observe directly, in contrast with the more local effects of land use, fishing, and
impoundments. Although hatchery stocking rates and high-seas fishing mortality
can be altered, purposeful manipulation of the ocean climate to counter natural
variations is currently beyond human capability. It is possible to respond appro-
priately to long-term, interdecadal environmental variation but this is difficult in
a management and conservation milieu of many short-term crises and large
interyear variability. A fishery and conservation management strategy can be
developed that is better attuned to the large spatial scale and long-term variability
in oceanic conditions relevant to Northwest Pacific salmon.
OCR for page 40
40
UPSTREAM: SALMON AND SOCIETY IN THE PACIFIC NORTHWEST
Interdecadal Variation in Ocean Climate
Evidence of wide-scale interdecadal variation in the Northeast Pacific and
the Gulf of Alaska is increasingly convincing (UNESCO 1992J. Over the period
of dependable records (from 1946 to the presents, the greatest anomalies in mean
decadal sea surface temperatures occurred in the decade 1977-1986. During that
period, lower sea-surface temperatures (more than 0.5°C below average' oc-
curred in the central North Pacific, and higher sea-surface temperatures (more
than 1.5°C above average) occurred along the West Coast of North America.
Those changes were associated with changes in many other climatic variables.
For example, the average annual North Pacific sea-level pressures from Novem-
ber through March (Trenberth 1990) exhibited a step decrease in about 1976.
Stream flows in many rivers increased in coastal Alaska but decreased in the
Pacific Northwest indicating that interdecadal climatic variations might influence
not only the ocean environment, but also inland conditions affecting salmon
habitat (Cayan et al. 1991~. The Pacific Northwest Index, a composite index of
climatic variation, incorporates the average annual coastal temperature, the aver-
age annual basin temperature, and snow depth in March. Variation in the index
from 1901 to 1985 in the Puget Sound basin (Ebbesmeyer and Coomes 1989) not
only shows warming in the late 1970s but demonstrates that interdecadal varia-
tions are regular features of the region during the twentieth century. Recognition
of the interdecadal patterns enhances our knowledge of seasonal and interannual
variations in the ocean environment (Wooster 19923.
Interdecadal changes have influenced the migration routes of adult salmon
returning to spawning streams and, more important, the physics and biology of
the ocean (UNESCO 1992' related to salmon production. Groot and Quinn
(1987) documented a change in the percentage of salmon approaching the Fraser
River from the northern passage around Vancouver Island versus the southern
passage. Only about 17% took the northern passage from 1953 to 1977, but about
40% did from 1977 to 1984. Changes in migratory route are accompanied by
differences in the timing of migrations as well (reviewed by Quinn 19901.
Predation of salmon at sea has been studied almost exclusively in nearshore
areas as smolts have been leaving and adults returning. A wide variety of fishes
and other animals eat salmon, and major predation by spiny dogfish (Squalus
acanthias) (Beamish et al. 1992) and pinnipeds, such as harbor seals (Phoca
vitulina) (Olesiuk 1993) and California sea lions (reviewed by Palmisano et al.
1993; see Box 2-1), can occur in some situations. Predation has apparently
eliminated the smelt production from a hatchery in southern Vancouver Island in
sets of years when warmer waters allowed mackerel (Scomber japonicusJ to
move north and prey on the smelts entering the sea (B. Riddell, pers. comm., June
19949.
Salmon at sea eat fish, crustaceans, and soft-bodied organisms in the upper
1-200 m; variation in diet among species (e.g., Beacham 1986) and years (Brodeur
OCR for page 41
SALMON GEOGRAPHY AND ECOLOGY
41
1992) has been noted. The abundance of those prey resources varies seasonally,
from year to year, over decadal time scales, and across broad regions of the North
Pacific Ocean (Frost 1983, Brodeur and Ware 19921. In analyses for the North
Pacific Ocean from 1965 to 1980 (Beamish and Bouillon 1993), 25% of the
variation in the abundance of copepods was statistically associated with the Aleu-
tian Low Pressure Index; from 1976 to 1980, copepods were 1.5 times more
abundant than from 1965 to 1975. Analyses by Brodeur and Ware (1992) suggest
that zooplankton biomass in the Alaska Gyre during the 1980s was more than
twice that in the late 1950s and 1960s. Because copepods are an important food
of salmon and many of their prey, their abundance is expected to affect the
welfare of salmon on the high seas. The parallel trends in the catches of pink,
chum, and sockeye salmon across the entire North Pacific Ocean suggest that
common events across wide areas influence salmon production (Beamish and
Bouillon 19933. The pattern in the total catches of those species by Canada,
Japan, Russia, and the United States corresponds with the Aleutian Low Pressure
Index (see Figure 7 in Beamish and Bouillon 19933; this suggests that the fish are
fluctuating in response to long-term ocean variations (Beamish and Bouillon
1993~. Beamish and Bouillon (1993) suggested (on the basin of meager data) that
the index and the catches of Pacific salmon were low at the turn of the century-
further evidence is needed to support their conclusion.
The relation of both salmon catches and zooplankton abundance in the North
Pacific Ocean to the Aleutian Low Pressure Index suggests that the interdecadal
changes in salmon abundance are mediated via the food web. Beamish and
Bouillon (1993) presented a rationale for relating the Aleutian Low Pressure
Index to salmon abundance via changes in survival upwelling, nutrient availabil-
ity and phytoplankton production, and trophic relationships to copepod abun-
dance. There is evidence that the increase in marine survival of chum and pink
salmon in the 1970s corresponded to an increase in the Aleutian Low Pressure
Index; Japan's salmon catch during the 1970s indicated that marine survival
might have increased by 200%-300~o (see Figure 9 in Beamish and Bouillon
19931.
The interdecadal changes in salmon abundance that are linked to climate
appear to reflect increases and decreases in survival. Year-class strength seems
to be set early in the ocean phases of salmon life history (Pearcy 19921. How-
ever, the actual source of mortality and the mechanism that causes survival to
change are not known. It has been speculated that they are linked to north-south
changes in the distribution of other marine fishes, such as hake (Merluccius
productus) or other taxa that prey on young salmon (Holtby et al. 19901.
Although salmon catches across the North Pacific Ocean appear to vary
together in time, there is evidence that salmon catches along the Alaska coasts
and the Washington-Oregon-California coasts are out of phase with each other
(Francis and Sibley 19911. The reason for this phase difference in salmon catches
is probably that low-frequency changes in the California Current and the Alaska
OCR for page 42
42 UPSTREAM: SALMON AND SOCKET IN THE PACIFIC NORTHWEST
Current are out of phase, as well. Francis and Hare (1994) based an explanation
for the phenomenon on suggestions by Hollowed and Wooster (19921. They
proposed that
1. there are two mean states of winter atmospheric circulation in the North
Pacific which relate to the intensity and location of the winter mean Aleutian
Low . . .;
2. oceanic flow in the Subarctic Current and the resultant bifurcation at its
eastern boundary into the California and Alaska Currents is fundamentally dif-
ferent in these two states;
3. the patterns in Alaska salmon production tend to indicate long interdec-
adal periods of oscillating 'warm' and 'cool' regimes: early 1920s to late 1940s/
early l950s (warm), early l950s to mid 1970s (cool), mid 1970s to present
(warm);
4. the hypothesized inverse behavior of the long-term production dynamics
OCR for page 43
SALMON GEOGRAPHY AND ECOLOGY
. ~,,.~''~' - 0'~ - '~""'~' - ~'~'1~''"'
,,., T ,~,= i: ,.
' ~.~. ~.~.~>'="~'~N'~'~"~" 4'!~' '"'
. ~, , .... ....
· ~ ~ - '''we'd' .~'',~.,,0~0 l. - 7~. m~ ,.._..............
·. . ' '~'~'~"~ 0'~'~'''"
........................................ ...............
· ~,,0~: - ~ . ~
... ~ ~ : .. I: ~ ;
..... .. ... .. ~.~..~
:..:::::::: ::::: :~ ::: ~ ~ an:
· .~ ~ -A ~ ~ ~
~ ~; )~ ~ ~ ; ~ :~ ~ ~ ~ ~ L ~ ~
· . - ~.~; ~ ~ ~ ~ ~ ~ ~-~ ~ :::
~.~.~.~.~.~.~.~. .~.-.~
~ :~ ~ ~.-..
~ ~ ~ ~ ~ do mI
....~.~.~....~..~.~..~ ~.~.
~ ~ t~ Ark Swat ~ ~ '$ 1 ll~85~
....................... c ~ ~ :~ 5542 i:
................ ..........
......................... . _ ~. ~
. . . . . . . . . . . . . . . . . . . . . . . ~ . ~ ~ . , . _ . . . ~ . . .
:.:.:.:.:.:.:.:.:.:.:.~:.:.~:.:.~:.~..~. :~.:.~.:.:~.::
. ~
::::::::::::~:~....~..:~: be.: - ~.~.~. ~ - ::::::
.~.~d ~ b~m imn ~-= w ~ ~ ~ hi.: .
· ~ ~ ;~ ~ :~ ~ 4~ ~ ~ ~ ~ ~ ~ ~ ~ ~2 1~ ~
y ~ ~ ~
... ~.~..~.~.~.~..~.~.~.~..~ -......
~ .... : : ::: :::
i.. ~-....
of the Alaska CuITent salmonids . . . and zooplankton . . . is related to effects of
these two states of winter atmospheric circulation on the dynamics of . . .
physical oceanographic systems . . . and, subsequently, on biological processes
at the base of the food chain.
4
'3
The impact of interdecadal oscillations apparently occurs in a salmon's first
year in the ocean (Francis and Hare 1994, Hare and Francis 19951. That conclu-
sion is based on time-series intervention analyses that identify years when "step
changes" occur in a long-term record of annual values. Step changes are sudden
increases and decreases in a data record that persist for a period of years and are
apparent in spite of large year-to-year variation. The time series analyzed were of
western-Alaska (Bering Sea) sockeye-salmon catches from 1920 to 1990, north-
ern Gulf of Alaska pink-salmon catches from 1935to 1990, winter air tempera-
tures at Kodiak Island, and the North Pacific index of the intensity of the Aleutian
Low.
Step changes were apparent in all four time series; in the air-temperature
OCR for page 44
44
UPSTREAM: SALMON AND SOCIETYIN THE PACIFIC NORTHWEST
data, a cooling step began in 1947 and a warming one in 1977. There was a lag
of 1 year between the step change in the climatic time series and that in the catch
of pink salmon, which migrate directly to the ocean after emerging from the
gravel. In comparison, a lag of 2 years was observed for the sockeye salmon,
which spend 2-3 years in freshwater before entering the ocean. Thus, the effects
of interdecadal climate changes are reflected in the fish catches and match the
time lags expected on the basis of when the salmon first enter the ocean. The
analyses also suggest that, at least for the coastal streams of Alaska that have not
been extensively degraded, the fish respond to changes in the ocean conditions
rather than to changes in conditions of the natal streams.
Density-Dependent Effects
A corollary to the influence of interdecadal changes in ocean conditions on
salmon survival during their first year at sea is how they are density-dependent.
Whether salmon at sea experience density-dependent growth or survival is con-
troversial. But it appears that salmon growth and therefore the age structure of
returning spawners can be affected by high densities at sea. That is exemplified
by systems with high natural abundance, such as sockeye salmon from Bristol
Bay, Alaska (Rogers and Ruggerone 1993), and regions with extensive artificial
propagation, such as of chum salmon from Japan (Ishida et al. 1993~. It also
appears that density-dependent growth reductions occur only in the early and
final periods of marine residence, when salmon are in coastal waters. Salmon
from a given region are thus competing primarily with each other; there is little
evidence of competitive interaction among population complexes from distant
regions, such as Japanese chum salmon and Bristol Bay sockeye (Rogers and
Ruggerone 1993~.
Beamish and Bouillon (1993) suggested that with the exception of Japanese
chum salmon, hatcheries were not important contributors to the increase in salmon
catches in the North Pacific during the 1970s and 1980s. In the Gulf of Alaska,
increased hatchery production of pink salmon contributed to the increase in
catches from the early to middle 1980s, but natural reproduction increased at the
same time by a factor of 3 to 4. The increases in Alaskan catches of chum and
sockeye were so great in the late 1970s that hatcheries could have contributed to
them only slightly. For Canadian fisheries, some increases can be attributed to
enhancement programs for sockeye and chum salmon, but these increases in
enhancement could not account for the marked regional increase in all northern
salmon populations, because Canada's contribution to the total catch of Canada,
Japan, Russia, and the United States was relatively small. Russian chum-salmon
catches increased from 1976 to 1990 even though releases from hatcheries were
reduced.
Beamish and Bouillon (1993) suggested that hatchery production strategies
should take into account the interdecadal changes occurring in ocean quality
OCR for page 45
SALMON GEOGRAPHY AND ECOLOGY
45
relative to salmon production. In general, hatchery production tended to track
wild production in the 1970s and 1980s but did not play a large role in the
increase in production. However, Beamish and Bouillon (1993) speculated that
it may not be an appropriate strategy to continue to release large numbers of
artificially reared smolts during a period of decreasing marine survival of salm-
on.... Declines in marine survival should be managed differently from de-
clines resulting from fishing mortality that is too high.
That is an interesting idea of great potential importance, but Beamish and Bouil-
lon provided no evidence that hatchery releases influence marine survival of wild
fish when ocean conditions are either good or poor. Nevertheless, density-depen-
dent effects, even between or among hatchery populations, may result in de-
creased returns, smaller sizes, and decreased value.
Larger Spatial and Temporal Scales
The emerging understanding of interdecadal changes in the ocean climate
and the related mechanisms that affect salmon at sea have implications that are
both exciting and disconcerting to scientists thinking about resource manage-
ment. Humans are beginning to understand what happens to salmon during the
majority of their lives the portion spent at sea. Although we know little of the
details, the new insights already demonstrate that variations in salmon abundance
are linked to phenomena on spatial and temporal scales that humans and human
institutions do not ordinarily take into account. Consider that the apparent effec-
tiveness of hatcheries might have resulted from favorable ocean and climatic
conditions in the era when the hatcheries were built; what looked like human
manipulation of the total number of salmon might have been only a reapportion-
ment among different populations. Or consider that the decline of some popula-
tions might be a direct result of introducing new hatchery populations into an
ocean pasture of limited capacity.
The scale of human endeavor often has been incommensurate with the scale
of salmon ecology. Some of our current policies are based on deep ignorance: it
is not reasonable to assume that ocean conditions vary in ways that are generally
uniform and random in their impacts on populations of salmon. Interdecadal
variations and the importance of the ocean phase should be incorporated into
human thought, planning, and actions in response to the effects of and attempts to
repair damage that occurred during the freshwater phases of the salmon lives.
The possible overriding effects of interdecadal changes in ocean conditions on
salmon, the results of freshwater salmon management, and the overwhelming
focus of human attention on the more-visible freshwater phases of the salmon
history combine to provide the key ingredients for surprises in the future.
Representative terms from entire chapter:
north pacific
