Assessment of the Risk of Invasion 

of National Forest Streams in 

the Pacific Northwest by Farmed 

Atlantic Salmon

Peter A. Bisson

United States 

Department of 

Agriculture

Forest Service

Pacific Northwest 

Research Station

General Technical

Report

PNW-GTR-697

November 2006
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Pesticide Precautionary Statement

This publication reports research involving pesticides. It does not contain recommendations for their use, nor does it imply that the uses discussed here have been 

registered. All uses of pesticides must be registered by appropriate state or federal 

agencies, or both, before they can be recommended.

CAUTION: Pesticides can be injurious to humans, domestic animals, desirable 

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pesticides and pesticide containers.

Authors

Peter A. Bisson is a research fish biologist, Forestry Sciences Laboratory, 3625 93rd

Avenue SW, Olympia, WA 98512, pbisson@fs.fed.us.

Cover Photo by Tom Iraci
Abstract

Bisson, Peter A. 2006. Assessment of the risk of invasion of national forest streams 

in the Pacific Northwest by farmed Atlantic salmon. Gen. Tech. Rep. PNWGTR-697. Portland, OR: U.S. Department of Agriculture, Forest Service, Pacific 

Northwest Research Station. 28 p.

This report describes the evidence for invasion of Pacific Northwest streams by 

Atlantic salmon (Salmo salar) that have escaped from marine salmon farms, and 

assesses the potential impact of farmed salmon invasion on native fishes inhabiting 

streams on National Forest System lands. The current risk to streams on National 

Forest lands in the Pacific Northwest from Atlantic salmon invasions appears to be 

low and is limited to a few areas in northwest Washington and southeast Alaska. 

However, long-term risks may be substantial if fish continue to escape from marine 

rearing pens or freshwater hatcheries. The two greatest threats appear to be that 

(1) Atlantic salmon could transmit a serious disease or parasite to native fishes, and 

(2) escaped salmon could eventually adapt to local conditions, leading to selfsustaining populations. If Atlantic salmon populations are eventually established, 

this species’ preference for swiftly flowing stream habitats could facilitate competition with currently at-risk species such as steelhead (Oncorhynchus mykiss). 

This could result in a pattern of expansion similar to that observed in other nonnative aquatic plants and animals, in which a prolonged early colonization period is 

followed by a rapid phase of exponential growth as breeding populations adapt to 

local conditions.

Keywords: Atlantic salmon, invasive aquatic species, salmon farms, Pacific 

Northwest streams.
Contents

1 Introduction

1 Overview of Environmental Issues Associated 

With Salmon Farming

2 Location of Salmon Farms on the Pacific Northwest Coast

3 History of Farmed Salmon Escapes

5 Evidence for Established Atlantic Salmon Populations 

on the Pacific Coast

9 National Forest Streams Potentially at Risk of 

Atlantic Salmon Invasion

9 Atlantic Salmon Life History, Habitat Preferences, 

	 	 and Potential Interactions With Pacific Salmon

9 Migration and Spawning

11 Juvenile Morphology and Rearing Habitat

12 Juvenile Migrations

14 Density Dependence and Interspecific Interactions

16 Food Habitats

16 Diseases and Parasites

18 Native Diseases and Parasites as Potential Barriers 

to Atlantic Salmon Invasion

19 Conclusion

20 Early Detection

22 English Equivalents

22 Acknowledgments

22 Literature Cited
Assessment of the Risk of Invasion of National Forest Streams in the Pacific Northwest by Farmed Atlantic Salmon



Introduction

Overview of Environmental Issues 

Associated With Salmon Farming

Commercial farming of Atlantic salmon (Salmo salar) in marine net pens was 

developed in Norway in the 1960s. Juvenile Atlantic salmon are spawned in fresh 

water and reared in freshwater hatcheries until they can adapt to seawater, after 

which they are transferred to open cages of metal or heavy fabric mesh in sheltered 

marine areas where they grow to marketable size. The obvious advantages of 

farmed Atlantic salmon include (1) efficient production and harvest systems and 

(2) a product of generally uniform size and quality, easily processed and delivered 

to customers. Additionally, salmon farming is touted as reducing the pressure on 

naturally spawning stocks of salmon during a time when their numbers are sharply 

dwindling. In 2005, the total worldwide production of farmed Atlantic salmon was 

estimated to be about 400 million fish (Atlantic Salmon Federation 2005).

Based on industrial fish culture techniques pioneered by the Norwegians, 

Atlantic salmon farms have prospered on several continents. Most farmed salmon 

are produced in countries with protected bays and cold marine waters, particularly 

Norway, United Kingdom, Chile, and Canada. According to the Food and Agriculture Organization of the United Nations, global salmon production (all species, 

both farmed and captured in the wild) has more than doubled in the last 15 years 

from less than 1 million metric tons to about 2 million metric tons, and nearly all 

of the increase has resulted from Atlantic salmon culture (Jones 2004). Coincident 

with this rise in production, salmon prices in global markets have plummeted even 

as consumption rates have increased. Naylor et al. (2003) reported that ex-vessel 

prices per pound for wild-caught salmon declined from 36 to 82 percent, depending on the species, from 1984 to 1992, and between 1988 and 2002 (when farmed 

salmon production was rapidly proliferating) prices dropped an astounding 54 to 

92 percent. The global salmon glut has seriously impacted local economies based 

on capture fisheries. In Alaska salmon fisheries, ex-vessel prices fell from $600 

million in 1992 to $150 million in 2002 (Naylor et al. 2003), and commercial fishing 

licenses that sold for hundreds of thousands of dollars in the early 1990s are now 

selling for $20,000 to $30,000. The salmon farming industry itself appears to have 

been a victim of its own success. Some corporations have reported operating losses 

owing to depressed prices, and intense competition within the industry has resulted 

in efforts to cut production costs.

In addition to economic issues, a number of environmental concerns have 

arisen in connection with salmon farming. In early 2004, organochlorine pesticide 

Global salmon 

production (all species, 

both farmed and 

captured in the wild) 

has more than doubled 

in the last 15 years 

from less than 1 million 

metric tons to about 

2 million metric tons, 

and nearly all of the 

increase has resulted 

from Atlantic salmon 

culture.
General Technical Report PNW-GTR-697

residues and other chemicals known to be carcinogenic were detected in farmed 

salmon at concentrations greater than found in wild salmon, particularly in salmon 

from European farms (Hites et al. 2004). Most of the contaminants, termed “persistent organic pollutants” because of their longevity, resulted from commercial feed 

that contained high concentrations of fish oils from abundant pelagic species, e.g., 

jack mackerel (Trachurus symmetricus), anchoveta (Engraulis spp.), and herring 

(Clupea spp.), exposed to agricultural, urban, and industrial runoff in near-shore 

marine waters. Accumulation of uneaten food, waste products, and drugs beneath 

net pens has also been linked to localized pollution and potential contamination of 

wild fish and shellfish in the vicinity of pens (Goldburg et al. 2001).

Two of the most significant environmental concerns attached to Atlantic salmon 

farming in the Pacific Northwest have been (1) the possibility that escaped salmon 

would interact with and potentially harm native fauna, and (2) that Atlantic salmon 

farms would serve as epicenters for parasite and disease transmission to wild 

salmonids. Over the past 15 years there have been repeated escapes of salmon from 

pen facilities. Most escapes occur during large storms, but the precise number of 

escapees is rarely known. Worldwide, hundreds of thousands of Atlantic salmon 

have escaped from culture facilities, and where they are farmed outside of their 

native range, that is, in the Pacific Northwest and Chile, they have the potential to 

become an invasive species (Naylor et al. 2005). Salmon farms in a number of locations have created conditions favorable to parasites that infest wild fish. Particularly 

troublesome have been outbreaks of parasitic copepods (sea lice [Lepeophtheirus 

salmonis]). These parasites can be treated in farmed salmon with drugs but have 

been linked to declines of wild fish that are attacked as juvenile salmon swim by 

net pens on their way to sea (Krkosek et al. 2005).

The objective of this analysis is to examine the evidence for population 

establishment by escaped Atlantic salmon in the Pacific Northwest, and to assess 

the potential implications of invasions for native fishes inhabiting National Forest 

System streams. In addition, the analysis will assess the vulnerability of anadromous salmonid populations using National Forest System lands to salmon farming 

impacts when fish have left their natal watersheds.

Location of Salmon Farms on 

the Pacific Northwest Coast

At present, approximately 140 salmon farms occur along the Pacific coast of North 

America. Most fish raised in the farms are Atlantic salmon, but some farms raise 

Chinook salmon (Oncorhynchus tshawytscha) or coho salmon (O. kisutch), both of 

which are native to the region. The majority of farms (127 in January 2005) are in 
Assessment of the Risk of Invasion of National Forest Streams in the Pacific Northwest by Farmed Atlantic Salmon

British Columbia, although there are a few salmon farms in Puget Sound and the 

Strait of Juan de Fuca–all owned by a single corporation.

Hundreds of federal, state/provincial, and tribal salmon hatcheries also produce 

fish for harvest along the Pacific coast from Alaska to central California, but commercial salmon farms are limited to Washington and British Columbia. In 1990, net 

pen salmon farming was outlawed in Alaska, where capture fisheries dominate the 

economy of many coastal towns. The coasts of Oregon and California do not possess the sheltered, cold-water fjords needed for net pen rearing, and salmon farming 

operations have not been economically viable there. Overall, the contribution of 

Washington State salmon farms to total Pacific Northwest farmed salmon production is quite small. Washington farmed salmon rank 18th out of 40 agricultural 

commodities in the state (Washington Fish Growers Association 2005), although 

fish farming is growing more rapidly than other agricultural commodities. In 

contrast, salmon farming has become a major force in British Columbia’s economy. 

Farmed salmon represent British Columbia’s most valuable agricultural export, and 

the province has become the world’s fourth largest farmed salmon producer (British 

Columbia Salmon Farmers Association 2005). Atlantic salmon make up 85 percent 

of the farmed fish produced in British Columbia, and approximately 90 percent of 

those fish are exported to the United States. Chile, however, remains the leading 

farmed salmon exporter to the United States at present.

History of Farmed Salmon Escapes

The majority of salmon that escape from farming operations do so during large 

storms that damage the pens and enabling some or all of the fish contained within 

them to swim free. In most cases, the job of reporting is left to the farm operator, 

who may be reluctant to report the true number of escaped fish. Therefore, the 

accuracy of reports is uncertain, and quite likely the number of fish claimed by 

salmon farmers to have escaped is underestimated. Severe storms have resulted 

in rare but massive escapes. In 1996, 1997, and 1999, escapes of approximately 

107,000, 369,000, and 115,000 fish, respectively, were reported from Washington’s 

salmon farms (Washington Department of Fish and Wildlife 2001). British Columbia records indicate that between 1991 and 2002, in total 452,049 Atlantic 

salmon escaped from farms, including freshwater hatcheries and marine net pens 

(Canada Department of Fisheries and Oceans 2003). Again, these figures include 

only reported escapes. As shown in table 1, in some years no escapes were reported 

despite maturing Atlantic salmon being captured in commercial and sport fisheries.

Elsewhere, major storms and pen construction failures have liberated even 

larger numbers of farmed salmon. Soto et al. (2001) reported that salmon farms 

The majority of salmon 

that escape from 

farming operations do 

so during large storms 

that damage the pens.
General Technical Report PNW-GTR-697



in southern Chile lost several million fish during heavy storms in 1994–95, and 

escapes of hundreds of thousands of fish have occasionally occurred in European 

salmon farms. Since that time, the industry claims to have increased the security 

of pens by improving their design, but a World Wildlife Fund Report (WWF 

2005), based on recent Norwegian government data, states that salmon escape rates 

from Norway’s farms (the world’s largest farmed Atlantic salmon producers) in 

2001–2004 still ranged from about 350,000 to 750,000 fish annually. The presence 

of large numbers of farmed salmon in rivers near the pens and in nearby capture 

fisheries supports the hypothesis that escaped fish may be more common than 

industry-supplied escape statistics suggest.

In addition to mass escapes during storms, some fish are stocked in net pens but 

never harvested in normal operations. These fish are termed “leakage” because it is 

assumed they jump out of the pens or swim through the mesh openings in the nets 

or cages. Volpe (2001) estimated that leakage accounted for 0.5 to 1.0 percent of the 

total annual production of Pacific Northwest salmon-rearing operations. For Atlantic salmon farms in Washington and British Columbia, such an escape rate would 

amount to tens of thousands of fish per year.

Although escape statistics are almost nonexistent for freshwater hatcheries 

where young Atlantic salmon are reared until transferred to marine pens, there is 

some evidence that young salmon have entered streams into which hatchery waters 

Table 1—Atlantic salmon escapes and recoveries in Washington State 

from 1990 to 2002 

Farm Escape Commercial Sport Hatchery

Year production numbers catch catch recoveries

1990 996,083 None reported 449 4 None reported

1991 1,362,671 None reported 1,012 17 None reported

1992 2,391,884 None reported 166 0 None reported

1993 1,728,195 None reported 225 25 None reported

1994 2,095,769 None reported 369 9 None reported

1995 1,897,874 None reported 200 7 None reported

1996 2,001,282 107,000 120 16 2

1997 2,133,791 369,000 2,204 99 53

1998 946,616 22,639 31 17 None reported

1999 1,211,997 115,000 14 3 25

2000 1,296,025 None reported 15 1 None reported

2001 1,497,695 None reported 6 14 None reported

2002 1,138,026 None reported 2 0 None reported

Source: Washington Department of Fish and Wildlife 2001 (data updated 2003).
Assessment of the Risk of Invasion of National Forest Streams in the Pacific Northwest by Farmed Atlantic Salmon

are discharged. In July 2003, approximately 250 juvenile Atlantic salmon, “some 

up to a foot long,” were observed in Scatter Creek, a Chehalis River tributary in 

western Washington that receives discharge water from a commercial Atlantic 

salmon hatchery (Washington Department of Fish and Wildlife 2003). At present, it 

is not known whether any of these fish survived to return to the Chehalis River as 

spawning adults.

Many Atlantic salmon that escape from pens are never seen again (Alverson 

and Ruggerone 1997), but some are caught by sport or commercial fishers in both 

marine and freshwater environments, or adult salmon are observed in rivers during 

spawning surveys (Atlantic salmon, like semelparous Pacific salmon, are autumn 

spawners). Few are reported relative to the large numbers that escape, but reported 

captures are likely underestimates of fish actually caught. Captures may not be 

reported to state agencies, or the fish may be confused with native salmonids.

Although the majority of escaped Atlantic salmon that are captured are taken 

close to where they originated, some fish are captured at relatively great distances. 

Soto et al. (2001) found that most escapees stayed near the net pens and continued 

to feed on food pellets that fell through the netting. However, Brodeur and Busby 

(1998) captured an immature Atlantic salmon near the Pribilof Islands in the Bering 

Sea, hundreds of kilometers from where it was presumably reared in British Columbia. Every year Atlantic salmon are taken incidentally in commercial fisheries in the 

Gulf of Alaska, an area that lacks salmon farms. Adult Atlantic salmon have also 

been caught or observed in many streams on Vancouver Island (Canada Department 

of Fisheries and Oceans 2003), as well as in Puget Sound and the Strait of Juan de 

Fuca. To date, there appear to be no published records of any adult Atlantic salmon 

originating from salmon farms having been captured in Washington or Oregon 

coastal streams, or in Columbia River tributaries.

Evidence for Established Atlantic Salmon 

Populations on the Pacific Coast

Evidence that escaped Atlantic salmon have established populations on the Pacific 

coast of North America is mixed. Atlantic salmon have been deliberately stocked in 

streams throughout the world for a century (MacCrimmon and Gots 1979, Waknitz 

et al. 2002), but successful establishment of anadromous (sea-run) Atlantic salmon 

populations outside their native range–northwestern Europe, Iceland, and northeastern North America–has been very rare. Table 2 gives examples of attempts to 

establish Atlantic salmon populations through deliberate introductions.

In North America, Waknitz et al. (2002) reviewed the success rate of deliberate introductions of Atlantic salmon in the United States. They reported that there 
General Technical Report PNW-GTR-697

have been at least 170 attempts to establish self-reproducing populations in 34 

states, none of which succeeded. Atlantic salmon were first brought to the Pacific 

Northwest in 1904 (MacCrimmon and Gots 1979) for recreational angling, including some fish stocked in mountain lakes. According to Waknitz et al. (2002), these 

attempts produced no naturally spawning populations, and Atlantic salmon stocking in Washington State was discontinued in the early 1990s. In Oregon, the best 

known location of a successful Atlantic salmon fishery is at Hosmer Lake (formerly 

Mud Lake), a high mountain lake in the Deschutes River subbasin of the Columbia 

River. Hosmer Lake was barren of fish until brook trout (Salvelinus fontinalis) were 

stocked in 1929, followed by Atlantic salmon in 1958. A successful Atlantic salmon 

fishery has resulted, but the population is maintained by hatchery spawning.

In British Columbia, approximately 7.5 million juvenile Atlantic salmon were 

released to establish self-reproducing populations between 1905 and 1934 (MacCrimmon and Gots 1979). With the possible exception of the Cowichan River 

on lower Vancouver Island (where nonnative brown trout Salmo trutta have also 

become established), there has been no evidence that viable populations resulted 

Table 2—Examples of attempts to introduce Atlantic salmon outside their native range

Source and year Destination Reason Status

Holland 1929 Indonesia Fill a “vacant niche” Failed

United States of Cyprus Aquaculture Failed, but efforts

America 1971 are continuing

United States of Australia Angling Hatchery maintained

America 1963–64, 1964–67

Multiple sources 1864–1995 New Zealand Angling Established in one lake 

and river in South Island

Canada 1904, 1970 Argentina Aquaculture, angling Successfully introduced into

L. Fagnano, Traful, Carrhue

Grande, and some southern rivers

United States of Brazil Angling Failed

America 1957

Germany 1935; Chile Angling, aquaculture Uncertain, but a number of

more recently from Norway established populations exist 

in lakes and rivers of southern

Chile and the Patagonian region

United Kingdom 1960 Falkland Islands Angling Failed

United Kingdom 1960 Kerguelen Islands Unknown Failed

Source: Welcomme 1988.
Assessment of the Risk of Invasion of National Forest Streams in the Pacific Northwest by Farmed Atlantic Salmon

from these early attempts. Until the late 1990s, therefore, Atlantic salmon had not 

become established in the Pacific Northwest, except for very few populations in 

mountain lakes that were maintained by artificial propagation.

The proliferation of Atlantic salmon farms in Puget Sound and British Columbia in the 1990s, coupled with repeated escapes of large numbers of farmed 

fish, initiated concerns that unintended populations of this species could invade 

watersheds where native salmon and steelhead were sharply declining (Alverson 

and Ruggerone 1997). The possibility that Atlantic salmon could become an 

invasive species harmful to native fauna led to targeted investigations of coastal 

streams in areas near where escaped fish had been caught or reportedly observed. 

Volpe et al. (2000) surveyed the Tsitika River in northeastern Vancouver Island, 

specifically looking for juvenile Atlantic salmon that had been produced by successful spawning of escaped farmed fish. In late summer 1998, 12 unidentified 

juveniles of the genus Salmo were captured, and another 28 were observed but not 

captured by snorkelers. All were seen in proximity to juvenile steelhead O. mykiss.

Mitochondrial DNA analysis subsequently confirmed the identity of the specimens 

as juvenile Atlantic salmon. Fish of two distinct size classes were represented in 

the sample, and seasonal variation in the growth rings on scale samples indicated 

stream rearing (including a winter growth check in older, larger specimens) rather 

than farm rearing. Thus, both age 0+ and age 1+ individuals appeared to be present 

in the river. Volpe et al. (2000) concluded that the most likely explanation for the 

presence of juvenile Atlantic salmon in the Tsitika River was natural reproduction 

of feral adults. The nearest Atlantic salmon farm was approximately 26 km from 

the river mouth, although it was impossible to determine the exact origin of the 

parents of juvenile Atlantic salmon found in the study. The authors stated: “Based 

on our observations, we suggest that Atlantic salmon may constitute an invading 

species” (Volpe et al. 2000: 902).

Evidence suggesting that Atlantic salmon had become established in a coastal 

stream was greeted with much concern within the conservation community and 

denials from the local aquaculture industry. The Canadian Department of Fisheries 

and Oceans set up an Atlantic Salmon Watch Program (http://www.pac.dfo-mpo.

gc.ca/sci/aqua/ASWP_e.htm) as a clearinghouse for information on Atlantic salmon 

sightings. The Web site includes information about sightings or captures of juvenile 

and adult Atlantic salmon from British Columbia, Alaska, and Washington from 

the early 1990s to 2002 (none of the tables contain more recent information). Alaska 

catches are reported by marine area. Washington catches are reported by marine 

catch area or by river. British Columbia data include marine captures, adults in 

fresh water, and juveniles in fresh water. At present there are no records of juvenile 

The proliferation of 

Atlantic salmon farms 

in Puget Sound and 

British Columbia in 

the 1990s, coupled 

with repeated escapes 

of large numbers of 

farmed fish, initiated 

concerns that 

unintended populations 

of this species could 

invade watersheds 

where native salmon 

and steelhead were 

sharply declining.
General Technical Report PNW-GTR-697

Atlantic salmon caught or observed in streams in Alaska or Washington, except 

for the fish observed near an Atlantic salmon hatchery in the Chehalis River basin 

of coastal Washington (see above). In an Internet fact sheet dated August 2001, the 

Washington Department of Fish and Wildlife states “To date, no naturally-produced 

juvenile or adult Atlantic salmon have been found in Washington, in spite of extensive monitoring of outmigrating Pacific salmon smolts in the streams and rivers of 

the state” (Washington Department of Fish and Wildlife 2001). Since 2000, Alaska 

Department of Fish and Game biologists have snorkeled select streams where adult 

Atlantic salmon have been sighted, including the Situk River (near Yakutat), Ford 

Arm (near Sitka), and Ward Creek (near Ketchikan). So far, no juvenile Atlantic 

salmon produced by spawning feral adults have been detected (Alaska Department 

of Fish and Game 2005).

Whether escaped Atlantic salmon have actually established breeding populations in British Columbia streams still remains uncertain. According to the Atlantic 

Salmon Watch Program database, juveniles were observed or captured in 12 

streams on Vancouver Island, but 9 of these streams likely held juveniles that escaped from net pens shortly after being transferred from hatcheries and subsequently moved back into fresh water. Only three streams (Adam and Eve River, Amor 

de Cosmos Creek, and Tsitika River) contained fish that showed characteristics of 

being the progeny of feral Atlantic salmon spawning (table 3). It is noteworthy that 

no Atlantic salmon juveniles were collected or observed in the three streams in 

Table 3—Freshwater observations of juvenile Atlantic salmon in British Columbia 

from 1996 to 2002

River/Lake 1996 1997 1998 1999 2000 2001 2002 Total

Adam River 3 3

Amor de Cosmos Creek 113 8 121

Cameleon Harbour Creeka 1 1

Carnation Creek a 1 3 1 2 7

Georgie Lake a 41 21 86 30 178

Keogh River a 1 2 3

Lake of the Mountains Creek a 3 4 7

Lois Lake a 13 13

Pye Creek a 1 1

Ritterdon Creek a 1 1

Stamp River a 3 3

Tsitika River 24 2 3 29

Total 54 26 114 147 15 3 8 367

aAtlantic salmon from these sites were believed to be escapees from hatcheries.

Source: Canada Department of Fisheries and Oceans 2003.
Assessment of the Risk of Invasion of National Forest Streams in the Pacific Northwest by Farmed Atlantic Salmon

2001 and 2002. Most of the juveniles were observed from 1996 to 2000, an interval 

that included a major El Niňo event accompanied by large storms that resulted in 

mass escapes from some farms. Volpe (2001) pointed out that less than 1 percent of 

British Columbia’s streams possessing potential Atlantic salmon habitat have been 

surveyed for this species, and the same is surely true for Alaska and Washington. 

Therefore, the current status of fully established populations of Atlantic salmon in 

the Pacific Northwest (if any exist) is unknown.

National Forest Streams Potentially at 

Risk of Atlantic Salmon Invasion

Based on the existing location of Atlantic salmon farms and records of escaped 

Atlantic salmon sightings, streams on four national forests may be at risk for this 

potentially invasive species: the Tongass and Chugach National Forests in Alaska, 

and the Olympic and Mount Baker-Snoqualmie National Forests in Washington. 

Given the large number of coastal streams in the Tongass National Forest, almost 

any stream to which anadromous salmonids have access can be considered vulnerable. Sightings of Atlantic salmon near the Chugach National Forest have been 

quite rare and limited to streams in the southern part of the forest. The most recent 

freshwater sighting was an adult Atlantic salmon captured in the southern Copper 

River delta in 2003,1 and it seems likely that the area of vulnerability to invasion 

is limited to Prince William Sound. In Washington, Mount Baker-Snoqualmie 

National Forest streams draining into Puget Sound may be at risk, and eastern and 

northern Olympic National Forest streams draining into Hood Canal and the Strait 

of Juan de Fuca are also at risk of invasion.

Atlantic Salmon Life History, Habitat Preferences, 

and Potential Interactions With Pacific Salmon

Migration and Spawning

Within their native range of the North Atlantic Ocean, Atlantic salmon travel over 

large marine areas. Unlike some species of Pacific salmon, which remain close to 

their natal river system after entering saltwater, Atlantic salmon move quickly to 

oceanic foraging grounds in late spring and summer (Hansen and Quinn 1998). 

Once at sea, Atlantic salmon exhibit considerable variation in the amount of time 

they remain before returning to spawn. In general, the longer they remain at sea, 

1Reeves, G. 2003. Personal communication. Research fish biologist, Forestry Sciences Laboratory, 3200 SW Jefferson Way, Corvallis, OR 97331.

Based on the existing 

location of Atlantic 

salmon farms and 

records of escaped 

Atlantic salmon 

sightings, streams on 

four national forests 

may be at risk for this 

potentially invasive 

species: the Tongass 

and Chugach National 

Forests in Alaska, and 

the Olympic and Mount 

Baker-Snoqualmie 

National Forests in 

Washington. 
General Technical Report PNW-GTR-697

10

the larger they become as spawning adults; large adult males can reach 1.5 m long 

and weigh up to about 35 kg, whereas the maximum size of females tends to be 

somewhat smaller (Hendry and Cragg-Hine 2003). Oceanic homing migration 

begins after 1 to 4 years, with older individuals tending to return earlier than 

younger ones. Timing of freshwater entry depends on the location of natal rivers. 

Hansen and Quinn (1998) reported that salmon can enter rivers in Scotland during 

any month of the year, similar to steelhead in the Pacific Northwest, but salmon 

from Norway enter rivers only from May to October. Like other anadromous salmonids, olfactory cues acquired as juveniles govern their migratory routes once in 

fresh water. For naturally spawned fish, most return to their home river or stream. 

Within a river basin there may be distinct runs of Atlantic salmon that return to 

different tributaries at different times, similar to some species of Pacific salmon. 

Stewart et al. (2002) found that timing and location of Atlantic salmon migration 

was partly under genetic control, suggesting that different breeding populations 

within a river system can adapt to local conditions.

Atlantic salmon are iteroparous, that is, they are capable of multiple spawning 

attempts; however, the incidence of repeat spawning is about 10 percent or less in 

most populations (Mills 1989). Adult Atlantic salmon are strong swimmers and 

excellent jumpers (the species name derives from the Latin salio, to leap). Vertical 

jumping distances have been reported up to 3.7 m, with horizontal jumping distances measured up to 5 m (Nova Scotia Department of Fisheries and Aquaculture 

2005). Migration distances are highly variable, ranging from a few kilometers to 

more than 500 km. Atlantic salmon may spawn in streams and rivers of almost 

any size, but there are relatively few records of lake spawning within anadromous 

populations. Maturing Atlantic salmon generally stop feeding during spawning 

migrations and instead draw on fat reserves built up prior to entering fresh water.

In both anadromous (sea-going) and adfluvial (lake-going) populations, spawning usually takes place in autumn, although in some rivers spawning may extend 

into early winter (Atlantic Salmon Trust 2006). Like virtually all salmonids, 

Atlantic salmon prefer to spawn in clean, well-aerated gravel. Hendry and CraggHine (2003) stated that egg and alevin (sac fry) survival is compromised when the 

level of sediment less than 2 mm diameter exceeds 20 percent of the spawning 

substrate by weight. The spawning requirements of Atlantic salmon are therefore 

quite similar to those of Pacific salmon.

Although females choose the locations of spawning sites, adult males will 

remain in suitable spawning areas and defend females and their redds against the 

advances of other males. Dominance hierarchies are normally size-based, and 

multiple spawning events can occur for both males and females (Mills 1989). 
Assessment of the Risk of Invasion of National Forest Streams in the Pacific Northwest by Farmed Atlantic Salmon

11

Aggressive behavior, including displaying, chasing, and biting, can also occur in 

both sexes, with the advantage usually going to the larger individual–size matters 

more to males than females–or to the individual that has already established a territory (Fleming 1998). Larger individuals are less likely to spawn again than smaller 

individuals, and females are more likely to exhibit repeat spawning than males. 

Hendry and Beall (2004) observed that adult salmon with greater fat reserves 

tended to exhibit more violent and protracted aggressive behavior than those whose 

fat reserves had been depleted, and they suggested that the condition of fish arriving at the spawning grounds influences reproductive success.

Because both Atlantic salmon and semelparous (1-time spawning) Pacific 

salmon breed during the same season and in similar locations, there is a possibility 

that they might interact. Evidence for aggressive encounters between spawning 

adult Atlantic and Pacific salmon is rare because their native distributions are not 

sympatric. However, in the Great Lakes region of the St. Lawrence River, Pacific 

salmon have been successfully introduced and may co-occur with native Atlantic 

salmon. Tributaries to Lake Ontario are used by naturally reproducing Atlantic 

salmon and several introduced Pacific salmon species, and the Ontario Ministry 

of Natural Resources states “There are some studies and anecdotal observations 

which suggest negative interactions between Chinook salmon and Atlantic salmon 

spawners” (Ontario Ministry of Natural Resources 2005). The extent to which these 

interactions reduce the fitness of one or both species has not been reported.

Juvenile Morphology and Rearing Habitat

Juvenile Atlantic salmon usually prefer relatively swift streams and rivers for rearing. Young salmon usually emerge from spawning areas in the spring, and newly 

emergent fry tend to remain near the bottom where they are sheltered from turbulent currents in overlying water (fig. 1).

Although Atlantic salmon juveniles tend to prefer fast water habitats, their body 

morphology and microhabitat preferences differ somewhat from those of juvenile 

steelhead, a species that also inhabits swiftly flowing streams, and learning to 

distinguish them in the field is essential for fish population monitoring. Atlantic 

salmon have relatively large pectoral fins, which they can angle downward to aid 

in holding against the bottom (fig. 1). Steelhead, on the other hand, usually occur 

higher in the water column and do not possess such large pectoral fins. Additionally, the two species can be distinguished in the field by comparing the number and 

shape of parr marks; the height, shape, and coloration of the dorsal fin; and head 

coloration. Paired photos of juvenile Atlantic salmon and steelhead fry (age 0+) and 

parr (age 1+) are shown in figure 2. Many juvenile Atlantic salmon also possess 

Juvenile Atlantic 

salmon usually prefer 

relatively swift streams 

and rivers for rearing.
General Technical Report PNW-GTR-697

12

small red spots near the lateral line—a trait never seen in steelhead or other Pacific 

salmon.

Distinguishing juvenile Atlantic salmon from young Chinook and coho salmon 

is somewhat easier. Juvenile Chinook and coho salmon tend to have deeper bodies, 

longer and more compressed parr marks, a narrower caudal peduncle, and more 

deeply forked tails (fig. 3) than juvenile Atlantic salmon. Additionally, juvenile coho 

salmon usually have a pronounced white band along the leading edge of the dorsal 

and anal fins. Like steelhead, young of both species tend to occur higher in the 

water column than Atlantic salmon.

Juvenile Migrations

During the freshwater phase of their life cycle, young Atlantic salmon are usually thought to be highly territorial, to exhibit strong site fidelity, and to rarely move 

around in streams in response to seasonal changes (Gerking 1959). Immediately 

after emergence, fry generally move to the edge of the stream where current velocities are more favorable for their small size. As they grow, juvenile Atlantic salmon 

tend to occupy deeper waters, although they continue to reside in or near swiftly 

flowing habitats (Gibson 1993, McCormick et al. 1998). Parr (age 1+) appear somewhat less prone to seasonal movements such as can be observed in the migration of 

juvenile coho salmon to riverine ponds in winter (Peterson 1982), but in severely 

fluctuating environments Mäki-Petäys et al. (2004) showed that size-mediated 

Figure 1—Atlantic salmon fry in a Vancouver Island stream. This fish 

was believed to be the offspring of feral Atlantic salmon spawning. 

John Volpe
Assessment of the Risk of Invasion of National Forest Streams in the Pacific Northwest by Farmed Atlantic Salmon

13

Figure 2—Upper left: Atlantic salmon fry. Upper right: steelhead fry. Lower left: Atlantic salmon 

parr. Lower right: steelhead parr. Photos: John Volpe (upper and lower left), David Blevens (upper 

right), and Russ Thurow (lower right).

Figure 3—Left: juvenile Chinook salmon. Right: juvenile coho salmon. Photos: Peter Bisson 

(left) and Canada Department of Fisheries and Oceans (right).

shifts in preferred depth, velocity, and substrate composition of Atlantic salmon 

parr can occur throughout the year. Some Atlantic salmon can rear in lakes rather 

than streams, and this ability has been used to explain the origin of nonanadromous 

populations (Berg 1985). Additionally, male Atlantic salmon parr can mature in 

streams and forego anadromy (Fleming 1996). In this respect, they differ from 

most Pacific salmon species in which freshwater maturation is quite rare (except for 

landlocked forms such as kokanee [O. nerka]).

Smolting takes place when the fish have reached approximately 10 to 20 cm 

long and often occurs between 2 and 4 years depending on the productivity of the 

stream. In the Northeastern United States, Baum (1997) reported that about four of 

five juveniles smolt after 2 years, with the balance smolting after 3 years. Marschall 
General Technical Report PNW-GTR-697

14

et al. (1998) summarized the evidence that rapidly growing parr often smolt at a 

relatively young age (2 year), or forego anadromy entirely and mature in streams. 

There is a positive association between growth rate, smolting and parr maturation, 

but faster growing, younger smolts are frequently smaller (because they are young) 

than slow-growing, older smolts. Large, older smolts typically enjoy higher marine 

survival rates than smaller smolts, as do some species of Pacific salmon. The age 

structure of Atlantic salmon populations is therefore complex and can be influenced 

by a variety of environmental and genetic factors related to tradeoffs between age, 

growth, and survival.

Density Dependence and Interspecific Interactions

Consistent with their territorial behavior as juveniles, there is often a negative relationship between population density, growth, and survival of young Atlantic salmon 

in lotic environments where rearing space is limited (Elliott and Hurley 1997, Grant 

et al. 1998). Nislow et al. (2004) found that age 0 salmon in a Connecticut River, 

New Hampshire, tributary grew more slowly in two low-flow years than in a highflow year where rearing space included expanded foraging locations. Growth rates 

were also negatively correlated with population density. Like most stream-dwelling 

salmonids, juvenile Atlantic salmon exhibit density dependence in response to food 

and habitat availability.

Within their native range of western Europe and eastern North America, 

farmed Atlantic salmon regularly escape and enter rivers where they encounter 

native salmon populations. Jonsson (1997) summarized the potential intraspecific 

impacts of farmed salmon on their native counterparts in fresh water. Major effects 

included competition for food, rearing habitat, and mates; altered predation 

regimes; and increased risks of diseases and parasites. Fleming et al. (2000) 

studied the population effects of farmed salmon in a Norwegian river and found 

that farmed fish were competitively and reproductively inferior to wild fish. Farmed 

salmon achieved less than one-third the reproductive success of native salmon, and 

the productivity of the native population was reduced by about 30 percent as a result of resource competition and competitive displacement. Fleming and Petersson 

(2001) reviewed the evidence of hybridization occurs between wild and hatcheryorigin Atlantic salmon and found that the fitness of naturally spawning populations 

was nearly always reduced through interbreeding between wild and hatchery adults. 

They concluded that Atlantic salmon from hatcheries posed significant ecological 

and genetic risks to wild populations.

The extent to which juvenile Atlantic salmon influence, or are influenced by, 

juveniles of other salmonid species is less well known. Atlantic salmon fry and parr 
Assessment of the Risk of Invasion of National Forest Streams in the Pacific Northwest by Farmed Atlantic Salmon

15

can display aggression toward other fishes (Keeley and Grant 1998, also see Fausch 

1998), but I was unable to find any evidence that high densities of Atlantic salmon 

juveniles resulted in reduced populations of other fishes. For example, Raffenberg 

and Parrish (2003) sampled two streams in Vermont where Atlantic salmon fry had 

been stocked to accelerate the recovery of naturally spawning populations. The 

streams contained native brook trout and introduced rainbow trout. No inverse relationships between Atlantic salmon abundance and trout populations were detected; 

in fact, the more productive sites, that is, those with increased benthic invertebrates, 

contained higher densities of both salmon and brook trout. The authors concluded 

that habitat partitioning permitted native species (Atlantic salmon and brook trout) 

to coexist without negative population consequences.

Because the habitat preferences of juvenile Atlantic salmon and juvenile 

steelhead overlap considerably (Hearn and Kynard 1986), a major concern about 

farmed salmon invading Pacific Northwest watersheds is that they would negatively 

affect already depressed steelhead populations. To test the effect of Atlantic salmon 

on steelhead in streams, Volpe et al. (2001) stocked experimental stream channels 

on Vancouver Island with low, medium, and high densities of both species and 

monitored agonistic behavior and growth. They found that juvenile steelhead displayed more aggressive behavior than juveniles from an Atlantic salmon farm, but 

steelhead were more than two times more likely to direct aggression at conspecifics 

than toward members of the other species. Atlantic salmon, although less aggressive overall, were more than two times more likely to attack juvenile steelhead 

than other Atlantic salmon. Young Atlantic salmon fared poorly when released into 

habitats already populated by steelhead, but when Atlantic salmon were released 

into the experimental channels first and had an opportunity to establish foraging 

territories prior to the introduction of steelhead, the salmon generally outcompeted 

steelhead. Prior residence, therefore, was an important factor in determining 

competitive outcomes between the two species (also see O’Connor et al. 2000). 

Volpe et al. (2001) argued that because Atlantic salmon spawn in late fall or early 

winter, two or more months prior to most steelhead, their progeny would be likely 

to have established feeding territories in streams before steelhead fry emerge from 

spawning gravels. They concluded that “a 2-month head start on the competition 

would prove to be a significant factor” in affecting the ability of Atlantic salmon to 

become established in streams where steelhead are present (Volpe et al. 2001: 205).

Food Habits

Juvenile Atlantic salmon are generalized invertevores, feeding mainly on aquatic 

insect larvae acquired from the benthos or drifting in the stream (Gibson 1993, 
General Technical Report PNW-GTR-697

16

Mills 1989). The majority of life history studies report that both fry and parr feed 

on seasonally available aquatic and terrestrial invertebrates (Cunjak 1992, Thonney and Gibson 1989) and I found few scientifically documented cases of juvenile 

Atlantic salmon in streams feeding on other fishes or on fish eggs, although it would 

seem reasonable to expect them to consume small fishes or eggs if presented with 

the opportunity. In Patagonia, adfluvial Atlantic salmon feed on aquatic invertebrates as young and switch to a diet of crustaceans and other fishes as they mature 

in lakes (Expediciones Chile 2005). Keeley and Grant (1998) found that Atlantic 

salmon fry initially fed on small chironomid larvae, but as they grew older and 

larger began taking proportionately larger food items from the drift. They suggested that diet preferences demonstrated that age 1+ and older fish were size selective 

in their foraging behavior, but they also found that young Atlantic salmon fed on 

smaller prey items than other salmonids of similar size.

Based on the majority of evidence, the likelihood seems low that juvenile 

Atlantic salmon would prey upon native fishes in Pacific Northwest streams. 

However, a diet of seasonally available aquatic and terrerstrial invertebrates would 

place them in direct competition for the food resources of native salmonids. The 

ecological threat, therefore, is that established populations of Atlantic salmon could 

act as competitors but probably not predators on salmon and trout.

Diseases and Parasites

Atlantic salmon are known to be vulnerable to a variety of diseases and parasites, 

particularly those in hatcheries or aquaculture facilities. Parasites from farmed 

salmon can spread to wild fish when escaped salmon enter rivers to spawn. One 

of the most serious problems for wild salmon in Norway has been a small monogenetic trematode Gyrodactylus salaris that has become established in many river 

systems and has significantly reduced, and sometimes extirpated, native salmon 

populations (Håstein and Linstad 1991).

Although Gyrodactylus infestations can cause high mortality rates in wild 

salmon that have been newly exposed to the parasite via escaped farmed salmon, 

some salmon stocks (as well as populations of other native European salmonids) 

appear to be resistant to heavy infestations. However, the widespread damage 

caused by Gyrodactylus has been so severe that most countries in western Europe 

require elaborate quarantine and disinfection procedures for all fish transfers that 

could spread the parasite. Additionally, a variety of internal parasites have been 

associated with Atlantic salmon of hatchery origin, but to date, none have been as 

serious as Gyrodactylus.

The ecological threat 

is that established 

populations of Atlantic 

salmon could act 

as competitors but 

probably not predators 

on salmon and trout.
Assessment of the Risk of Invasion of National Forest Streams in the Pacific Northwest by Farmed Atlantic Salmon

17

Atlantic salmon are also vulnerable to many viral, bacterial, and fungal diseases. Outbreaks of these diseases are apparently rare in naturally spawning salmon 

populations, but can be common in fish raised in a high-density aquaculture environment (U.S. Fish and Wildlife Service 1999). Many of the diseases can be treated 

in the hatchery by antibiotics and antiviral vaccinations, or by bathing fish in a 

disinfecting solution. Transfer of viral, bacterial, or fungal diseases from farmed to 

wild salmon undoubtedly occurs, but epizootics in wild populations directly traceable to farmed salmon are less common than parasite outbreaks. Nevertheless, the 

virus causing infectious salmon anemia has spread from Norwegian aquaculture 

facilities to a number of rivers in Europe, and more alarmingly, has also spread to 

several locations in eastern North America (Cipriano 2002). In addition to causing 

millions of dollars of lost salmon production in western Europe, the virus is known 

to infect Chinook salmon, coho salmon, rainbow trout, and several char species.

Parasites and diseases affecting Atlantic salmon in fresh water, and the potential transfers of pathogens from farmed salmon to native species, have not received 

much research attention in the Pacific Northwest. On the other hand, heavy infestations of both hatchery and wild fish by a marine copepod parasite, the sea louse 

Lepeophtheirus salmonis, has been well documented in British Columbia (Morton 

et al. 2004). Sea lice are ectoparasites whose early life history stages thrive near 

marine salmon farms where uneaten food and fish waste can accumulate. Freeswimming life history stages of sea lice (copepodids) can attach themselves to 

wild fish swimming in the vicinity of net pens and at high infestation rates can be 

debilitating or lethal.

Morton et al. (2004) found that 90 percent of the juvenile pink salmon and 

chum salmon (O. keta) sampled in the Broughton Archipelago of coastal British 

Columbia, the site of many of the province’s salmon farms, possessed sea lice at a 

density of greater than 1.6 g lice/g host mass—the purported lethal level for sea lice 

infestation—and declines of these species have been directly associated with the 

proliferation of nearby farming operations. In contrast, Morton et al. (2004) found 

that juvenile salmon sampled from areas without salmon farms possessed almost 

no sea lice. So serious has the sea lice problem in British Columbia become that a 

public forum on salmon farming (Leggatt Inquiry) recommended that all net cage 

salmon farms be removed from coastal British Columbia waters (David Suzuki 

Foundation 2001).
General Technical Report PNW-GTR-697

18

Native Diseases and Parasites as 

Potential Barriers to Atlantic Salmon Invasion

A question that has received little scientific investigation is the extent to which 

parasites and diseases endemic to the Pacific Northwest might act as natural 

barriers to invasions of nonnative salmonids such as Atlantic salmon. Because 

Atlantic salmon are vulnerable to a wide variety of pathogens, and because their 

populations may be strongly affected by exposure to a new pathogen (see the 

discussion of Gyrodactylus above), it is possible that parasites and diseases to 

which Pacific salmon have adapted over time my be highly deleterious to Atlantic 

salmon and may prevent or hinder their colonization of Pacific Northwest streams.

There is at least one instance in Washington and Oregon where a nonnative 

species has failed to become established because of an endemic parasite. Brook 

trout, a species from eastern North America, have been widely stocked in both 

streams and lakes in the Pacific Northwest but have established breeding populations primarily in montane lakes. They are conspicuously absent from low-elevation 

coastal streams despite a century of sanctioned and clandestine releases. At present, 

only two breeding populations of brook trout are known from coastal streams in 

Washington and Oregon: a small tributary of the Willapa River in Washington, 

and a tributary of the upper Millicoma River in Oregon. Both streams possess high 

waterfalls and the brook trout are found only above these falls. Coincidentally, both 

streams contain the snail Juga plicifera below the falls but not above. 2 This snail 

species hosts an intermediate stage (redia) of a digenetic trematode Nanophyetus 

salmincola (a flatworm parasite) that itself harbors a rickettsia-type pathogen that 

causes “salmon-poisoning” disease in dogs. The rickettsia are not toxic to fish, but 

another life history stage (cercaria) of the flatworm carrier leaves its snail host and 

encysts in the skin and other tissues of salmon. Infested snails can spread the parasites to salmon (Baldwin et al. 1967), and high infestations of the encysted Nanopyetus in young salmon can lead to poor swimming performance and kidney damage 

(Foott et al. 1996). The absence of brook trout where Nanophyetus-hosting snails 

occur and the presence of brook trout where snails are absent suggests that this nonnative salmonid does not tolerate the endemic parasite and has been prevented from 

becoming established in coastal streams where the snail host is present.

2The occurrence of Juga below Millicoma Falls was noted by P. Reimers, personal 

communication, formerly of Oregon Department of Fish and Wildlife, P.O. Box 5430, 

Charleston, OR 97420. Juga was found below the falls on Fall Creek, a tributary to the 

Willapa River, by P. Bisson (author) and B. Fransen, Weyerhaeuser Company, P.O. Box 9777, 

Federal Way, WA 98063.

A question that has 

received little scientific 

investigation is 

the extent to which 

parasites and diseases 

endemic to the Pacific 

Northwest might act 

as natural barriers to 

invasions of nonnative 

salmonids such as 

Atlantic salmon.
Assessment of the Risk of Invasion of National Forest Streams in the Pacific Northwest by Farmed Atlantic Salmon

19

Can their vulnerability to native parasites and diseases be one of the reasons 

why Atlantic salmon have not established more breeding populations in Pacific 

Northwest watersheds? This topic deserves additional investigation. Additionally, 

selection for native disease or parasite tolerance in Atlantic salmon from farming 

operations, if it occurs, may lead to greater invasive potential over time. Further 

research should shed light on the ability of endemic pathogens to limit the invasive 

capability of this species.

Conclusion

This paper has examined the evidence for escaped Atlantic salmon from marine 

farming operations or freshwater hatcheries becoming established in Pacific Northwest streams. Evidence suggests that Atlantic salmon have established in only a few 

streams on Vancouver Island; however, the potential for additional invasion exists 

with continued escapes from holding facilities. The ecological consequences of 

feral Atlantic salmon populations have not been thoroughly explored in this region, 

in part because established populations are rare and limited to a small area. Atlantic 

salmon spawn at approximately the same time as several species of Pacific salmon, 

and their substrate preferences are similar to those of native salmon and steelhead. 

Breeding adult Atlantic salmon may therefore compete with fall-spawning Pacific 

salmon for spawning sites. Likewise, juvenile Atlantic salmon share rearing habitat 

affinities with species that occur in swiftly flowing streams, particularly steelhead, 

resident rainbow and cutthroat trout, and char. Their tendency to be closely associated with the stream bottom may allow a certain amount of microhabitat partitioning with resident and anadromous salmonids, but their food habitats broadly 

overlap, suggesting that abundant Atlantic salmon fry and parr may compete 

with native fishes for scarce food and habitat resources. Because Atlantic salmon 

spawn in late fall or early winter, young will emerge from spawning gravel prior to 

spring-spawning species. One laboratory study indicates that early fry emergence 

may give Atlantic salmon an opportunity to establish foraging territories before 

steelhead and resident trout fry emerge.

Farmed Atlantic salmon can be vectors of exotic diseases and parasites that, at 

least in Europe, have had devastating impacts on naturally spawning fish populations. Although freshwater parasite and disease introductions have not been associated with Atlantic salmon farms in the Pacific Northwest, heavy infestations of sea 

lice near farming sites have been associated with declines in early sea-going Pacific 

salmon juveniles, particularly pink and chum salmon. It is also possibe that the 

failure of Atlantic salmon to establish breeding populations in streams where 

escaped farm salmon are known to have entered may be related to their susceptibilAtlantic salmon have 

established in only 

a few streams on 

Vancouver Island; 

however, the potential 

for additional invasion 

exists with continued 

escapes from holding 

facilities.

Atlantic salmon share 

rearing habitat affinities 

with species that 

occur in swiftly flowing 

streams, particularly 

steelhead, resident 

rainbow and cutthroat 

trout, and char.
General Technical Report PNW-GTR-697

20

ity to native parasites and diseases—a situation that could change over time as 

natural selection favors more disease or parasite-resistant individuals.

The relative risk of Atlantic salmon becoming an invasive species of concern 

can therefore be partitioned into short (<5 years) and long-term (>5 years) time 

horizons. The current risk to national forest streams from Atlantic salmon invasions 

is low and limited to a few forests in northwest Washington and southeast Alaska. 

However, the long-term risks may be substantial if fish continue to escape from 

marine rearing pens or freshwater hatcheries. The two greatest risks appear to be 

that (1) Atlantic salmon may introduce a serious pathogen to native populations, and 

(2) escaped salmon will eventually adapt to local conditions as selection favors the 

survival and reproduction of a few individuals. Salmonids in general are capable of 

rapid evolution because their gamete production is relatively large, they have ample 

opportunity to hybridize, they are occasionally capable of polyploidy, and they 

have somewhat flexible life histories. Despite a long history of failure to establish 

Atlantic salmon from single or a few deliberate introductions, it seems possible 

that continuous recruitment of fish escaping from farming operations may eventually lead to locally adapted stocks. At that point, the species may rapidly become a 

dangerous invasive—a pattern that is often seen in other aquatic plants and animals 

where a prolonged early colonization period is followed by a rapid phase of exponential growth.

Table 4 presents potential short- and long-term risks of Atlantic salmon invasion, based on the evidence gathered from peer-reviewed scientific literature, 

agency reports, environmental and aquaculture industry publications, and the 

Internet.

Early Detection

Current programs to monitor the establishment of Atlantic salmon in the Pacific 

Northwest are inadequate. The province of British Columbia remains engaged in 

surveying streams for this species, but the total number of streams being sampled 

is still only a fraction of those where Atlantic salmon could invade. The states of 

Alaska and Washington also have invested minimal effort in surveying for this 

species, and it appears that the majority of the annual records consist of sport 

catches, which are not very reliable. Records of juvenile Atlantic salmon often 

consist of snorkel observations, although it is quite difficult to be certain of a 

species identification, especially when fish are small and water clarity is poor. 

Actual captures remain the best method of detecting Atlantic salmon, and confirmation is best carried out by meristic and morphometric analysis supplemented 

with genetic identification.

Current programs 

to monitor the 

establishment of 

Atlantic salmon in the 

Pacific Northwest are 

inadequate.
Assessment of the Risk of Invasion of National Forest Streams in the Pacific Northwest by Farmed Atlantic Salmon

21

Table 4—Summary of major short-term (<5 years) and long-term (>5 years) risks of escaped Atlantic Salmon 

from salmon farms becoming an invasive species in freshwater ecosystems in the Pacific Northwest

Ecological Strength of

risk category Short term Long term empirical evidence

Competition for spawning Low Low to moderate. Spawning Only mentioned in one

sites between adult Atlantic sites would have to overlap Canadian study; however, 

and Pacific salmon spatially and temporally. there are relatively few

Not considered a significant locations where Atlantic

problem in eastern North America salmon and Pacific salmon

where Pacific salmon have spawn together.

been introduced.

Competition for rearing Low Moderate to high for species Documented in at least one

space in fresh water that inhabit swiftly flowing controlled laboratory study.

streams or possibly lakes, if 

Atlantic salmon become

established.

Competition for food Low Moderate. The degree of Primary evidence comes

resources in fresh water competition depends on food from studies of sympatric 

availability and forage site farmed and native Atlantic 

selection. Atlantic salmon may be salmon in Europe, but to date 

less aggressive than native salmonids, there is no empirical evidence 

but their larger size in spring may for interspecific competition 

give them an advantage in defending between Atlantic salmon and 

the best feeding sites. Pacific salmon.

Predation on native fishes Low Low. Juvenile Atlantic salmon Food habitats of Atlantic

rarely eat other fishes in streams. salmon juveniles have been 

Landlocked populations in lakes well studied in their native

may be piscivorous. range.

Disease/parasite introductions Low Moderate to high. Pathogens in Widespread documentation 

farms and hatcheries may become of disease and parasite 

resistant to current treatments and outbreaks caused by escaped

spread more easily to native species. farm salmon.

Improvements in monitoring programs could occur by implementing a statistically valid sampling design in streams where Atlantic salmon could potentially be 

present. A rotating panel sampling regime such as the Enviroinmental Protection 

Agency’s Environmental Monitoring and Assessment Program is an example of a 

statistically sound protocol for selecting monitoring sites. Additionally, many 

agencies, tribes, and environmental organizations operate smolt traps to monitor 

salmon production in Pacific Northwest river basins. Training field technicians to 

identify Atlantic salmon and collect any that turn up in the traps would take advantage of existing (and continuously funded) sampling programs.
General Technical Report PNW-GTR-697

22

Should juvenile Atlantic salmon be found in a stream, the watershed can

immediately be targeted for intensive surveys to determine the distribution of 

juveniles within the system and their potential impact on resident and native anadromous species. Depending on the results of these studies, the stream may become 

a candidate for an Atlantic salmon eradication program. Although this action may 

seem extreme, experience with other nonnative salmonids in this area (e.g., brook 

trout) suggests that early detection and removal is far easier than having to combat a 

widespread invasion after the fact.

Acknowledgments

I thank the Pacific Northwest Research Station for support, and particularly the 

Focused Science Delivery Program for a grant to complete this analysis. This paper 

benefitted from the helpful suggestions of Tom Quinn, Keith Nislow, Kelly Burnett, 

and Steve Wondzell.

English Equivalents

When you know:	 Multiply by:	 To find:

Millimeters (mm) 0.039 Inches

Centimeters (cm) .394 Inches

Meters (m) 3.281 Feet

Kilometers (km) .622 Miles

Grams (g) .0352 Ounces

Kilograms (kg) 2.205 Pounds

Metric tons (mT) 1.102 Short tons

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