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Reasons for Listing and Current Threats

In determining whether to list, delist, or reclassify a taxon under the ESA, five threat factors are evaluated, including:

  • Factor A.  the present or threatened destruction, modification, or curtailment of its habitat or range;
  • Factor B.  overutilization for commercial, recreational, scientific, or educational purposes;
  • Factor C.  disease or predation;
  • Factor D.  the inadequacy of existing regulatory mechanisms; and
  • Factor E.  other natural or manmade factors affecting its continued existence.

Threats, categorized into the above factors, and the subsequent reasons for listing Atlantic salmon in Maine have been identified and extensively analyzed (National Research Council, 2004; Fay, et al., 2006).  The 2009 listing rule (74 FR 29344, 2009) highlighted the threat of Dams (Factor A), Regulatory Mechanisms Related to Dams (Factor D), and Marine Survival (Factor E) as being significant factors affecting the continued survival and recovery of the species:

Threats Associated with Factor A:

Significant Threats


Salmon smolts and post spawning adults travel downstream to the ocean from spawning and rearing habitat and encounter the same dams their parents experienced on upstream migration to spawning habitats.  Atlantic salmon face routes that pass through, over, or around the dam and facilities.  For electricity-generating dams, salmon can travel over the spillway of the dam, through a downstream fish passage facility if the dam has one, or through power-generating turbines.  Mortality can occur no matter which path Atlantic salmon take, but higher mortality rates occur if fish pass through the turbines.

Direct, indirect, or delayed mortality from entrainment into dam turbines, passage via spillways, or fish bypass has been shown to be an important factor affecting Atlantic salmon.  Dams can also cause indirect mortality.  Smolts injured or disoriented by dams may become more vulnerable to predators after passage (Mesa 1994).  Descaling can also impair osmoregulation, possibly leading to estuarine mortality (Zydlewski et al. 2010).  Lack of flow cues at dam reservoirs can increase the time in the impoundment and thus delay migration and increase predation (Holbrook et al. 2011).

Measuring indirect mortality from dams is difficult, but recent studies are approaching the problem. Holbrook et al. (2011) estimated the survival of outmigrating Atlantic salmon smolts through a large reach of the Penobscot River (up to Weldon dam) in 2005 and 2006.  This study provides a snapshot of total survival through the river, including all causes of mortality (e.g., direct and indirect mortality from dams, predation, disease, fishing, and stress from handling during the research and assessment).  An extensive acoustic array in the river, coupled with release of smolts with sonic transmitters, allowed researchers to separate out survival in specific reaches, including those with dams and those without dams.  High rates of mortality were observed in three other reaches:  Howland dam reach, West Enfield dam reach, and Milford dam reach.  The estimated survival of tagged hatchery smolts through these three reaches with dams was as low as 52 percent, whereas survival in reaches without dams exceeded 95 percent.  Although these data do not definitively reveal sources of mortality, these losses are likely attributable to the direct and indirect effects of the dams (e.g., physical injury, predation).  Shepard (1991) found that for the three lower Penobscot River dams (Milford, Great Works, Veazie) and the intervening habitat, net smolt survival varied between 44 and 94 percent.  For the lower four dams (West Enfield, Milford, Great Works, and Veazie, or for those fish choosing the Stillwater Branch route, (i.e., West Enfield, Stillwater, Orono, and Veazie) the survival rate was between 38 percent and 92 percent.  Given that many Penobscot salmon must pass through at least three dams and sometimes as many as seven, mortality from dams in any single year is likely to be significant.

In addition to the complete or partial blockage to upstream and downstream passage and direct, indirect or delayed mortality associated with passage through dams, dams have a number of additional negative ecological effects on Atlantic salmon.  Dams create impoundments that inundate the natural stream and river habitat and cause sediment deposition that can cover important rearing and spawning habitat.  Impoundments create large pools of water in which water temperatures can increase above preferred Atlantic salmon temperature levels.  These impoundments and associated habitat changes can become preferred habitat for warm water exotic species that prey on juvenile Atlantic salmon.  Impoundments can cause migratory delays, which, in turn, can cause poor synchrony of physiological tolerance to salinity (McCormick et al. 2009), thereby increasing estuarine mortality (Ferguson et al. 2006).  These effects may be exacerbated by climate change that may also alter predator/prey assemblages by decreasing qualitative habitat features that benefit salmon while concurrently increasing habitat features that benefit predators and competitors.  For additional information, see Fay et al. (2006), and appendix 8 in Fay et al. (2006), and the 2009 GOM DPS Atlantic salmon listing rule (74 FR 29344).

In conclusion, the direct, indirect, and delayed downstream mortality associated with dams and the ecological effects of dams are a significant threat to the recovery of the GOM DPS of Atlantic salmon.

 Secondary Stressors

Habitat Complexity

 Some forest, agricultural, and other land use practices have reduced habitat complexity in the range of GOM DPS Atlantic salmon.  Historic timber harvest practices have reduced the abundance and diversity of large wood (LW) and large boulders from many rivers.  Large wood is important for Atlantic salmon during several life history stages.  Nislow et al. (1999) found that survival of salmon fry in small streams in Vermont was strongly correlated with the availability of low-velocity microhabitats, and that the addition of LW (length of LW greater than bank full width) increases the availability of these microhabitats. Large wood may be even more important for older juveniles, because they use stream cover, including LW, particularly during winter (Cunjak et al. 1998).  In winter, salmon may require habitat that provides adequate shelter from adverse physical conditions and protection from predators.  Thus, availability of high quality winter habitat may influence salmon survival during this critical life stage, LW may increase overwinter survival in Pacific salmon by increasing habitat complexity (Quinn and Peterson 1996, Solazzi et al. 2000), and the same mechanism may apply for Atlantic salmon.

Water Quantity

Direct water withdrawals and groundwater withdrawals for crop irrigation and commercial, and public use can directly impact Atlantic salmon habitat by depleting streamflow (Dudley & Stewart, 2006) (MASTF 1997, Dudley and Stewart 2006, Fay et al. 2006).  Subsequently, reduced stream flow can reduce the quantity of habitat, increase water temperature and reduce dissolved oxygen.  The cumulative effects of individual water withdrawal impacts on Maine rivers is poorly understood; however, it is known that adequate water supply and quality is essential to all life stages and life history behaviors of Atlantic salmon, including adult migration, spawning, fry emergence, and smolt emigration (Fay et al. 2006).

Water Quality

Atlantic salmon likely are impacted by degraded water quality caused by point and non-point source discharges. The MDEP administers the National Pollutant Discharge Elimination System (NPDES) program under the CWA and issues permits for point source discharges from freshwater hatcheries, municipal facilities, and other industrial facilities. Maine’s water classification system provides for different water quality standards for different classes of waters (e.g., there are four classes for freshwater rivers, all of which are found within the GOM DPS range); however, these standards were not developed specifically for Atlantic salmon. Some portions of the GOM DPS are in areas with the highest water quality classification where water quality standards are the most stringent. These standards become progressively less stringent with each lower water classification. Additionally, permits allow an area of initial dilution or mixing zone where water quality requirements are reduced. Salmon in or passing through such zones would be exposed to discharges below water quality standards. The impacts to salmon passing through these zones are unknown. We are concerned that water quality standards for Class A, B, and C waters and mixing zones may not be sufficiently protective of all life stages of Atlantic salmon, particularly the more sensitive salmon life stages (e.g., smolts). Even where water quality standards are believed to be sufficiently protective, there are circumstances and conditions where discharges do not meet water quality standards. For example, there are documented cases in class C waters where dissolved oxygen standards (the lower bound of which is 5.0 ppm) were not met. This occurred in portions of the mainstem Androscoggin River, and in the East Branch of the Sebasticook River and Sabattus River (MDEP, 2008). When dissolved oxygen concentrations are less than 5.0 ppm, adult salmon breathing functions become impaired, embryonic development is delayed, and parr growth and health are impacted; conditions become lethal for salmon at dissolved oxygen concentrations less than 2.0 ppm (Decola, 1970).

When water quality reaches levels that are harmful to salmon, it is a stressor to the GOM DPS. Non-point source discharges such as elevated sedimentation from forestry, agriculture, urbanization, and roads can reduce survival at several life stages, especially the egg stage. Sedimentation can alter in-stream habitat and habitat use patterns by filling interstitial spaces in spawning gravels, and adversely affect aquatic invertebrate populations that are an important food source for salmon. Acid rain reduces pH in surface waters with low buffering capacity, and reduced pH impairs osmoregulatory abilities and seawater tolerance of Atlantic salmon smolts. A variety of pesticides, herbicides, trace elements such as mercury, and other contaminants are found at varying levels throughout the range of the GOM DPS. The effects of chronic exposure of Atlantic salmon, particularly during sensitive life stages such as fry emergence and smoltification, to many contaminants is not well understood. Fay et al. (2006) provide a discussion of water quality concerns in section 8.1.3. For these reasons, non-point source pollution, particularly sedimentation and acid rain, is a stressor to the GOM DPS. In summary, we have determined that degraded water quality is a stressor on the GOM DPS because of the known situations when water quality did not meet standards and was at levels that negatively impact salmon and because of the impacts of non-point source pollution, particularly sedimentation and acid rain.

Threats Associated with Factor B:

Significant Threats

No significant threats associated with Factor B were identified at the time of listing.

Secondary Stressors

 Fish Harvest

As summarized in the 2009 listing rule, overutilization for recreational and commercial purposes was a lesser stressor that contributed to the historical declines of the GOM DPS.  Intercept fisheries in West Greenland and St. Pierre et Miquelon, by-catch in recreational fisheries, and poaching result in direct mortality or cause stress, thus reducing reproductive success and survival.  Due to the continued involvement of the International Council for the Exploration of the Sea (ICES) and NASCO, the scientific advice from ICES to NASCO has been that there is no harvestable surplus of the mixed stock off West Greenland available for commercial harvest.  As a result, since 2002 within the West Greenland Commission of NASCO there has been agreement for no commercial export and to restrict the fishery at Greenland to an internal-use-only fishery.

Recreational angling of many freshwater species occurs throughout the range of the GOM DPS, and the potential exists for the incidental capture and misidentification of both juvenile and adult Atlantic salmon (Fay et al. 2006).  Juvenile Atlantic salmon may be easily misidentified as brook trout, brown trout, or landlocked Atlantic salmon, while adult salmon may be confused with adult landlocked Atlantic salmon or brown trout (Fay et al. 2006).  Direct or indirect mortality may result even in fish that are released as a result of injury or stress.

Threats Associated with Factor C:

Significant Threats

No significant threats associated with Factor C were identified at the time of listing.

 Secondary Stressors


Disease outbreaks, whether occurring in the natural or hatchery environment, have the potential to cause negative population-wide effects.  Atlantic salmon are susceptible to numerous bacterial, viral, and fungal diseases.  Bacterial diseases common to New England waters include bacterial kidney disease (BKD), enteric redmouth disease (ERD), cold water disease (CWD), and vibriosis  (Mills, 1971; Gaston, 1988; Olafsen & Roberts, 1993; Egusa, 1992);.  Fungal diseases such as furunculosis can affect all life stages of salmon in both fresh and salt water, and the causative agent (Saprolignia spp.) is ubiquitous to most water bodies.  The risk of an epizootic event occurring during fish culture operations is greater because of the increased numbers of host animals reared at much higher densities than would be found in the wild.  A number of viral diseases that could affect wild populations have occurred during the culture of Atlantic salmon, such as infectious pancreatic necrosis, salmon swimbladder sarcoma virus, infectious salmon anemia (ISA), and salmon papilloma (Olafsen & Roberts, 1993).  ISA is of particular concern for the GOM DPS because of the nature of the pathogen and the high mortality rates associated with the disease.  Most notably, a 2001 outbreak of ISA in Cobscook Bay led to an emergency depopulation of all commercially cultured salmon in the bay.

Parasites can also affect salmon.  There are more than 30 parasites of Atlantic salmon, of which the sea louse is one of the more common (Fay et al. 2006).  Sea lice are common and infestations can occur on wild fish in some areas where Atlantic salmon farming is concentrated (Fay et al. 2006).  A severe infestation can result in scale loss and flesh exposure.  Furthermore, sea lice may be vectors for diseases, particularly ISA (Nylund et al., 1994) as lice move from fish to fish (Fay et al. 2006).

Federally managed conservation hatcheries adhere to rigorous disease prevention protocols and management regulations; these must be continued.  These protocols and regulations are designed to prevent the introduction of pathogens into the natural and hatchery environments; prevent and control, as necessary, disease outbreaks in hatchery populations; and prevent the inadvertent spread of pathogens between facilities and river systems.


As summarized in the 2009 listing rule, the impact of predation on the GOM DPS is important because of the imbalance between the very low numbers of adults returning to spawn and the increase in population levels of double-crested cormorants, striped bass, seals, and nonnative predators, such as smallmouth bass.  Increasing levels of predators combined with decreasing abundance of alternative prey sources has likely increased predation mortality on juvenile Atlantic salmon, especially at the smolt life stage.

Threats Associated with Factor D:

Significant Threats

Regulatory mechanisms related to dams

Atlantic salmon require access to suitable habitat to complete their life history.  As described under Factor A, dams within the range of the GOM DPS impede access to much of the suitable habitat that was historically available. 

As explained in the 2009 rule (74 FR 29344, 2009), hydroelectric dams in the GOM DPS are licensed by FERC under the Federal Power Act (FPA).  At the time of listing there were 19 FERC-licensed dams in the Androscoggin watershed (16 impassable), 18 in the Kennebec watershed (15 impassable), and 23 in the Penobscot (12 impassable).  It should be noted that the numbers of impassible dams may include dams with fish ladders that are ineffective at passing Atlantic salmon that would allow for their survival and recovery.

Current licenses for many dams, though not all, contain a reservation provision under FPA section 18 (16 U.S.C. 797) that could allow fishways to be prescribed by the Services (16 U.S.C. 811).  However, exercise of that authority requires administrative proceedings before the FERC and the Services.  The FERC maintains that, for the remainder of the projects whose licenses do not contain reserved authority, reopening these licenses may be dependent upon the success of a petition to the FERC to exercise its own reserved authority.  The Services maintain that the listing of a species, designation of critical habitat, or the availability of any new information should trigger a re-initiation.  Regardless, if “take” is occurring at these facilities, such incidental take needs to be authorized under section 7 or 10 of the ESA.  The habitat degradation and ecological impacts caused by these dams cannot be addressed by the Services’ prescriptive authority under section 18 of the FPA, but may be under FPA section 10(j) (16 U.S.C. 803) recommendations.

The majority of dams within the GOM DPS do not generate electricity, and therefore do not require either a FERC exemption or a Maine Department of Environmental Protection (MDEP) water quality certification.  These dams are typically small and historically were used for a variety of purposes, including flood control, log drives, mill working, storage, recreation, and processing water.  Most of these facilities do not have fish passage.  Before salmon were listed, lack of fish passage and other impacts to salmon could be addressed only through State law, as noted previously.  Further, although the USFWS worked in Maine on salmon passage for decades before the DPS was listed, to a large extent the fishways the services approved were either not effective or were not maintained properly.  Overall, the inadequacy of existing regulatory mechanisms relating to dams is a significant threat to the GOM DPS 

Regulatory mechanisms for non-FERC dams are also insufficient to provide for fish passage. Because most non-FERC dams predate the Clean Water Act (CWA), section 404 of which regulates the discharge of dredged or fill material into waters of the United States, State law 12 M.R.S.A section 12760 is the only statute other than the ESA dealing with fish passage at these structures.  However, this law requires an administrative process and hearing only if requested by the dam owner.  For the State to require fish passage under this statute, a finding that fish can be restored “in substantial numbers” and the habitat above the dam “is sufficient or suitable to support a substantial, commercial or recreational fishery” must occur.  This statute has been used to require fish passage at only one dam in Maine and remains untested in the courts and at the administrative level (74 FR 29344).  It should be noted that construction of any new barriers would be subject to CWA section 404 guidelines regarding water degradation if they significantly impair aquatic life movement.

 Secondary Stressors

No significant threats associated with Factor D were identified at the time of listing.

Threats Associated with Factor E:

Significant Threats

Marine survival

Given a broad geographic range, extensive life history variation, and diverse management regimes, factors that influence survival in the ocean are considered the primary driver of variability in Atlantic salmon abundance trends across the North Atlantic. Upon entering nearshore waters, post-smolts move rapidly to common marine feeding and wintering areas for 1 to 2 years before returning to natal river to spawn, all while being subjected to natural and anthropic mortality sources including environmental stresses, predation, starvation, disease, and direct and indirect harvest.

Generally, Atlantic salmon populations in Europe are more productive than in North America and northern populations are more productive than southern populations.  However, salmon populations have been declining throughout the North Atlantic range, particularly since the early 1990’s when a significant decline (e.g. phase shift) in marine productivity occurred. Population declines of other species in the North Atlantic also occurred at this time. The hypothesized cause of the change in productivity is large scale climate forcing factors that altered thermal, salinity, and oceanographic regimes, which altered the flow of energy through the ecosystem. The resulting increased mortality due to these processes have been particularly acute for the two sea winter components of populations whereas the abundance of one sea winter adults (i.e. 1SW returns) have remained relatively stable over time. Thus the second year at sea is a hypothesized survival bottleneck for many populations, particularly for southern populations given their demographic reliance on a high proportion of two-sea-winter females (i.e. 2SW returns). Approximately 100% of US origin females return after two winters at sea and increased mortality for this life history strategy can have major consequences for the population dynamics of the US stock complex

 Although decreased marine survival resulting from the regime shift is considered to be a major driver of population abundance, there are many additional factors that may influence marine survival on regional scales. For example, thermal and osmotic stress in the early marine phase, especially during the freshwater to marine transition, is known to influence survival through direct and indirect effects associated with decreased predator avoidance or foraging success. Fish health (disease, infections, and parasites) may also influences marine survival and may be naturally occurring or of increased prevalence due to anthropogenic activities such as salmonid aquaculture.  Additionally, indirect latent and cumulative impacts from hydroelectric facilities are also known to decrease marine survival directly and indirectly through the absence of ecosystem processes provided by co-occurring healthy diadromous species complexes. However, the exactly mechanisms of these interactions are not fully understood.

 Additional anthropic activities are also potential sources of mortality during the marine phase. Although direct fisheries for Atlantic salmon have been greatly reduced or eliminated, mixed stock fisheries targeting Atlantic salmon off the coast of St. Pierre et Miquelon, Labrador, and West Greenland remain.  These fisheries have the potential to harvest post-smolt, 1SW maturing, and 1SW non-maturing Atlantic salmon en route to marine wintering areas, feeding areas, and natal rivers prior to spawning. The potential for indirect harvest of Atlantic salmon as bycatch in other fisheries is thought to be low given the lack of overlap with major commercial pelagic fisheries. However, it remains a potential mortality source, especially as new fisheries develop or if existing fishery dynamics change in time and space. Other possible sources of mortality at sea include energy development projects, dredging or dumping of dredge spoils, and offshore development projects although the potential impacts of these threats have yet to be quantified.

 Impacts from dams in freshwater and poor survival in the ocean are considered the primary impediments to U.S. Atlantic salmon recovery. Despite this, a comprehensive understanding of all the impact these factors have on Atlantic salmon populations is limited and the marine phase has historically been considered a ‘black-box’. Although significant advances have been made in our understanding of the marine phase of Atlantic salmon and in identifying the magnitude of spatiotemporal-specific survival bottlenecks, our understanding is imperfect and clear management actions to increase marine survival have yet to be identified. Further work is needed by scientists and managers to identify actions that can increase the marine survival of this endangered species. 

Secondary Stressors

Depleted Diadromous Communities

Dam building played an important role in reducing Atlantic salmon and other co-evolved diadromous species.  The Atlantic salmon is one of 12 native diadromous species in Maine:  Atlantic shad, alewife, blueback herring, sea-run brook trout, rainbow smelt, shortnose sturgeon, Atlantic sturgeon, striped bass, sea lamprey, and Atlantic tomcod are anadromous.  The American eel is the only catadromous species.

Conservative estimates of the historical numbers of the diadromous fishes that co-evolved with Atlantic salmon are in the millions: 1,000,000 alewives in the Penobscot in 1867; 1,200,000 alewives per year in 1 tributary of the Kennebec in the early 1800s; 2,472,000 alewives in the Damariscotta in 1896; 400,000 blueback herring in the Kennebec in 1880; 2,000,000 American shad in the Penobscot prior to the 1830s; and 3,500,000 rainbow smelt in the Penobscot in 1887 (Saunders et al. 2006).

Damming rivers, thus preventing migration to former spawning grounds, was a major factor in the decline of Atlantic salmon, sturgeon, river herring (blueback herring and alewife), and shad (Moring, 2005; Limburg & Waldman, 2009).  Many co-evolved diadromous species have experienced dramatic declines throughout their ranges, and current abundance indices are fractions of historical levels.  

Rangewide population declines are dramatic:  American shad, 97 percent; alewife, 99 percent; blueback herring, 99 percent; rainbow smelt, 99 percent; Atlantic sturgeon, 91 percent; Atlantic salmon, 96 percent; and American eel, 72 percent (Limburg & Waldman, 2009).  Similarly, in Maine current population levels are orders of magnitude smaller than historical population levels (Saunders et al., 2006). 

This dramatic decline in diadromous species has negative impacts on Atlantic salmon populations.  At historical levels, the alewife, blueback herring, American shad, rainbow smelt, and sea lamprey likely provided several important benefits for Atlantic salmon, such as providing alternative prey for predators of salmon (i.e., prey buffering) (Saunders et al. 2006), serving as food for juvenile and adult salmon (Cunjak et al. 1998), nutrient cycling (Durbin et al., 1979; Nislow & Kynard, 2009), and habitat conditioning (Saunders et al. 2006).  Habitat conditioning refers to sea lamprey, whose spawning movements remove sediment and algae from stream rocks, which may improve habitats for Atlantic salmon spawning (Guyette et al., 2014, Hogg et al, 2013, Gardner et al., 2012).

Because diadromous fish populations have been significantly reduced, ecological benefits from marine derived nutrients (MDN), habitat conditioning, prey buffering, and alternative sources of food for Atlantic salmon are significantly lower today compared to historical conditions.  These impacts may be contributing to decreased survival through (1) reduction of prey for reconditioning kelts, (2) increased predation risks for smolts in lower-river and estuarine areas, and (3) increased predation risks to adults in estuarine and lower river areas.  Although these impacts do not occur in the open ocean, the demographic impact to the species occurs after smolt emigration, and is thus a component of the marine survival regime.

Removing dams and providing access to spawning habitat will significantly help restoration of all diadromous species in the GOM DPS, which should, in turn, aid in the recovery of Atlantic salmon.  The ecological interplay and interdependence of Atlantic salmon and other diadromous species is still being investigated, but at present the lack of a robust diadromous community is a lesser stressor on the GOM DPS.

Artificial Propagation

The conservation hatchery programs at Craig Brook and Green Lake National Fish Hatcheries (NFH) are vital to preserving individual and composite genetic stocks until freshwater and marine conditions improve for wild adults to reach stable recovery numbers. Currently, progeny produced from wild and captive broodstock are released into their rivers of origin as eggs, fry, parr, and smolts.  In addition, surplus adult broodstock are returned to their river of origin.  Information on the numbers and life stages of Atlantic salmon stocked in the Gulf of Maine DPS can be found in the annual Atlantic salmon assessment committee report (e.g. USASAC, 2016).

As reviewed in Fay et al., (2006), captive propagation and maintenance of broodstocks can be used to sustain or supplement threatened or endangered fish populations (Flagg & Nash, 1999)0. Though potentially effective at maintaining or increasing the population size, there is potential for altering unique genetic characteristics of the natural population (Berejikian & Ford, 2004). Mating strategies used in hatchery propagation can reduce genetic variability inherent in populations through artificial reductions in the number of spawning adults through reproductive variation (Withler, 1988). Artificial selection may alter population specific life history or genetic traits that may both alter the genetic characteristics of the captive population in relation to the wild source population, or result in decreased ability of the population to survive in the natural environment (Berejikian & Ford, 2004). Therefore, implementing hatchery practices that minimize artificial selection are important to maintain population-specific genetic characteristics and within-population genetic diversity.  In an effort to minimize the genetic risks associated with maintenance of captive broodstocks, a broodstock management plan (Bartron M. L., et al., 2006) was developed with the goal of maintaining genetic diversity throughout the hatchery management process, including estimating genetic diversity for each captive broodstock.  


As reviewed in Fay et al. (2006), Atlantic salmon that escape from farms and hatcheries pose a threat to native Atlantic salmon populations (Naylor, et al., 2005). Because captive reared fish are selectively bred to promote behavioral and physiological attributes desirable in captivity (Hindar, Ryman, & Utter, 1991; Utter, Seeb, & Seeb, 1993; Hard, et al., 2000). Experimental tests of genetic divergence between farmed and wild salmon indicate that farming generates rapid genetic change as a result of both intentional and unintentional selection in culture and that those changes alter important fitness-related traits (McGinnity, et al., 1997; Gross, 1998). Consequently, aquaculture fish are often less fit in the wild than naturally produced salmon (Fleming, et al., 2000). Annual invasions of adult aquaculture salmon have the potential to disrupt local adaptations and reduce genetic diversity of wild populations (Fleming, et al., 2000). Bursts of immigration also disrupt genetic differentiation among wild Atlantic salmon stocks, especially when wild populations are small (Mork, 1991). Natural selection may be able to purge wild populations of maladaptive traits but may be less able to if the intrusions occur year-after-year. Under this scenario, population fitness is likely to decrease as the selection from the artificial culture operation overrides wild selection ( (Hindar, Ryman, & Utter, 1991; Fleming & Einum, 1998), a process called outbreeding depression. The threat of outbreeding depression is likely to be greater in North America where aquaculture salmon have been based, in part, on European Landcatch strain.

In addition to genetic effects, escaped farmed salmon can disrupt redds of wild salmon, compete with wild salmon for food and habitat, transfer disease or parasites to wild salmon, and degrade benthic habitat (Windsor & Hutchinson, 1990; Saunders R. L., 1991; Youngson et al., 1993; Webb, et al., 1993; Clifford et al., 1997).  Farmed salmon in have been documented to spawn successfully, but not always at the same time as wild salmon (Lura & Saegrov, 1991; Jonsson et al., 1991; Webb et al., 1991; Fleming et al., 1996). Late spawning aquaculture fish could limit wild spawning success through red superimposition. There has also been recent concern over potential interactions when wild adult salmon migrate past closely spaced cages, creating the potential for behavioral interactions, disease transfer, or interactions with predators (Lura & Saegrov, 1991; Crozier, 1993; Skaala & Hindar, 1997; Carr et al., 1997; DFO, 1999). In Canada, the survival of wild postsmolts moving from Passmaquoddy Bay to the Bay of Fundy was inversely related to the density of aquaculture cages (DFO, 1999)

Concerns about aquaculture continue in the GOM DPS, but recent advances in containment and marking of aquaculture fish offer more control over these threats and reduce the risk of negative impacts of aquaculture fish on the GOM DPS.  Permits issued by the Army Corps of Engineers (ACOE) and MDEP require genetic screening to ensure that only North American-strain salmon are used in commercial aquaculture; marking to facilitate tracing fish back to the source and cause of the escape; containment management plans and audits; and rigorous disease screening.  Aquaculture is a lesser stressor on the GOM DPS; however, these measures do not eliminate the risk aquaculture fish pose to wild Atlantic salmon but serve to reduce the potential for negative impacts.  It is important to note that at this time equally protective requirements regarding salmon aquaculture do not exist on the Canadian side of the border.  Fish held in Canadian cages, or those that may escape from Canadian cages, can still pose disease, genetic, and ecological risks to U.S. Atlantic salmon.


As reviewed in Fay et al. (2006), prior to 1800, the resident riverine fish communities in Maine were relatively simple consisting of brook trout, cusk, white sucker, and a number of minnow species. Today, Atlantic salmon co-exist with a diverse array of non-native resident fishes including landlocked salmon, brown trout, largemouth bass, smallmouth bass, chain pickerel, and northern pike (MDIFW 2002). The range expansion of non-native fishes is important given evidence that niche shifts may follow the addition or removal of other competing species (Fausch, 1998). For example, in Newfoundland, Canada, where fish communities are simple, Atlantic salmon inhabit pools and lakes which are generally considered atypical habitats in systems where there are more complex fish communities (Gibson, 1993). Use of lacustrine habitat in particular, can increase smolt production (Matthews et al., 1997). Conversely, if salmon are excluded from these habitats through competitive interactions, smolt production may suffer (Ryan, 1993). Even if salmon are not completely excluded from a given habitat type, they may select different, presumably sub-optimal, habitats in the presence of certain competitors (Fausch, 1998). Thus, competitive interactions may limit Atlantic salmon production through niche constriction (Hearn, 1987). Competition is a lesser stressor to the GOM DPS because it can exclude salmon from preferred habitats, reduce food availability, and increase predation.


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