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New and Emerging Threats


Current threats to Atlantic salmon include both the primary and secondary factors identified in the final rule (74 FR 29344, 2009). In addition, this recovery plan identifies undersized or poorly designed and installed road stream crossings, climate change and the Greenland intercept fishery as emerging significant threats.

Emerging Threats Associated with Factor A:

Road stream crossings

Road stream crossings were identified at the time of listing as a lesser stressor, which in conjunction with other lesser stressors constituted a significant threat to the species.   In the draft recovery plan, the threat of road crossings is identified as constituting a significant threat to the species.  Road stream crossings are found throughout the GOM DPS.   In the Gulf of Maine DPS, 10,169 road crossings have been assessed for fish passage effectiveness.  Of these, there are 3,259 impassible culverts, 3,677 culverts that are a partial barrier to fish passage, and another 1,803 where passage effectiveness is unknown (A. Abbott, personal communications, 8-2017).  Most of those road crossing barriers are found on the smaller first and second order streams within a watershed.  Corrugated metal, plastic or cement culverts, rather than bridges or bottomless arch culverts, are frequently installed at road crossings to reduce costs.  Undersized culverts create hydraulic barriers that sever habitat connectivity within the range of the GOM DPS.  Improperly placed and undersized culverts create fish passage barriers through perched outlets, increased water velocities, or insufficient water flow and depth within the culvert.  Poorly placed or designed road stream crossings reduce access to habitat necessary for Atlantic salmon spawning and rearing and alter stream processes including transport of sediment and materials.

Road stream crossings that restrict movements within and among suitable habitats on first and second order tributary streams have a significant impact on parr production.   In a study on the Sheepscot River, Sweka et al. (2007), found that smaller tributary streams contributed more individuals to the total outmigrating smolt population than larger mainstem habitats.  Sweka and Mackey (2010) found a similar relationship throughout Maine Atlantic salmon rivers in which parr density decreased with increasing cumulative drainage area.  They modeled this relationship using quantile regression to illustrate an upper bound (90th percentile) of parr density that could be expected in a stream reach of a given cumulative drainage area.  They estimated that approximately 83 percent of predicted parr production in the GOM DPS occurs in the small drainage areas reaches with drainage areas less than 100 km2 (90 percent from habitat with drainage areas less than 209 km2, and 95 percent from habitat with drainage areas less than 411 km2). Many of these smaller drainage areas constitute 1st and 2nd order streams where road crossing barriers are most often found. 

Emerging Threats Associated with Factor B:

Mixed-stock fisheries

Commercial fisheries for Atlantic salmon within the United States have been closed since 1947; however, small but significant fisheries continue within the species’ migratory corridor off the coast of Canada and Greenland.  To effectively engage in issues requiring international collaboration such as these distant water fisheries, the United States maintains a presence at the North Atlantic Conservation Organization (NASCO) and International Conference for the Exploration of the Seas (ICES).  The United States is a signatory to the “Convention for the Conservation of Salmon in the North Atlantic Ocean” which entered into force in October 1983, creating NASCO to ensure that the burden of Atlantic salmon conservation was shared by both States of Origin and Distant Water Countries. NASCO promotes the conservation, restoration, enhancement, and rational management of salmon stocks in the North Atlantic Ocean through international cooperation.  NASCO has six members, which include Norway, the United States, European Union (EU), Canada, the Russia Federation, and Denmark (in respect of the Faroe Islands and Greenland). The United States is represented at NASCO by scientists and managers from NOAA Fisheries as well as staff from the Department of State, other Federal and non-federal agencies, and private sector advisors.   NMFS’ role is to work to reduce impacts to U.S. stocks from distant water fisheries, and seek to hold ourselves and other countries accountable for the protection and conservation of Atlantic salmon.  NMFS scientists compile and analyze data on the status of the GOM DPS and take this information to the International Council for the ICES Working Group on North Atlantic Salmon. This group takes and analyzes data from throughout the North Atlantic to provide scientific advice to NASCO. NMFS scientists coordinate and participate in the international sampling effort for the Greenland fishery.

Intercept fisheries (adult fish captured in nets while in transit to or from their feeding grounds in the North Atlantic or on their feeding grounds in the North Atlantic) have posed a significant challenge to recovery of the GOM DPS.  For instance, the reported catch estimate for the West Greenland fishery in 2014 was 57.8 tons; given the potential for under-reporting for the 2014 fishery at West Greenland, total catch in Greenland that year may have been higher.

In response, a new regulatory measure for the intercept mixed stock salmon fishery at West Greenland was adopted at the 2015 annual meeting NASCO, effective through 2017.  Although this measure does not include a stated catch limit for the fishery, Greenland unilaterally set a 45-ton quota for the 2015 to 2017-time period.  The new regulations maintain the prohibition on exports of Atlantic salmon from Greenland and will require Greenland to implement stronger monitoring, control, and reporting requirements.  The new measures include enhanced licensing requirements for fishermen, such as annual catch reporting to maintain a license and in-season catch reporting, that will allow Greenland to swiftly close the fishery if and when the catch limit is reached.  They also ensure that if any overharvest of the unilateral catch cap occurs in a particular year, it will result in an equal reduction in the catch limit for the following year and will preclude any under-harvest from carrying forward to a future year.  It should be noted that these regulations are subject to periodic review and revision.

Populations of United States origin salmon are also harvested by St. Pierre and Miquelon (an offshore territory of France located off the coast of Newfoundland).  Although smaller in scale than the West Greenland fishery, this fishery operates outside any international management regime, as France (with respect to St. Pierre and Miquelon) has refused to join NASCO as a party.  Moreover, the domestic management regime in place does not effectively limit what can be caught.

U.S. origin salmon are also harvested in Labrador, Canada.  There are two types of subsistence net fisheries in Labrador that authorize the harvest of Atlantic salmon: resident subsistence trout fisheries that permit some by-catch of salmon; and aboriginal food, social and ceremonial (FSC) fisheries that allow direct harvest of Atlantic salmon.  In recent years, the fishing season and mesh sizes in the various fisheries have been modified in an effort to reduce the capture of large salmon (NAC(16)3, 2016).  Carcass tags are required for all harvested salmon in these fisheries.  Carcass tag allocations are set by DFO for each group which limits the total harvest of salmon which can be taken.   All sales of salmon are prohibited. The majority (roughly 80%) of the subsistence food fishery harvest occurs in estuaries with roughly 20% of that harvest occurring in coastal areas (NAC(16)3, 2016).  Recent genetic information presented by Bradbury et al. ( (2014; 2015), and ICES (2015) suggests that salmon of U.S. origin accounted for a very low level of the harvest in that fishery (approximately 0.6% from 2006 to 2011 and <0.1% from 2012 to 2014).  Even though the relative proportion of U.S.-origin salmon in that fishery is low, the effect to U.S. returns is still an important consideration.  For example, Bradbury et al. (2015) reported exploitation rates of U.S.-origin salmon to range from 1.04 to 4.20% from 2006 to 2011.  


Emerging Threats Associated with Factor E:

Climate Change

Fay et al. (2006) and NRC (2004) summarize the potential impacts of climate change on Atlantic salmon. At the time of listing in 2009, although there was reasonable certainty that climate change was affecting Atlantic salmon in the GOM DPS, there was uncertainty surrounding specifically how and to what extent.  Since listing, new and emerging science has helped us gain a better understanding of these effects and just what the ramifications are for salmon.  Recent information indicates that climate change is having significant impacts on the ecosystems that Atlantic salmon depend on and subsequently having significant impacts on the overall survival and recovery of Atlantic salmon (Mills et al., 2013). Following is a synopsis of the effects of climate change, and the new and emerging science that has elevated its concern for Atlantic salmon. 

Since the 1970s there has been a historically significant change in climate (Greene et al., 2008). Climate warming has resulted in increased precipitation, river discharge, and glacial and sea-ice melting (Greene et al., 2008). The past 3 decades have witnessed major changes in ocean circulation patterns in the Arctic, and these were accompanied by climate associated changes as well (Greene et al., 2008). Shifts in atmospheric conditions have altered Arctic ocean circulation patterns and the export of freshwater to the North Atlantic (Greene et al. 2008, IPCC 2006). With respect specifically to the North Atlantic Oscillation (NAO), changes in salinity and temperature are thought to be the result of changes in the earth’s atmosphere caused by anthropogenic forces (IPCC, 2006). 

Global climate change can affect all aspects of the salmon’s life history as entire ecosystems can shift rapidly (compared to evolutionary timescales) from one state to another, altering habitat features through increases in sea surface temperatures (IPCC, 2001); changes in frequency of seasonal cycles of phytoplankton, zooplankton and fish populations in the marine environment ( (Greene & Pershing, 2007); changes in freshwater hydrologic regimes; and altering the timing and frequency of river ice flows.  All of these factors can significantly alter the ecosystem in which salmon have become adapted by effecting environmental cues that stimulate migration, spawning and feeding activities. 

Friedland et al. (2005) summarized numerous studies that suggest that climate mediates marine survival for Atlantic salmon as well as other fish species. Recent analyses of bottom water temperatures found that negative NAO years are warmer in the north and cooler in the Gulf of Maine (Petrie, 2007). Positive NAO years are warmer in Gulf of Maine and colder in the north (north of 45° N) (Petrie 2007). Strength of NAO is related to annual changes in diversity of potential predators: at southern latitudes, there are more species during positive NAO years (Fisher et al., 2008). The effect is system-wide where 133 species showed at least a 20 percent difference in frequency of occurrence in years with opposing NAO states (Fisher et al. 2008).

In a recent study, Mills et al. (2013) was able to associate a major decline in Atlantic salmon abundance to a series of oceanic changes across multiple levels of a salmon’s ecosystem as a result of changing climate conditions.   Her results suggest that climate driven environmental factors and warmer ocean temperatures resulted in poor trophic conditions constraining the productivity and recovery of Atlantic salmon populations in the North Atlantic.  Though all Atlantic salmon in the North Atlantic are affected by the changes in trophic conditions, the effects on populations dominated by 2 sea-winter fish (such as the GOM DPS) appears greater than populations dominated by 1 sea-winter fish.  This suggests that there is a greater cumulative effect of poor trophic conditions on 2 sea-winter fish as a result of longer residence times at sea.  Mills’ study goes on to suggest that the impacts to Atlantic salmon are most associated with salmon’s ecosystem response to warming rather than the direct impacts of warming itself.   These effects include changes to phytoplankton and zooplankton communities that salmon’s principle prey species, capelin, feed on.   Subsequent to these changes, the size, distribution and behavior patterns of capelin has shifted making them less available for salmon to prey on, subsequently reducing the overall fitness and survival of Atlantic salmon.

Within the freshwater range of Atlantic salmon, water temperature is one of the most important environmental factors affecting all forms of aquatic life in rivers and streams (Annear, et al., 2004).  In addition to climate change, water temperature can be influenced by changes in riparian cover, dams, alterations in stream channel morphology (Annear, et al., 2004), waste water discharge, and urban development.  Among rivers within the GOM DPS, records extending back to the early 1900’s indicate that spring runoff has become earlier, fall ice-on is later, and there are fewer days of total winter ice on Maine rivers (Dudley & Hodgkins, 2002).  In support of these observations, a combination of land-surface and sea surface air temperature data shows an overall increasing trend in annual air temperatures for New England between the period of 1901 to 2000, with the greatest seasonal warming rates occurring in the winter months December, January and February as indicated by a period of record extending from 1976 to 2000 (IPCC 2001).  Several studies indicate that small thermal changes may substantially alter reproductive performance, species distribution limits, and community structure of fish populations (Pankhurst & Van Der Kraak, 1997; McCormick et al., 1999; Rahel et al., 1996; McCarthy & Houlihan, 1997; Welch et al., 1998; Schindler, 2001).  Changes in fish community structure can alter predator/prey assemblages by decreasing qualitative habitat features that benefit salmon while concurrently increasing habitat features that benefit predators and competitors. 

Temperature is especially important for Atlantic salmon given that they are poikilothermic (i.e. their body temperatures and metabolic processes are determined by temperature).  Temperature can be a stimulant for salmon migration, spawning, and feeding (Elson, 1969).  Thermal changes of just a few degrees celsius can critically impact biological functions of salmon including: metabolism (McCarthy & Houlihan, 1997; Somero & Hofmann, 1997; Reid et al., 1998), reproductive performance (Pankhurst & Van Der Kraak, 1997), response to contaminants (Reid et al., 1997) and smolt development (McCormick et al., 1998).  Unnatural changes in water temperatures may also affect growth, survival and migration timing of Atlantic salmon in freshwater, the survival and timing of migrating smolts in the estuarine environment, and the survival of juveniles soon after entering the marine environment (National Research Council, 2004).  Juanes et al., (2004) examined migration timing data from the Connecticut River drainage and from drainages in Maine and Canada and found a shift towards earlier peak migration dates across systems, correlating with long term changes in temperature and flow that may represent a response to global climate change.  For migrating smolts, the interrelatedness of water temperature and photoperiod may be extremely important to consider.  One of the concerns with climate change is the rate at which water temperatures increase could conceivably regulate the window of opportunity in which smolts can successfully transition from freshwater to saltwater.  McCormick et al. (1998) suggested that smolts experiencing delays in migration, such as those that occur at dams, may have lower survival rates if they are unable to reach saltwater within the migration window.  One possible explanation for this reduced survivorship is that a shortened migration window due to increased temperatures could conceivably result in increased predation pressure as more smolts are forced to migrate over a shorter period of time.

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