Once salmon eggs are extruded by the female, goldeneyes, brook trout, and Atlantic salmon parr may feed on them (White 1939a). Fry and parr are preyed upon by brown trout, brook trout, eels, burbot, northern pike, chain pickerel, largemouth and smallmouth bass, yellow perch, belted kingfishers, herons, mergansers, barred owls, otter, and mink (White 1936; White 1939a; White 1939b; Godfrey 1957; Warner 1972; Larsson 1985; Amiro 1993; Kalas et. al. 1993).
During the smolt stage, physiological changes occur that allow Atlantic salmon to make the transition from freshwater to saltwater. The process of smoltification occurs during spring, at a time when juvenile salmon or smolts migrate to the ocean. In New England, smolts encounter lakes and ponds, dams, water diversion structures and canals. These structures and areas provide habitat for predators and may also delay the migration of smolts, and increase their vulnerability to predation (Ruggles 1980; Saunders 1960). Smolts may be preyed upon by pickerel, smallmouth bass, northern pike, burbot, red-breasted merganser, ospreys, and black-backed gulls (White 1939b; Blair 1956; Barr 1962; Warner 1972; Larsson 1977, 1985; van den Ende 1993).
During their seaward migration, smolts enter estuaries and may not exit to the sea immediately (Fried et al. 1978; Danie et al. 1984). Extended residence in estuaries increases their vulnerability to predators (Bley 1987). Estuarine predators include striped bass, cod, American pollock, whiting, garfish, double-crested cormorant, European cormorant, and harbor seals (Carlin 1954; Bigelow and Schroeder 1953; Thurow 1966; Rae 1969, 1973; Hvidsten and Mokkelgjerd 1987; Hvidsten and Lund 1988; Barrett et al. 1990; Greenstreet et al. 1993; Massachusetts Cooperative Fish and Wildlife Research Unit unpublished data). There is evidence to suggest that predation by cormorants in the Machias River estuary may adversely affect salmon in the river. Meister and Gramlich (1967) provided evidence of predation by cormorants on salmon in the estuary. They documented that double-crested cormorants consumed an estimated 8,000 tagged smolts during the period 1966-1970. The potential impact of striped bass predation on Atlantic salmon was discussed at the 1999 Annual Meeting of the U.S. Atlantic Salmon Assessment Committee. Evidence was presented that striped bass are now spawning in the Kennebec River and it was hypothesized that spawning could expand to other northern river systems. It was also noted that striped bass seem to be arriving in New England waters earlier in the spring and more fish may even be overwintering in New England (USASAC 1999). Results from a study on the Merrimack River were presented and provided evidence of striped bass consuming Atlantic salmon just below the Essex Dam. In 1997, stomach content analysis was conducted on 41 striped bass, and 32 salmon smolts were documented and another 28 were suspected. Only 16 of the 389 striped bass stomachs analyzed in 1998 contained salmon smolts. The difference between the two years may be explained by the timing and availability of river herring as an alternative prey species (USASAC 1999). In spite of the fact that the period of transition from freshwater to life in the sea is probably one of the most critical episodes in the life history of Atlantic salmon, comparatively little information is available about their behavior and factors affecting survival during this life stage (Hislop and Shelton 1993).
From the time they leave the river and estuary to the end of their first winter at sea, Atlantic salmon are termed post-smolts. Hislop and Shelton (1993) refer to the work of Jarvi (1989) who found that during the initial period of adjustment in the ocean, post-smolts are under physiological and osmotic stress, and both their tendency to shoal and the speed with which they react to predators are suppressed. Sea trout as well as gadoid fishes such as cod, saithe and pollock are known to feed on post-smolts (Rae 1966, 1967, 1969; Hvidsten and Mokkelgjerd 1987; Hvidsten and Lund 1988). Pelagic seal populations such as harp seals, which typically feed on small schooling fish such as capelin, herring and mackerel, may represent a significant post-smolt predation factor as well. These seals move north and south with the ice edge.
Atlantic salmon grow rapidly while in the ocean; an increase in size reduces vulnerability to predators. Little is known about the predator-prey interactions involving salmon in the high seas. Many of the documented cases of predation in the ocean show that benthic feeders including shark, skate, ling, and Atlantic cod prey on Atlantic salmon (Hislop and Shelton 1993).
Hislop and Shelton (1993) report that marine mammals including harbor seals, gray seals, harp seals, and ringed seals may be the only significant predators of maturing salmon (salmon returning to natal rivers) in home waters. Among all seals, the gray seal is of greatest concern to fishers and fish farmers due to encounters with Atlantic salmon, salmon nets, and salmon farms (Anthony 1994; Rae and Shearer 1965; Rae 1960). The population of North Atlantic gray seal in U.S. waters increased from about 30 in the early 1980's to about 500-1000 animals in 1993 (NOAA unpublished data). A recent spring survey in Massachusetts counted 2035 gray seals. In the EEZ of the eastern U.S. abundance is likely increasing, but the actual trend is not known. The number of harbor seals along the New England coast has increased nearly fivefold since 1972. The estimated number of harbor seals in New England waters was 28,810 based on aerial survey and haul-out counts conducted in summer 1993 along the coast of Maine (Kenney and Gilbert 1994). The harp seal population has also increased dramatically to approximately three million animals in 1990 (NOAA unpublished data).
The predator-prey interactions involving salmon are complex. Anthony (1994) explores the theoretical beneficial aspects of predator control programs that may minimize impacts to salmon in riverine and coastal environments. Such analyses are complicated by the fact that these predator-prey systems were historically in balance or dynamic equilibrium. Reestablishing this balance requires consideration of the numerous predator and prey species that interact in food webs and function within very large ecosystems. Atlantic salmon abundance and the number and type of predators may vary annually in rivers, estuaries, and marine environments. Hislop and Shelton (1993) suggest that it may be unrealistic to believe that it will ever be possible to address the problem of predation in the open ocean.
The most well known freshwater external parasites of Atlantic salmon are the gill maggot, Salmincola salmonea, the freshwater louse, Argulus foliaceus, and the leech, Piscicola geometra. Gyrodactylus salaris is an ectoparasite that has, in the last decade, resulted in serious problems for Atlantic salmon populations in Norway (Johnsen and Jensen 1991, Bakke et al. 1990). Hastein and Linstad (1991) report that this parasite is a major disease problem in Norwegian salmon rivers, and has caused almost total eradication of young salmonids in some rivers. Farmed fish are amenable to treatment. Bakke (1991) reports that G. salaris now occurs in Russia, Finland, Sweden and Norway. There is evidence to suggest that susceptibility to G. salaris varies among stocks, and water temperature is an important variable with respect to reproduction and transmission of this parasite. In Norway the parasite is now reported in 34 rivers and about 35 hatcheries and its distribution in wild salmon populations is associated with the stocking of fish from infected hatcheries (Johnsen and Jensen 1991). Internal parasites include trematodes (flukes), cestodes (tapeworms), acanthocephalans (spiny-headed worms) and nematodes (round worms) (Mills 1971; Bley 1987; Hoffman 1967; Jones 1959).
Once in the sea, Atlantic salmon lose their freshwater parasites but acquire others from the marine environment. The variety of parasites may increase for Atlantic salmon in the sea. For most ocean fishes the increase is related to the variable food source, the assortment of intermediate hosts found in the ocean, the vast area of migration which increases exposure, the tendency of fishes to school in the ocean during various life stages, and/or the increase in size of the host body (Polyanskii and Bykhovskii 1959).
The sea louse, Lepeophtheirus salmonis, is one of the more common ocean parasites of Atlantic salmon. With severely infested fish, often the skin is loose, and flesh may be exposed. In Norway, the level of sea lice infestation on wild fish in some areas where Atlantic salmon farming is concentrated, has been found to be ten times greater than in areas where there are no farms (NASCO 1993).
The most important vertebrate parasite is probably the sea lamprey, Petromyzon marinus. The impacts of sea lamprey on Great Lakes fishes and introduced salmonine species is well documented, but there is a paucity of information regarding its effect on sea-run Atlantic salmon (Mills 1971). Sea lampreys are anadromous and enter New England rivers in the mature stage, in spring, when Atlantic salmon are migrating to home waters (Scott and Crossman 1973).
Atlantic salmon are susceptible to numerous bacterial, viral, and fungal diseases. The more common bacterial diseases to New England waters include furunculosis, bacterial kidney disease (BKD), enteric redmouth disease (ERM), coldwater disease (CWD), and vibriosis (Mills 1971; Gaston 1988; Olafsen and Roberts 1993; Egusa 1992). Furunculosis can be a problem in both the freshwater and marine life stages of Atlantic salmon. It is so widespread that no natural waters with resident fish populations are considered to be free of it. Because of the high incidence of this pathogen in some Atlantic salmon rivers in the U.S., many returning mature salmon carry it (Gaston 1988). Furunculosis can be treated in hatchery populations through the administration of antibacterial medicated feed and/or intraperitoneal (IP) injections. Control measures include commercial vaccines and surface disinfection of eggs with iodophore. Furunculosis can be a source of significant mortality in wild populations if river water temperatures become unusually high for extended periods.
Bacterial kidney disease is a chronic infection of salmonine fishes in culture environments. The bacterium is vertically transmissible even with egg disinfection measures, and once established, it can be difficult to control and virtually impossible to cure. Control in hatcheries depends on ensuring that eggs and smolts are from non-infected stocks; control in farms requires that fish be nutritionally fit (Olafsen and Roberts 1993; Gaston 1988; Egusa 1992). Although present in Canada as well as the US, there has not a high frequency of occurrence of BKD in the Northeast. Its occurrence in federal and most state trout hatcheries in New England has been limited.
Enteric redmouth disease (ERM) is caused by the bacterium Yersinia ruckeri. It occurs in salmonids throughout Canada and much of the U.S. and has been documented in cultured as well as captive sea-run Atlantic salmon in Maine and Connecticut (Gaston 1988). Generally this disease results in sustained low-level mortality, but large scale epizootics can occur if chronically infected fish are stressed during hauling, or exposed to other poor environmental conditions. This disease is amenable to treatment in hatcheries using medicated feeds or, for recaptured wild adults, intraperitoneal injections. Control in cultured populations is accomplished through commercially available vaccines and surface disinfection of eggs.
Coldwater Disease, caused by the bacterium Flavobacterium psychrophilum, has recently been found to be a potentially serious problem to Atlantic salmon in New England waters. Ongoing studies by the Biological Research Division of the USGS at their Leetown Science Center have shown that the pathogen induces pathology and subsequent mortality among juvenile Atlantic salmon, and that the pathogen is vertically transmitted from carrier sea-run adults to offspring via the eggs. Intra-ovum CWD transmission influences egg quality and affects early life stage survival (personal communication, Rocco Cipriano).
Vibriosis occurs in many species and is likely ubiquitous in marine and estuarine waters. In infected salmonine species, red necrotic or boil-like lesions occur in the musculature. Hemorrhages may occur in the viscera, and the intestinal track becomes inflamed. Typically, outbreaks and the level of severity escalate with an increase in water temperature. There have been recent reports of cold water vibriosis infection in farmed Atlantic salmon in Norway and Scotland. The infection occurs during winter at water temperatures below 9o C, and resembles the condition referred to as "Hitra disease" in Norway (Gaston 1988). A commercially available vaccine is utilized extensively in the salmon aquaculture industry to reduce losses to Vibriosis.
Atlantic salmon exhibit a limited number of viral diseases in culture; common ones include infectious pancreatic necrosis (IPN) and salmon papilloma (Olafsen and Roberts 1993). IPN is endemic in New England and in the Canadian Maritime Provinces. The IPN virus has generally not been found to be a serious source of mortality in Atlantic salmon in North America but has caused serious mortality in cultured European stocks. The disease can not be treated effectively in the hatchery and avoidance is the most effective control mechanism. Salmon papilloma or pox is a benign condition that can occur on wild and farmed fish in the first or second year of life.
Infectious Salmon Anemia (ISA)/Hemorrhagic Kidney Syndrome (HKS) was found in Canadian (New Brunswick) net pen sites in the Bay of Fundy in 1996. This was the first occurrence of this virus in North America although it had been in Norway since 1984 and has subsequently been detected at a number of sites in Scotland and the Shetland Islands. The Scottish and Shetland outbreaks of ISA have been linked to a single primary source and the spread of the disease has been associated with farming practices and interfarm transfers. An investigation is ongoing in Canada in an attempt to identify the source of the virus. There is currently no treatment or effective preventive vaccine available for this disease. Norway and Scotland have pursued a strategy of eliminating the disease by slaughter of infected fish, long-term fallowing of infected sites and, since effluent from processing plants and transport barges was identified as a high risk for spread of the disease, treatment of slaughter effluent. Known occurrences of the disease have been limited to aquaculture operations. Mortalities associated with ISA have been high in Canada and similar eradication management measures were initially adopted in response to the presence of the disease, including destroying infected fish, removing all fish from these infected zones and financial compensation to growers. More recently, Canada appears to have moderated their strategy from eradication to containment (reduction or elimination of financially compensated destruction). The disease was detected in 1998 at two land-based facilities in Nova Scotia that have no obvious ties to the infected New Brunswick sites. Some US pen sites in Cobscook Bay are close enough to fall within the ISA virus positive "quarantine zones" in New Brunswick waters, so there is great concern over the potential for this disease to infect US aquaculture stocks.
The ISA virus has not been found in any wild salmon populations to date, though over 1,000 wild salmon have been tested in Canada and the US to date. In 1998 the USFWS began monitoring captured sea run salmon mortalities for ISA virus and it has not been detected. The aquaculture industry in Maine has also completed a testing program and reports that it is not present in farm fish. The Maine Fish Health Advisory Board, consisting of disease specialists from state and Federal agencies, the University of Maine and private aquaculture has reviewed the information from Canada, prepared an action plan for detection of the ISA virus in Maine, and recommended against the importation of smolts from ISA positive zones in Canada. Although ISA has not been observed as a problem for wild stocks, there is great concern as it directly affects pre-spawning adults.
In 1998, a lethal retrovirus was detected in wild Atlantic salmon that had been captured as parr in the Pleasant River and reared at North Attleboro National Fish Hatchery (NANFH) in Massachusetts. In 1995 (180 parr), 1996 (80 parr) and 1997 (164 parr) were held in isolation at the North Attleboro National Fish Hatchery and a private hatchery in Deblois, Maine for the purposes of rearing the fish to sexual maturity, spawning them, and returning progeny back to the Pleasant River. Mortalities began in two of three rearing units holding these salmon at North Attleboro in 1997 and continued in 1998. (Salmon in the third unit were never found to contain the virus or exhibit symptoms.) Necropsy revealed massive tumors in the swimbladder. Pleasant River fish at Deblois were also found to be positive for the virus, though no disease was present and no mortality occurred. Cornell University scientists identified the causative agent as a cancer-causing retrovirus known as Salmon Swimbladder Sarcoma Virus. This disease and a presumptively causative retrovirus were first reported from sub-adult farmed Atlantic salmon in Scotland (Duncan 1978: McNight 1978) and it was named SSSV by Wolf (1988). The disease has not been reported from Scotland since, and the relationship between this and the Maine retrovirus has not been determined.
Virus-positive fish from North Attleboro were moved to a quarantine facility at the USGS-Biological Resources Division facility in Leetown, WV, to obtain detailed information on the pathogenicity of the virus, and the remaining stocks at North Attleboro and Deblois hatchery were destroyed. A non-lethal test for detection of this virus was developed by Cornell and testing of wild salmon stocks from other Maine rivers held at the Craig Brook National Fish Hatchery in Maine was carried out. Of 510 salmon of various ages from five rivers, 7 were found to be carriers of SSSV. These infected fish came from three rivers; Machias (1 adult, 4 smolt), East Machias (1 adult), and Narraguagus (1 smolt). Samples from the Sheepscot and Dennys were negative for the virus. No fish at Craig Brook NFH has ever demonstrated symptoms of the disease in the seven years wild stock have been held at that hatchery. However, the virus has demonstrated that it can cause lethal disease in salmon under the conditions existing in the Massachusetts hatchery. Results of this preliminary testing of captive Downeast Rivers wild stocks at CBNFH exhibiting no signs of disease indicates that the virus may be widespread at a low level in the environment. Expressions of the disease such as observed at North Attleboro may only occur under extremely adverse environmental and/or nutritional conditions.
A togavirus isolated in tissue culture has been detected in Atlantic salmon from farms in Maine and New Brunswick. The virus appears to be in New Brunswick and has been found in the Cobscook Bay area of eastern Maine. There has been no disease found associated with this virus at present, but it is monitored as part of the routine health inspection process for aquaculture operations in Maine. Most salmon encounters fungi during their various life stages. Saprolegnia is the only fungal disease of Atlantic salmon, and is primarily found in adult males. It invades the epidermis and is associated with the presence of high levels of androsteroids (Olafsen and Roberts 1993; Gaston 1988).
Interactions between wild Atlantic salmon and other salmonids are mostly limited to interactions with brook trout and, occasionally, brown trout. Interactions with these species indicate that their habitat use varies between allopatric and sympatric populations (Gibson 1973; Randall 1982). The result of interactions and shifts in habitat use are related to food availability; when food was scarce, segregation increased (Gibson 1973). Competition appears to play an important regulatory role shortly after fry emerge from redds, when fry densities are at their highest (Hearn 1987). These interactions may cause Atlantic salmon and brook/brown trout populations to fluctuate from year to year. Since these species co-evolved, wild populations should be able to coexist with minimal long-term effects (Hearn 1987; Fausch 1988). The fact that species have co-evolved suggests that their coexistence involves a dynamic interaction which can affect population structure. Information is presented below regarding species that share rivers within the DPS with Atlantic salmon. In most cases, conclusions cannot be drawn regarding the effects and magnitude of competition between these species and Atlantic salmon as no data is currently available.
Smallmouth bass and landlocked salmon inhabit headwater lakes in the Dennys River watershed. Largemouth bass were illegally introduced to headwater lakes in the Dennys River watershed. Brook trout are found in the main stem and are known to frequent the estuary. Other fish, birds, and mammals are found in the estuary and may compete with salmon for forage or space. Stocking of landlocked salmon in Meddybemps Lake may impact anadromous Atlantic salmon. Splake are also stocked into Old Stream on the Dennys River and have the potential to impact Atlantic salmon and their habitat.
Landlocked salmon are found in the East Machias River watershed. The river and estuary have a diverse assemblage of fishes, birds, and mammals that may compete with salmon for food or space.
The Machias River and estuary have a diverse assemblage of fishes, birds, and mammals that may compete with salmon for food or space. Landlocked salmon, pickerel, and smallmouth bass are found in the watershed.
With the exception of brook trout and landlocked salmon, there are few primary competitors or predators that would affect Atlantic salmon during their riverine residence in the Pleasant River. The river and estuary have a diverse assemblage of fishes, birds, and mammals that may compete with salmon for food or space.
There are few species that are likely to compete with salmon in the Narraguagus River. Landlocked salmon are found in lakes within the drainage but predation and competition is not considered to be a major threat to sea-run salmon stocks. The river and estuary have a diverse assemblage of fishes, birds, and mammals that may compete with salmon for food or space.
Warmwater fishes inhabit headwater ponds and may be found in the Ducktrap River. The river and estuary have a diverse assemblage of fishes, birds, and mammals that may compete with salmon for food or space.
Landlocked salmon and warmwater fishes including pickerel and smallmouth bass are present in the Sheepscot River drainage. Splake are stocked into Sheepscot Pond. Landlocked salmon are also present in Sheepscot Pond as are naturally reproducing populations of brown trout. The river and estuary have a diverse assemblage of fishes, birds, and mammals that may compete with salmon for food or space.
Fish diseases have always represented a source of mortality to Atlantic salmon in the wild, though the threats of major loss due to disease are generally associated with salmon culture. The level of threat from disease has remained relatively static until the last three years. Three recent events that have increased our concern for disease as a threat to the DPS include 1) the appearance of ISA virus in 1996 on the North American continent within the range of possible exposure of migrant DPS salmon, the discovery in 1998 of the retrovirus SSSV within the DPS population, and the new information available in 1999 on the potential impact of CWD on salmon. The ISA virus a causes fatal disease among sub-adult and adult salmon in aquaculture environments and can be laterally transmitted through salt water over distances of several kilometers. This situation represents a new potential threat to wild salmon but is difficult to gauge, as the virus has not been found in Canadian wild stocks where the disease is prevalent.
The discovery of SSSV within four wild populations in the DPS and at least one occurrence of disease observed in a hatchery situation represents a new potential disease threat to the DPS. The presents of this pathogen, apparently for several years, in other hatchery situations without the occurrence of any disease makes it difficult to assess the degree of risk to the DPS. The recent findings relative to the impact of CWD at early life stages in salmon and the transmissibility of the pathogen from adult to egg, even when eggs are disinfected by standard hatchery procedures, raises the potential threat of the long-known pathogen to a higher level, but more information will be necessary to fully assess its impact on the DPS. Our assessment of the overall threat to the DPS from disease is increased by these recent developments.
The nature of these three specific developments in terms of direct loss to the DPS from disease in the wild is difficult to assess, but circumstances to date suggest that direct mortality may not be the major threat to the DPS. However, there is an indirect threat through the impact of these diseases on the river-specific fish cultural program implemented on five rivers to enhance maintenance and recovery of these imperiled populations. The impacts of ISA, SSSV, and CWD appear to focus on the fish cultural environments. They can pose a significant new hurdle to the enhancement program's ability to function effectively, thereby significantly degrading a major tool and strategy for recovery. The level of threat to the perpetuation and recovery of the DPS from salmon disease has significantly increased in the past three years.
Interactions between wild Atlantic salmon and other salmonids are mostly limited to interactions with brook trout and, occasionally, brown trout. The rivers and estuaries of the DPS have diverse assemblages of fishes, birds, and mammals, some of which are contemporary introductions and did not co-evolve with Atlantic salmon. Some of these species may compete with salmon for food or space. The effects and magnitude of competition by these species is not known. The introduction or transfer of other salmonids, especially landlocked salmon and brown trout, under management by the state fishery agencies should be done cautiously, with a conscious effort to avoid negative impacts to DPS populations.