Harbor Seal

Jonas Teilmann , Anders Galatius , in Encyclopedia of Marine Mammals (Tertiary Edition), 2018

Summary

Phoca vitulina , the harbor seal, also known as the common seal, belongs to the family unit Phocidae, or true seals, within the order Carnivora. Based on genetics, harbor seals are placed in a clade with their closest relative, the spotted seal as well as the greyness seal and ringed seal, Caspian seal and Baikal seal. Harbor seals inhabit temperate and Arctic regions inside the Northern Hemisphere in both the Atlantic and Pacific Oceans. Iii subspecies are recognized: first in the Pacific ( P.v. richardii), second in the Atlantic (P.v. vitulina), and third in a freshwater lake system on the Ungava Peninsula in eastern Canada (P.v. mellonae). Harbor seals live in coastal habitats, simply oftentimes enter freshwater rivers and inlets to forage. Although they show relatively high-site fidelity, they may also movement several hundred kilometers offshore to forage for days, returning to the aforementioned or nearby haul-outs.

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Ecosystem Structure

Robert B. Spies , ... Gordon H. Kruse , in Long-term Ecological Change in the Northern Gulf of Alaska, 2007

Offspring Constraints and Strategies

Harbor seal pups are unusual amidst pinnipeds and other mammals in existence born with substantial fat stored in blubber. This insulative layer, along with the lack of lanugo (the temporary white birth glaze of well-nigh phocids responsible for early on terrestrial thermoregulation), makes them capable of swimming and diving soon afterwards birth and certainly provides some immediate short-term energy reserves. Pups may too occasionally accompany their mothers on foraging trips from an early stage (due east.k., Bowen et al., 1999). Such precocial beliefs, coupled with a curt maternal dependency, demands that pups be well developed at birth and that females rapidly transfer energy prior to weaning. At birth, harbor seal pups weigh about eleven kg (about 13% of maternal mass) and comprise 11–14% fatty stored in blab (Bowen et al., 1992; Hammill, 2003). During the 24-day suckling catamenia, the pups grow rapidly, doubling or tripling their weight. In the most comprehensively studied population (Sable Island, Nova Scotia, Canada), harbor seals are weaned at nearly 24 kg and contain 38 to 40% fat (Muelbert et al., 2003). However, data from a 4-year menstruum (1997–2000) in Prince William Sound suggests that harbor seals there are weaned both heavier (29 kg) and fatter (42% fat; Iverson et al., 2003). Although harbor seal pups begin to feed within days of weaning, they lose body mass for at least iv–half-dozen weeks before they are able to reach positive free energy balance (Muelbert et al., 2003). Hence the ample blubber reserve at weaning insulates the pup and provides an energy resource during the days and weeks it takes to acquire to fodder independently and efficiently after it is abruptly weaned. Thus, harbor seal pups have several strategies that probable enhance survival. They are buffered at nascency by sizable fat stores deposited in utero and are also buffered during lactation if their female parent can efficiently transfer milk energy rapidly. They are weaned abruptly with piddling foraging feel and only their blubber energy stores to rely on. However, unlike most other phocids, harbor seal pups have already acquired substantial pond skills and probable besides some foraging skills by weaning and, thus, may have an reward over other newly weaned phocid pups. Nonetheless, acquiring these skills early perhaps places them at greater risk of being eaten by aquatic predators such as sharks (e.g., Bowen et al., 1999, 2003) or killer whales.

At birth, sea lion pups weigh about twenty kg (about 8% of maternal mass) with little fat stored in blab. This nascency mass is greater than that of harbor seals on an absolute ground, but relative to maternal size is about forty% less than that of harbor seals. Many are weaned eight–12 months afterwards at about 95 kg, a proportional weight gain twice that of harbor seals but over a time menstruation of most 14 times longer. These values are all increased in sea lions that do non wean for ii years or longer, as has recently been reported in sea lions from Southeast Alaska (Chiliad. Pitcher, pers. comm.). Ocean lion pups grow much more slowly than harbor seal pups, in part because milk energy content and milk intake are lower and in part because nursing bouts are less frequent and interrupted, owing to the need for mothers to forage for extended periods during lactation. Every fourth dimension the female parent leaves to forage, the bounding main lion pup fasts, losing a substantial portion of the free energy it just gained from nursing. This yo-yo pattern of energy intake and loss results in extremely deadening growth rates, which are comprised primarily of lean tissue growth rather than just fattening every bit in the example of harbor seal and other phocid pups. Nevertheless, sea lion pups may incorporate every bit much as 27% fatty at 10 months of historic period (Rea et al., 2003), providing them some energy resources during their gradual weaning process. Although sea lion pups grow slowly, they have the luxury of a prolonged (∼1–three years) and undecayed food source (female parent), while they learn the difficult lessons of how to feed themselves. Past weaning, they have developed substantial os and muscle mass and are highly coordinated, compared to most newly weaned phocid seals.

Sea otter pups are built-in at near 2 kg, or about 8% of their mother's mass (Kenyon, 1969). At weaning, a sea otter pup weighs almost 12 kg (Riedman and Estes, 1990), most half the mass of an average woman and representing a sixfold increase from birth weight. This compares to about a threefold gain in the harbor seal pup and a fourfold gain in the ocean lion pup past weaning. This difference in relative growth probably reflects contributions from prolonged maternal investment and the undeveloped stage of the sea otter pup at birth compared to the relatively precocial state of pinniped offspring at nascence. In contrast to the sea panthera leo, and even the harbor seal, a sea otter pup is in constant clan with its female parent until weaning, nursing upwardly to half dozen times per twenty-four hour period (Riedman and Estes, 1990). While a harbor seal pup is a practiced diver the day later nativity, a sea otter pup may not become a expert diver until 3 to 4 months of age. At that place is good evidence that the dietary specialization for specific prey species exhibited by some developed female sea otters is taught to their offspring (Estes et al., 2002). Thus, sea otter pups benefit from prolonged and rather intense association with their mother. During this time, they not only are provided nutrients in the form of milk and captured casualty, but they also larn to groom themselves, dive, and obtain nutrient independently.

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Pinnipeds

Thomas A. Jefferson , ... Robert L. Pitman , in Marine Mammals of the Earth, 2008

Threats and status

Many harbor seals live in shut proximity to big populations of humans and are exposed to high levels of industrial, urban, and agricultural pollutants. Both chronic oil spills and discharges, and episodic big scale spills cause direct mortality, and take long term impacts on harbor seal health and their environs.

Harbor seals live in coastal areas in the middle of some of the near heavily fished waters on World, and equally a result at that place are entanglement issues likewise every bit furnishings on the food bondage they depend on for their casualty. In that location are besides conflicts with smaller, localized fisheries, and historically in that location take been organized population reduction programs and bounties for taking seals.

Mass die-offs from viral outbreaks have claimed thousands of harbor seals. In the belatedly 1980s more than 18,000 harbor seals are estimated to take been killed by a phocine distemper virus (morbillivirus). Exposure to diseases from terrestrial carnivores, including human being pets, creates an increased risk of exposure to infectious disease. Immunosuppression from chronic exposure to pollutant contaminants probably contributes to harbor seal susceptibility to diseases.

Despite the fact than most harbor seals live in relatively close proximity to humans, their population levels are more often than not not well-known. Combining contempo estimates yields a world-wide population of 300,000 to 500,000 animals. The subspecies P. v. stejnegeri of the western Pacific (approximately 7,000), and P. v. mellonae (120–600) of the rivers and seal lakes of the lower Ungava Peninsula, Canada, may be the subspecies most at risk due to low population numbers. Isolated populations in Svalbard and the Baltic Sea, both in the hundreds, are also dangerously low.

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Long-Term Change

Robert B. Spies , ... Gordon H. Kruse , in Long-term Ecological Alter in the Northern Gulf of Alaska, 2007

4.9.1 Harbor Seals

Harbor seals are the nearly widely distributed pinniped in the world, occurring in both the North Pacific and North Atlantic oceans. Their range is near continuous around the rim of the North Pacific from San Ignatio Lagoon, Mexico (27°N) to Hokkaido, Japan (43°Northward), and extends into the eastern Bering Sea every bit far northward equally Kuskokwim Bay (60°Due north) ( Fig. iv.47). Today, at that place are some 36,000 harbor seals in the northern GOA between Kayak I. and the Copper River Delta in the e to Imitation Pass at the finish of the Alaska Peninsula in the west (Boveng et al., 2003). This total is much lower than in the heart of the past century considering of the eradication programs and commercial harvests described in Section 3.5, and unexplained losses since the early 1970s (Fig. four.48).

Figure 4.47. Harbor seal distribution (in yellow) in the Northward Pacific Sea.

Figure 4.48. Trends in harbor seal populations on Tugidak Is. and in Prince William Sound.

 Data from Pitcher (1990), Frost et al. (1999), and Jemison et al. (2006). Copyright © 2006

These more recent declines are all-time documented at Tugidak I., a small island southwest of Kodiak I. (in the Kodiak Archipelago), which was formerly the largest harbor seal rookery in the Gulf of Alaska, mayhap the largest in the earth. The affluence of seals there prior to 1964 and the inception of the commercial harvest was estimated to have been approximately 20,500 (Pitcher, 1990). A population simulation model indicated that the commercial harvests of some 16,000 pups in 1964–1972 should take caused a 30% refuse to nearly 15,000 animals. Following protection, numbers should have stabilized and so begun to grow during the mid to tardily 1970s. Yet, the abundance of harbor seals at Tugidak I. in 1976, when systematic counts were initiated, was only most 9300, or just 60% of the modeled estimate. The population continued to collapse at a abrupt charge per unit of 19% per year through the end of the decade, before slowing to nigh 7% per yr in the 1980s and early on 1990s (Bullpen, 1990; Jemison et al., 2006). By 1993, at that place were fewer than 800 seals left at Tugidak I. Between 1993 and 1998, the population recovered somewhat (nearly ten% per year) before leveling off at about 1400 animals (Fig. iv.48).

The plummet of harbor seals at Tugidak I., at least the unexplained portion from the early 1970s onward, and subsequent partial recovery appears to fairly correspond trends at nearby rookeries on Kodiak I., where seals declined by an boilerplate of 66% betwixt the mid-1970s and early 1990s, and have since then increased by nearly xl% (Lewis et al., 1996; Pocket-size et al., 2003). The status of seals on Kodiak I. prior to 1975 is not known because systematic counts were not made. Nonetheless, despite increases at several sites in recent years, the overall abundance of harbor seals in the Kodiak Archipelago remains at just xv–35% of that in the mid-1970s. And, compared to 50–60 years agone, today's abundance is certainly a much smaller percentage. Some sense of the magnitude of this difference can exist gained from the fact that the electric current abundance of harbor seals at Tugidak I. is still less than 10% of that prior to exploitation and the ensuing unexplained declines.

Besides, the decline of harbor seals in the northwestern GOA typifies trends farther west. On Otter I. (Pribilof Is., east Bering Sea), they declined by about 40% between 1974 (start count) and 1978 and by an additional 70% between 1978 and 1996 (Jemison et al., 2006); seals declined at a rate of about three.5% per year along the n side of Alaska Peninsula between 1975 (kickoff count) and 1995 (Withrow and Loughlin, 1996); and despite an annual increase in Nanvak Bay (Bristol Bay, east Bering Body of water) of about 2.one% per year from 1990 to 2000, the maximum count in 2000 was just 20% of the highest count in 1975 (get-go count) (L. Jemison in Small-scale et al., 2003). Systematic counts of harbor seals have not been made in the Aleutian Islands, simply conspicuous declines have been observed by scientists working there during the past 30 years (J. Estes, unpublished obs.).

Harbor seals in eastern and central PWS declined also – by 63% between 1984 and 1997 and by about 3% per year since then (Frost et al., 1999; Ver Hoef and Frost, 2003). The lack of systematic counts prior to 1984 makes information technology impossible to know whether the refuse began before then, maybe concurrent with declines in the Kodiak I. region. Notably, the decline in PWS was underway by the time of the 1989 oil spill, and the magnitude of the pass up from 1984 to 1990 (50%) was similar to that in the aforementioned time catamenia at Tugidak I. (60%). Numbers in PWS continued to decline afterwards 1990, but increased at Tugidak I.

Scientists believed initially that about 320 harbor seals were killed in PWS past oil from the spill (Frost et al., 1994). That judge was based on changes in numbers at 25 haulout sites along a standard trend-count route established in the early 1980s. Subsequent monitoring of seals at those sites led to the farther conclusion that the population in the sound connected to decline through the 1990s (Frost et al., 1999).

Still, this interpretation was challenged based on more widespread surveys that included additional areas of PWS, particularly the western sound and glacial haulout sites (Hoover-Miller et al., 2001). These scientists contend that there is no evidence of meaning direct mortality of seals by oil in 1989, and that surveys in 1989 and since and then along the original trend-count route failed to account for the mobility of seals – they suggest that many seals likely moved to other parts of the audio to avoid oiled habitats. In support of their belief, they betoken out the large interannual variability in numbers of seals at individual haulouts, suggesting curt-term local movements, and that since 1991 numbers of seals at glacial sites have tended to increase while numbers at land sites have tended to decrease. Still, studies of radio-tagged seals in the spill expanse did not show developed movements into the northern fjord areas of Prince William Sound where counts accept been increasing. As well, dead harbor seals often sink, as may have happened following the spill.

In contrast to the widespread declines of harbor seals throughout the northern and western GOA, Aleutian Is., and Bering Sea since the 1970s, trends in abundance in southeastern Alaska and British Columbia have been variable but generally reverse. In Glacier Bay in northern southeastern Alaska, a dramatic increase occurred between the mid-1970s and mid-1980s, followed by declines beginning in the early on 1990s (Mathews and Pendleton, 1997, 2000). Harbor seals near Ketchikan in southern southeastern Alaska increased rapidly from the early on 1980s through the mid-1990s, then slowed through the finish of the decade, whereas seals near Sitka increased niggling if any over this aforementioned period (Small et al., 2003). In British Columbia, harbor seals had been reduced to about 9000–10,500 by 1970 because of irresponsible compensation programs and heavy commercial harvests, particularly in the 1950s and 1960s. Following protection in 1970, harbor seals increased at the rate of about 12% per year (approximately the maximum intrinsic growth rate), and they numbered 75,000–88,000 by 1988 (Olesiuk et al., 1990; Olesiuk, 1999). The growth rate so slowed in the early 1990s, and the population numbered about 108,000 past the end of the decade (Olesiuk, 1999).

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The Exxon Valdez Oil Spill

Stanley D. Rice , ... Jeffrey W. Short , in Long-term Ecological Change in the Northern Gulf of Alaska, 2007

Seals

Harbor seals were also susceptible, for in PWS they did not avert oil slicks, or oil-coated intertidal algal mats. In May 1989, 81% of 585 harbor seals observed in oiled areas of PWS were coated with oil, and many were heavily contaminated ( Frost et al., 1994). Only fourteen dead seals, more often than not pups, were recovered in PWS later the spill considering carcass retrieval and counts were difficult – dead seals often sink. Some of these carcasses could be from natural causes. Coupled with a 40% decline in harbor seal abundance in the 5 years before the spill, modeling of population trends was needed to estimate the mortality due to oil rather than natural factors. Using the counts from almanac aerial surveys of haulout sites, Frost et al. (1994) estimated that 300 harbor seals died from the spill. That the number was not college may exist due to a survival strategy – a layer of blab that buffers them against hypothermia. So, despite having their pilus coated with oil, seals could still stay warm, providing some time for recovery from acute exposures.

Harbor seals in the spill area were also, nevertheless, lethargic. They are normally wary on land and enter the ocean when people approach or when shipping fly over them at low altitudes. In the starting time 2 to 3 months after the spill, many oiled seals could be approached on human foot to within a few meters and were described every bit sick or unusually tame (Lowry et al., 1994). In contrast to this unusual beliefs, interactions between oiled mother-pup pairs appeared normal – they seemed to remain bonded and pups nursed, even on heavily oiled mothers. The physical condition of pups appeared normal equally well, indicating that nursing and milk provisioning were successful.

Monoaromatic hydrocarbon fumes are suspected of affecting seals, only there were no direct measurements of these volatile hydrocarbons made just above the slicks where the seals breathe. Of 27 seals collected and immediately examined, all had several types of external and internal lesions that were consistent with oil vapor exposure (Spraker et al., 1994). In virtually cases, the lesions were mild and likely reversible. Lesions in the encephalon of some seals, however, may explain the disorientation and lethargy noted in numerous heavily oiled seals in early summertime (Lowry et al., 1994), and may have been responsible for the deaths of the most severely affected animals. Alive oiled seals taken to rehabilitation centers did not display behavioral or clinical signs of organ dysfunction, and 15 of 18 animals survived to release (Williams et al., 1994). It was articulate from field observations that many seals with oiled pelts survived and lost the oil rather chop-chop once they did not straight contact oil.

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Harbor Seal and Spotted Seal

John J. Burns , in Encyclopedia of Marine Mammals (Second Edition), 2009

B Molt

In harbor seals, the molt generally occurs during mid-summertime to early fall, within ii or 3 months of the pupping season ( Bigg, 1981). During the molt, seals booty out more frequently than at any other time of the year except for the pupping season. There are differences in timing among age and sex cohorts. Usually yearlings brainstorm and end the molt earliest, followed by subadults, then adult females, and concluding, adult males. There is overlap among these general historic period groups. Throughout their all-encompassing range the molt occurs after cessation of the breeding season. Accordingly, it occurs later in the yr in the late breeding populations such as those in Europe, British Columbia, and Puget Sound.

Spotted seals of the Okhotsk and Bering seas molt mainly in late spring (Burns, 1970; Trukhin, 2005). Pups, as mentioned, take the color and pelage pattern of adults after their lanugo is shed. Older seals brainstorm the molt immediately later on the breeding flavor and show an overlapping historic period-related sequence like to that of harbor seals. The period of intensive molt is during May and June, during which time the body of water water ice is retreating chop-chop and deteriorating. In areas where the ice disappears early, or in minimal ice years, the molt is completed on shore haulouts and at sea.

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Ecotoxicological Biomarkers and Accumulation of Contaminants in Pinnipeds

Kristina Lehnert , ... Ursula Siebert , in Marine Mammal Ecotoxicology, 2018

Liver Function

A study using master hepatocytes from harbor seals investigated the impact of PCBs on cell viability ( Korff et al., 2009). In a two-step biopsy perfusion method (Reese and Byard, 1981; Clement et al., 2001), hepatocytes were isolated from fresh liver tissue of harbor seals from the Northward Sea. During cultivation, the hepatocytes were exposed to environmentally relevant contaminants with known (hepato-) toxic qualities (PFOS and an Aroclor mixture) in concentrations corresponding to those found in wild harbor seal tissue. Cell viability and maintenance was measured by evaluating the activity of mitochondrial dehydrogenases (XTT assay), the membrane integrity (LDH release), and urea synthesis (Korff et al., 2010). Although the main aspects of prison cell viability and the specific metabolism of principal seal hepatocytes were non reduced by the investigated pollutants, urea synthesis decreased slightly during tillage (Korff et al., 2010). This finding shows that primary hepatocytes may exist a cell culture model in which furnishings of pollutant exposure can be monitored closely under controlled experimental weather. In subsequent proteome analyses, 10 proteins from approximately 160/gel modified their poly peptide expression levels (Behr et al., 2008). This showed that protein expression patterns enable discriminating between pollutant-incubated cells and negative control cells. Some of the upregulated proteins were shown to vest to the cytochrome P450 enzyme grouping (Behr et al., 2008).

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Salmonids*

D. Mills , in Encyclopedia of Ocean Sciences (Second Edition), 2001

Predation

Marine mammals recorded predating on Pacific salmon include harbour seal (Phoca vitulina), fur seal (Callorhinus uresinus), Californian sea lion (Zalophus grypus), humpbacked whale (Megaptera novaeangliae) and Pacific white-sided dolphin (Lagenorhynchus obliquideus ). Pinniped scar wounds on sockeye salmon, caused by the Californian sea king of beasts and harbor seal, increased from 2.eight% in 1991 to 25.nine% in 1996 and on spring-run chinook salmon they increased from 10.5% in 1991 to 31.8% in 1994.

Predators of Atlantic salmon postsmolts include gadoids, bass (Dicentrarchus labrax), gannets (Sula bassana), cormorants (Phalacrocorax carbo) and Caspian terns (Hydroprogne tschegrava). Adult fish are taken by a number of predators including the grey seal (Halichoerus grypus), the common seal (Phoca vitulina), the bottle-nosed dolphin (Tursiops truncatus), porbeagle shark (Lamna cornubica), Greenland shark (Somniosus microcephalus) and ling (Molva molva).

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Coronaviruses in Aquatic Organisms

H. Schütze , in Aquaculture Virology, 2016

20.3.ane.one Harbor Seal Coronavirus (Unassigned)

The simply probable case of coronavirus infection of harbor seals was reported past Bossart and Schwartz (1990). In 1987 three seals (Phoca vitulina) housed at the Miami Seaquarium were affected past an acute necrotizing enteritis. Ii of the seals died without showing any clinical signs; however, the third exhibited marked leukocytosis, aridity, hypernatremia and hyperchloremia. Pathological sections from all iii seals revealed all-encompassing focal bronchoalveolar hemorrhage and edema, with severe diffuse pulmonary congestion. Moderate-to-astringent lymphoid depletion was detected in the spleen and in visceral and peripheral lymphnodes. Coronavirus-specific antigens were detected in the intestinal mucosa. Immunofluorescence staining with antibodies against TGEV, FIPV and canine coronavirus (CCoV) (all alphacoronaviruses) yielded positive results, whereas immunofluorescence staining for bovine coronavirus (BCoV) (a betacoronavirus) was negative. Cross-reactivity among coronaviruses is limited to closely related species (de Groot et al., 2012b). Based on antigenic cross-reactivity, the coronavirus infecting the harbor seals most probably belonged to the genus Alphacoronavirus. All the same, because none of the ICTV demarcation criteria were met, the pathogen was not assigned to the genus.

A short sequence derived from a coronavirus infecting another harbor seal, which encoded the RNA-dependent RNA polymerase, has besides been published (NCBI Genbank Acc. No. FJ766501). It is causeless that further studies will confirm the presence of related viruses in harbor seals.

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Stress and Behavior

Heribert Hofer , Marion L. East , in Advances in the Study of Behavior, 1998

1. Case Written report two: The Mass Die-Off of N Bounding main Harbor Seals in 1988

The mass die-off of North Bounding main harbor seals in 1988 is an interesting case that demonstrates the difficulties of identifying factors underlying population catastrophes and separating cause and result. A massive research endeavour that continues to this day has led to the publication of many papers. Despite this endeavor, the crusade(s) of the 1988 mass die-off accept still not yet been conclusively identified, although a number of contributing factors take emerged. Progress was directly related to the introduction of experimental techniques, only historical reviews and comparisons with other populations and dice-offs were too important.

In 1988, around 18,000 harbor seals died in European waters. The showtime casualties were noted in Danish and Swedish waters in Apr 1988, and past autumn 1988 seals in Kingdom of norway, Federal republic of germany, the Netherlands, and the United Kingdom were affected (Dietz et al., 1989). The firsthand cause of death was identified as phocine distemper virus (PDV) (Cosby et al., 1988). However, this does not necessarily provide an explanation of the spatial and temporal pattern of mass mortalities. Where did PDV come from and why did it crusade mass mortalities? Were other environmental factors involved? Several hypotheses were proposed (Simmonds, 1991). These were not mutually exclusive and focused on different aspects of the mass dice-off:

1.

Higher than average monthly temperatures may crusade seals to leave the water and aggregate on land in unusually loftier densities. Large aggregations and loftier densities may have facilitated widespread and rapid manual of pathogens (Lavigne and Schmitz, 1990). There was indeed a correlation between temperatures and several seal mass die-offs known to have occurred in the twentieth century, including the 1988 dice-off (Lavigne and Schmitz, 1990).

two.

Individuals that carried the virus may have originated from infected seal populations that "invaded" a PDV-naive population, thereby triggering mass mortality (Heide-Jørgensen et al., 1992). These invaders may have been either harp seals, Phoca groenlandica, from the Barents Sea that were known to have "invaded" Scandinavian waters in 1987 (Harwood and Grenfell, 1990), harbor porpoises, Phocoena phocoena (Kennedy et al., 1988), or a variety of seal species from Greenland, Canada, or the Usa (come across Simmonds, 1991). Analysis of serological samples demonstrated that the North Sea harbor seal population was PDV-naive and that a variety of other marine mammals carried PDV (Simmonds, 1991; Heide-Jørgensen et al., 1992). There are bug assigning the origin of PDV to a item marine mammal population because the data on the geographic progress of the seal die-off and possible contact times of putative carrier candidates exercise non match well, and there are no information on bodily contact rates betwixt various species (Simmonds, 1991). It was doubted that the virus could accept been effectively transmitted under field conditions because information technology requires rather specific conditions to survive transmission, but the virus must take spread well because Danish farmed mink, Mustela vison, became infected with PDV in 1989 (Heide-Jørgensen et al., 1992). Thus, the origin and mode of transmission of PDV in harbor seals is currently not known.

iii.

An exceptional bloom of the alga Chrysochromulina polylepsis in early 1988 may accept produced high concentrations of toxic substances that may have stressed seals and suppressed their allowed systems, which in turn may have reduced resistance to pathogen infection and caused quick mass mortality (Lavigne and Schmitz, 1990). However, there are discrepancies betwixt the timing and spatial spread of the algal bloom and the initiation of the seal die-off, the potential toxicity of the algal bloom is unknown, and evidence on links between toxic algal blooms and previous marine mammal dice-offs is contentious (Lavigne and Schmitz, 1990; Simmonds, 1991).

4.

Marine pollution, specially organochlorines such as PCBs, may have exacerbated the consequences of pathogen infection and thereby initiated mass mortality (Simmonds and Johnston, 1989). When Lavigne and Schmitz (1990), Simmonds (1991), and Heide-Jørgensen et al. (1992) published their papers, prove of the possible damage of immunocompetence by contaminants such every bit organochlorines and the fitness consequences of contaminants was largely circumstantial and based on observational data in other marine mammals. Lavigne and Schmitz (1990) therefore proposed that this explanation contributed little explanatory power to other factors such as elevated temperatures and very high population densities. Similarly, Skaare et al. (1990) concluded that observed organochlorine and heavy metal concentrations from harbor seals in Norway gave no support to suggestions that organochlorines and heavy metallic pollution may have been straight involved in the observed seal deaths. Still, impairment of fecundity by organochlorines had been demonstrated by observational studies in Dutch harbor seals (Reijnders, 1980) and feeding experiments with Dutch harbor seals fed on PCB-contaminated fish (Reijnders, 1986). Information technology was already known that exogenous toxins such as PCB and other organochlorines seemed to interfere primarily with the endocrine system causing changes similar to those present in hyperadrenocorticism in gray seals, Halichoerus grypus, and ringed seals, Phoca hispida (Bergman and Olsson, 1986). St. Aubin and Geraci (1986) identified the zona glomerulosa equally an organ through which a stress response to contaminants or other environmental stimuli might exhaust adrenal hormone reserves or desensitize the adrenal cortex to other physiological stimuli.

Data on blood biochemistry and thyroid hormone levels from relatively uncontaminated individuals became bachelor in 1995 (Schumacher et al., 1995), studies on the historical progress of stress-related morphological changes were conducted in 1994 (Olsson et al., 1994), and the starting time experiments to investigate potential links between contaminants and impairment of immune office were published in 1994 (de Swart et al., 1994). More is now also known virtually historical changes in levels of contaminants in seal populations and what proportion of contaminants are transferred between mothers and pups. Basic blood biochemcial parameters of seals changed betwixt areas containing polluted and unpolluted prey and were not affected by captivity weather condition if the animals were held in captivity for a long time (9 months, Schumacher et al., 1995). Baltic greyness seals and ringed seals suffered from a disease circuitous described equally a principal lesion in the adrenals causing secondary reactions in various other organs, including skull bone lesions. The incidence of skull bone lesions has increased in Baltic seals since World War Two, indicating the presence of unnatural stress factors. Analytical results and pathological findings suggested a particular class of contaminants that include PCBs to be the most likely candidate to instigate the disease circuitous (Olsson et al., 1994). Finally, long-term experiments have demonstrated that immune part in harbor seals was impaired if they were fed fish from polluted waters (de Swart et al., 1994). Contaminated food not only changed cellular immunity, information technology as well impaired virus-specific immune responses and thus caused immunosuppression (de Swart et al., 1993, 1995a; Ross et al., 1995). Exposure to contaminants may therefore have had an agin effect on the defense force against virus infections, affecting the severity of viral infections, survival rates, and the spread of infections during recent epizootics (Ross et al., 1996b). Farther studies produced interesting show on vertical transmission of contamination betwixt mothers and pups. Female Baikal seals, Phoca sibirica, transferred about 20% of their total DDTs and 14% of their full PCBs to the pup during lactation (Nakata et al., 1995). Curt-term fasting typical for lactating mother harbor seals did non beal immunosuppression in animals with high burdens of organochlorines (de Swart et al., 1995b), simply lymphocyte functionality and total immunoglobulin M levels were reduced in mothers at the end of lactation (Ross et al., 1993). Pups at birth and females tardily in lactation may therefore be more susceptible to infection by viral and bacterial agents than other population segments. Perinatal exposure to environmental contaminants represented a greater immunotoxic threat than exposure as juvenile or developed (Ross et al., 1996a).

This spate of recent experimental and historical studies suggests that environmental pollutants may accept contributed to the severity and extent of distemperlike infections in seals and dolphins in recent years (Osterhaus et al., 1995).

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