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  • Abbey Doherty

Seal Mortality Gulf Of Maine, 2018


It is the summer of 2018 and the hot months of July and August are rapidly approaching. Throngs of tourists begin to flood the beaches of southern Maine. Nothing is unusual, except for one thing. Beginning in June, there were numerous reports of dead seals and strandings along the beaches, but it was not a cause for concern at the time. However, as the weeks passed, the number of dead or stranded seals skyrocketed. By the end of August, hundreds of dead seals were found along the Gulf of Maine coast, and those were just the ones reported. This phenomenon puzzled scientists across the country, the tourists, and the citizens of the area.

The unusual seal mortality event occuring in the summer of 2018 in the Gulf of Maine.

Marine Mammals staff taking samples and observations of dead seal found off the coast of Maine. [1]


Seal Behavior

Seals are mainly distributed in the cooler shelf waters of the Northeast in the United States.[1] They follow a southward migration movement from the Bay of Fundy to northern New England waters in the winter, and a northward movement from southern New England to Maine and eastern Canada prior to their pupping season.[1] Both the gray and harbor seals inhabit the Gulf of Maine waters, and their pupping season occurs during mid-May.[1] The average seal’s haul-out occurs on tidal ledges in Northern New England. However, closer to the Nantucket Sound, seals tend to favor rocky and sandy habitats. These haul-out behaviors are strongly determined by the environmental conditions, which include tide, air temperature, time of day, wind speed, and precipitation. Human disturbance can also affect the haul-out location of seals. Seals are considered a top predator in the Gulf of Maine and are opportunistic feeders for fish and other crustaceans.

History of Seal Mortality

Seals have had a relatively long history of being “impacted by anthropocentric and natural sources.”[1] These factors include: disease, predation, pollutants, and abandonments.[1] Seal mortality has also been linked to human activity. Seals are seen as competition to fisheries because they consume the fish in the area, and were also sought after for their skins and furs. For this reason, seals were routinely killed by fishermen in the Gulf of Maine. Both harbor and gray seals had been nearly eradicated in Maine and Massachusetts prior to 1972.[2] Beneficially, in 1972, the Marine Mammal Protection Act was put into place, prohibiting the killing of these seals, which in turn increased seal populations thus forward.[1]

History of PCB’s

As stated before, seals are top predators in the Gulf of Maine. For this reason, they bioaccumulate any pollutants or material deposited in the ocean. Contaminants are known to suppress the immune systems of organisms and makes it hard to fight off any disease or cope with adversity.[3] Polychlorinated biphenyls (PCBs) are one example of a contaminant that has impacted the entire marine ecosystem. PCBs are man-made toxic chemicals that ended up being deposited in bodies of water by means of spills, improper disposal, runoff, and other leaks.[4] These PCBs are harmful as they are incredibly persistent in the environment by attaching to sediments and particles which enable them to spread long distances.[4] PCBs were banned in 1979 by the U.S. Environmental Protection agency, but they still remain in the freshwater and oceanic bodies today.[3] Specifically, rivers in the Gulf of Maine were home to many mills that deposited this harmful chemical into the ocean prior to the ban, which contributes to the presence of PCBs today. Since a seal's lifespan is around 30-35 years, these pollutants bioaccumulate over time and are stored in the fat of organisms. This puts nursing pups at risk because they are receiving concentrated levels of this toxin through their mother milk.

Current Seal Mortality

Annually, 1,000 seals in the Gulf of Maine are killed due to accidents such as boat strikes and commercial fishing operations. This statistic is likely to increase as the human population increases and, consequently, there is more of an overlap in the use of coastal habitats.[1] However, this is not necessarily the focus of the current seal mortality seen in the summer of 2018. Generally there are only about a couple dozen seals found dead during the summer, and it usually is due to accidents or bullet wounds from illegal killings.[2] However, by the end of the August of 2018, over 838 seals were found: 177 which were live, and 661 which were dead.[1] The National Oceanic and Atmospheric Administration (NOAA), “Declares unusual die off for… seals in the Northeast.”.[1] This is because many of the carcasses found were moderately to severely decomposed and ranged between all different ages, and live seals have been showing poor signs of health.[1]

Problem and Seals as Bioindicators

Main problem: Seals during the summer of 2018 died at an alarming rate. This was very unusual compared to the typical behavior and mortality of both harbor and gray seals in the Gulf of Maine. Scientists’ research has been aimed at finding an answer behind this die off. It is important to get to the root of the problem because seals are a bioindicator of the overall health of the ocean.[2] The unusual and increasing mortality of these seals raises concern about the health of the Gulf of Maine, and the future of other organisms.

Marine Mammals staff taking samples and observations

of dead seal found off the coast of Maine. [2]


“We have to look at the whole picture when you talk about a large die-off like this.”[4] -Dr. Susan Shaw

Graph of seal strandings in Maine, New Hampshire, and Massachusetts. Clear spike is shown in month of August, 2018. [2]

When speculations began that there may be an unusual die off among seals in the Gulf of Maine, scientists started running tests on both dead and live seals, and other factors such as water quality. Originally, scientists were puzzled due to the varying ages and decomposition of the seals, and the seemingly normal water quality of the Gulf of Maine. There did not appear to be a clear solution. However, eventually test results came back. With these results came varying conclusions.

The first conclusion is that there was a “deadly outbreak of distemper.”[2] Distemper is a viral disease that weakens the immune system. Many of the live seals washing up were coughing and showed signs of lethargy. This aligns with distemper as it causes lung infections and seizures by attacking the seals brain tissue.[3]

However, distemper is not the only disease that came back positive: “Some of the first batch of samples from seals that stranded in Maine, New Hampshire, and Massachusetts… have tested primarily positive for either the Avian Influenza or Phocine Distemper virus.”[1] These preliminary results were from laboratories at Tufts University and University of California, Davis. Each seal found in the sampling had signs of influenza, however, most of them had overlapping symptoms of phocine distemper. This led scientists to come to the conclusion that both diseases contributed to the massive seal die off in the summer of 2018. Distemper can weaken the immune system, which makes it easier to catch Influenza.[2] Dr. Goldstein, a professor at University of California, Davis, says, “Some seals have also been found to have the flu, though it is not clear whether the compound of infections are killing them, or whether the distemper is reducing the animals immunity and making them vulnerable to the flu.”[3] She also mentions, “Infections are more likely to spread at this time of year when the seals are living in close quarters.”[3] This is why it is important to look at the whole picture when analyzing a die off such as this one.

It is clear that there were likely multiple factors that lead to the seal mortality event, and it is very likely that both Distemper and Influenza played a role. However, it does not end there. As mentioned previously, PCBs are still present in the Gulf of Maine. Despite the PCB ban, the contamination is persistent and remains present because the chemical is durable, and it makes its way up the food chain into large predators such as seals.[4] It is speculated that their immune system has been compromised by these toxins, making them susceptible to other diseases.[4] The Portland Press Herald states, “We find this in young animals. They are immune-suppressed from birth… when some pathogen comes along like this, they are very susceptible to becoming very sick and dying very quickly.”[4] Because contaminants like PCBs bioaccumulate, they are most concentrated and present in adult top predator marine animals. This means that when mother seals give birth, by feeding their offspring, they are also feeding them these PCBs that are accumulated and stored in fat tissue, which exposes the newborns to this toxin. New offspring already have the trouble of learning to survive, and now they are also faced with contaminants that reduce their chance of being able to survive. Since the immune system of the seals is comprised, and during the summer months since they are all located close together, it is easy for them to catch diseases and result in a die off such as what was seen in the summer of 2018. The most recent findings by the NOAA state that the virus was determined to be phocine distemper. They continued research after the summer and found that going into the colder months, the amount of seal strandings had decreased. Seacoast Science Center rescue manager Ashley Stoke says, "As the temperature drops, the virus can't duplicate as quickly, so it can't spread as quickly”[5]. Stoke also makes an advisory statement for the future: “If there's something we can do to prevent it or if we start to see a trend, it just helps show us what's going on in the grand scheme of those animals rather than just simply taking measurements and disposing of them”[5]. It is evident that even with the investigations already done, it is important to continue research in order to get the whole picture and enact a proper preventive action course.

This graph shows the cause per year of marine mammal mortality from the year 1991 to 2018. The cause of the seal mortality seen in 2018 was an equal result of biotoxins, infectious disease, and unknown/pending results. [3]


Considering the nature of this issue is so recent, there have not been any conclusions made on how to proceed. Since the seal mortality has not necessarily conclusively been linked to human activity, it is hard to determine a solution in which humans are able to make a positive impact. However, it is plausible that the most likely cause of the seal mortality was a combination of the PCB’s, phocine distemper, and influenza. Because of these results, we are able to use this mortality event as a warning and potentially a foreshadowing of future die offs to occur. Since seals are a bioindicator of the ocean’s health, this die off does not bode well for the Gulf of Maine. Although the PCB ban was decades ago, we still see that it is negatively impacting marine life in the gulf. This should serve as a warning to humans that if we continue to pollute the waters, whether it be through chemical pollution, fossil fuels, or trash in the ocean, another die off of this magnitude is bound to happen. The Gulf of Maine is already the fastest warming water of its kind [1], and if pollution continues to increase, organisms will have no choice but to move to different waters, or they will not survive. Beyond the fact that the seal mortality this summer was tragic, it also should be alarming to the human population. Not only do top predators like the seals play a large role in the marine food web, but they also add to biodiversity. If other species populations decline at the rate seen in the summer of 2018 for the seals, the overall biodiversity of the Gulf will decline, impacting both the environment, and the fisheries that depend on the biodiversity of the Gulf of Maine to sustain themselves. Moreover, this does not only pertain to the Gulf of Maine. This can be seen as a global concern. Everywhere across the globe, environments are being polluted, and oceans are warming. It is almost definite that an event like this will occur again. With that in mind, it is also not easy to come to a solution because the situation is multifactorial. Limitations to helping the problem include legislation, individual impacts, and the fact that it is such a broad problem. Legislation may be limiting because it is hard to get laws passed, such as the Marine Mammal Protection Act, that are effective in reducing harm done to these marine animals. Individual impacts may exacerbate the stress on these animals, such as adding pollution to the oceans via everyday activities (driving, plastic use, etc). This is such a broad problem because there is not one simple contributing factor to account for, so it makes the problem feel inaccessible. However, it is still important the human stakeholders of the Gulf of Maine take a harder look at the impacts of the seal mortality issue in order to account for the overall damages done to the environment, and attempt to prevent or lessen the probability of this happening again.



1. “Ecology of the Northeast US Continental Shelf.” Northern Shrimp - Status of Fishery Resources off the Northeastern US,

2. Online, Seacoast. “Seals Were Once Nearly Wiped out from the Gulf of Maine. Here's How They Were Brought Back.” Bangor Daily News, Bangor Daily News, 2 July 2018,

3. Writer, Peter McGuire Staff. “Decades of Chemical Pollution Suspected in Maine's Seal Die-Off.” Press Herald, 20 Aug. 2018,

4. “Polychlorinated Biphenyls (PCBs).” Illinois Department of Public Health , Feb. 2009,


1. NOAA. “Seals in New England Test Positive for Avian Flu and Distemper.” NOAA Fisheries, 12 Sept. 2018,

2. Trotter, Bill. “Seals Are Dying at Alarming Rates, and the Virus That's Killing Them Could Affect Your Pets.” Bangor Daily News, Bangor Daily News, 12 Oct. 2018,

3. Weintraub, Karen. “Hundreds of Seals Are Dying on the New England Coast.” The New York Times, The New York Times, 7 Sept. 2018,

4. Writer, Peter McGuire Staff. “Decades of Chemical Pollution Suspected in Maine's Seal Die-Off.” Press Herald, 20 Aug. 2018,

5. Ropeik, Annie. “Distemper Remains Likely Cause of Gulf of Maine Seal Die-Off.” New Hampshire Public Radio,


1. “Gulf of Maine Warming Faster Than Most Bodies of Saltwater, Research Finds | Ocean Leadership.” Consortium for Ocean Leadership, 28 June 2016,


1. Writer, Peter McGuire Staff. “Decades of Chemical Pollution Suspected in Maine's Seal Die-Off.” Press Herald, 20 Aug. 2018,

2. Associated Press. “NOAA Declares Maine Seal Die-off as 'Unusual Mortality Event'.” Press Herald, 25 Dec. 2018,

3. NOAA. “Marine Mammal Unusual Mortality Events.” NOAA Fisheries,


Abby Doherty is an 18 year-old. resident in Kennebunk and a recent graduate of Kennebunk High School. Abby is currently enrolled in Drexel University, Philadelphia where she is majoring in Film Production. Her hobbies include filming, lacrosse, surfing, and social activism! In Abbie’s words, “I plan on continuing to be a community member and activist in the next chapters of my life!”

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