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The Harbour seal (Phoca vitulina), also known as the common seal, are true seals. True seals do not have ears. They are mainly found in the Northern Hemisphere ranging from the Atlantic and Pacific Ocean to the Baltic Sea. This makes them the most wide ranging seals of all pinnipeds. Commonly brown, gray, or tan, these seals have a distinct V-shaped nostril. These marine mammals are one of the many that are not near extinction. This is mainly due to reproduction, habitat, and diet.
Reproduction is significant because of the amount of time the seals spend away from their pups and the time they keep the pups safe from predators is critical. There are between 400,000 to 500,000 harbour seals living today, with the number increasing at a slow rate. Reproduction within populations is normally polygamous, meaning a single male can mate with more than one female. The male harbor seal is first to begin its mating process by chasing, biting, or holding the female. Once the female responds, both of them head into the water where they mate. Mating usually occurs in the same place every year, but the mates are different.
Sexually matured males weigh an average of 75 kilograms, while females do not mature until they are 50 kilograms. Maturity in a male occurs between the ages of three and seven. On the other hand, female's range from two to six for sexual maturity. During the 1992-1994 breeding season, M. Lance and his colleagues constructed an experiment where they tested the amount of foraging and reproduction of harbour seals. Out of the many seals experimented with, 31 adult males were planted with time-depth recorders (TDR's) to determine the diving differences off the Sable Island in Nova Scotia. In late May, the beginning of the breeding season, the males were captured, planted with the TDRs, and weighed. At the end of June, all of the seals were recaptured to remove the TDRs and were reweighed. The results projected were that the males made deeper dives in the beginning of the breeding season, which was more than 20 meters. Later in the mating season, dives became much shallower on an average of 8-12 meters. One difference between these dives was not only the depth, but also the shape. Deep dives had more of a uniform shape, with rapid flat bottomed rates fluctuating up and down. Shallow water dives were more variable in shape. Deep water diving for male harbour seals was found more commonly during the daytime, whereas shallow water dives happened in the late hours of the night because females were not around. Normally, larger male harbour seals have an advantage in the mating process since they can spend less time displaying their courtship in shallow waters and more time offshore where most females are located.
The mating season occurs from the late spring, throughout the fall, and ends at the beginning of the winter. Females give birth only once per year, with the gestation period more than humans; humans have a period of nine months, where harbour seals have an eleven month gestation period. When harbour seal pups are born, they weigh close to 11-12 kilograms and already have the ability to crawl and swim. In Scotland, female harbour seals were studied in the summer of 1988, 1989, and 1991 to determine the distribution and activity during the breeding season. During the summer, eight females with pups appeared in three different locations; Beauly, Cromart, and Dormoch Firths. The experiment consisted of the female seals to be safely extracted from the waters, weighed, and placed with radio tracking devices. In 1988, catches were determined during anytime of the day, while in 1989 and 1991, they used specific times for six straight weeks every day. The conclusion of the experiment resulted in normal seals ranging about 45 kilometers from the foraging site, but the range for female seals caring for their pups was significantly lower, especially during the stages of the lactation period. There was also a relationship between the distances away from pups to body size of the females as well. The larger the females, the less time they would spend feeding, while smaller females spend more time. This is significant because the larger the female, the less time they would spend away from their pups. Therefore the females would be able to take care of their pups and keep them safe from any predators. Ultimately this will result in an increased population.
Harbour seals are located off both the east and west coast of the United States. On the east coast, they are typically found in Canada, and migrate down to Massachusetts and New York. They are rarely found near Virginia or North Carolina. On the other hand, the west coast consists of seals found in Baja, California and as far as the Bering Sea. The habitat they live in has a major role because they need specific aspects of the Earth, such as rocky banks, to keep them safe and for them to be able to feed. Harbour seals explore the best geographic location to find food for them and their pups. They need a specific habitat to survive. Normally, the habitat needs to consist of temperate coastal with rocks, reefs, beach, and drifting ice as places to haul out and mate. They are considered marine mammals because they depend on a significant amount of time in the water to find prey. However, the reason they need the land is to rest, regulate their body temperature, and socializing which may lead to mating.
Austen C. Thomas, a biologist from West Washington University, believed that harbour seals were very attracted to herring holding habitats where the fish roamed around and their spawning habitats. This notion came from the fact that herrings use prey pulses which are large aggregations in time and place that are very predictable and abundant. He believed that the herring habitats are important aspects to the population of harbour seals. As herrings arrived and spawned, Thomas predicted there would be a seasonal shift of seals with occasional hotspots. The seals voluntarily migrated to the herring holding habitat and spawning habitat to feed. The experiment took place in the Puget Sound George Basin (PSGB) located in the British Columbia of Canada to Washington in the United States. This location was set because it is very diverse in marine life, including a massive population of herrings, and has nearly 30,000 harbour seals. Also in the PSGB, the National Marine Fisheries Service has declared that harbor seals are a risk factor to the local herring stocks. With this in mind, nine harbour seals from an island in Washington were planted on their dorsal side with a time depth recorder; a global positioning system. The dorsal side was chosen because it has the best satellite connection possible. Out of the nine seals, only one foraged within the holding and spawning habitat, while the others foraged in different locations. It is suggested that harbour seals are the reason that there is a decline in herrings within the island.
Another experiment was done between the months of June and August over a three-year span from 2005-2007. Off of the Gulf of California, 15 islands were examined, seven were documented to have harbour seals breed, and the remaining had no known seal habitat ever. Sampling sites, which included rock peninsulas and inlets, were pointed out at each island. Between one to nine sites were found at each island, which came out to be a total of 59 sites. Out of these sites, only 26 were occupied and the rest were vacant. Such aspects that were looked at included the type of the color of substrates, how curved the shorelines were, and how east it was for seals to access pools of water and shade. Logistic regression models were used to explore how habitat characteristics explained sea lion occupancy patterns. Since the temperature is increasing everyday because of global warming, it was concluded that all of the characteristics of the islands play a huge role in the ability for the seals to maintain body temperature and heat stress. These results helped understand and protect such islands that were unused, which could be later used as seal habitats. There were more thermally favorable areas where it was east to monitor and keep the current areas within the seal populations safe.
The diet consists of many foods. After only four weeks of nursing from their parents, pups will have traveled long distances and are weaned away from their mothers. As pups rely on their parents for food, young harbor seals feed on shrimp before entering and adult diet. As they grow up and become adults, their diet consists of fish, crustaceans, and mollusks. Which of these do they prefer and which has the biggest impact on their rate of survival?
The diet of these seals has been studied thoroughly in the past. One of those studies consists of determining the intake of food by looking at the feces of the seals. Off the coast of Mousa, Scotland, the experiment was done during the month of June through September over a four-year span from 1994-1997. The harbour seals in this area enjoyed many different species of fish. This assortment included whiting (Merlangius merlangus), herring (Clupea harengus), sandeel (Ammodytidae), and garfish (Belone belone). Over the span of this experiment, data was collected by weight of the amount of each prey. Whiting comprised of nearly 16-34%, herrings were 12-28%, sandeels had 7-18%, and garfish had 7-22% of the weight. Sandeels consisted of 51-60% of the harbour seals diet, which herrings had 8-48%. The average size of fish eaten was larger than that reported in comparable studies from other experiments. Harbour seals appear to have selected larger sandeels and whiting than the average size available to the area. The estimated consumption of fish by the 500 seals on Mousa ranged from 152-195 tons.
In some places of the world, harbour seals eat so much that they endanger other species. This can be seen in the Pacific Northwest, specifically in the State of Oregon. To examine predation of salmonids, sampling was taken by telemetry, which allows remote measurements of reporting information, and by molecular genetics. After 700 hundred hours of observations, an estimated 1,161 adult salmonids was consumed during the daytime over a span of 19 kilometers. Using an acoustic telemetry array, 21 out of 56 seals exhibited behavior that specialized in feeding on salmonids. Those seals spent most of their time in the riverine of the area. With the genetic analysis of the 116 salmonid structures recovered from 11 seal fecal samples showed that Coho salmon comprised of one half of the fish consumed. Results concluded that the seals consumed 21% of the premature Coho salmon population. The predations of the seals with the salmonids as their pretty were seen more upriver and at night., but what was significant was that a small portion of the seals did it. This study will help understand the predation of seals and provide important aspects such as risk assessments and aid conservation efforts for the population of salmonids. Although these aspects help harbour seals thrive and stay off the extinction list, there has been a decline in the population.
Decline of Harbour Seals
In December 1979 and October 1980, more than 400 harbour seals died along the New England Coast. It was later determined that the deaths occurred due to an acute pneumonia associated with the influenza virus. The virus replicated in mammals and causes mild respiratory disease.
The influenza B virus is a human pathogen whose origin and possible reservoir in nature are not known. This virus was isolated from a naturally infected seal and was found to be infectious to seal kidney cells in vitro. Sequence analyses and serology indicated that the influenza virus is closely related to strains that circulated in humans four to five years earlier. Retrospective analyses of sera collected from 971 seals showed a prevalence of antibodies to the influenza B virus in 2% of the animals after 1995 and none present before then. This animal reservoir harboring the virus has circulated in the past and may pose a threat to humans.
Another virus, the Phocine Distemper Virus (PDV) is a paramyxovirus that is pathogenic to many pinniped species, especially seals. It was first identified in 1988, when it caused nearly 18,000 deaths of harbour seals, and 300 grey seal deaths all along the northern European coast. The latest outbreak of PDV occurred in May of 2002. There was a high and unusual mortality rate off the shores of Anholt, Denmark. Out of 900 seals, 182 died with signs of respiratory and nervous disease. Shortly after, about 440 seals were found dead off the coast of Denmark, 100 more seals in Sweden, and 10 in the Netherlands. Necropsies were done to seven seals (four adults, one sub adult, and two juveniles) in various states of decomposition. Tissue samples (lung, kidney, bladder, and brain) were examined for morbillivirus nucleic acid by reverse transcriptase polymerase chain reaction. The tissue samples were found to be positive. Serum samples were tested for morbillivirus specific immunoglobulin M (IgM) antibodies. The tested seals had the IgM antibodies, showing recent infection. These findings along with the PDV infections indicate that they are the cause of mortality rates in harbour seals in Northern Europe.
Harbour seals are very abundant in the world and are found all over the Northern Hemisphere. Their survival rates are based on reproduction, habitat, and diet. Reproduction is significant because the seals time is allocated to if they are away or near from their pups. When the seals are away they are most likely searching for food, however when the seals are near, they are keeping the pups safe from harm and predators. The habitat has a major role because they need specifics aspects of the Earth to live. This includes rocky banks to keep them safe and so they can find food. Lastly, their diet is essential for all works of life. Harbour seals search for the best location for food, not only for them, but for their pups as well. Although reproduction, habitat, and diet continue the growth of the population of seals, sometimes nature takes its course and kills many of them. For example, influenza A and B have killed many of the species, but the most important and deadly has been the Phocine Distemper Virus (PDV). PDV has killed over 20,000 seals total during its first outbreaks in 1988, and its latest outbreak in 2002. Harbour seals are safe from being extinct, but when will the next outbreak of Phocine Distemper Virus take place, and will it take out all the harbor seals this time around?
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