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On Seals

Updated: Oct 1, 2020

Common Seals at the Shannon Estuary

Seals are among the best loved marine mammals. Predominantly perceived as cute and cuddly, these animals belong to a group called pinnipeds, which means fin-footed, and have been around for a long time. It is thought that today’s seal species have all evolved from one common otter-like ancestor some 15-20 million years ago. Since then the 34 seal species known worldwide have adapted perfectly to their marine environment. While seals spend a considerable amount of time on the shore resting, especially during their annual moulting, they are built for the water. To reduce water drag a seal’s bodies is streamlined, the true seals or phocids have even lost their outer ears, and a thick layer of blubber enhances this body shape and provides insulation as well. As a result seals are excellent and agile swimmers. They can reach speeds of up to 25 knots (almost 50 kmh) which is created from mechanical motions of the rear end while the front and back flippers are being used to steer and perform sudden course changes. This speed and agility are necessary to successfully go after fish, their main food source. Just like their distant relatives, the whales and dolphins, seals have to dive to catch their prey. A dive can last between 5-15 minutes during which the animal can reach depths of up to 70 meters. Seals exhale on diving and the necessary oxygen is stored in the muscles in the form of myoglobin and in the blood in the form of haemoglobin. While all mammals possess these oxygen binding proteins, diving species like cetaceans and seals show a much higher concentrations of myoglobin and haemoglobin. In addition seals have about twice the blood volume of terrestrial mammals, can shut down blood supply to non-essential organs and slow down their heart rate to 4-15 beats/minute.

Grey Seals on Rathlin Island

While seals have excellent eyesight, their eyes are large with a cornea that can balance out the refractive index of water and the retina can handle very low light conditions, it is now thought that the seals’ whiskers are their main tool to detect movement and therefor prey in the water. Studies have shown that seals can pick up movement up to 180 meters away, in comparison a dolphin’s echo location only works to about 110 meters. Another very recent discovery is seals, grey seals in this case, clapping their flippers underwater which according to researchers produces a surprisingly loud sound. This is most likely a communication technique used either to attract possible mates or warn competitors and just the latest example of how much we still need to learn about the world around us.

Ireland has two native seal species. The bigger of the two is the grey seal. Males (bulls) can weigh up to 300 kilogram and measure up to 3 meter in length. Females (cows) are smaller, weighing up to 150 kilogram and reaching a length of up to 2 meters. The coat of the bulls is usually darker than the one the females are wearing, however cows with a dark brown coat are not unheard of. The common seal in comparison is much smaller and weighs only up to 130 kilograms and doesn’t grow longer than 2 meters. Again males are bigger than females, but the difference is not as distinct as it is in grey seals.

Grey Seal bull and a smaller and lighter coloured female on Rathlin Island

The biggest visual difference between the two species apart from size is the head. Grey seals show a characteristic dog-like snout and the nostrils are arranged parallel. Common seals feature a short snout and the nostrils appear distinctively V-shaped. The two species are often referred to as the dog and the cat of the sea and indeed grey seals have a rather dog-like appearance while the common seal resembles a cat.

A further difference is the reproduction cycle and the seals’ preferred surroundings. Common seals stay close to the mainland and settle in sheltered bays, estuaries and even harbours (which gave them their alternative name harbour seal) where they give birth in summer from June onward and mate again soon after. The pubs are born with a fully waterproof coat and are able to swim perfectly within a few hours after being born. Grey seals on the other hand give birth later in the year between September and December and seek out remote areas particular on offshore islands. Here the pubs are born with a white, fluffy coat that is not waterproof. This means the pub has to stay on land until it sheds this first coat which happens after around 6 weeks. During this time the mother is always nearby so even if a grey seal pub appears alone, chances are a female animal is somewhere nearby. Grey seals are also known to wander far and wide. A UK study has shown that travelling grey seals can cover up to 100 km/day and animals tagged in north-east England and eastern Scotland travelled as far as the Orkney Islands, Shetland Islands and Faroe Islands as well as offshore into the eastern Atlantic and the North Sea. A similar Irish study that followed a number of seals from their haul-out site on the Great Blasket Island revealed similar results. Four of the eight tagged seals repeatedly made trips to Scotland while the other four only made it as far as the Galway and Mayo coast. Overall it seems that the travel behaviour of grey seals is highly individualised. While some animals seem to travel long distances form haul-out site to haul-out site others stay committed to the same site although their trips can take them far away from the site for days or even weeks. Common seals seem to be less inclined to travel and their foraging trips on average don’t exceed a range of 50 km.

Grey Seal pup during the moulting process

Grey seal spy-hopping

Seals feed mainly on fish which is the cause for ongoing problems between seals and fishermen. Seals have few natural enemies and especially in Irish waters only the occasional orca might take the odd seal. Commercial hunting however, especially during the 18thcentury, brought seal populations close to extinction. In 1914 the grey seal protection act was introduced which was extended to all seal species in 1970. Managed seal culls were carried out in the 1960s and early 1970s after protests by fishermen, but the culls were ceased again in 1978.

Since then populations have recovered. A survey in 2017/2018 counted 4007 common seals (risen from 2955 in 2003) and 3698 grey seals (risen from 1309 in 2003) and a new count is expected this year. While these numbers are encouraging, the growing seal population already causes renewed calls for a seal cull from some fishing communities on Ireland’s west coast. Seals might take fish from the nets but blaming the animals for dwindling “fish stocks” is more than unreasonable. Apex predators, a position on top of the food chain the Irish seals can claim, are controlled by the available food and as long as seal populations are thriving logic dictates there is enough fish. Supporting this logic are some fishing communities in the south-east of the country who support seal conservation and actively feed the animals with scraps and other leftovers from their catch. The real problem that causes many fishermen in Ireand to struggle doesn’t banana pose on the Great Blasket, it sits more likely in an office in Dublin or Brussels…

To learn more about seals and seal conservation please visit the Seal Rescue Ireland website.

Carsten Krieger, March 2020


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