The Seals

Some interesting things about seals…

Grey Seals are amongst the rarest species of seals in the world. The UK population represents 38% of the world’s and 95% of the EU’s population.

Life Span

  • Grey seal pups are born weighing 14kg. At around 4 weeks old they are left by their mother to fend for themselves. 50% of pups die within the first year.
  • Seals mature at about 4 years old. Up until then they are referred to as juveniles.
  • Male Grey Seals (bulls) are thought to live to 25 years old. Females (cows) to 30.
  • The oldest recorded female lived to 46 years old.

Size

  • Grey Seals are the biggest wild land breeding mammal in the UK.
  • Males grow up to 250 kg in weight and 2.5 metres long, females are smaller at up to 150 kg in weight and 1.8 metres long.

Diving

  • Seals exhale before diving and store oxygen in their muscle (myoglobin) and blood (haemoglobin). They collapse their lungs at depths over 25m. They shut down blood supply to non-essential organs and slow the heart rate by one third.
  • Seals typically dive for 5-10 minutes, to depths of 30-70 metres. Although they can dive deeper and for longer.

Senses

  • Sight: Seals eyes are specially adapted to allow them to see underwater and on land.
  • Sound: Seals have well-developed hearing both underwater and on land. Their hearing is similar to that of a human.
  • Smell: Seals must close their nostrils tightly before diving. Smell is important to seals on land, it alerts them to the presence of a threat and is used as a way of recognising other seals.
  • Touch: Seals highly sensitive whiskers detect vibration. There are cases of blind seals surviving suggesting that for fishing the whiskers are more important than sight.

Diet

  • Seals are opportunistic feeders. They feed on fish, crustaceans, cephalopods.
  • Seals must come above water to digest their food in an oxygen rich environment.

Breeding

  • Males (bulls) reach sexual maturity at 5-6 years. They often have to wait longer in order to be strong enough to fight off other males on breeding grounds. Generally, bulls are polygynous (have more than one mate in a season).
  • Females (cows) reach sexual maturity at 3-5 years and have a single pup each year.
  • Female seals have an embryonic diapause (delayed pregnancy). Mating immediately after weaning their pup, the fertilised egg remains dormant for 3 months whilst she builds up fat reserves lost whilst suckling her pup. The egg then becomes attached and develops for a further 8 months. Females will then return to give birth between October and December (in this area although it varies around the uk) to the same pupping ground. The Farne Islands is the nearest breeding ground to St. Mary’s. Though not a breeding ground 95% of the seals that make use of the island are post weaned juveniles, with many newly weaned pups likely to have travelled from the breeding colony at the Farne Islands and some from the Isle of May using the island for vital rest.

Hauling Out

  • Grey seals spend around one third of their time on land, coming ashore to rest, moult and digest food. Coming onto land is known as “hauling out”. St. Mary’s Island is an important haul out location.
  •  Ideally seals will haul out onto rocks as the tide goes out and stay there until the next tide floats them off. Anything that disturbs them before this will affect their daily routine.

Impact of disturbance

What is a disturbance?

Anthropogenic disturbance refers to any change in the seals behaviour as a response to human presence or activity.

How is disturbance measured?

To quantify the reaction of hauled-out seals to human presence SMSW uses Mortenson (1996) scoring system to identify the intensity of the seals reaction. The definitions within this system are as follows:

Level 1 – alerting: head orientation of the seal towards the direction of any source of disturbance.

Level 2 – movement: any movement of one or more seals from their resting position away in any direction from the disturbance source or towards the water’s edge.

Level 3 – flushing: one or more seals moving from haul-out sites into the water.

Effects of disturbance

Seals are very vulnerable to disturbance, which upsets their routine of feeding and digestion. Seals that see a lot of people in one day can be severely impacted by these interruptions with knock on effects to their health, vigilance and energy budget.

Injury: On St. Mary’s Island when the tide goes out it can leave seals stranded in high places. Scrambling across rocks can lead to exhaustion and ripped out claws. For pregnant females this can be fatal for both mother and pup. Falls from a height can cause life threatening injury.

Stress: Seals may not always return to the water – some will stay where they are but will be constantly aware of your presence. In this state of heightened alertness, and will likely feel stress in the same way a human does, showing an increased heart rate, cortisol and adrenalin levels.

Seals swimming close to the shore may be looking for somewhere to come out. Not being able to get out the water will have serious health and welfare implications.

Death: Seals under one-year old, who are already struggling to teach themselves to feed, have thin blubber layers and finely balanced energetics for whom disturbance can tip the balance. Being forced into the water can result in fatality.

During moulting (spring/summer) seals need to raise their body temperature to moult their old fur, this can only be done on land. During this time, they do not feed properly they are weak and their immune system is compromised. Seals can suffer hypothermia and organ failure due to sudden exposure to cold water.

Sometimes it is necessary for a seal to recuperate from illness or injury. Being flushed into the water in a poor state of health could lead to death.