The Seals of Chichester Harbour - Answers

12 March 2021 | Posted in Guest blogger , mammals , Marine
The Seals of Chichester Harbour - Answers
© Alexander Mustard / 2020VISION

Thank you very much for joining John Arnott for his 'Seals of Chichester Harbour' talk. It'll be available to view on the Sussex Wildlife Trust catch-up page for a limited time here.

Over to John -

There were so many interesting comments and questions I’m afraid I haven’t been able to answer them all.  Some of you have had fantastic encounters with seals, both here in Sussex and much further afield and I enjoyed reading about those.

If you do visit Chichester Harbour and walk around Thorney Island to view the seals on the west side, I forgot to say that it is a military garrison.  So if you miss the car parking spot and drive too far down Thorney Road you will be met by young people with guns who will politely turn you away.  When you walk along the coastal footpath you will have to go through a security fence with a gate controlled by the army guardhouse.  Just press the button and they will click the gate open.  There is a camera trained on the gate in case you look dangerous – I always give it a wave but I don’t know if anyone at the other end appreciates that.

If you are travelling from some distance it’s always worth checking the Chichester Harbour Conservancy website (or ring during office hours 01243 512301) about any temporary footpath closures before you leave.  Thanks to Sue for the reminder about that.

Lily: How sharp are their teeth? (Lily - 8)

Well Lily, all the cheek teeth of the seals we have in this country are pointy as that is the best way to hold slippery fish.  The seals don’t chew their food the way we do so don’t have flat topped teeth like the ones you have in your cheeks.

Although our seals have pointy cheek teeth they are not as sharp as needles.  They don’t need to be that sharp because the muscles in the seal’s jaws can bite down hard on the fish.  If it is a small fish they swallow it whole.  If it is too big for that they hold half the fish in their teeth then use the claws on their front flippers to hook into the other half of the fish and then jerk their heads back to tear off a chunk they can easily swallow.  You could try to copy that using a slice of bread cut into the shape of a fish BUT you would still have to chew the bread before you can swallow it safely.

In the Antarctic there’s a type of seal called a Crabeater seal.  Its cheek teeth are the most highly developed of any other seal species.  Each tooth divides into several branches which are curled over as well as being pointy.  These teeth then form a sieve for filtering out krill (which is a type of shrimp), so they don’t eat crabs at all!  Ask someone to find a picture on the internet.  If your teeth were like that it would be very difficult to get your toothbrush into all the little gaps.  Luckily seals don’t have to use toothbrushes.

Anne and Sue: Do both breeds intermix and mate with each species?  Do you get hybrids between the two species?

Grey and Common seals are extremely unlikely to hybridise.  Their mating cycles and strategies are so different that they don’t come into season at the same time in the UK.  Common seals mate when females have weaned their pups, i.e. roughly July to early August.  Grey seals also mate when the females have weaned their pups, in this case from mid-September at the earliest in the south-west to as late as mid-January on the east coast of England.

I found the following information in the book Seals of the World by Judith E King (1983).  This book was published before the modern development of molecular genetics and gene sequencing led to the explosion of revised taxonomic relationships we are ‘enjoying’ now.

Around the World, hybridisation between seal species is not commonly seen.  There are records of possible hybrids between Antarctic fur seals and Subantarctic fur seals on Marion Island in the South Indian Ocean.  Both species breed there and bulls of each species have been seen with cows of the other species included in their harems.  Some individuals have also been seen with physical characteristics of both species.  (There is a more recent paper on this published in 2006 which notes that 17-30% of the pups on Macquarie Island, between New Zealand and Antarctica, are hybrids of these two species together with New Zealand fur seals – a complex ménage à trois).

On Sable Island, Nova Scotia, a bull Grey seal was seen to attempting to mate with several young Common seal females but they struggled too much and he was unsuccessful (1975).

In captivity there are several records of interbreeding between various fur and sea lion species but if pups were produced they were either born dead or didn’t survive for more than a few days or weeks.  Obviously, captivity is an artificial situation.  Sometimes species of fur seal and sea lion kept in the same enclosure would attempt interbreeding even though they would never meet in the wild because of different geographical distributions.

Michael: Are parasites a big problem for our seals and marine ecosystems?

Seals do have parasites but because of their amphibious existence external parasites are not so common.  One species in particular has adapted to the amphibious life of its hosts, the seal louse which goes by the fantastic name of Echinophthirius horridus.  The only known host is the Common seal.  High infestations may compromise the seal’s diving ability and hence its ability to successfully capture prey.  More significantly, the seal louse is an intermediate host for the seal heartworm Dipetalonema spirocauda, an internal parasite which can be fatal for Common seals.

There are a variety of internal parasites such as tapeworms, flukes and roundworms.  The most infamous is the roundworm known as the Cod worm Phocanema decipiens.  Cod worms live as adults in the guts of seals (Grey and Common in the UK) and release eggs in the faeces of the seal host.  These are eaten by bottom living Copepods (small crustaceans about the size of a grain of rice) where they develop into larvae.  They then progress up the food chain, via other bigger crustaceans, until they end up in the guts of commercial fish such as Cod and Haddock.  The worm-like larvae are mostly found in the sheets of tissue (mesenteries) that support the intestines of the fish, so are mostly removed when the fish is gutted before eating.  However some form cysts in the muscles of the fish, especially the ‘cheeks’ of muscle surrounding the intestinal cavity – and these are the bits we eat.  Normal cooking kills these worms (they can grow up to 4cm) so they are harmless to us unless we eat raw fish in Japanese sushi, when they can make us ill.  The main problem is that infestations of Cod worm make the fish unmarketable as people don’t fancy the idea of eating fish that has not-so-little yellowy worms waving at them.  Properly frozen fish will kill the worms.  Minus 20 Centigrade for at least 24 hours is the legal requirement in the UK for fish intended to be eaten raw or lightly cooked (Food Standards Agency website).

Since seals, particularly Grey seals, are the primary hosts, there have been calls for seal culls, especially in Canada when infestation rates were high.  The hope was that by reducing the number of Grey seals a vital link in the life cycle of Cod worm would be removed, thus reducing or removing infestations in commercial fish.  Unfortunately it seems there is not a directly proportional link between the number of seals and the numbers of Cod worms.

Michael and Ron & Sue and Lance: What are the main causes of mortality for seals?  Natural predators (in Chichester Harbour)?

There’s quite a list which includes causes such as abandonment of pup by mother, starvation, death caused by storms, illness (Phocine Distemper Virus was a major killer in 1988/9 and again in 2002), injury and predation.  There are not many predators, such as Orcas, though sharks may feed on pups and Grey seals are known to attack and eat Common seal pups.  A male Grey seal on the Isle of May in the Firth of Forth was renowned for eating Grey seal pups.  For that he earned the name of Hannibal (after Hannibal Lecter).

There are no natural predators in Chichester Harbour, though I have watched anxiously as a bull Grey seal happens to shift closer to a Common seal mum and pup.  The two species seem to get along peaceably with each other.

Interactions with humans can cause seal mortality.  Threats include entanglement in fishing gear, collisions with boats, shooting on offshore Salmon farms (illegal in Scotland as of 01.02.21 owing to a ban in America next year on imports of Salmon from countries which harm seals in the production of Salmon – an export business worth £180 million a year to the Scottish Salmon industry), disturbance, pollution (especially toxic chemicals concentrating up the food chain to seals) and frisbee necklaces.  Climate change is another factor, with the increased frequency and ferocity of autumn storms washing many Grey seal pups off their beaches when they then drown or are battered on rocks.  If the mothers survive but haven’t finished producing milk for their dead pups, they get mastitis and their hormones are thrown out so they don’t mate that year and there is consequently no pup the following year.  Increased rainfall in autumn has caused cliff collapses in the south-west, which has buried Grey seal mothers and pups under tons of rubble.


Grey Seals © The Wildlife Trusts

John: Do seals ingest plastics in their hunting for food?

Yes, that’s pretty certain.  What isn’t known is what effect this will have on their health.  Humans also ingest plastics and again, we don’t yet know what the long term effects will be on our health.

Andrew: How do walrus, fit in the evolution of seals to have both sets of distinctive traits, (ears as well as longer forelimbs)?

Yes, the Walrus shares some features with True seals and others with Eared seals.  It used to be thought that seals evolved from two ancestors, a bear-like one (fur seals, sea lions and walruses) and an otter-like one (true seals).  The latest evidence points to a single ancestor, a bear-like one, which evolved into an otter shape as it developed an amphibious way of life.  This then evolved into all the modern seals (fur seals, sea lions, true seals and the walrus).

The oldest seal ancestors date from roughly 50 million years ago.  They separated into the eared seals plus walruses in one group and the true seal in another group 33 million years ago.  Then the walruses separated from the eared seals almost 28 million years ago.  So walruses remained more closely related to eared seals for longer than the true seals did.  Now only one species of walrus has survived (two other species are extinct).

No-one can explain why Walrus retained some true seal features along with its eared seal ones.

Nathaniel and Elzbieta: Are there any conflicts between humans and seals in Chichester?  Should tourists’ visits be controlled?

The only known potential conflicts with humans in Chichester Harbour are through visitors by boat to their haul outs.  I’ve watched boat visitors from the shore as well as coming across them during our own boat surveys.  They have all behaved in a respectful manner towards the seals, though there is a clear tendency to get too close to them.  This isn’t malicious and once they understand the consequences they are very willing to back off.  The problem is getting the message out there into the boating community.  The talk I gave you was based on a talk I have been giving to the sailing clubs in Chichester Harbour.  That is linked with the Code of Conduct too.

The Chichester Harbour Conservancy office was enclosing copies of the Code of Conduct with mooring licences.  The Code is under review at the moment.

Visiting Grey seal pupping rookeries is very popular in the autumn.  At Horsey and Winterton in north-east Norfolk there are volunteer seal wardens and roped walks to help visitors enjoy the seals in a sustainable way.  At Forvie National Nature Reserve at the mouth of the Ythan River in Scotland there have been many incidences of disturbance when the Grey seals are moulting on the beach.  It is illegal in Scotland under the Marine (Scotland) Act 2010 to deliberately or recklessly disturb seals.  Yet Ythan Seal Watch group are struggling to educate human visitors.  So far there has been no prosecution by either NatureScot or the police despite many complaints by groups such as Ythan Seal Watch.

Neill: Do paddleboarders and canoeists disturb the seals?

Yes, sometimes, even though they behave in a responsible way around the seals.  The movement of paddles while canoeing seems to upset the seals.  Also, there is a theory that a canoeist looks like the big dorsal fin of an Orca with the canoe representing the whale’s body.

Paddleboarders can worry the seals because they look like people walking on the water.  A person in a boat doesn’t seem nearly as threatening to a seal.  However, the chief cause of disturbance is just people getting too close, no matter what water craft they are using.

Ron & Sue and Pam: Why aren’t the electronic tags recoverable?  We think they should be as it’s marine pollution.

Yes, I agree.  Decaying batteries under water is not good and who knows what the other constituents of the tags were.  However it was the technology available at the time.  I guess the scientists had to weigh up the benefits to seal conservation of the data they obtained against the environmental cost of five lost tags, in the context of all the other rubbish that ends up in our seas.  Tough call and we can all make our own minds up on that.

I don’t know whether modern tags are recoverable.  Even if they are, it may be that research organisations would struggle to afford the cost of collecting them by boat from vast areas of the ocean.  I’ll see if I can find out what the situation is with modern telemetry tags on seals and if possible I will update the information here.

Thea, Jacqui, Brian, Don, Nick, Andrew, Joan, Wendy and Gabrielle: Where do the grey seals breed/come from?

Popular question!  So far as I can ascertain, the nearest breeding colony for Grey seals to the west of Chichester Harbour is on the Mew Stone at the mouth of the River Dart and at Peartree Point south of Start Point, both in south Devon.  From there they breed in scattered locations around the Cornish coast in secluded bays and coves, and then further on round Wales, north-west England and into Scotland.

The nearest breeding colony to the east is round the corner and up to north-east Norfolk at Horsey.  Also further on to the north Norfolk coast at Blakeney Point.  These are amongst the largest breeding colonies in England and are based on long sandy or shingle beaches.

To the south across the Channel there are a few breeding colonies in northern France, e.g. a few miles east of Dieppe and in Brittany to the west.

These will be the areas that the juvenile Grey seals come from, and maybe go back to as well.  It would be great to tag the Grey seals but finding funding for a repeat project is out of the question.  The tagging project I described cost in the region of £25,000, though that included boat and shore-based surveys as well.

Colin, Diane and Lesley: How long do they live?

If they survive through to adulthood a Grey seal might expect to live for 25 years for a male and 35 years for a female.  For a Common seal life expectancy would be 20 years for a male and 30 years for a female.  Males don’t live so long, it is thought, because of the stresses of competing with other males and securing compliant females during the breeding season.

Jill and Kirsty: What type of nikon ? is it a coolpix?

Yes, I use an old Nikon Coolpix P510.  Other bridge cameras are available!

Thea: Do you have a photo ID catalogue for Greys even though you think they are transient?

Yes, we do maintain a photo ID catalogue for Grey seals and will be sharing it with the Cornwall Seal Group Research Trust.

Janet and Tatiana: What effect will offshore windfarms have on seals?

From reports I’ve read it seems that windfarms may be providing new foraging areas for seals, Harbour porpoises and dolphins.  After the initial disruption of construction, the bases of the turbines become in effect artificial reefs.  As seaweeds and sessile invertebrates start to colonise they seem to kick start a whole new wildlife community, including fish and of course marine mammals.

Of course, there are still concerns about collisions of birds (and bats) with the blades, and with the visual intrusion.  It occurs to me that commercial fishing boats are not likely to risk operating amongst those turbines, especially the larger trawlers.  So they could become underwater refuges to aid fish population recovery?  That’s just me speculating, it’s not really my field.

Capturing energy from tidal movements and currents using underwater turbines is a different matter.  Collisions would be likely as faster currents in restricted areas are often used as foraging areas by smaller marine mammals because fish are more concentrated.  However, physical barriers such as bars or nets would prevent this, provided they didn’t become encrusted with marine organisms which impede the flow of the current to the turbines!  Again, I have no expertise in this area.

Phil: The grey seals at Donna Nook in Lincolnshire where we used to live have their pups in December/January. Do different colonies around the UK breed at different times? Or do Common and Grey Seals breed at different times of the year?

Expectant Grey seal mums spend the summer feeding and building up their blubber reserves before giving birth in the autumn.  The Grey seal pupping season in the UK as a whole lasts from the very end of August to the early part of January.  However in any one location it only lasts for about 10 weeks.  So what’s going on?

With Grey seals, pupping starts earlier in the south-west of England (from the end of August) and gets progressively later clockwise round Britain, starting early November in Norfolk.  This is because of the earlier spring flush of biological productivity in the south-west, which spreads north and east over the following weeks.  Expectant mothers are able to start gaining weight sooner in the south-west than they can on the east coast of England, and this has a knock-on effect on the timing of the pupping season.

The reason expectant mums need to build up the energy reserves is because they have to fast for about 18 days while their pup is being suckled.  Grey seal pups don’t swim well because of their long white furry coats so the mothers mostly stay protectively by their pups on the shore, pumping an estimated 30,000kcal of fat-rich milk daily into their pup, while at the same time going without food themselves.

Common seal pups on the other hand can swim within hours of birth, so they are able to accompany their mothers to sea.  That means the mothers can feed during the four week lactation period, so don’t have to build up their fat reserves first, unlike the Grey seal mums.  It also means that the pups can be born during the summer in June and July.


Common/Harbour Seal © The Wildlife Trusts

Justy: Gestation period same for grey and common seals?

Yes, the same – about 8 months.  Mating takes place after pups are weaned in both species.  The fertilised egg remains in suspension in the womb and doesn’t implant and develop for about three months.  This results in pups being born at the right time of year, in summer for Common seals and autumn for Grey seals.  See the previous question for more on this.

Karen: Do common seals give birth in the water or on shore?

Both Greys and Commons give birth on the shore, sometimes at the water’s edge.  After a short delay the afterbirth is produced and this attracts gulls and maybe crows which feed on it.

Peter: What does the male do after mating? Does he stay around at all or do they come together again next year?

With both Grey and Common seals the males stay by or on the pupping beach (known as a rookery) until all available females have finished weaning and have mated.  Grey males adopt a harem of up to ten or so females and will fight other males if necessary to keep them away.  Common males wait offshore for females to pass by, when the males will display by calling, porpoising, rolling, slapping water, bubble blowing and generally showing off to attract a female to mate.

Males take no part in the rearing and care of the young.  After the mating season is over, males and females resume normal life (eating and sleeping) and while they will continue to rest on the shore together this isn’t coordinated in the way it is during the pupping, breeding and moulting seasons.  So we see fewer seals at any one time.

Colin: Do grey and common seals compete with each other eg for food. Which dominates?

Very little is known about this.  They probably do compete but not directly by being aggressive towards another seal.  When out foraging both species are solitary (except for Common seal mothers accompanied by their pup for four weeks).  So the effect of any competition for food probably depends more on the relative abundance of the two species within an area.

On the east coast of Scotland and around the Orkney and to some extent the Shetland isles, there have been dramatic declines in Common seal numbers.  The Sea Mammal Research Unit have set up a project to find out why and one possible cause being investigated is possible competition with Grey seals.

In a direct conflict between a Grey and a Common seal of equivalent ages, I’d back the Grey to win every time!  Greys are bigger than Commons, females Greys marginally so but adult Grey males are 50% more massive than female Greys.

Caroline: Why do seals arch their backs like a u shape?

There is less blubber over the head and hind flipper/tail regions, so seals of either species will raise these areas off cold wet mud or water in order to keep themselves warmer.  This results in a banana shape and is called just that – the banana pose.

If they get too hot they can dilate blood vessels in their hind flippers and open them wide into the air to cool off.  If necessary they can always take a quick dip.

Robin: Surveys shown are mid summer. What are the numbers like at this time of year (March)?

The Common seals will still be in Chichester Harbour but in lower numbers at the haul outs as they are not so coordinated outside the pupping, mating and moulting seasons (June to September).

The Grey seals will also still be there but in unpredictable numbers throughout the whole year.  Greys are coordinated at their pupping sites during the pupping and mating seasons but as I explained, pupping and mating doesn’t occur in Chichester Harbour.  Like Common seals, Grey seals also spend more time out of the water when moulting in the period January to March or April.

We do not continue our counts over the winter because the weather is less predictable and we will get a better understanding of the fortunes of the Common seal population if we survey them during the summer.  Our main interest is in the Common seals because they are the only breeding population on the south coast of England.

The Grey seals are probably just passing through but we are interested in them because the numbers have risen slowly and steadily over the years.  They are probably dispersing from their successful breeding colonies down the east coast of England where numbers of pups have been increasing.  To be honest, we don’t know where they come from but Grey seals are known to be long-distance travellers from tagging projects elsewhere in Britain and France.

If anyone is near the main haul out in Chichester Harbour over the winter we will note seal numbers but it is only on an occasional basis.


Grey Seal pup © Dave Kilbey

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