Natur Cymru
A full version of this article appears in the
magazine.
Do you have slides and photographs from decades ago that
might match with seals still around today? If you have past images
of seals with recognizable pelage patterns we will be pleased to
try to match them. m.mcmath@ccw.gov.uk
or 01248 387175.
Around the British Isles there are three somewhat separate grey
seal populations, although there is some mixing of the juveniles.
The Welsh seals belong to a south-western group which has slightly
differing breeding habits from those in Scotland and on North Sea
coasts. In Wales up to 40% of pups are born on boulder beaches at
the back of sea caves, with most of the remainder in coves or
gullies often with little space above the reach of waves on spring
tides. Mothers, pups and attendant bulls spend more time in the
shallow water than other populations. Pups from about five days old
can be seen swimming and playing in the water, on their own or with
their mother, but rarely with other pups. This may be an advantage
in that the pups are experienced swimmers, perfectly at home in the
water by the time they are weaned – or earlier if sea conditions
require them to swim sooner. The breeding season in Wales is
earlier in the autumn than further north. Nevertheless, pup
mortality in a cove on Ramsey, where space is restricted, was about
twice that on a low lying Orkney island. Recent genetic work
supports conclusions, from the behavioural differences, that the
south-western group of seals is relatively distinct.
Weight at weaning is vital
A very important finding from the early weighing and tagging
work was that the weight seal pups reach before weaning, moulting
and going to sea independently, is crucial to their survival. There
is a high natural mortality rate in the first year of around 50%,
and at weaning few pups below a critical weight of around 40kg
ultimately survive. There is some recent evidence that pups at
places where suckling is frequently interrupted by visitors on
coastal paths grow more slowly and are weaned at lower weights.
Nevertheless, by judicious field craft it is possible to get into
positions to observe and enjoy the activity of the seals and their
pups without disturbing them.
Comprehensive direct counts of all the adult seals around Wales
would be very difficult. It is slightly easier to count numbers of
the white coated pups, though even this can be quite challenging on
the Welsh coast where they are mainly born in small coves and
caves. The most frequently used method for estimating total seal
populations is by a complex process of extrapolation from the
numbers of pups. The single pups are weaned in only about 18-25
days, so the spread of birth dates requires several counts or
allowance for those not encountered on a single site visit. From
mortality rates of different age classes and life-tables,
allowances can be made for the probable numbers of immature animals
and for the mature cows not giving birth.
Using the ratio of bulls to cows in various age classes, these
can be added to give whole population estimates within margins of
error. A west Wales / southern Irish Sea population has been
estimated at around 5000-6000. At Skomer Marine Nature Reserve pup
production and survival to weaning has been monitored for many
years, and indicates that the population is now stabilising rather
than continuing to slowly increase after the end of millennia of
human exploitation.
Pelage patterns
The old saying that “a leopard does not change
its spots” also applies to seals. Once the pups have moulted their
initial white coat they maintain the same complex and individually
distinct patterns of light and dark patches of their pelage (fur)
for the rest of their lives. This means that individuals can be
recognised, particularly from the markings on their head and neck,
which can be photographed even when the seals are bobbing about in
the sea. This helps to understand movements, behaviour and
ultimately the dynamics of whole populations. In grey seals pelage
patterns are most obvious on the cows, which normally have dark
patches of differing shapes and sizes on a lighter grey background.
Digital photography, with some computerised adjustment to allow for
slight differences in head orientation, allows matching to be made
using photo-identification catalogues. A seal that happened to be
photographed as a juvenile on Bardsey in the summer of 1985 has
subsequently been matched to a mother that has given birth to pups
on the same island over a run of recent years, including 2011. Code
numbers are used in the catalogue, but names based on patch shapes
can help when viewing them – the Bardsey seal is called
“Hammerhead” from a patch of this shape on the neck. Grey seals are
relatively long-lived and she must now be about 27 years old. The
bulls (males) are generally darker with some lighter spots on a
dark background and when fully mature may be virtually black, so
individuals are rarely matched using the current computerised
software: however it is possible to recognise individual males by
eye. At Skomer drawings of wounds and scars over several decades
allowed individual bulls to be followed over time.
No place like home
Individual seals often repeatedly return to the same favoured
haul-out sites after fishing trips that may last several days and
satellite tracking shows that they will travel 50 to 100 miles to
favoured grounds. Photo-id has shown that many experienced cows
return year after year to pup not just at the same coves, but
sometimes at more or less the same spots in the coves. Studies
around parts of Wales have now been going just long enough for
young seals first photographed as moulted pups to be returning to
breed. Some of these have been observed with their own pups quite
near to where they were born themselves.
In spite of over 70 years of studies of Welsh seals there is
always more to learn. Technologies open new avenues for insights
about populations as a whole, and improved understanding about the
detail of social interactions and site dependency. Spotting seals
brings pleasure to many visitors to the Welsh coast, and who knows
what value they add to the economy.
Mandy McMath is Senior Marine Vertebrate
Ecologist at the Countryside Council for Wales.