Charting Progress 2. Chapter 3. Healthy and Biologically Diverse Seas: Seals

3.5.1 Key points
i. Introduction
Two species of seal live and breed in the UK.
About 36% of the world’s population of grey
seals reside in the UK; almost 90% of these are
in Scotland. About 4% of the world’s population
or 30% of the European subspecies of harbour
(also known as common) seal are found in the
UK with 80% of these in Scotland. Although
both species can be seen all round the UK
coast, they are considerably more abundant in
some areas than others. Changes in local seal
population size and distribution may be related
to changes in the marine environment. Under
the Conservation of Seals Act 1970, the Natural
Environment Research Council has a statutory
obligation to provide the UK Government
with ‘…scientific advice on matters related
to the management of seal populations’. A
major component of this advice is up-to-date
information on the size and distribution of UK
seal populations. Both grey and harbour seals
were given protection under the Conservation
of Seals Act 1970 and under the EU Habitats
Directive. In Scotland, the Conservation of Seals
Act has been replaced by the Marine (Scotland)
Act 2010, which has extended seal protection
measures.
Five species of Arctic seal occasionally visit the
UK coast.
ii. How has the assessment been
undertaken?
UK seal populations are monitored by different
organisations using a variety of techniques. Grey
seal populations are assessed through estimation
of pup production at traditional breeding
colonies. Pup production is monitored annually
at colonies that contribute an estimated 85% of
pups born in the UK. Most colonies are surveyed
using aerial photography, others through ground
counting. The great majority of colonies in Wales
and south-west England are not included as they
are extremely difficult to survey and cannot be
surveyed frequently because grey seals breed in
caves or on beaches at the foot of remote cliffs.
Harbour seal populations are also monitored
mostly by aerial survey, although at a different
time of their annual cycle since they do not
aggregate when breeding. In south-east England
and parts of Scotland they are monitored
annually. In the remainder of Scotland they are
monitored at about four to five year intervals. In
Northern Ireland, counts are monthly. Grey seal
pup production has been monitored since the
early 1960s; harbour seals have been monitored
since the late 1980s. Both grey and harbour
seals are probably more numerous now than
in the historical past, when they were locally
hunted and/or harvested.
iii. Current and likely future status of seal
populations
In the UK, grey seals are considerably more
numerous than harbour seals. After decades
of increase, total grey seal pup production
appears to be levelling off in the UK and is now
increasing at only a small number of colonies.
Harbour seal numbers have declined dramatically
in Shetland, Orkney and the east coast of
Scotland since 2001, by over 50%. There has
been a smaller decline in the Outer Hebrides but
numbers on the west coast of Scotland have
remained relatively stable. The causes of these
declines are not yet known. Two outbreaks
of phocine distemper virus (PDV) affected the
harbour seal population in eastern England with
50% dying in 1988 and 22% dying in 2002.
In marked contrast to European harbour seal
populations which showed an immediate and
rapid recovery, recovery in the eastern England
508 Healthy and Biologically Diverse Seas
population after the 1988 outbreak was delayed
for three years and has yet to begin following
the 2002 outbreak. PDV outbreaks are likely
to recur in the future but it is not possible to
predict the proportion of the population that
might be affected, which populations are most
vulnerable (besides eastern England) or precisely
when outbreaks will occur.
iv. What is driving change?
The previous increase in grey seal pup
production is at least in part due to the increased
availability of breeding sites following the
abandonment of human settlements on remote
islands, including the automation of lighthouses.
The current reduction in the rate of increase
is thought to be due to density dependent
factors affecting the population as a whole. It is
not yet clear whether factors affecting survival
are more important than factors affecting
fecundity. Causes of the declines in harbour seal
populations in north and east Scotland remain
unclear. Contributing factors could be either
natural or anthropogenic or both and include:
competition with grey seals, predation by killer
whales, unregulated shooting and declines in
important prey species (e.g. sandeels).
It is difficult to predict future trends in UK
seal populations with certainty. Grey seal pup
production appears to be stabilising, so the
grey seal population is likely to stabilise over
the next decade or so. It is far harder to predict
what harbour seal populations in north and east
Scotland will do, given recent declines and the
lack of any obvious cause(s). In east England,
PDV outbreaks are likely to recur in the future.
The observed recent declines in north and east
Scotland populations are not related to the
outbreak of PDV in 2002 as none of the harbour
seals sampled in areas of decline had been
exposed to PDV.
v. What are the uncertainties?
While there are uncertainties in estimating
the size of the total grey seal population from
the annual estimates of pup production, the
procedures used to estimate pup production
are considered to be robust. Through the
1970s, 1980s and early 1990s, seal research
was focussed heavily on grey seals, with
harbour seals only coming to attention in
1988, after the first PDV outbreak. With the
recently documented declines, harbour seals
are becoming increasingly important. Most of
the pressure assessment is based on expert
judgement but it is difficult to assess the effects
of different pressures on seal populations. This
applies to both species.
vi. Forward look
The Scottish Government and Scottish Natural
Heritage have funded a number of projects
investigating the declines in harbour seals in
north and east Scotland. Increasing renewable
energy production may impact marine mammal
populations, requiring additional information in
certain areas. Harbour seal monitoring frequency
in Scotland is infrequent compared with grey
seal monitoring.