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

3.5.1 Key pointsi. IntroductionTwo species of seal live and breed in the UK.About 36% of the world’s population of greyseals reside in the UK; almost 90% of these arein Scotland. About 4% of the world’s populationor 30% of the European subspecies of harbour(also known as common) seal are found in theUK with 80% of these in Scotland. Althoughboth species can be seen all round the UKcoast, they are considerably more abundant insome areas than others. Changes in local sealpopulation size and distribution may be relatedto changes in the marine environment. Underthe Conservation of Seals Act 1970, the NaturalEnvironment Research Council has a statutoryobligation to provide the UK Governmentwith ‘…scientific advice on matters relatedto the management of seal populations’. Amajor component of this advice is up-to-dateinformation on the size and distribution of UKseal populations. Both grey and harbour sealswere given protection under the Conservationof Seals Act 1970 and under the EU HabitatsDirective. In Scotland, the Conservation of SealsAct has been replaced by the Marine (Scotland)Act 2010, which has extended seal protectionmeasures.Five species of Arctic seal occasionally visit theUK coast.ii. How has the assessment beenundertaken?UK seal populations are monitored by differentorganisations using a variety of techniques. Greyseal populations are assessed through estimationof pup production at traditional breedingcolonies. Pup production is monitored annuallyat colonies that contribute an estimated 85% ofpups born in the UK. Most colonies are surveyedusing aerial photography, others through groundcounting. The great majority of colonies in Walesand south-west England are not included as theyare extremely difficult to survey and cannot besurveyed frequently because grey seals breed incaves or on beaches at the foot of remote cliffs.Harbour seal populations are also monitoredmostly by aerial survey, although at a differenttime of their annual cycle since they do notaggregate when breeding. In south-east Englandand parts of Scotland they are monitoredannually. In the remainder of Scotland they aremonitored at about four to five year intervals. InNorthern Ireland, counts are monthly. Grey sealpup production has been monitored since theearly 1960s; harbour seals have been monitoredsince the late 1980s. Both grey and harbourseals are probably more numerous now thanin the historical past, when they were locallyhunted and/or harvested.iii. Current and likely future status of sealpopulationsIn the UK, grey seals are considerably morenumerous than harbour seals. After decadesof increase, total grey seal pup productionappears to be levelling off in the UK and is nowincreasing at only a small number of colonies.Harbour seal numbers have declined dramaticallyin Shetland, Orkney and the east coast ofScotland since 2001, by over 50%. There hasbeen a smaller decline in the Outer Hebrides butnumbers on the west coast of Scotland haveremained relatively stable. The causes of thesedeclines are not yet known. Two outbreaksof phocine distemper virus (PDV) affected theharbour seal population in eastern England with50% dying in 1988 and 22% dying in 2002.In marked contrast to European harbour sealpopulations which showed an immediate andrapid recovery, recovery in the eastern England508 Healthy and Biologically Diverse Seaspopulation after the 1988 outbreak was delayedfor three years and has yet to begin followingthe 2002 outbreak. PDV outbreaks are likelyto recur in the future but it is not possible topredict the proportion of the population thatmight be affected, which populations are mostvulnerable (besides eastern England) or preciselywhen outbreaks will occur.iv. What is driving change?The previous increase in grey seal pupproduction is at least in part due to the increasedavailability of breeding sites following theabandonment of human settlements on remoteislands, including the automation of lighthouses.The current reduction in the rate of increaseis thought to be due to density dependentfactors affecting the population as a whole. It isnot yet clear whether factors affecting survivalare more important than factors affectingfecundity. Causes of the declines in harbour sealpopulations in north and east Scotland remainunclear. Contributing factors could be eithernatural or anthropogenic or both and include:competition with grey seals, predation by killerwhales, unregulated shooting and declines inimportant prey species (e.g. sandeels).It is difficult to predict future trends in UKseal populations with certainty. Grey seal pupproduction appears to be stabilising, so thegrey seal population is likely to stabilise overthe next decade or so. It is far harder to predictwhat harbour seal populations in north and eastScotland will do, given recent declines and thelack of any obvious cause(s). In east England,PDV outbreaks are likely to recur in the future.The observed recent declines in north and eastScotland populations are not related to theoutbreak of PDV in 2002 as none of the harbourseals sampled in areas of decline had beenexposed to PDV.v. What are the uncertainties?While there are uncertainties in estimatingthe size of the total grey seal population fromthe annual estimates of pup production, theprocedures used to estimate pup productionare considered to be robust. Through the1970s, 1980s and early 1990s, seal researchwas focussed heavily on grey seals, withharbour seals only coming to attention in1988, after the first PDV outbreak. With therecently documented declines, harbour sealsare becoming increasingly important. Most ofthe pressure assessment is based on expertjudgement but it is difficult to assess the effectsof different pressures on seal populations. Thisapplies to both species.vi. Forward lookThe Scottish Government and Scottish NaturalHeritage have funded a number of projectsinvestigating the declines in harbour seals innorth and east Scotland. Increasing renewableenergy production may impact marine mammalpopulations, requiring additional information incertain areas. Harbour seal monitoring frequencyin Scotland is infrequent compared with greyseal monitoring