Seals
Assessment of progress towards the achievement of Good Environmental Status for seal biodiversity.
Extent to which Good Environmental Status has been achieved
The UK has achieved its aim of Good Environmental Status for grey seals in the Greater North Sea and Celtic Seas. There was a significant increase in the abundance of harbour seals in West Scotland where the majority of harbour seals are located, but their status in other parts of the Celtic Seas is uncertain. Harbour seals in the Greater North Sea have not yet achieved Good Environmental Status.
How progress has been assessed
In the UK Marine Strategy Part One (HM Government, 2012) the UK set out the following “Characteristics of Good Environmental Status” for Biodiversity:
“At the scale of the Marine Strategy Framework Directive Sub-Regions, and in line with prevailing conditions, the loss of biodiversity has been halted and, where practicable, restoration is underway:
The abundance, distribution, extent, and condition of species and habitats in UK waters are in line with prevailing environmental conditions as defined by specific targets for species and habitats.
Marine ecosystems and their constituent species and habitats are not significantly impacted by human activities such that the specific structures and functions for their long-term maintenance exist for the foreseeable future.
Habitats and species identified as requiring protection under existing national or international agreements are conserved effectively through appropriate national or regional mechanisms.”
The extent that Good Environmental Status, as articulated by the European Commission (2010), had been achieved, was assessed using targets set out for seals in the UK Marine Strategy Part One (HM Government, 2012). These targets were used to assess the status of the two species of seal that are resident in UK waters: the Atlantic grey seal (Halichoerus grypus, referred to, hereafter, as ‘grey seal’) and the harbour seal (Phoca vitulina, referred to, hereafter, as ‘common seal’).
The status of grey seals was assessed against two targets: (i) population size and (ii) population condition, using pup production as an indicator of the latter. The status of harbour seals was assessed against only a single target for population size. Pup production of harbour seals was not assessed because the breeding behaviour and locations of harbour seals makes pup production very difficult to monitor, compared to grey seals. Indicators relevant to the targets were then agreed, in cooperation with OSPAR, which used data collected by monitoring programmes in the UK and by neighbouring countries in the Greater North Sea. Only UK data were used in the OSPAR assessment of seals in the Celtic Sea because data from Ireland were not made available. The results of each indicator assessment in each spatial assessment unit were scored and averaged for each sub-region to evaluate the extent that the targets have been met. The status of both species would need to be consistent with the achievement of Good Environmental Status, for it to be achieved for seal biodiversity (Figure 1).
Figure 1: Schematic representation of the integration of indicators and targets to assess progress towards Good Environmental Status for seals in each UK Marine Strategy Framework Directive Sub-Region. ‘D1’ indicates Marine Strategy Framework Directive Descriptor 1 (Biodiversity).
Progress since 2012
Grey seals have continued to increase in numbers since monitoring began.
In the Greater North Sea, harbour seal numbers have increased in English colonies, but have continued to decline in Scottish colonies. Research into the potential causes of the decline has been ongoing since 2011. While the causal mechanisms for the decline have not yet been identified, several factors have been ruled out including: fisheries bycatch, pollution, loss of habitat, entanglement in marine debris, and legal shooting. In the Celtic Seas, harbour seal numbers have increased in West Scotland, but there are inconclusive declines in two other assessment units and insufficient data in another. In response to the observed declines, survey effort has been increased around the UK coast, which will provide more data to inform future trends, with some North Sea populations surveyed annually.
Outstanding issues
The reasons for the harbour seal decline are unclear but could include naturally occurring pressures such as interactions with grey seals (predation and competition), predation by killer whales in the Celtic Seas, and exposure to toxins from harmful algae.
Further information
Research is underway in Scotland to investigate potential causes of the harbour seal declines. While several factors have been ruled out as primary causes (e.g., fisheries bycatch, pollution, loss of habitat, entanglement in marine debris and legal shooting), investigations are ongoing into the remaining potential causes such as interactions with grey seals (competition and predation) and exposure to toxins from harmful algae. In addition, research will continue to investigate the life history parameters (e.g. survival and birth rates) and population dynamics of seals in areas of contrasting population trajectories. This will be through focused photo-identification studies and necropsies of stranded dead seals to improve our understanding within these populations.
Achievement of targets and indicators used to assess progress
A summary of indicator assessments and achievement of targets in the Greater North Sea is shown in Table 1, and for the Celtic Seas in Table 2. In both tables downward arrows without an asterisk indicate a significant decrease, downwards arrows with an asterisk indicates an inconclusive decrease, and upwards arrows indicate a significant increase.
Table 1. A summary of indicator assessments and achievement of targets in the Greater North Sea.
|
Target |
Assessment unit: |
North Coast and Orkney |
Shetland |
Moray Firth |
East Scotland |
NE England |
SE England |
South England |
Species status |
|
Population size: in all of the indicators monitored, there should be no statistically significant decrease in the abundance of seals caused by human activities. |
Harbour seal trend |
↓ |
↓ |
↓* |
↓ |
↑ |
↑ |
Not applicable |
Harbour seal status appears not to be consistent with Good Environmental Status |
|
Number of harbour seals (2014) |
1938 |
3039 |
733 |
194 |
90 |
4681 |
20 |
||
|
Grey seal trend |
↑ |
Grey seal status appears consistent with Good Environmental Status |
|||||||
|
Population condition: there should be no statistically significant decline in seal pup production caused by human activities |
Grey seal production trend |
↑ |
↓* |
↑ |
↑ |
↑ |
↑ |
Not applicable |
|
|
Number of pups (2014) |
23758 |
3875 |
1600 |
5027 |
0 |
||||
* an inconclusive increase/decrease
Table 2. Summary of indicator assessments and achievement of targets in the Celtic Seas.
|
Target |
Assessment unit: |
Northern Ireland |
Southwest Scotland |
West Scotland |
Western Isles |
SW England |
Wales |
NW England |
Species status |
|
Population size: in all of the indicators monitored, there should be no statistically significant decrease in the abundance of seals caused by human activities. |
Harbour seal trend |
↓* |
Insufficient data |
↑ |
↓* |
Not applicable |
Not applicable |
Not applicable |
Harbour seal status is uncertain. |
|
Number of harbour seals (2014) |
948 |
834 |
13,878 |
2,739 |
|
5 |
10 |
||
|
Grey seal trend |
↑ Number of grey seals = 34,400 (2014) |
Grey seal status appears consistent with Good Environmental Status. |
|||||||
|
Population condition: there should be no statistically significant decline in seal pup production caused by human activities. |
Grey seal production trend |
Not applicable |
Insufficient data |
↑ |
↑ |
Not applicable |
↑ |
Not applicable |
|
|
Number of pups (2014) |
100 |
|
4,054 |
14,316 |
250 |
1,650 |
0 |
||
Further information
Harbour seal status assessment
The status assessment of harbour seals was based on progress towards a single target (HM Government, 2012):
Population Size: “At the scale of the MSFD sub-regions abundance of seals is not decreasing as a result of human activity: in all of the indicators monitored, there should be no statistically significant decrease in abundance of marine mammals caused by human activities.”
In the Celtic Seas, the extent to which the UK target for population size has been achieved for harbour seals remains uncertain. There was a significant increase in abundance in west Scotland where the majority of seals are, but there was inconclusive evidence of declines elsewhere.
In the Greater North Sea, the UK target for harbour seal abundance has not been met. Harbour seal abundance is stable or increasing along the English North Sea coast but has declined along the coast of the Scottish North Sea. The causes of these declines are currently unknown, although some potential factors such as fisheries bycatch have been discounted as contributing to the decline.
Evidence to support the harbour seal status assessment
The supporting evidence for this target evaluation and the extent that corresponding criteria in European Commission (2010; 2017) have been achieved comes from the assessment of the indicator: seal abundance and distribution.
This indicator assessment has been undertaken as part of the OSPAR Intermediate Assessment (OSPAR Commission, 2017a). The European population of harbour seals has been subdivided into a number of assessment units. The size of these units reflects the short distances that harbour seals travel from their haul-out sites on land to forage at sea. The UK assessment included only those assessment units in UK waters.
The abundance indicators use estimates of harbour seal numbers from monitoring programmes that count seals on land during the annual moult (August). These figures are the minimum estimates of harbour seal numbers. Trends in the abundance of harbour seals were assessed within each assessment unit. The target for population size in each unit was not met if a) seal abundance during 2009 to 2014 declined by more than an average of 1% per year, and/or b) seal abundance has not decreased by more than 25% since the baseline year (1992 or the start of the time series, if later).
Harbour seal abundance targets were met in West Scotland (Celtic Seas) and in the English part of the North Sea, where numbers are stable or increasing. Targets were not met in the Scottish part of the North Sea (East Scotland, Orkney, and Shetland), where harbour seal abundance has shown a marked and prolonged decline. The reason(s) for these declines are unclear but a major research programme is underway to investigate potential causes. While the causal mechanisms for the decline have not been identified, several factors have been ruled out as primary causes and these include fisheries bycatch, pollution, loss of habitat, entanglement in marine debris and legal shooting. Research will continue into the remaining potential causes, including predation (by grey seals and killer whales, competition with grey seals), and exposure to toxins from harmful algae. Elsewhere, the assessment of harbour seal abundance was inconclusive.
Grey seal status assessment
The status assessment of grey seals was based on progress towards two targets (HM Government, 2012):
Population Size: “At the scale of the MSFD sub-regions abundance of seals is not decreasing as a result of human activity: in all of the indicators monitored, there should be no statistically significant decrease in abundance of marine mammals caused by human activities.”
Pup production: “At the scale of the MSFD sub-regions, marine mammal productivity is not significantly affected by human activities: there should be no statistically significant decline in seal pup production caused by human activities.”
The status of grey seals in both the Celtic Seas and the Greater North Sea is consistent with Good Environmental Status. Both targets for population size and population condition (pup productivity) have been met. Abundance and productivity of grey seals have both increased significantly since the UK initial assessment (HM Government, 2012) and also over the longer-term, since the early 1990s. This improvement in UK waters is mirrored in the wider grey seal population of the north-east Atlantic. This population is likely to be recovering from a time when it was significantly depleted by human activities. Hunting, pollution, and overfishing (competition) are all likely to have reduced populations in the recent past and may still have an influence now. These improvements cannot carry on indefinitely as natural carrying capacity (or equilibrium) will eventually be reached.
Assessment of the target on grey seal population size
The UK target was met for grey seal abundance across its range in the UK, where the species has been increasing.
Evidence to support the assessment of grey seal population size
The supporting evidence for this target evaluation and the extent that corresponding criteria in European Commission (2010; 2017) have been achieved comes from the assessment of the indicator: seal abundance and distribution.
This indicator assessment has been undertaken as part of the OSPAR Intermediate Assessment (OSPAR Commission, 2017a). In contrast, to harbour seals, grey seals forage across a much wider area, so their abundance was assessed in a single assessment unit covering the whole of the Greater North Sea (using data from other North Sea countries) and the UK parts of the Celtic Seas (data from Ireland were not available).
Because the single large assessment unit for grey seals covers colonies beyond UK waters, the total abundance of grey seals was modelled using both summer counts of grey seals and counts of pups at grey seal colonies during the autumn and winter. Trends were assessed over the short-term (2009 to 2014) and the long-term (1992 to 2014, for most areas). The target for population size was not met if a) seal abundance during 2009 to 2014 declined by more than an average of 1% per year, or b) seal abundance has not decreased by more than 25% since the baseline year (1992 or the start of time of the series, if later).
The population model predicted that the grey seal population in the assessment unit covering both the Greater North Sea and Celtic Seas has grown over both the short and long-term. The number of colonies occupied by breeding grey seals were stable over both the short and long-term, where the animals occur and such data were available.
Assessment of grey seal pup production
The UK target was met: the number of grey seals born each year in both the Greater North Sea and the Celtic Seas has increased substantially, since 1992, and has continued to rise in recent years (from 2009 to 2014).
Evidence to support the assessment of grey seal pup production
The supporting evidence for this target evaluation and the extent that corresponding criteria in European Commission (2010; 2017) have been achieved comes from the assessment of the indicator: grey seal pup production.
This indicator assessment has been undertaken as part of the OSPAR Intermediate Assessment (OSPAR Commission, 2017b). The UK assessment included only those assessment units in UK waters.
Grey seal pups were counted at major breeding sites (‘colonies’) using aerial photographic survey methods; and where these are not possible, ground counts or boat-based counts. Multiple counts per colony were generated, spread across the breeding season (September to February, depending on colony location). In the UK, pup counts are converted to total pup production at each colony by using an established statistical model that describes how the number of pups at the site varies over the season. Both types of estimates (maximum count and modelled pup production) were used in this assessment.
Trends in pup production were assessed in a number of assessment units throughout the UK and in neighbouring countries in the Greater North Sea (as used in the assessment of harbour seal abundance). Trends in each unit were assessed over the short-term (2009 to 2014) and the long-term (1992 to 2014, for most areas). The target for population condition was not met if; a) annual pup production during 2009 to 2014 declined by more than an average of 1% per year, or b) pup production has not decreased by more than 25% since the baseline year (1992 or the start of time series, if later).
Grey seal pup production has increased in both the long and short-term, within all assessment units where there were breeding sites and sufficient data to carry out the assessment. As there was no decline in pup production in these areas, the targets for pup production have been met. The annual rate of increase in pup production has quickened during 2009 to 2014, compared to the mid-2000s (UK Initial Assessment; HM Government, 2012) in all assessed areas, except in Shetland.
Pup counts in Shetland have suggested a decline in pup production there during 2004 to 2014, which was not evident during the Initial Assessment (HM Government, 2012) based on counts from the mid-2000s. However, there is low confidence that the results from Shetland represent an actual decline, because the counts varied substantially from year to year. This variance is likely the result of difficulties in counting pups in Shetland.
Grey seal pup production increased rapidly following the end of culling in the 1970s, and 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 increases in pup production had slowed by the time of the initial assessment, probably due to density-dependent factors affecting the population as a whole.
Moving forward
Determining the impact of human pressure is key to assessing progress against the UK target. Regular surveys will continue around the UK coast to monitor population abundance and trends. In addition, regions of decline will continue to be surveyed more frequently to establish population trends and abundance. A seal bycatch indicator will be developed.
References
Acknowledgements
European Commission (2010) ‘Commission Decision of 1 September 2010 on criteria and methodological standards on Good Environmental Status of marine waters’ (notified under document C (2010) 5956) (Text with EEA Relevance) (2010/477/EU). Official Journal of the European Union L232, 2.9.2010, pages 14-24 (viewed on 5 July 2018)
European Commission (2017) ‘Laying down criteria and methodological standards on good environmental status of marine waters and specifications and standardised methods for monitoring and assessment’ Commission Decision (EU) 2017/848 of 17 May 2017, repealing Decision 2010/477/EU. Official Journal of the European Union L 125, 18.5.2017, pages 43-74 (viewed on 5 July 2018)
HM Government (2012) ‘Marine Strategy Part One: UK Initial Assessment and Good Environmental Status’ (viewed on 5 July 2018)
OSPAR Commission (2017a) ‘Seal abundance and distribution’ OSPAR Intermediate Assessment 2017. (viewed on 5 July 2018)
OSPAR Commission (2017b) ‘Grey Seal Pup Production’ OSPAR Intermediate Assessment 2017. (viewed on 5 July 2018)