Grey Seal Pup Production

D1 - Biological Diversity

D1.3 - Population condition

In the Greater North Sea and in parts of the Celtic Seas, the number of grey seals born each year has increased substantially since 1992 and has continued to rise in recent years (2009–2014).

Area Assessed

Printable Summary

Background

This indicator assesses trends in the number of grey seal pups born at breeding sites in the Greater North Sea and in the United Kingdom part of the Celtic Seas. Grey seals gather to breed at long-established colonies located on islands, sand banks and mainland coastlines around Europe.

As higher predators, seals can be used as an indicator to reflect the state of the marine ecosystem. Grey seal pup production is influenced by many factors such as disease, competition with other species, changes in the distribution and abundance of prey, disturbance, and interactions with fisheries. Grey seals were hunted into the 20th Century, and as a result have disappeared entirely from some areas, but are now protected in most areas of Europe.

Seals have been hunted both illegally and legally for a long time and it is not possible to know the undisturbed state, nor the current carrying capacity that could be attained alongside protection from illegal hunting. Except for hunting, no straightforward link has yet been identified between pup production and human activity, although several human activities may, at least in part drive, change in pup production. If changes are detected, this signals the need to investigate the cause and to determine whether management measures are required.

Atlantic grey seal (Halichoerus grypus) pup (courtesy of Arran Bee )

Grey seal pup production was assessed in the OSPAR Quality Status Report (QSR) 2010 by means of an OSPAR Ecological Quality Objective (EcoQO) in the Greater North Sea. The EcoQO for no decline of greater than 10% in grey seal pup production over a five-year running mean was assessed as met in all areas of the Greater North Sea with relevant populations and where data were reported.

The data used to construct this Indicator were also used in part, to estimate grey seal abundance for the Indicator on Seal Abundance and Distribution. As such, these two indicators are to some extent related.

Methods

Indicator metric and data collection

Grey seal (Halichoerus grypus) pups are counted at major breeding sites (‘colonies’) usually using aerial photography survey methods (Figure a); where these are not possible, ground counts or boat-based counts may be used instead (e.g. Stringell et al., 2014). Multiple counts per colony are generated, spread across the breeding season (September to February, depending on colony location).

Figure a: Aerial survey image of grey seal colony (courtesy of Sea Mammal Research Unit)

Most Contracting Parties count pups several times during the pupping season and the maximum count is taken as a minimum estimate of ‘pup production’, that is, the total number of pups produced over the season as a whole. The maximum pup count will always under-estimate pup production because pups are born and leave the colony at different times. In the United Kingdom, pup counts are converted to total pup production at each colony. This is carried out using an established statistical model that describes how the number of pups at the site varies over the season (SCOS, 2014). Both types of estimate (i.e. maximum count and modelled pup production) are used in this assessment. From calculations across Scottish colonies, maximum pup counts represent 0.82 (95% confidence interval, 0.81–0.83) of total pup production. Hereafter, both types of estimate are referred to collectively as ‘pup production’.

The assessment in the Celtic Seas did not include Ireland because the density of Irish data available was not sufficient to enable a comprehensive assessment of change in abundance. In Ireland, the status of the grey seal is considered to be ‘Favourable’ given the current knowledge of population size, distribution, ecology and prevailing pressures on these species (NPWS, 2013).

Colony survey frequency varies by ‘Assessment Unit’ (AU) (Figure A) and ranges from annual to about five-yearly. Details of seal monitoring programmes for each AU are given in Table a.

Table a: Seal monitoring programmes.
OSPAR indicator Seal abundance and distribution Grey seal pup production
Species Grey seals (ALL AGES) Harbour seals (ALL AGES) Grey seals (PUPS)
Metric August counts (in conjunction with harbour seal moult counts) Moult counts (spring) August moult counts Pup Counts (number of pups present at any one time) Pup production (total number of pups produced during entire breeding season)
UK Northern Ireland Single aerial survey approx every 5 years Single aerial survey approx every 5 years SMRU partial survey 2002, 2005; Boat-based monthly surveys of Strangford Lough
Southwest Scotland Single aerial survey approx every 5 years Single aerial survey approx every 5 years No formal monitoring as population very low
West Scotland Single aerial survey approx every 5 years Single aerial survey approx every 5 years Repeat aerial survey, annual to 2010 biennial thereafter
Western Isles Single aerial survey approx every 5 years Single aerial survey approx every 5 years Repeat aerial survey, annual to 2010 biennial thereafter
Orkney & North coast Single aerial survey approx every 5 years Single aerial survey approx every 5 years Repeat aerial survey, annual to 2010 biennial thereafter
Shetland Single aerial survey approx every 5 years Single aerial survey approx every 5 years Ground count, annual since 2004
Moray Firth Repeat aerial survey, annual Repeat aerial survey, annual Repeat aerial survey, annual to 2010 biennial thereafter
East coast Scotland Single aerial survey, annual Single aerial survey, annual Repeat aerial survey, annual to 2010 biennial thereafter
Northeast England No formal monitoring as population very low No formal monitoring as population very low Ground count, annual
Southeast England Repeat aerial survey, annual Repeat aerial survey, annual Ground count, annual
South England
Southwest England
Wales
Northwest England
France French North Sea & Channel coasts Combination aerial and land surveys, approx 2-3 per month Combination aerial and land surveys, approx 2-3 per month Combination aerial and land surveys, approx 2-3 per month Monthly ground surveys
Denmark Wadden Sea Repeat aerial survey, annual Repeat aerial survey, annual Repeat aerial survey, annual Repeat aerial survey, annual from 2008
Limfjord Repeat aerial survey, annual Repeat aerial survey, annual from 2015 Repeat aerial survey, annual Repeat aerial survey, annual from 2015
Kattegat Repeat aerial survey, annual Repeat aerial survey, annual Repeat aerial survey, annual
Netherlands Wadden Sea Repeat aerial survey, annual Repeat aerial survey, annual Repeat aerial survey, annual Repeat aerial survey, annual from 2001
Dutch Delta Monthly aerial survey Monthly aerial survey Monthly aerial survey Monthly aerial surveys have not yeilded a credible pup production in the area
Germany Wadden Sea (inc Helgoland) Repeat aerial survey for harbour seals with recording of incidental grey seal sigthings,annual Repeat aerial survey, annual Repeat aerial survey, annual Repeat aerial (Lower Saxony + Hamburg, from 2016 onwards also Schleswig-Holstein) or ground/ship based (Schleswig-Holstein, Helgoland) survey, annual from 2008
Norway Norwegian west coast, south of 62N Single aerial survey approx every 5 years Ground count at least every 5 years
Northern Skagerrak and Oslo Fjord Aerial survey, annual

Spatial scope: assessment units (AUs)

Despite their ability to travel long distances, individual mature grey seals of both sexes are usually faithful to particular breeding sites, and may return to within 10–100 m of individual breeding locations (Pomeroy et al. 2000). The Greater North Sea and UK parts of the Celtic Seas were divided into 21 AUs  to provide a description of changes in pup production at a ‘local’ scale (Figure b). Direct impacts on pup production, such as disturbance, persecution and food availability may be more detectable at these smaller scales.

It is important to consider that AUs do not represent demographically independent populations. Additionally, survey coverage and monitoring effort is higher where breeding grey seals are most abundant. In many AUs monitoring is undertaken in specific areas by local organisations and does not form part of synoptic surveys. See Table a for details of monitoring programmes in each AU.

The AUs are the same as those used for the harbour seal abundance and distribution assessment under the indicator on Seal Abundance and Distribution. The assessment units in the Greater North Sea are broadly similar to those previously defined as OSPAR EcoQO sub-units.

Figure b: Assessment units for grey seal pup production.

Key: 1 Northern Ireland, 2 South-west Scotland, 3 West Scotland, 4 Western Isles, 5 North coast and Orkney, 6 Shetland, 7 Moray Firth, 8 East Scotland, 9 North-east England, 10 South-east England, 11 South England, 12 South-west England, 13 Wales, 14 North-west England, 15 French North Sea and English Channel Coast, 16 Belgian coast and Dutch Delta, 17 Wadden Sea, 18 Limfjorden, 19 Kattegat, 20 Northern Skagerrak and Oslo Fjord, 21 Norwegian west coast, south of 62°N.

Baselines

Seals have been hunted both illegally and legally for a long time and it is not possible to know the undisturbed state, nor the current carrying capacity that could be attained alongside protection from illegal hunting. Time series data for grey seal pup production do not provide an indication of a time when seal populations were not impacted or what the values would have been. It is therefore not possible to identify a baseline representing un-impacted conditions. The method adopted for this assessment assesses grey seal pup production data using both a fixed baseline and a rolling baseline with associated assessment values.

A fixed-baseline year of 1992 was used, which is the baseline year used by some European Union Member States for seals under the Habitats Directive (Council Directive 92/43/EEC). For AUs where grey seal pup production data for 1992 were not available, the start of the time series was used as the baseline.

Bearing in mind that the arbitrarily assigned baseline does not necessarily reflect a state without impacts, it is not possible to assess the status of seals in relation to the concept of ‘favourable conservation status’ under the European Union Habitats Directive. Instead, trends in the seal population are assessed. A short-term rate-based assessment value was also adopted that uses a rolling baseline (Method 1; OSPAR, 2012). Here, the average annual rate of population change over consecutive six-year periods (the reporting cycle for the European Union Marine Strategy Framework Directive; MSFD) was assessed against the next period. The rolling baseline provides a means to indicate change in population size compared with the previous six-year assessment period, rather than relying solely on an historical fixed baseline, which probably reflects a point in time when the population is already depleted due to anthropogenic pressures.

A potential issue with this type of quantitative trend thresholds, known as ‘shifting baselines’ is that each successive assessment uses a different starting point as the basis for comparison. This could result in a substantial cumulative decrease occurring over more than one six-year assessment period not being flagged as a problem, because in each six-year period the rate of decline remained below the assessment value (OSPAR, 2012).

Use of the two types of baseline and associated assessment values seeks to provide an indicator that would warn against both a slow but long-term steady decline (the problem of ‘shifting baselines’ associated with only having a rolling baseline) and against a recovery followed by a subsequent decline (potentially missed with a fixed baseline set below reference conditions).

Assessment values

OSPAR Intermediate Assessment 2017 Indicator Assessment values are not to be considered as equivalent to proposed European Union MSFD criteria threshold values, however they can be used for the purposes of their MSFD obligations by those Contracting Parties that wish to do so.

Indicator assessment values were set as a percentage deviation from the baseline value (Method 3; OSPAR, 2012).

Associated with the above baselines, two assessment values were used to assess grey seal pup production in each AU:

  • Assessment value 1: No decline in grey seal pup production of >1% per year in the previous six-year period (a decline of approximately 6% over six years).
  • Assessment value 2: No decline in grey seal pup production of >25% since the fixed baseline in 1992 (or closest year).

For assessment value 2: in cases where a shorter timescale than the period to 1992 is assessed, a 25% decline since the baseline is therefore not equivalent to a 25% decline in those AUs where data do extend to 1992. For example, a 25% decline since 2003 describes a more rapid contraction in population size than a 25% decline since 1992.

Using the two assessment values together would provide a means to identify a need for management measures to promote population growth (i.e. recovery) and to identify subsequent slowing of population growth as carrying capacity is approached.

Assessing change in pup production

Pup production estimates from all colonies within a given AU were summed to generate an overall assessment for that area. Quantitative assessments of trends in pup production were only completed for those AUs where three or more annual data points were available.

In addition to the assessment of the Wadden Sea (AU17), a separate assessment was conducted on the Dutch part of the Wadden Sea, where the time series began in 2001, compared to 2008 for other parts of the Wadden Sea. Earlier data exist, both for Dutch and German waters, but these were collected using different methods and so are difficult to compare with data collected subsequently.

Grey seal pup production estimates were assessed in relation to a baseline set at 1992 (or the start of the time series where this was after 1992), and by calculating the average rate of annual population change within the last reporting round (2009–2014).

To determine the change in pup production since the baseline year, generalised linear models or generalised additive models were fitted to the sum of pup production data within an AU with a quasi-Poisson error distribution and log link using all available annual survey data for the period 1992–2014. The percentage change in pup numbers since the baseline year (Formula A; Δabundance) and 80% confidence intervals were calculated from fitted values. Although no formal hypothesis testing was conducted, 80% confidence intervals were calculated to reflect the choice to set the significance level, α, equal to 0.20 or 20%.

Formula A: Calculation of long term trend in abundance

where A is the count fitted by the model in the baseline year and B is the count fitted by the model in the most recent survey year.

To estimate the annual change in pup production over the previous six-year reporting round, a linear trend was fitted to the sum of all available data in each AU for the reporting round 2009–2014. GLMs were fitted to the sum of pup production data within an AU with a quasi-Poisson error distribution and log link. Annual growth rate (%) and 80% confidence intervals were estimated for each of the AUs. Although no formal hypothesis testing was conducted, 80% confidence intervals were calculated to reflect the choice to set the significance level, α, equal to 0.20 or 20%.

Indicator metrics and associated confidence intervals were assessed against two thresholds: the average annual growth rate (%) and 80% confidence intervals within the last reporting round were less than -1% per year, and the percentage change in abundance since baseline year (Δbaseline) and 80% confidence intervals were less than -25%. If the 80% confidence intervals encompassed the threshold, the assessment was classified as ‘inconclusive’.

Statistical power of detecting change in pup production

There are many reasons why pup counts could vary, aside from representing true changes in pup production, These include variation in weather and a recent disturbance at a colony. It is therefore advisable to examine the variability in survey counts and to incorporate this variability into trend estimates. The International Council for the Exploration of the Sea (ICES) Working Group for Marine Mammal Ecology (WGMME) (ICES, 2014) provided general advice on the need to understand the statistical power of monitoring programmes. In the present context, statistical power is the percentage confidence in not missing a significant decline. Statistical power depends on the sample size (number of surveys), the level of statistical significance set (α-level), variance in the counts, and the magnitude of the trend, that is, -1% and -25%. The ICES WGMME (ICES, 2014) recommended that monitoring should achieve a minimum of 80% power – which equates to a 20% chance of making a Type II error (i.e. the frequency with which a true decline would not be detected). The same group also recommended that the threshold for detection of a statistically ‘significant’ trend be relaxed from the traditional α=0.05 to α=0.20. The α parameter, or statistical significance level, equates to the probability of concluding that a statistically significant trend exists when it does not (Type I error). A α value of 0.2 and power of 80% means there is equal probability of making an incorrect conclusion (either Type I or Type II error) about the detection of a trend.

Current monitoring programmes vary in the level of statistical power achievable. To carry out a full study of retrospective power to detect changes in the observed population trends, detailed information about the between- and within-year variability in all survey counts would be necessary. A full assessment of power was not undertaken here but confidence intervals (CIs)  provide a measure of confidence in the assessment. Simply said, they describe the frequency with which the true, unobservable, population parameter (here, the mean count) could be expected to fall within the intervals described by an upper and lower confidence limit. Where the confidence intervals encompass the assessment value the data do not provide conclusive evidence for the calculated value being above or below the assessment value.

Results

In the Greater North Sea and parts of the Celtic Seas, grey seal pup production has increased over both the long term and short term, in all assessed areas where there were breeding sites and sufficient data to carry out the assessment (Figure 1 and Figure 2). There is no decline in pup production in these areas, in line with both assessment values. Although the data available for Shetland (AU6) were not sufficient to allow an assessment, pup counts appear to indicate a long-term decline in pup production during the period 2004–2014.

Grey seal pup production increased rapidly in Greater North Sea Assessment Units (AUs) during the period 2009–2014. It is likely that the increase in pup production in the AUs in continental Europe (e.g. the Wadden Sea, AU17) is being driven by the immigration of animals from the large colonies to the north of the United Kingdom.

Growth rates in pup production were lower in West Scotland (AU3) and the Western Isles (AU4). This may be due to these areas approaching carrying capacity.

Changes in grey seal pup production were assessed up to 2014 in the Greater North Sea and in parts of the Celtic Seas within British waters. Grey seals breed along the coast of Ireland, but corresponding data are not available because the data density could not support an assessment.

The confidence rating for the methodology was considered to be moderate/low. The confidence rating for data availability was considered to be moderate.

Long-term change was assessed from a baseline year of 1992 (or later for some time series). Short-term change concerns the period 2009–2014. The analysis looked at the extent of long-term or short-term decline. No declines were detected.

Figure 1: Change in grey seal pup production during the period 2009–2014, assessed against no decline greater than 1% per year

The numbers in each circle refer to the respective ‘Assessment Unit’ (see key)

Figure 2: Change in grey seal pup production during the period 1992–2014, assessed against no decline greater than 25%.

The numbers in each circle refer to the respective ‘Assessment Unit’ (see key). Although 1992 was used as the baseline year, in some Assessment Units a later year was used (given in parentheses below).
Assessment Units: 1 Northern Ireland, 2 South-west Scotland, 3 West Scotland, 4 Western Isles, 5 North coast and Orkney, 6 Shetland (2004), 7 Moray Firth (1997), 8 East Scotland, 9 North-east England, 10 South-east England, 11 South England, 12 South-west England, 13 Wales, 14 North-west England, 15 French North Sea and English Channel Coast, 16 Belgian coast and Dutch Delta, 17 Wadden Sea (2001 for the Netherlands and 2008 for Germany and Denmark), 18 Limfjorden, 19 Kattegat, 20 Northern Skagerrak and Oslo Fjord, 21 Norwegian west coast, south of 62°N (2000).

The assessment results presented in Figure 1 and Figure 2 are derived from the values for rates of change in pup production presented in  Table b.

Table b: Short-term (2009–2014) and long-term (baseline–2014) change in grey seal pup production in each Assessment Unit

Table B Key: T1: Growth rate 2009–2014 refers to the annual growth (%) in counts between 2009 and 2014. T2: Δbaseline refers to the change in numbers since the baseline year. Upper and lower 80% confidence intervals with upper and lower limits are given for both assessment values based on modelled growth rates (upper CIT1, lowerCIT1 and upper CIT2, lowerCIT2 respectively). YearsT1 and YearsT2 refers to the number of years of pup count data in the periods 2009–2014 and 1992–2014 respectively. Observed change in pup production since the baseline year, not from modelled data, is denoted by Observed Δbaseline. The baseline year refers to 1992 or the start of the time series. ‘Most recent count’ refers to the most recent estimate of total pup production (and is not necessarily in 2014). ‘Relative size’ refers to the most recent estimate of total pup production expressed as a proportion of the total for all AUs. aInformation collated by the Wadden Sea Secretariat includes pups born in the Netherlands and Germany (very few pups born in Denmark); bDutch portion of Wadden Sea only (longer data series); cNumbers represent maximum number of pups counted; in other AUs the number given is modelled pup production (i.e. the total number of pups born in the season).

The values in Table b were estimated from trend models that were fitted to observed annual pup production within each Assessment Unit (AU), where there were sufficient time series data to do so (see Methods section). The observed and modelled estimates of annual pup production are plotted for each AU in Figure b.

Figure c: Modelled trends in grey seal pup production in each Assessment Unit.

Points denote observed annual pup production, coloured according to four periods between 1992 and 2014: 1992–1996 (pink), 1997–2002 (purple), 2003–2008 (orange), 2009–2014 (green). Modelled trends in annual pup production are shown for the period from start of the time series (baseline year) to 2014 (dash line) and for 2009–2014 (dotted and dashed line); the 80% confidence interval around each trend is shown as a greyed area on the plot. Assessment Units: 3 West Scotland, 4 Western Isles, 5 North coast and Orkney, 6 Shetland, 7 Moray Firth, 8 East Scotland, 9 North-east England, 10 South-east England, 13 Wales, 17 Wadden Sea, 17a Wadden Sea – Netherlands only (longer time series), 21 Norwegian west coast, south of 62°N.

Perhaps most striking of the trends in Figure c was the continued rapid growth in grey seal pup production in Greater North Sea AUs. Here, it was also apparent that while total pup production in mainland AUs (e.g. the Wadden Sea, AU17) is small compared to that in the major areas to the northwest of the United Kingdom, they are also experiencing rapid growth similar to those areas in the Western North Sea (Figure c). Brasseur et al. (2015) described the recovery and continued growth of the largest grey seal population in mainland Europe (Wadden Sea, AU17) in more detail, noting that recolonisation was most likely by animals from the western North Sea colonies. The slower growth rates of colonies the Celtic Seas (e.g. West Scotland, AU3; Western Isles, AU4) has also been observed in more detailed analyses of United Kingdom grey seal pup production data and it has been suggested that these areas may be approaching carrying capacity (SCOS, 2014).

Assessment of confidence

There is moderate/low confidence in the methodology used in this assessment. There is consensus within the scientific community regarding this methodology therefore the confidence could be considered moderate, however as the method has been developed specifically for this assessment and not been more widely used it is considered moderate/low.

The assessment is undertaken using data with a spatial coverage and temporal extent that is mostly sufficient for the area assessed but gaps are apparent in certain areas. As such, confidence in data availability is moderate.

Conclusion

Grey seal pup production has increased over both the long term and short term in all assessed areas in the Greater North Sea (except Shetland) and in the parts of the Celtic Seas covered by this assessment.

The results for pup production should be considered alongside those for the common indicator on seal abundance and distribution. While both indicators show an improvement in the condition of the grey seal population in the North-East Atlantic, the population is likely to be recovering from a time when it was significantly depleted by human activity. Hunting, pollution and overfishing are all likely to have reduced populations in the recent past and might still be having an influence. The natural carrying capacity for the number of grey seals in the North-East Atlantic is not known. Even without pressure from human activities it is clear that pup production cannot keep increasing indefinitely and will reduce as the natural carrying capacity is reached.

Knowledge Gaps

There are several knowledge gaps that need to be addressed to improve this assessment for it's next iteration. There are currently insufficient data points in some of the Assessment Units (AUs) (Figure 1). Pup production is only a partial indicator of population condition, which is also affected by female fecundity, pup survival and body condition and this aspect of the indicator could be further developed. There is also a lack of information on human impacts on pup production and how adult grey seals move between AUs, including how this impacts populations.

It was not possible to make a quantitative assessment in all 21 Assessment Units (AUs) for several reasons. One being that grey seals may not breed in sufficient numbers in all AUs to warrant regular monitoring; in such cases the assessment area was considered ‘not applicable’ to this assessment (Figure 1). The remaining assessment gaps were due to a lack of sufficient annual data points to perform a quantitative assessment of the time series. Infrequent monitoring may be because there are relatively few animals breeding there and / or may be related to the difficulty in assessing breeding sites during the autumn and winter. Continued support for the long-term seal monitoring programmes in place within Contracting Parties will ensure that sufficient data are collected to provide quantitative assessments of grey seal pup production for those areas. The aggregation of data across national programmes in the North-East Atlantic provides a possibility for meta-analyses that place regional / national population trends into the wider regional context.

Pup production when used on its own is not a meaningful indicator of population condition. An ideal measure of population condition would include information on female fecundity and pup survival. Such demographic parameters are only available where long-term and detailed monitoring of colonies exists (e.g. Isle of May in Scotland). Another good indicator of population condition would be changes in body condition (e.g. blubber thickness).

From the present data it is not possible to exclude human influence on pup production. Even without pressure from human activities it is clear that pup production cannot keep increasing indefinitely and will reduce as the natural carrying capacity is reached. Further demographic studies could help predict the natural carrying capacity for the number of grey seals in the North-East Atlantic and provide an indication of likely future trends in pup production, with and without impacts from human pressures.

A demographic link between AUs is possible, but the extent of movement of adults between colonies and how this influences local pup production is at present poorly understood. Further international studies on the movements of individual seals could help better understand this and even prevent unintended disturbance of these inter-colony exchanges.

ICES 2014. Report of the working group on Marine Mammal Ecology (WGMME). 10-13 March, 2014 Woods Hole, Massachusetts, USA. ICES CM 2014/ACOM: 27, 230pp

NPWS (2013). The Status of Protected EU Habitats and Species in Ireland Overview Volume 1. Unpublished Report National Parks & Wildlife Services. Department of Arts, Heritage and the Gaeltacht Dublin Ireland.)

OSPAR, 2012. MSFD Advice Manual and Background Document on Biodiversity. Version 3.2. OSPAR Biodiversity Series. Available at: http://www.ospar.org/documents/dbase/publications/p00581/p00581_advice%20document%20d1_d2_d4_d6_biodiversity.pdf

Pomeroy, P. P., Twiss, S. D. and Redman, P. 2000. Philopatry, site fidelity and local kin associations within grey seal breeding colonies. Ethology: 899–919.

SCOS. 2014. Scientific advice on matters related to the management of seal populations, 2014. Currently available on the SCOS website at http://www.smru.st-and.ac.uk/documents/2259.pdf

Stringell, T., Millar, C., Sanderson, W., Westcott, S. and McMath, M. 2014. When aerial surveys will not do: grey seal pup production in cryptic habitats of Wales. Journal of the Marine Biological Association of the United Kingdom. 94: 1155-1159.

Brasseur, S., van Polanen Petel, T., Gerrodette, T., Meesters, E., Reijnders, P., Aarts, G. 2015. Rapid recovery of Dutch grey seal colonies fuelled by immigration. Marine Mammal Science. 31: 405-426.

SCOS. 2014. Scientific advice on matters related to the management of seal populations, 2014. Currently available on the SCOS website at http://www.smru.st-and.ac.uk/documents/2259.pdf