Grey Seal Pup Production

Grey seal pup production was assessed in the Greater North Sea in the 2010 OSPAR Quality Status Report (QSR) by means of an Ecological Quality Objective (EcoQO). 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 local populations and where data were reported.

Grey seal pup production was then assessed again in the OSPAR Intermediate Assessment (IA) undertaken in 2017, which focused on the Greater North Sea and UK parts of the Celtic Seas. Using further developed threshold values (see Assessment Methods) based on those initial OSPAR EcoQO’s, grey seal pup production 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.

While the methods between the IA 2017 and the QSR 2023 remain the same, the QSR 2023 expands upon those OSPAR Regions assessed within the IA 2017, now encompassing Region II, the whole of Region III, out to the Marine Strategy Framework Directive (MSFD) boundary, as well as the addition of Region I. The expansion into the Arctic Waters (Region I) may allow for more latitudinal variation in the examination of indicator trends, particularly concerning the effects of environmental change. The expansion is in line with the OSPAR NEAES 2030 S5.O4: By 2025 at the latest OSPAR will take appropriate actions to prevent or reduce pressures to enable the recovery of marine species and benthic and pelagic habitats in order to reach and maintain good environmental status as reflected in relevant OSPAR status assessments, with action by 2023 to halt the decline of marine birds.

When linked and compared with suitable pressure indicators, assessments of pup production trends within and across OSPAR Regions can provide key insights into the drivers of observed changes and help inform appropriate management of those pressures especially where there is movement between Regions. However, it should also be noted that the already depleted state of grey seals across the North-East Atlantic at the start of this indicator’s assessment baseline year (1992), the trends since this likely reflect already depleted colonies, recovering from low numbers.

Pup production is only a partial indicator of the wider grey seal population condition. The outputs from this indicator are largely a secondary proxy for the grey seal element of the M3: Seal Abundance and Distribution indicator which assesses grey seal abundance on a single AU basis, spanning Regions I – III. It is suggested that the outputs of this indicator are used in conjunction with those from M3 to provide a more fine-scale, and temporally accurate, understanding of grey seal trends across the North-East Atlantic.

Overview

Assessments of changes in abundance and distribution were made within discrete geographical areas of coastline, or ‘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). OSPAR Regions I, II and III were divided into 25 AUs (Figure a) to provide a description of changes in pup production at a ‘local’ scale where direct impacts on pup production, such as disturbance, persecution and food availability may be more detectable. The AUs do not necessarily represent demographically independent populations and survey coverage and monitoring effort is higher where breeding grey seals are most abundant. The AUs reflect a balance between population structure evidence and feasible monitoring sites for both harbour and grey seals within Contracting Parties. Monitoring of grey seal pups is conducted by the same organisations, and at a similar scale to that data provided for M3: Seal Abundance and Distribution and so match those AUs used for conducting assessments on harbour seals, and grey seal distribution within the M3 indicator. The AUs in the Greater North Sea are broadly similar to those previously defined as OSPAR EcoQO sub-units. See M5 Coordinated Environmental Monitoring Programme (CEMP) Guideline for further information on the development of these units.

Key: 1. South-West Scotland, 2. West Scotland, 3. Western Isles, 4. North Coast & Orkney, 5. Shetland, 6. Moray Firth, 7. East Scotland, 8. North-East England, 9. South-East England, 10. South England, 11. South-West England, 12. Wales, 13. North-West England, 14. Northern Ireland, 15. Ireland, 16. French North Sea & Channel Coast, 17. Belgium Coast and Dutch Delta, 18. Wadden Sea, 19. Limfjorden, 20. Kattegat, 21. Iceland, 22. Skagerrak, 23. Norway (Hvaler – Stad), 24. Norway (Stad – Vesterålen), 25. Norway (Troms – Finnmark).

Grey seal pup monitoring in the North-East Atlantic

Atlantic grey seals pup in the autumn or winter and moult in early spring. The frequency of surveys during these periods varies across OSPAR Contracting Parties due to differences in the total number of resident animals, funding, geography, and historical development of the monitoring programmes. Grey seal (Halichoerus grypus) pups are counted at major breeding sites (‘colonies’) usually using aerial photography survey methods (Figure b); 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 conducted, spread across the breeding season (August to February, depending on colony location).

 

For many colonies in Scotland (including all the major ones), pup counts are used to estimate total pup production, that is, the total number of pups produced over the season at each colony using an established statistical model that describes how the number of pups at the site vary over the season (Russell et al., 2019).  Similarly, Ireland and Iceland also estimate pup production using multiple counts (Ó Cadhla et al., 2013; Granquist & Hauksson, 2019)

All other Contracting Parties count pups several times during the pupping season and provide peak count values as an index of pup production. The peak count gathered from monitoring will always under-estimate pup production because pups are born and leave the colony at different times and the breeding season is longer than any one pup stays on the colony. For some smaller colonies or stretches of coast where pups are born, often a single count is used as an index of pup production. Hereafter, both types of estimates are referred to collectively as a ‘pup production index’.

Colony survey frequency varies by ‘Assessment Unit’ (AU) and ranges from annually to about five-yearly. In many AUs monitoring is undertaken in specific areas by local organisations and does not form part of synoptic surveys. Details of grey seal pup production monitoring programmes for each AU are given in Annex 2 of the M5 CEMP Guidelines.

Data Collation

Following a data call in February 2021, all Contracting Parties were asked to provide data on an AU-scale of peak grey seal pup count and/or total pup production estimates for the period 1992 (or earliest year) –2019 (or latest year). Data were received from the UK, Ireland, France, Belgium, Germany, the Netherlands, Denmark, Sweden, Norway, and Iceland.  

Data received from Ireland and Norway were too sparse to conduct analysis on (<5 data points). Data submissions from these countries have been supplemented by descriptive evidence summaries to support the indicator.
Greenland noted that not enough annual counts have thus far been conducted in the country, and that seal presence is infrequent and limited. Similarly, for the Faroe Isles, only grey seals breed there and counts only commenced in 2018. Therefore, assessments were not able to be generated for these countries and instead, descriptive data have been provided to supply an overview of seal presence within these waters.
Although numbers from these noted countries are considered as marginal compared to those larger, frequently monitored haul-out sites and colonies in neighbouring countries, they are recorded in an OSPAR framework for completeness.

Grey Seal Pup Production Threshold Values and Baselines

Two threshold values were used to assess grey seal pup production in each AU in relation to baselines described below. The use of the two threshold values aims to provide an indicator that would warn against both a slow but long term steady decline and against a recovery followed by a subsequent decline (potentially missed with a fixed baseline set below reference conditions). The two threshold values together would be able to act as a trigger for investigation of any necessary management measures to promote recovery.

Threshold Value 1:

• No decline in seal abundance of >1% per year in the previous six-year period (this is approximately 6% over 6 years).

This uses a rolling baseline (Method 1; OSPAR, 2012) based on the most recent six-year period, seeking to identify if seal populations are maintained, with no decrease in population size with regard to the (short term) baseline (beyond natural variability (<1% per year)) and to identify if efforts are needed to restore populations, where they have deteriorated due to anthropogenic influences, to a healthy state.
To estimate the annual increase or decrease in the number of animals counted within the most recent six-year reporting round, the fitted trend abundance in 2014 was compared against that of 2019.
To avoid the problem of shifting baselines (See M5 CEMP Guidelines) when using the rolling baseline applied in Threshold Value 1, an assessment value relating to a fixed, historic baseline is also needed (Threshold Value 2).

Threshold Value 2:

• No decline in seal abundance of >25% since the fixed baseline in 1992 (or closest value).

The baseline chosen (1992) relates to that used by some Member States for reporting under the European Union Habitats Directive (Council Directive 92/43/EEC) (or if such data are not available, the start of the data series). Testing shows that there is sufficient monitoring to assess against this threshold value with confidence.  It should however be noted that if data are not available from 1992, and a shorter timescale is assessed, the 25% decline since the baseline is not equivalent to those AUs where data do extend to 1992 (i.e., a 25% decline since 2003 would describe a more rapid contraction in the population than a 25% decline since 1992).
To estimate the annual increase or decrease in the number of animals counted within the long term time period, the fitted trend abundance in 1992* was compared against that of 2019.
*Data as far back as 1992 were not available in all AUs; the first year of data thereafter was used. Indicator assessment values were set as a deviation from the baseline value (Method 3; OSPAR, 2012).

Appropriateness of Historic Baseline (1992)

The fixed 1992 (historical) baseline reflects a point in time when populations were already subject to anthropogenic pressures including culling or natural pressures, so this baseline year most likely represents a time when populations had not yet recovered from severe depletion.
Seals have been historically hunted both illegally and legally and it is not possible to know the undisturbed state, nor, for some areas, the current carrying capacity that could be attained alongside protection from illegal hunting and absence of exposure to anthropogenic activities. Time series data for grey seal pup production do not provide an indication of a time when seal populations were not impacted and what it would look like in terms of abundance and distribution. It is therefore not possible to identify a baseline representing unimpacted conditions. This also leads to challenge in assessing the status of seals in relation to the concept of a “favourable conservation status”.

Analysing Changes in Pup Production

As per the Intermediate Assessment 2017 (IA 2017), grey seal pup production was assessed based on either estimated pup production or peak counts using either generalised linear or additive models. It is deemed appropriate to use both types of pup data as the same method was consistently used at each site for the duration of the assessment period.

Generalised linear or additive models (GLMs, GAMs; Wood 2011) were fitted to count data on a log scale using negative binomial error (or a Poisson error distribution if necessary) as part of both threshold values of pup production. All analysis was conducted within R (R Core Team, 2021). Percentage change (mean and 80% confidence intervals) were estimated for each AU over the short and long term assessment periods; 80% confidence intervals were calculated to reflect the choice to set the significance level, α, equal to 0,20 or 20% (Formula A). If the 80% confidence intervals encompassed the threshold, the assessment was classified as ‘inconclusive’.

All assessments have included data that were available before (from 1984) and after (to 2020) the standard assessment window (1992-2019) to generate more robust population trends for analysis in the AUs. These data have been included to improve the robustness of the models and indicate more accurate trends. For AUs for which there were a limited number or temporal extent of counts for the AU, when possible, a more comprehensive time series from a subset of the AU (>75% of pups) was used as a proxy to allow robust trend fitting.

Quantitative assessments of trends in pup production were only completed for those AUs where more than four annual data points were available. For AUs for which there were a limited number or temporal extent of counts for the AU, when possible, a more comprehensive time series from a subset of the AU was used as a proxy to allow robust trend fitting.

As previously noted, grey seal pup production is continuing to increase rapidly in eastern England (AUs 6 –9), including the Channel, Dutch Delta and Wadden Sea areas (AUs 16-18) both since the baseline year and short term, between 2014 and 2019 (Figure 4). The increase in pup production in these continental Europe AUs is, in part, driven by the immigration of animals from the large colonies to the north of the United Kingdom (Brasseur et al., 2015). Continued increases within eastern England AUs is hypothesised to, in part, be driven by recruitment of females born elsewhere, from areas that have perhaps already reached carrying capacity in the north of the UK (Brasseur et al., 2015; Russell et al., 2019).

Pup production trend analysis within the northern region of the Celtic Seas AUs (2 – 4) and Orkney demonstrated rapid and sustained increases against the historical baseline year, but in more recent years there has been a declining rate of growth and levelling off in pup production. It is likely that these AUs have reached their carrying capacity for grey seals (Thomas et al., 2019).

Analysis outputs from Iceland demonstrate that the AU did not achieve the long or short term threshold value and grey seal pup production continues to decline. In 2019, a new regulation was enacted to support the recovery of both grey and harbour seals. This regulation bans hunting but does allow for exemptions to carry out “traditional hunt” – of which there are thought to be few. Prior to this, though it was acknowledged that seals were being removed for both subsistence and to reduce interactions with fisheries, no legislation on seal hunting was in place and no obligation to record removals was in place in Iceland. However even since the introduction of legislation it is considered that the risk from by-catch in Icelandic fisheries may pose a greater threat to the species’ recovery in the country (Granquist & Hauksson, 2019; Granquist, 2021).

Surveys and model-based assessments using Norwegian grey seal count data have historically indicated an increasing grey seal population in Norway. However, more recent surveys within AU 24 between 2014 and 2015 showed a decrease in grey seal pup production. Counts within AU 25 were either slightly reducing or stable in 2016 when compared to counts from 2006. Grey seal pup production in Norway (Stad – Vesterålen) (AU 24) was at the same low level in 2018. However, within this AU, pup production in Lofoten (Nordland) had almost doubled in 2020 compared to the low level in 2015 (NAMMCO, 2021).

Confidence Assessment

Assessment methods largely follows those used in IA 2017, with some minor amendments as outlined in the CEMP guideline. Confidence in the data availability is considered to be moderate as there was mostly sufficient spatial and temporal coverage to carry out assessments at the scale of the AUs, some gaps are still apparent. Using this method, the statistical power to detect a grey seal population rate of decline of 1% per year was low (around 20%) in some AUs, especially where monitoring occurs on a less than annual basis. There is consensus within the scientific community regarding this methodology, though some questions remain, 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 could be considered more moderate / low.

It was not possible to make a quantitative assessment in all 25 Assessment Units (AUs) for several reasons. This may partially be due to the original delineation of the AUs being based largely around those appropriate for harbour seal monitoring by many Contracting Parties and so do not necessarily account for the spatial distribution of those areas used specifically for breeding by grey seals. This leaves several AUs as N/A simply due to there not being known breeding colonies in the region, or of sufficient size to conduct cost effective monitoring on them.

The remaining assessment gaps were due to a lack of sufficient annual and inter-annual data points to perform a quantitative assessment of the time series. Infrequent monitoring is often a result of funding dependency, but also may be because there are relatively few animals breeding in an area and / or may be related to 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.

As noted in Background, pup production when used on its own is not a meaningful indicator of population condition. An ideal measure of population condition could include information on female fecundity and pup survival. However, demographic parameters are only available where long term and detailed monitoring of colonies exists (e.g., Isle of May in Scotland), and have limited use at larger scales due to movement of both pups and adults between colonies. The potential development of formal analysis linking this indicator assessment and that of the M3 common indicator on Seal Abundance and Distribution could prove beneficial.

From the present data it is not possible to exclude human influence on pup production. Even without pressure from human activities, pup production cannot keep increasing indefinitely and will stabilise as carrying capacity is reached. Further demographic studies could help predict 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.

Grey seals exhibit increased site philopatry once they reach the breeding age, and some breeding sites themselves only exist for the season before individuals redisperse across the North-East Atlantic (Brasseur et al., 2015; Russell et al., 2019). Recovery of grey seals throughout the North-East Atlantic is largely UK centric. As AUs in the UK begin to reach carrying capacity and as individuals reach breeding age they redistribute along coasts of continental Europe. Further research into the level and extent of exchange between UK and continental Europe and Arctic countries would benefit understanding of potential spread of human impacts in areas.

Power to detect the growth rate specified in assessment value 1 (decline of >6% between 2014 and 2019) was retrospectively calculated using the formulae from Thomas (1997) where λ (the non-centrality parameter) is a function of the specified effect size (here, the assessment growth rate of -1% per year), the sum of squares and variance estimated from the fitted model. The total sample size was calculated as the product of the number of years of survey data and the typical number of replicate surveys performed in the Assessment Unit (AU). Using this method, the statistical power to detect a grey seal population rate of decline of 1% per year was low (around 20%) in some AUs, especially where monitoring occurs on a less than annual basis. This assessment value may need to be revised for future assessments and / or monitoring programmes improved to enhance the power to detect trends. In the present assessment, the confidence intervals provide a robust assessment of uncertainty in the magnitude of observed trends.

Seal colonies, particularly during the breeding season are experiencing increasing levels of human disturbance both on land and at sea from activities such as, but not limited to walkers, tour boards, paddle boats, jet-skis and recreational swimming activities (Strong & Morris, 2010; Andersen et al., 2012; Granquist & Sigurjonsdottir, 2014; Granquist & Nilsson, 2016; Bellman et al., 2019). Disturbance signs can range from increased alertness to complete ‘flushes’ of colonies into the water. The extent to which these activities impact pupping sites via stress responses and ultimately the success of breeding females is not clearly understood. The increasing frequency of reported disturbance events across the North-East Atlantic, and the unquantified impacts on population status warrants further research.

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