Indicator Name: Abundance and distribution each of



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Common Indicator Technical Specification

M-3: Abundance and distribution each of harbour and grey seals

  1. Indicator

Name: Abundance and distribution each of harbour and grey seals

Code: M-3

OSPAR Threatened and Declining Species included in indicator: None

State of methodological development:

Development step

Defined

Indicator metrics

Yes

Ecosystem components attributed (species/habitat types)

Yes

Atlantic Grey Seal (Halichoerus grypus), Harbour Seal (Phoca vitulina)

Applicability to sub-regions

Yes

Assessment scales

Yes

Monitoring parameter

Yes

Monitoring frequency

Yes



  1. Appropriateness of the indicator

Biodiversity component: Marine Mammals

MSFD criteria: 1.1 Species Distribution;1.2 Population Size

MSFD indicator: 1.1.2 Distributional pattern within range; 1.2.1 Population abundance

Sensitivity to specific pressures

Relevance to management measures

Practicable

Applicable across region

Consensus among CPs

Low

Non-specific – indicator of state that responds to multiple pressures



Low

Has previously been adopted as an EcoQO

Celtic Seas and Greater North Sea only

High

Number of Contracting Parties (CPs) intending to use the indicator or something similar1: Greater North Sea (n=8) = 7; Celtic Seas (n=3) = 3.

Marine mammals, including seals, are top predators, and comprise an important part of marine biodiversity. As such, they are included in the Indicative list of characteristics (Table1 in Annex III of the MSFD) that are referred to in Articles 8, 9, 10 and 11 on Assessments (of GES), Defining GES, Environmental Targets and Monitoring Programmes, respectively. Harbour seals and grey seals are listed in Annex II of the Habitats Directive, and are implicitly referred to in the Indicative list of characteristics, as “species which are the subject of Community legislation”. Indicators of seal population status are a relevant aspect of defining GES and assessing progress towards GES under Descriptor 1 – Biological Diversity, of the MSFD.

A number of human activities may, at least in part, be drivers of trends in abundance and changes in distribution, but there is no clear link between the population status of seals and human activities. The indicator would serve to trigger the investigation of possible cause and effect relationships as a basis for measures.



  1. Parameter/metric

Harbour seal:

  1. Population size – a measure of ‘minimum population size’ is made from counts of harbour seals on land at haul-out sites during moult. This proxy for population size is an underestimate of the true population size as it includes only those animals hauled out at the time of counting. This metric was previously used to construct the EcoQO on harbour seals.

  2. Distributional pattern – percentage change in occupancy of spatial units (e.g. subareas, or grid cells) between two time periods.

=

where A is the number of spatial units (e.g. subareas, or grid cells) in an Assessment Unit occupied by harbour seals during reference period A, B is the number of units occupied in a subsequent period B, and N is the total number of spatial units within the Assessment Unit. For subsequent assessments in addition to the previous assement period (rolling baseline) a fixed baseline can also be used, set using the original assessment period (fixed baseline).



  1. Shift in occupancy - an index to describe the overall shift in the distribution of harbor seals between spatial units over time.

where A is the number of spatial units (e.g. subareas, or grid cells) occupied by harbour seals during reference period A and B is the number of units occupied in a subsequent period. A&B is the number of units occupied in both periods.

For subsequent assessments in addition to the previous assement period (rolling baseline) a fixed baseline can also be used, set using the original assessment period (fixed baseline).

The index value is between 0 and 1: a value of 0 indicates that there has been a complete shift in the spatial units occupied; a value of 1 indicates there has been no shift



Grey seal:

  1. Population size – population is to be estimated from a Bayesian demographic model (SCOS, 2014) fitted to pup production/counts and informed, where available, by independent estimates of population size from grey seal moult counts or counts of grey seal made during the harbour seal moult. The assessment will be made for a single Assessment Unit covering the Celtic Seas and Greater North Sea. This analysis will be supported by any available estimate of the proportion of time grey seals spend hauled out during the survey window that CPs are able to provide.

  2. Distributional pattern – percentage change in occupancy between two time periods for a given spatial unit:

=

where A is the number of spatial units (e.g. s, or grid cells) in an Assessment Unit occupied by seals during reference period A, B is the number of units occupied in a subsequent period B, and N is the total number of spatial units within the Assessment Unit.

For subsequent assessments in addition to the previous assement period (rolling baseline) a fixed baseline can also be used, set using the original assessment period (fixed baseline).


  1. Shift in occupancy - an index to describe the overall shift in the distribution of grey seals between subareas or grid cells over time.

where A is the number of spatial units (e.g. subareas, or grid cells) occupied by grey seals during reference period A and B is the number of units occupied in a subsequent period. A&B is the number of units occupied in both periods

For subsequent assessments in addition to the previous assement period (rolling baseline) a fixed baseline can also be used, set using the original assessment period (fixed baseline).

The index value is between 0 and 1: a value of 0 indicates that there has been a complete shift in the spatial units occupied; a value of 1 indicates there has been no shift..



  1. Baseline

Ideally baselines should equate to reference conditions, i.e. set at a point in the historical time-series when human impacts were considered to be negligible. However such historical abundance and distribution data are not available and our data time series provides no indication of when an unimpacted state might have occurred nor what the values would have been.

It would also be unrealistic to expect to be able to achieve reference conditions again as they reflect a past level of negligible human impact (by definition) which cannot now be restored, given for instance large-scale coastal developments and tourism. Reference condtions would also not reflect changes due to natural drivers such as climate.

An alternative approach would be, for now, to set the baseline at recent values of abundance and distribution, noting that the baseline could later be changed to a more meaningful value as knowledge allows.

Under the Habitats Directive, in the absence of an objective ‘favourable reference level’ for grey seal pup production, the baseline value defaults to that in 1992, the starting year of the Directive. In Assessment Units where other more suitable baseline values are available, bespoke baselines can be set. Where data for 1992 is not available, the start of the data time series is used.

Recently the ICES WGMME highlighted the problems in setting baselines to the time series and proposed that trend-based targets, such as the EcoQOs on grey seal pup-production, which do not require comparison to a baseline, should be adopted for the common indicators (ICES 2014a).

ICES (2014b) expressed the importance of maintaining a quantified time element in the target, rather than relying on the phrase “long-term” suggested in an earlier iteration of this technical specification.

However, OSPARs ICG COBAM in their advice manual on biodiversity targets and indicators for MSFD (OSPAR 2012), noted that there is a potential problem with the quantatative trend targets, known as ‘shifting baselines’. ‘Shifting baselines’ recognises that each successive target assessment is comparing slightly different sets of consecutive data points. This could allow an indicator to continually decline at a slower rate than the target, so much so that after many years, the population may have declined substantially without missing its targets.

To address the points above two baselines will be used to assess grey and harbor seal abundance and distribution.



The M3 baselines are:

  1. A ‘fixed’ reference level (1992 or nearest).

  2. A ‘rolling’ baseline of the previous reporting round (6 years).



  1. Target setting

The ICG-COBAM expert group on marine mammals has previously suggested the following target be applied separately to each species:

Maintain populations in a healthy state, with no decrease in population size with regard to the baseline (beyond natural variability) and restore populations, where deteriorated due to anthropogenic influences, to a healthy state“.

ICES (2014b) provided general advice on the need to understand the statistical power of monitoring programmes before targets are set under MSFD in relation to that monitoring. It is not advisable to set targets that demand a higher statistical precision than can be met within a feasible monitoring programme. The statistical power of current monitoring programmes vary. In areas such as the Wadden Sea, comprehensive coordinated survey efforts throughout the year provide some of the most robust estimates of seal population size. In other areas, however, the survey area is too large or complex to feasibly enable such comprehensive surveys (e.g. many parts of the Scottish coast) and the power to detect change in these regions is reduced. To reflect these differences, changes in seal abundance will be reported along with 95% confidence intervals associated with the change observed.

The targets below should be assessed in each Assesment Unit (AU) proposed for each species.



The M3 targets are:

  1. No decline in Abundance of > 1% per annum in the previous 6 year period (rolling baseline).

  2. No decline in Abundance of > 25% since the fixed baseline at the start of the Habitats Directive in 1992 (or closest value).

A similar target was suggested for seal distribution but as changes in seal distribution are currently difficult to detect and assess from abundance surveys, this aspect of the indicator will be considered as a ‘surveillance indicator’: the metric(s) will be described but not quantitatively assessed against a target.
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