Thresholds and Status evaluation

Good status reflected through the distribution of seals in the Baltic Sea is based on concepts developed for the conservation of seals. The concept for defining threshold values to indicate good status is derived from the general management principle in the HELCOM Recommendation 27/28-2, which states that the distribution is to allow breeding seals to expand to suitable breeding distribution in all regions of the Baltic Sea.

Good status is achieved when the threshold values for all considered parameters are achieved. Good status is achieved when 1) the distributions of seals are close to pristine conditions (e.g. 100 years ago), 2) or where appropriate when all currently available haul-out sites are occupied (modern baseline), and 3) when no decrease in area of occupation occurs (Thresholds figure 1). Three different parameters of distribution are given for all species of seals:

  1. Breeding distribution on land or ice, the threshold value is achieved when available sites are occupied
  2. Distribution on land/ice for resting/moulting, the threshold value is achieved when available sites are occupied.
  3. The area of occupancy, which includes sea areas used for transport and foraging, the threshold value is achieved when seals are not hindered in executing these activities.

 

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Thresholds figure 1: Good status is achieved when distribution of seals is close to pristine conditions (e.g. 100 years ago), or where appropriate when all currently available haul-out sites are occupied (modern baseline), and when no decrease in area of occupation occurs.

Threshold values are defined for the exact haul-out sites for each species as shown in Results figure 6 for grey seals, and Results figure 9 for Baltic harbour seals. The haul-out sites of ringed seals are dependent on the annual sea ice extent and described in Results figure 3. 

The following criteria are used to evaluate whether the threshold value is achieved or failed:

  • The distribution of breeding sites for each management unit of harbour seals are evaluated against pristine conditions. The threshold value is achieved when all previously used sites are colonized, and distribution is not diminishing.
  • The distribution of haul-out sites used for resting and moult of harbour seals are almost identical to the distribution of breeding sites.  The threshold value is achieved when all previously used sites are colonized.
  • Grey seals are facultative land breeders that switch between breeding on land and ice, where ice is favoured if available (J├╝ssi et al. 2008). The threshold value is achieved when available land breeding sites are colonized, and distribution is not diminishing.
  • Grey seal haul-out sites used for resting and moulting may differ considerably between breeding sites, as moulting and resting sites can be locked in by ice and thereby inaccessible during breeding. The threshold value is achieved when available haul-out sites are colonized and not diminishing.
  • Ringed seals breed in lairs constructed in snow covered broken and consolidated ice. The sizes of the breeding areas display substantial inter-annual variation. The threshold value is achieved when the long-term breeding area is stable or not diminishing due to direct human activities
  • Ringed seals rest and moult on ice if available. During ice free conditions ringed seals haul out on rocks or small islands. The threshold value is achieved when ringed seals have access to all available haul-out sites and the numbers of haul-outs are not diminishing.
  • For the area of occupancy the threshold value is achieved when seals have access to all feeding grounds and they can move freely among haul-out sites and the feeding grounds.

The modern baseline approach is applied when pristine conditions cannot be achieved due to irreversible long-term environmental changes (e.g. sandbanks used for haul-out have vanished), or factors such as multi-fold increased human exploitation of fish stocks that will persist for the foreseeable future. Since the environment has changed over the past century, and formerly used haul-out sites have disappeared in the Southern Baltic, current distributions are evaluated against colonization of currently available haul-out sites. This type of a modern baseline should be defined so that the species will thrive and persist in the future.

Especially in cases where a modern baseline is applied, the additional criterion for evaluating whether good status is achieved 'distributional range is not diminishing' can be applicable for populations above the limit reference level (LRL). The LRL has been agreed in HELCOM to be set at 10,000 individuals per management unit, understanding that the haul-out fraction during moult surveys is 70%.

This HELCOM core indicator is comparable to the OSPAR common indicator M-1; 'Distributional range and pattern of harbour and grey seal haul-outs and breeding colonies', which also applies a modern baseline approach. The difference between the OSPAR 'common indicator' and the HELCOM 'core indicator' is that the latter also encompasses the range of seals at sea during foraging and transport.