Monitoring Requirements

Monitoring methodology

HELCOM common monitoring relevant for the seal population trends is documented on a general level in the HELCOM Monitoring Manual under the sub-programme: Seal abundance.

HELCOM monitoring guidelines for seals were adopted in 2014 and currently all monitoring guidelines are being reviewed for inclusion in the Monitoring Manual.

The three regularly occurring seal species in the Baltic Sea, harbour seal (Phoca vitulina), ringed seal (Pusa hispida) and grey seal (Halichoerus grypus) are monitored at their haul-outs during their annual moulting and pupping seasons, with the aim of estimating the abundance and trends (moulting counts) and pup production (pupping counts). Grey seals and harbour seals are counted on their land haulouts during moulting time. Ringed seals are surveyed using transect sampling methodology during moult on the ice. In most areas, the monitoring is performed using aerial surveys, where the seal haul-outs are photographed during the relevant periods in areas where there is a significant occurrence of seals.

Detailed descriptions of the survey methodology and analysis of results are given in the HELCOM monitoring manual (Galatius et al. 2014).


Current monitoring

The monitoring activities relevant to the indicator that are currently carried out by HELCOM Contracting Parties are described in the HELCOM Monitoring Manual in the Monitoring Concept Table.

Sub-programme: Seal Abundance

Monitoring Concept Table

Current monitoring covers all haul-out sites presently used by seals in the Baltic Sea and is considered to be sufficient to cover the needs of the indicator except for southern ringed seals. See description in the Assessment Requirements of the HELCOM Monitoring Manual.


Description of optimal monitoring

The monitoring strategy is optimal for harbour seals which are surveyed three times annually during the moulting period, and increased effort would not significantly improve results (Teilmann et al. 2010). The same is true for ringed seal surveys on ice in the Bothnian Bay, where a minimum fraction of 13% of the ice area is surveyed. Increasing survey effort would only marginally affect the precision of estimates (Härkönen & Lunneryd 1992). Also the coordinated grey seal surveys would be only marginally improved by increased effort. The planned decrease in monitoring frequency for grey seal by Finland might lower the confidence of the evaluation in future years.

However, two management units could benefit from modified methodology:

Limfjord harbour seals

The fjord was separated from the North Sea by land until the 1820s and genetic analyses indicate different populations in the eastern and western fjord, with the eastern fjord being predominantly inhabited by the original population of the fjord and the western fjord predominantly inhabited by immigrants from the North Sea / Wadden Sea (Olsen et al. 2014). A study determining the relative abundances of the two populations, the level of interbreeding and the habitat use of seals with genetic signature is necessary for evaluation in this area.

Southern ringed seals

Since ice cover has been diminishing over the past decades, monitoring of ringed seals on ice in the Archipelago Sea, The Gulf of Finland, and Estonian coastal waters including the Gulf or Riga has only been possible during a few years over the past 20 years. However, before the aerial surveys started, ringed seals were counted on land in August, when they returned to the coast after having spent most of the summer foraging at sea (e.g. Härkönen et al. 2008). Such data is available from the Gulf of Finland, where numbers of counted ringed seals amounted to 300 animals in 1992 (Härkönen et al. 1998), whereas only 100 ringed seals were observed in the same area in 2014 (Verevkin pers. com.). Consequently, the method of surveying ringed seals hauled out on rocks would be an appropriate alternative method for southern ringed seals.