Winter periscope mode and sandbanks instead of ice: how seals rest in the southern Baltic

Today, in the southern Baltic, stable sea ice forms only sporadically. Although the winter of 2026 brought unusually severe frosts and partial coastal freezing, ice is no longer the structural basis of seal winter ecology in Polish waters.

In the Polish Exclusive Economic Zone, three seal species are recorded: grey seal (Halichoerus grypus), harbour seal (Phoca vitulina) and ringed seal (Pusa hispida). In the southern Baltic, however, the grey seal is by far the dominant and most frequently observed species.

– In the Polish part of the Baltic Sea we have three seal species, but in the south we most often encounter grey seals – says Capt. Konrad Kończak of Rejsy na FOKI – GROM Mikoszewo.

From ice to sandbanks: changing winter habitats

In the northern Baltic, stable ice cover still supports pupping and resting. In the south, where winters are milder and ice is unreliable, this ecological function has largely shifted to coastal sandbanks and shallow deltaic zones.

The most important site in Poland is the Mewia Łacha nature reserve at the mouth of the Wisła (Vistula River). This dynamic estuarine system, shaped by fluvial sediment transport and ongoing deposition, creates extensive sandbars and shoals that now act as key haul-out sites.

Environmental characteristics of the Vistula mouth include:

• continuous formation and reshaping of sandbanks through sedimentation processes,
• mixing of freshwater and marine waters, increasing nutrient availability and productivity,
• rich feeding grounds for both benthic and pelagic fish species.

– The Vistula mouth benefits seals because fish flowing down the river effectively arrive right under their snouts – notes Konrad Kończak. – It does not replace offshore foraging trips, but it certainly improves prey availability in the coastal zone.

Hauling out on sandbanks allows seals to conserve energy and maintain thermoregulation. Their thick blubber layer and dense fur provide insulation, yet regular emergence from the water remains essential for moulting and for the physiological condition of juveniles.

Periscope mode and year-round behavioural strategy

In conditions of unstable or absent ice, seals increasingly rely on what observers describe as a “periscope mode” posture—remaining submerged with only the head above the surface.

This behaviour enables them to:

• limit exposure to wind and surface cooling,
• monitor surroundings for disturbance or threats,
• maintain access to breathing openings in fragmented ice fields.

Periscope positioning is not limited to winter. It forms part of a broader, year-round behavioural strategy. Neurophysiological research indicates that automatic surfacing for respiration is regulated by hemispheric brain mechanisms, allowing partial rest while maintaining vigilance. This dual-state adaptation supports energy efficiency and situational awareness in dynamic coastal environments.

Sandbanks as the new winter platform

Although extreme winters such as that of 2026 may temporarily restore partial ice cover, long-term trends suggest that sandbanks and shallow estuarine systems are becoming the primary winter resting structures in the southern Baltic.

Observations confirm that seals occasionally move upstream along the Vistula, following fish migrations—even during daytime hours.

– Sometimes seals travel upriver after fish. Such situations are observed even in broad daylight – adds Konrad Kończak.

The southern Baltic population demonstrates high behavioural plasticity. Instead of relying solely on stable ice platforms, seals increasingly exploit geomorphologically dynamic coastal habitats. Deltaic bathymetry, sediment plumes and shallow sandbanks now function as ecological substitutes for historical ice fields.

In this evolving winter landscape, seals remain a permanent element of the region’s marine biodiversity—resting on shifting sandbanks, scanning the horizon in periscope mode, and adapting to a Baltic Sea defined less by ice and more by sediment and hydrodynamics.