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Summary

Introduction

This report documents the capture and tagging of ten harbour and grey seals from the Rødsand seal sanctuary, Southeast Denmark. The investigation provides information on site fidelity, migration and Kernel home range of the seals prior to the construction of the wind farm. It also serves as a baseline study for future determination of changes in habitat selection by seals during and after construction of the wind farm.

Movements and Kernel home ranges

Geographic locations were obtained both from land and at sea. The track-lines together with the Kernel home ranges are presented individually for four harbour seals and six grey seals. From these figures it is clear that the harbour seals remained within 50 km of the tagging site year-round, while grey seals made extensive movements up to 850 km away from Rødsand to Sweden, Germany, Estonia and Latvia. These differences in dispersal patterns were reflected in the calculated Kernel home ranges, where seals that dispersed farther from the tagging site had large estimates of area use (or home range). The average Kernel home range (95% fixed Kernel) of the harbour seals was 394 km2 ranging from 237 to 709 km2, whereas the corresponding Kernel home range was 130 times larger for grey seals namely 51,221 km2 ranging from 4,160 to 119,583 km2 for five out of the six grey seals. Although only few (seven) positions were obtained within the wind farm area, the calculated Kernel home range of all four harbour seals and four out of six tagged grey seals extended into the wind farm area. All the tagged harbour seals stayed year-round in the Rødsand area, whereas, the grey seals on average only remained in the area for 17.8% (range: 2.6 - 58.3%) of the monitored time.

These findings imply that the Rødsand locality is more important for harbour seals than for grey seals. Grey seals appear to have alternative feeding and haul out sites for the major portion of the year. Therefore disturbances near Rødsand, especially during late winter and early spring, when the grey seals probably leave the area to breed and mate in the Baltic Sea, will have higher impact on harbour seals compared to grey seals.

Haul-out and dive data

Haul-out and dive data were only sampled from a pregnant harbour seal and a subadult female grey seal. The grey seal spent about 40% of its time on land while the harbour seal spent only 22% of the time on land. Both spent 17-18% of time at the surface (0-2 m). The grey seal frequented areas with greater depths than the harbour seal, which stayed locally in the shallow waters near Rødsand. The average dive depth for the grey seal was 44 m (range: 12-82 m) and 9 m (range: 6-12 m) for the harbour seal. Although the harbour seal exploited an area with much shallower water it made significantly longer dives. The grey seal dived almost 10 vertical km per day, while the harbour seal only dived 3.5 vertical km per day. The adult female harbour seal only hauled-out from sunrise until early evening with up to 60% of the time on land during late morning. The subadult female grey seal hauled-out during all hours of the day but also peaked around late morning and midnight with almost 40% of the time on land.

A number of recommendations are made for future additional work. Hence, satellite telemetry (including dive recorders) and GPS/GSM technique should be continued during and after the planned construction work, to obtain information on the effects of the construction activity and presence of the wind farm. Improved capturing techniques should be used, as the seals at Rødsand are very nervous compared to other areas. The tagging should preferably be conducted in September and use the obtained moulting information to improve data collection.

Full report in pdf. format (1169 KB)
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Helle Thomsen

01.11.2007


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