Seals 2011 Common Seals in the Wadden Sea in 2011 Common Wadden Sea Secretariat, Wilhelmshaven November 2011 1999  / 2000 / 2001 / 2002 / 2003 / 2004/05 / 2006 / 2007 / 2008 / 2009 / 2010 / 2011 (PDF format) 1999 / 2000 / 2001 / 2002 / 2003 / 2004 / 2005 / 2006 / 2007 / 2008 / 2009 / 2010 / 2011 (html format) Aerial Surveys of Harbour Seals in the Wadden Sea in 2011: - Solid increases in total number as well as pups by the Trilateral Seal Expert Group (TSEG) (07 November 2011)   Introduction In 2011 the coordinated aerial surveys for the harbour seal counts of the Wadden Sea were performed as usual. The counts are synchronised between the three Wadden Sea countries: Denmark, Germany and the Netherlands in order to obtain a single estimate for the size of the entire Wadden Sea population, and the number of pups born. As seals are counted when hauling out on sandbanks, counts are carried out when low-tide occurs during midday. Results and Interpretation Although this year’s growth was less than the average since the PDV epidemic in 2002, the counts from 2011 gave a solid increase in the maximum number of harbour seals hauled out during the moult in August (24,118 and an increase of 9% from last year). The number of pups born in 2011 followed the same trend, during the peak in pupping in June, a total of 7,044 pups were counted, equivalent of an increase of 8% over last year’s count. The total count of 24,118 harbour seals in the Wadden Sea was composed of 3,105 in Denmark, 8,493 in Schleswig-Holstein, 4699 in Lower Saxony and Hamburg and 7,821 in the Netherlands. The general increase of 9% is unevenly distributed; in Denmark there was an increase of 9%, in Schleswig-Holstein the increase was just 2%, the counts in Lower Saxony and Hamburg dropped by 9% while numbers in the Netherlands increased by 34. The 34% increase in the Netherlands cannot be attributed to internal recruitment, given an estimated maximum annual growth rate of 13% for harbour seals (Härkönen et al. 2002). This probably means that a substantial proportion of the harbour seals that hauled out in Lower Saxony and Hamburg during last year’s moult have moved to the Netherlands and was counted here in 2011. A similar indication of displacement was observed in last years’ counts, where a growth of 21% was seen in Schleswig-Holstein. The relatively low numbers in Lower Saxony/Hamburg and Schleswig-Holstein may also conceal higher actual numbers as only one August-survey was conducted in each of these areas. In Schleswig-Holstein, the survey was conducted in early August, while there is a trend of the past years to obtain the maximum count in mid-August. The increase in pupping over last year’s pup-count was driven by higher numbers in Denmark (699; +23.9%) and Schleswig-Holstein (3294; +14.7%), while the pup count was close to last year’s in Lower Saxony and Hamburg (1606; -2.5%) and the Netherlands (1445; 0.0%). Evidently, the much higher abundance of harbour seals during the moult in the Netherlands was not reflected in the pup counts. One can only speculate on factors affecting the number of animals on land, such as disturbance, the necessity for the seals to feed further away limiting the amount of time to haul out, or a change in the age and sex composition of the population. Seals are known to haul out more when moulting. However animals of different age and sex do not moult at the same time (Härkönen et al. 1999). Changes in the composition of the population would therefore result in shifts of the peak numbers. As it is not possible to discern the different age and sex classes during the aerial surveys, this cannot be tested.   Based on results from recent years, it has been speculated that the population was approaching the carrying capacity of their habitat and growth was beginning to slow down (TSEG 2009). Even though this years’ increase is larger than last years’ low growth (3%, TSEG 2010), there is a trend for decreasing growth rates between years since the epidemic in 2002. The estimate for the total Wadden Sea harbour seal population - correcting for the animals not observed whilst in the water (TSEG 2009) - would result in an estimated population size of 35,500 harbour seals.   References Härkönen T, Harding KC, Heide-Jørgensen MP (2002) Rates of increase in age-structured populations – a lesson from the European harbour seals. Canadian Journal of Zoology 80: 1498-1510. Härkönen T, Harding KC, Lunneryd SG (1999) Age- and sex-specific behaviour in harbour seals Phoca vitulina leads to biased estimates of vital population parameters. Journal of Applied Ecology 36: 825-841 TSEG (2010) Aerial Surveys of Harbour Seals in the Wadden Sea in 2010: Strong increase in pups, slight increase in total number? Trilateral Seal Expert Group (TSEG) (10 March 2010), Common Wadden Sea Secretariat, Wilhelmshaven, http://www.waddensea-secretariat.org/news/news/Seals/Annual-reports/seals2010.html [access 22 October 2011] TSEG (2009) Aerial Surveys of Harbour Seals in the Wadden Sea in 2009: Growth of the harbour seal population slowing down? Trilateral Seal Expert Group (TSEG) (15 October 2009), Common Wadden Sea Secretariat, Wilhelmshaven, http://www.waddensea-secretariat.org/news/news/Seals/Annual-reports/seals2009.html [access 03 March 2011]   Trilateral Seal Expert Group (TSEG) Sophie Brasseur, IMARES, Texel, The Netherlands Thomas Borchardt, LKN Schleswig-Holstein, Nationalparkverwaltung, Germany Richard Czeck, Nationalparkverwaltung Niedersächsisches Wattenmeer, Niedersachsen, Germany Lasse Fast Jensen, Fiskeri- og Søfartsmuseet, Esbjerg, Denmark Anders Galatius Jørgensen, Department of Bioscience, Aarhus University, Denmark Sven Ramdohr, LAVES Cuxhaven, Niedersachsen, Germany Ursula Siebert, ITAW, University of Veterinary Medicine Hannover, Germany Jonas Teilmann, Department of Bioscience, Aarhus University, Denmark       Figure 1: Number of Counted Seals in the Wadden Sea since 1975           Home | Trilateral Cooperation | News/Service | Management | Monitoring | Interregional Cooperation