Stranding data of Common Seals (Phoco vitulina) and Grey seals (Halichoerus grypus) in The Netherlands between 2009 and 2013

Abstract

Stranding data of 158 common seals (Phoca vitulina) and 22 grey seals (Halichoerus grypus) were collected from February 2009 till December 2013 by Utrecht University, dept. Pathobiology. Seals were found dead on the coast line or died within 24 hours in rehab. Stranded seals were collected by rehabilitation and education center “Ecomare” on Texel and by the Institute for Marine Resources (Imares) on Texel. The number of collected seals increased from 23 in 2009 to 71 in 2013. This has no association with the population size living in the Dutch waters (common seals P = 0.177; grey seals P=0.3745). In Osinga et al (2012) an association between population and stranded seals was found9. The total monthly stranding rate for common seals peaked in January(n=20), May(n=21) and July (n=20). Stranding rate in January peaked due to more stranded adult seals (n=9). Stranding rate peaked in December(n=7) for grey seals, due to more stranded juvenile animals. There is no significant difference between male and female stranding for both species (0.15 ≤ P ≤ 0.20; P=0.20). In the study of Osinga et al (2012) they found frequently more male stranding than female stranding in grey seals9. In this study, mainly juvenile animals (n=52/180) stranded for both species. Most common seals (n=109/158) stranded on the coast line of Texel. Most grey seals (n=11/22) stranded on the coast line of the North Sea. This is a bit strange because there are no sandbanks in the North Sea which the grey seals can haul on. A few seals (n=3) stranded at an inland location. Certain by-catch was seen three times between 2009 and 2013. Possible by-catch was seen on an average of four cases per year, except for 2013 when there were 18 cases of possible by –catch. The average of four cases per year was seen by Osinga et al (2012) as well9. In this study, by-catch was more frequent in juvenile animals (n=17), unlike Osinga et al (2012) were by-catch was more frequent in subadult and adult9. In previous studies, Van Haaften (1982), by-catch was more frequent in young animals13.