In the south eastern part of the North Sea, three marine mammal species are considered to reproduce regularly. Besides the harbour porpoise (Phocoena phocoena) as the only cetacean species (Gilles et al., 2016), two phocid seals occur frequently in these waters (CWSS, 2017).
The smaller of the two is the harbour seal (Phoca vitulina) which is currently the most abundant in the Wadden Sea (Galatius et al., 2017). Harbour seals have a broad distribution with subspecies also occurring in the Pacific (Phoca vitulina richardii) and even in freshwater habitat (Phoca vitulina mellonae). They show little sexual dimorphism with female adult individuals from the Atlantic having an average size between 140 and 146 cm and weighing between 67 and 83 kg (Teilmann & Galatius, 2017). Males of this region have an average size between 153 and 156 cm with an average weight between 75 and 104 kg (Teilmann & Galatius, 2017).
The harbour seal population in the Wadden Sea has since the beginning of coordinated aerial surveys increased continuously from approximately 3,840 seals during moult in 1975 to 25.036 counted individuals in 2017 (Galatius et al., 2017). Irrespective of the positive net growth since 1975, the population has experienced severe mortality events with two phocine distemper virus (PDV) outbreaks which have caused a sudden population decline of around 57 % in 1988 and 47 % in 2002 respectively and an influenza A (H10N7) outbreak in 2014 which caused a moderate decline of up to 10 % locally (Bodewes et al., 2015; Härkönen et al., 2006).
Interestingly, throughout the last years the constant increase of the population has slowed down and seemingly even stopped (development of count data between 2013 and 2017: -1 %;
see Figure A. 1). If this pattern persists, indicating a density dependent decrease of growth or if the observed development is within the naturally occurring fluctuation of the population needs to be seen during the coming years.
The second seal species occurring in German waters is the grey seal (Halichoerus grypus).
Grey seals occur throughout the Atlantic with three distinct populations, namely the Eastern Atlantic, the western Atlantic and the Baltic Sea population (Hall & Russell, 2017). In contrast to harbour seals, grey seals have been locally extinct due to human exploitation and only started to recolonise the Wadden Sea in the end of the 1960s (Czeck & Paul, 2008). Since their return the population has constantly increased to 5,445 counted seals during moult in 2017 (Brasseur et al., 2017). The two biggest grey seal colonies in German waters are on the Kachelotplate between the islands Borkum and Juist (422 individuals in 2017) and on Helgoland (616 individuals in 2017) (Brasseur et al., 2017). For details on the population development of the two seal species in recent years, also see Figure A. 2 & Figure A. 1 in the Supplementary materials. Opposed to harbour seals, grey seals show a strong sexual dimorphism. Whereas adult female individuals from the North Sea region average in size of up
to 200 cm weighing between 105 to 186 kg, adult males in our region grow up to 230 cm with a weight of up to 310 kg (Hall & Russell, 2017; Jefferson et al., 2008).
Harbour porpoises occur throughout the continental shelf waters of the Northern Atlantic, the North Pacific, the Black Sea as well as the Baltic Sea and are regarded as an abundant species in most of its range (Hammond et al., 2008). Within a large scale survey which was conducted in 2016 throughout European Atlantic waters, the abundance of porpoises was estimated to be around 466,569 animals (Hammond et al., 2017). Porpoises feed on a variety of prey species including fish and cephalopods but prey preferences are considered variable depending e.g. on region or season (Santos et al., 2004). They too show sexual dimorphism with the females growing on average to 160 cm and weighing around 60 kg whereas the males only grow to around 145 cm weighing on average 50 kg (Bjørge & Tolley, 2009).
Even though no significant decrease in abundance is evident for any of the three marine mammal species frequently occurring in the North Sea, anthropogenic induced threats like habitat degradation, noise and chemical pollution or bycatch persist (IJsseldijk et al., 2018).
With the aim to reduce the number and degree of threats, all three species are integrated in different legislative frameworks assigning different levels of protection (Santos & Pierce, 2015).
All three species are considered opportunistic top predators in the ecosystem and mainly utilise a diverse range of different fish species (Andreasen et al., 2017; Das et al., 2003; Hammond
& Wilson, 2016; Sharples et al., 2012). Besides this piscivorous prey resource, reports of seals preying on water birds as well as infrequent reports of cannibalism by grey seals have been published (Bedard et al., 1993; Lucas & McLaren, 1988; Tallman & Sullivan, 2004).
The phenomenon of grey seal predation on marine mammals
In 2011 the first incidences of lesions on a harbour porpoise hypothesised to have been induced by a grey seal, were recorded (Haelters et al., 2012). This study reported of two porpoise carcasses showing severe lesions of unknown aetiology. To elucidate the cause of the detected lesions, the measurements of the wounds taken during the necropsies were compared to skull measurements and inter-teeth distances of grey and harbour seals. Results strongly suggested the grey seal as a potential perpetrator. Despite a first scepticism in the scientific community (Haelters et al., 2015), more publications reporting of similar cases followed (Bouveroux et al., 2014; Jauniaux et al., 2014; Stringell et al., 2015). Investigations which were initiated as a consequence of the first reports, showed the potential extent of the phenomenon with at least 17 % of investigated porpoise carcasses from the Dutch coast being
on the Isle of May (Bishop et al., 2016; Brownlow et al., 2016). Until now, spatially limited studies could show, that the suspected rate of this behaviour in certain areas may well have measurable negative effects on the local part of the population with unknown consequences for the ecosystem (Brownlow et al., 2016; Leopold et al., 2015b). As a result of the first reports of instances of grey seal predation on marine mammals in German waters, two projects funded by the Ministry of Energy, Agriculture, the Environment, Nature and Digitalization, Schleswig Holstein were initiated and conducted in collaboration with different institutes of the University of Veterinary Medicine Hannover, Foundation under the lead of the Institute for Terrestrial and Aquatic Wildlife Research (ITAW) in Büsum.
The aim of these projects was to investigate the phenomenon of grey seal predation on marine mammals in detail with respect to pathological, behavioural and ecological aspects.
This dissertation was conducted within the frame of the above mentioned projects and results will be presented accordingly.