Blubber thickness variation in grey, harbour and ringed seals A proxy for seal health and an indicator of energetic constraints Photo taken by Ken Howard. Used with permission Andre Hermansson Degree project for Master of Science in Biology Animal Ecology, 30 hec, vt 2015 Department of Biological and Environmental Sciences University of Gothenburg Supervisor: Karin Hårding Examiner: Lotta Kvarnemo Abstract HELCOM has chosen seals’ nutritional status as one of several so called ‘core indicators’ that shall be monitored in all Baltic countries to document changes in the environment. The parameter measured to describe seal nutritional status is blubber thickness. The seals included in this study were grey, harbour and ringed seals either shot, caught in fishing gear or found dead along the Swedish coast of the Baltic Sea and the Swedish West coast 2002-2012. The blubber content as percentage of body mass (%fat), the so called LMD-index, was calculated. This was correlated with several other body condition indices taken from the literature, with the aim of finding out how well they could describe blubber content and thus, indirectly nutritional status. The seasonal and interannual variation in blubber thickness were also investigated. The sternum blubber thickness, the most widely used measuring site of blubber thickness in pinnipeds, was found to correlate well with %fat in all three species. Thus, suggesting that it is a good indicator of total blubber content/nutritional status. Dorsal and sternum blubber thicknesses were also found to correlate well with each other, suggesting that sternum blubber thickness can substitute the dorsal measure, by multiplication with a factor 1.114, for calculating the %fat. In agreement with previous studies, a seasonal variation in blubber thickness, with a decrease in spring and increase in fall/winter was found. Interestingly, a significant decrease in the autumn-winter %fat was found over the time period 2002-2012 for sexually mature grey seal females. This trend could pose a future problem for both individual survival and reproduction. The underlying cause is as yet unknown, but may be related to decreasing fish stocks, dietary changes and population density related factors. Total blubber content in kilograms of juvenile grey seals in autumn varied widely among individuals. Simple energetic modelling suggested that these differences corresponded to substantial differences in the amount of ingested food between fatter and leaner seals of the same cohort and these differences may prove to be insurmountable given the limited time available for feeding before their first winter. This may reflect differences in lactation and early foraging success. Thus, a good early start in the life of the juveniles, with proper lactation and good foraging success seems to be crucial. Introduction Study species The Atlantic grey seal (Halichoerus grypus) is a large, fish-eating phocid seal that inhabits temperate coastal waters of the North Atlantic and the Baltic Sea (Abt et al. 2002). In the Atlantic it is a land-based colonial breeder, with large sexual dimorphism and a polygynous mating system (Worthington Wilmer et al. 1999). Female grey seals come ashore once a year to give birth to and rear a single cub (Sparling et al. 2006). The Baltic grey seal population is different from the Atlantic population in several ways, the most obvious being the ability to shift breeding habitat depending on sea ice availability, thus it is a facultative land/ice breeder. It has been shown that pup survival, weaning weight and health are significantly increased on ice as compared to land, most likely because of lower exposure to crowding and pathogens on ice (Jüssi et al. 2008). The Baltic grey seal breeds in early spring, with pupping peaking in late February to early March (Thompson and Härkönen 2008a). The lactation period is around 17 days long and during this period the female does not go to sea to feed but instead relies on stored energy reserves in the form of blubber. Thus, maternal energy reserves is a major factor determining cub survival rates and female fitness. Females forage at sea for 7 months between the moulting and breeding seasons in order to accumulate enough fat reserves for the next breeding season (Sparling et al. 2006). After a century of bounty hunting and decades of low fertility due to environmental pollutants (Harding et al. 2007), the Baltic grey seal population has now been on the rise since 1990, with an annual rate of 5.8-8.5 %. This recent increase in population size has caused problems with local fisheries (Harding et al. 2007; Kauhala et al. 2015) which has led to the reintroduction of hunting in Sweden since 2001, with an annual quota of around 200 seals (Kauhala et al. 2015). The harbour seal (Phoca vitulina) is a smaller phocid seal (Bowen et al. 1992). Its diet is very diverse, ranging from various fish species to cephalopods and crustaceans. It is one of the most widespread pinniped species, inhabiting coastal waters, bays, rivers, estuaries and intertidal areas of the Northern Hemisphere, ranging from temperate to polar climate. The harbour seal is considered a promiscuous or weakly polygynous species (Thompson and Härkönen 2008b). Like the grey seal and other phocids, the females give birth on land, lactation is brief and weaning abrupt. However, unlike most phocids, the new-born cubs are large relative to the mother’s size and follow their mother to sea only hours after birth, and the females often feed during lactation (Bowen et al. 1992). The Swedish harbour seal has several distinct populations, around the island of Öland, on the Swedish West Coast and the southern Baltic (Bäcklin et al. 2013). Along the Swedish West Coast regional genetically distinct stocks can be identified (Olsen et al. 2014). The populations in the Baltic and Kattegatt have suffered several disease related crashes in the past but are now on the rise, increasing at an approximate rate of 12 % per year, however the rate has been declining in recent years (Härkönen et al. 2013). The ringed seal (Pusa hispida or Phoca hispida) is a small phocid seal, possessing a very thick blubber layer. Its diet consists mainly of small schooling fish species, e.g. herring and sprat, although some variation exists, depending primarily on prey availability and energy content. It has a chiefly circumpolar distribution throughout the Arctic Basin and adjacent seas, but some subspecies are also found in lakes further south (e.g. the Saima seal and the Ladoga seals). The Arctic ringed seal is a typical ice-seal, breeding, moulting and resting on sea ice. The Baltic population however, use ice for breeding and moulting, but is forced to haul out on land during the summer due to a lack of sea ice. In the spring females give birth to a single pup in lairs made in the ice and snow. Lactation lasts a bit over a month on average and mating occurs towards the end of this period, similar to other phocids (Kovacs 2008). Due to over-harvesting the Baltic ringed seal population declined drastically during the past century (Kokko et al. 1997; Kovacs 2008). With the exception of the Bothnian Bay, the populations in the rest of the Baltic have shown either no increase or even a decline since the 1990s. In addition to the past hunting, other major threats include bycatches in fishing gear, pouching, infectious diseases and the predicted future reductions in sea ice availability due to climate change (Kovacs 2008; Sundqvist et al. 2012) and various anthropogenic pollutants such as organochlorines and heavy metals (Nyman et al. 2002; Kovacs 2008). Study area The data used in this study is from seals shot, caught in fishing gear or found dead along the Swedish coast of the Baltic Sea and the Swedish West coast between the years 2002 and 2012. The grey seal data is considered applicable for the whole Baltic Sea population, as grey seals are very mobile across the Baltic Sea basin. The ringed seal data on the other hand may apply only to the Bothnian Bay, where most of the samples are from. The harbour seal data is applicable to the Swedish West coast. Blubber Blubber, a layer of lipid-rich tissue between the epidermis and the underlying muscles is a fundamental part of the mammalian adaptation to the aquatic lifestyle. It can constitute 15–55 % of total body mass in marine mammals. Its functions include not only thermoregulation but also storage of metabolic energy (Noren et al. 2014), buoyancy regulation (Webb et al. 1998) and hydrodynamic drag reduction (by modifying the streamlined shape of the body)(Fish 2000). The thickness of the blubber layer is important not only for individual survival but also for reproduction, being very important for lactation and cub survival (Harding et al. 2005; Stephens et al. 2014; Bowen et al. 2015). Blubber thickness in adult seals has been shown to vary seasonally, with a decrease during reproduction, lactation and moulting in spring followed by a subsequent increase when autumn arrives (Nilssen et al. 1997; Hauksson 2013). Lactation in phocid seals is brief and often involves maternal fasting with concomitant drastic reductions in the mother’s energy reserves, the energy being transferred to the rapidly growing pup, and is ended with abrupt weaning when the mother departs to feed at sea. Maternal fat reserves may decline by as much as 79 % (e.g. harbour seal, P. vitulina) during the lactation period (Bowen et al. 1992). The amount of fat stored by each animal is influenced by external conditions such as abundance of prey items, parasite load, environmental toxins and other stressors and thus could serve as an indicator of body condition that correlates with habitat quality and reproductive rate (Noren et al.