Kyhn. L.A., Beest, F.V. & Galatius, A. 2022. Spæktykkelse hos tre danske havpattedyr 2019-2021. Overvågning udført for Miljøstyrelsen. Aarhus Universitet, DCE – Nationalt Center for Miljø og Energi, 42 s. - Teknisk rapport nr. 253.
http://dce2.au.dk/pub/TR253.pdf
This report describes data for blubber thicknesses from Danish populations of harbor porpoises (Phocoena phocoena), harbor seals (Phoca vitulina) and gray seals (Halichoerus grypus), collected in the years 2019-2021 under the project 'Monitoring Blubber thickness in Danish marine mammals', and then data is compared statistically with data collected in the period 1988-2017. Data from 2018 are included in the most recent period 2019-2021, even though this period was between the current and the previous three-year project period. The purpose of the project is to provide data for use in assessing the condition of Danish marine mammals, including for use in indicators under the Marine Strategy Directive under the auspices of HELCOM and OSPAR. In order to be able to account for the development in blubber thickness over time, blubber thickness data from previous periods have been used for the respective management areas for harbour seals (western Baltic, Kattegat, Limfjord and the Wadden Sea), gray seals (Baltic and North Sea) and harbor porpoises (Belt seas and the North Sea). A large part of this report builds on the previous reporting of the precursor for this project (Galatius et al., 2018), as not much new knowledge has come to the field since then.
A threshold value for harbor seals at exponential growth in the Kattegat of 38 mm blubber for shot and live seals has previously been proposed (Galatius et al., 2018), and it has been proposed that this threshold also applies to the small populations in the Limfjord and the Baltic Sea, where it is difficult to obtain sufficient data to estimate a norm for blubber thickness (Galatius et al., 2018). Based on this threshold value, the stocks in the Limfjord, Kattegat and the southern Baltic Sea were not in GES (Good Environmental Status). The threshold value does not apply to the North Sea, where a systematically lower blubber thickness has been found. For harbor seals, a decrease in blubber thickness has been observed in the Kattegat since the last period, which is probably due to the size of this stock approaching the carrying capacity of the environment, but there is as yet no basis for proposing a threshold value for harbor seal populations that have reached environmental carrying capacity, as we do not have a data base for such populations.
In the smaller populations of harbor seals in the western Baltic Sea and the Limfjord, no significant developments are seen in the assessment period 2018-2021 compared to previous data. No harbour seals from the Wadden Sea population have been measured.
For gray seals, significantly thinner layers of blubber in the Baltic Sea collected for this project are seen than the set thresholds for good environmental condition in the HELCOM indicator for blubber thickness. This is probably due to the colder sea temperature in the northern Baltic Sea compared to the southeastern Baltic Sea, which requires a thicker layer of blubber. Compared to previous periods, a thinner layer of fat in gray seals is seen both in the south-eastern Baltic Sea and in the North Sea, although not significantly.
For harbor seals from both the Belt Seas and the North Sea, no significant development is seen between the current assessment period and older data, but as with the two seal species, a significant difference is seen in the blubber thickness between the Belt Seas and the North Sea. Thus, for all species, there is a thinner layer of fat on the animals from the North Sea than on animals from the Belt Sea. This is probably due to the fact that water temperatures in the North Sea are higher in winter due to the Gulf Stream.
For all three species, in addition to the time variation, there is also a variation in the collected blubber thicknesses in relation to many factors such as time of the year, sex, individual size and population status in relation to the carrying capacity of the environment.
The data collected will be part of international cooperation under the auspices of HELCOM to set threshold values for good environmental condition.
