International Day of the Seal is March 22, so we’ve collected all of our research thus far on harbour seals in one article for you below!
Tracking Harbour Seals and Monitoring Sound Exposure
It may surprise you that that seals are commonly found in the Elbe river: their haul-out sites are located in the intertidal zone, close to the main waterway or in branches. This environment, however, is highly exposed to vessel traffic to & from the busy Port of Hamburg: Germany's largest port by volume and the third busiest port in Europe.
As part of the Saturn project, we're monitoring three species of marine mammals to better understand their responses to shipping noise. State-of-the-art miniature sound and movement recording tags (D-TAGs) are an important tool for uncovering the baseline (undisturbed) behaviour of wild marine mammals and to gain a first understanding of responses to anthropogenic underwater noise. To gather field data on marine mammal behaviour — and as a function of real-world exposure to underwater radiated noise (URN) from ships — the Institute for Terrestrial and Aquatic Wildlife Research at the University of Veterinary Medicine Hannover (TiHo) deployed D-TAGs on harbour seals for the first time in the world in this riverine and estuarine habitat.
SATURN facilitates multidisciplinary investigations by veterinarians & biologists to study the effects of shipping noise. While tagging, the health status of harbour seals is assessed by clinical investigations, supplemented with analysis of blood & swab samples (see image below). Then, after a preprogrammed period (22 days), the tag detaches from the animal and is then recovered using the Argos satellite system to get an approximate position (within a radius of a few kilometres), followed by UHF tracking.
Analysing the data from these DTAGs can help us understand 1) how often porpoises and harbor seals are exposed to vessel noise that triggers behavioural changes leading to negative offsets in energy balance, and 2) what the drivers of such responses are in terms of vessel noise levels and characteristics.
Left: SATURN project researchers collect a swab sample to assess the health status of a seal during tagging. Right: the track of a seal through the Elbe river in Germany.
In the image above, the seal track shows that it remained within the Elbe river for all 22 days and visited Hamburg Port multiple times. DTAGs simultaneously capture noise exposure, behaviour and contextual metadata: the colour of the dots illustrate the sound levels in the 2kHz decidecade band (a measure of noise exposure), and we see a wide range of exposure levels that shows high potential for SATURN to find different responses to these noise exposures. We have already developed an automated tool to detect intervals of elevated noise in the sound recordings. This was applied to quantify vessel noise exposures from previous deployments of DTAGs on nine harbour seals from the Wadden Sea. The results clearly show that seals in industrialised waters are exposed repeatedly to vessel noise and that exposures cannot be predicted reliably from AIS data.
SATURN project researchers Maria Morell (left) and Joy Boyi (right) conducting analyses at TiHo.
Analysing Inner Ears to Detect Hearing Loss
To quantify the impact of underwater radiated noise on the health of porpoises and seals, we have analyzed at TiHo the inner ears of stranded (deceased) marine mammals to detect potential cases of permanent hearing loss. We have optimized a new labeling protocol of the inner ear and have collected individuals from several age classes found in the Baltic, North Sea and Elbe river. In addition, we have reviewed TiHo’s database on the pathological results of stranded individuals, and blood and swab results of all seal catches of the last 30 years. We provided updated age- and length specific estimates on general animal health, reproductive capacity and hearing capacity, which can influence animal survival and reproduction.
Last year, Maria Morell and the TiHo team published a case of a neonate harbor seal in the journal Animals, where we found lesions in the inner ear, likely congenital. This information will be important to establish a baseline knowledge on “natural” congenital malformation of the inner ear of harbor seals, to further differentiate from potential damage caused by exposure to factors (including noise) that the individuals might encounter during their lifetime. We also describe the protocol to extract and perfuse the ears in harbor seals that will be implemented in SATURN.
In addition, SATURN will provide updated age- and length specific estimates on general animal health, reproductive capacity and hearing capacity, which will influence animal survival and reproduction. We have reviewed TiHo’s database on the pathological results of stranded individuals, and blood and swap results of all seal catches of the last 30 years, and already selected interesting cases and targeted organs to be categorized in more depth.
At TiHo, we will also combine clinical health parameters with behavioural recordings in harbour seals to determine how health influences behaviour, and vice-versa. In previous projects we have established immune-relevant and more recently endocrine-related genes in the blood of seals from the North and Baltic Seas, recently published at the journal Aquatic Toxicology, which will be implemented in SATURN. We are currently developing a novel combination of molecular biomarkers in blood to assess noise-related hearing, endocrine and immune responses in harbour seals.
Image from “Selective Inner Hair Cell Loss in a Neonate Harbor Seal (Phoca vitulina)” in the journal Animals https://www.mdpi.com/2076-2615/12/2/180.