Bonus Projects

The Baltic Seal - BONUS BIO-C3

Join the adventures of our Baltic Seal in this journey through his life history, and the history of the seal populations, in the Baltic Sea, and discover how their interactions with humans affect the ecosystem. As a postdoc trying to model the ecological role that the seals have in the Baltic Sea, especially in relation to fisheries, I spend much of my working time in front of a computer. This is my way to get closer to the Baltic Sea water and its fauna.
30.06.2016 11:25

Time to change the jacket

David Costalago

Our friend, like all other grey seals, stayed on a rock for the entire moulting time, which is normally a couple of weeks in April-June. Although seals can be extraordinarily fast in the water by moving their bodies sideways, they are definitely not agile runners, and during moulting time they move as little as possible, and try to avoid having to get into the water. This behavior – individuals gathering, or hanging out together, on land (or ice) for a period of time in large and noisy groups- is very typical in seals and other pinnipeds, and is termed ‘hauling-out’, and the groups of seals are called haul-outs (except when the hauling-out has a reproductive purpose, when the groups are known as rookeries). What the young seal of our story would get from hauling-out, besides resting and staying dry while moulting, might be thermal regulation, predator avoidance and social interaction.

Why do they need to moult anyway if the process involves stopping feeding for up to 6 weeks? Fur is like the seals waterproof jacket, and thereby helps to keep them at a constant and warm temperature. It is composed of 2 layers of hair: 1) the bottom layer, consisting of wooly, curly, short hairs that are more numerous than the top layer and maintain a layer of dry air next to the skin to repel water, providing thermal insulation; 2) the top layer consists of longer, generally coarser hair and provides the visible coat patterns that we can see, while at the same time protecting the seal from ultraviolet radiation. With time, fur gets thinner and weaker and thus less efficient at insulating, and hormonal changes induce seals to go on land to start the moulting process.

This implies, of course, that our young friend will be starving and if he didn’t manage to store enough energy prior to moulting, he will likely have an urge to eat something as soon as he renews his fur. Being his first moult, little could he expect that it would be so hard for him. The seal was tremendously hungry…

Grey seals are not very picky with what they eat. However, the Baltic Sea waters are not particularly biodiverse, neither do they contain a huge amount of fish, unlike other more productive regions of the oceans. Therefore, to fill up his belly with the 6 kg of food that our juvenile seal requires every day, he will have to spend a significant amount of time and energy searching for nutritious fish (although who would reject a tasty octopus when it comes too close).

Our friend has witnessed how some other young seals, still lacking some experience in the matters of the hunt, recklessly decided to make incursions into fishing traps that humans had set for fish. On some occasions, some of these temerarious teenagers, if unable to ripping off the net, could never find their way out of the trap. So, he considered it wiser to try and hunt his own food. Herring, cod and sprat are his favourite meals, and sprat and herring in particular are normally more plentiful than other fish species in the Baltic. However, fish populations in general, and especially these three highly appreciated species, are not in peak abundance these days. That relative scarcity of food resources for seals might generate dynamics in the ecosystem that will potentially affect not only seals, but all components in the Baltic food-web, from humans to plankton. And that deserves another post. Or two.


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30.06.2016 13:16
Blend
Great write up David, looking forward to the next one
30.06.2016 13:37
David Costalago
Thanks Blend!
01.07.2016 10:12
Interested reader
Wow, 6 kg of food a day, what a hard job!