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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.

08.08.2016 13:12Is there a seal problem? David Costalago

Let’s leave now our seal alone while he rejoices with a tasty meal and a joyful underwater play, and let’s focus a bit on ourselves.

We now know, among other things, what our seal friend likes to eat, and how much. We have also learnt that fish is not what it used to be in the Baltic. One proof of that is that fishers nowadays are capturing less tons of fish than, for example, 30 or 40 years ago. If we give a closer look to our friend’s favourite snacks, cod and herring, we see that their populations in the Baltic Sea have decreased substantially over the last decades (see Figures below), especially in the so-called Baltic Proper, where most of the Baltic grey seals live. Cod, in particular, suffered a very dramatic reduction of its population during the 1980s. These years coincide with the time when the seals in the Baltic started to recover. 

trends SSB cod-herring_Post 5
Grey seals in the Baltic region had been drastically reduced by the bounty hunting (did I already mention that there was an estimated population of 100000 grey seals in the Baltic Sea one hundred years ago?). Then, the hunting ceased in the 1940s. However, contrarily to what it was expected, the population continued decreasing until the late 1970s. This last reduction of the numbers of seals has been attributed to pollution. Some chemicals, mainly the infamous DDT and PCB, that had been used for agriculture and industrial purposes ended up in the oceans, and it has been proven that - in addition to having other disastrous effects- they were affecting the reproductive capacity of the female seals. So, only after these pollutants were definitely banned (late 1970s), the seals could start to increase their numbers, although still very far from what the population was at the beginning of the 20th century.

Thus, the relatively rapid recovery in the Baltic seal populations since the 1980s raised concerns in the fisheries sector and, since then, frequent calls have been made for culls of grey seals, claiming that seal predation is the main reason behind the diminished fish stocks and that a reduction in the number of seals would produce an increase in the amount of commercially landed fish. As a result, the seal hunting was reinstated in 1997 in Finland and, soon after that, in Sweden. In theory, the hunting does not intend to reduce the population as much as it tries to avoid the damage that some seals can inflict to the fishing gears or the catches. As we learnt in the last post, some seals would dare to try and “steal” food from within fishing traps or from nets. These seals, though, are generally in poorer health condition than the average seal, suggesting that the incursions to fishing gears are normally the result of a desperate situation for a seal. Or perhaps it is just because it is an (apparently) easy catch for them. Whatever the case is, and acknowledging that the harm that seals provoke to the fishing gears (or to the fish already caught in nets) might have important economic repercussions at a local level, we do not yet know to what extent the seals are actually affecting fisheries resources in the whole Baltic Sea.

There are other potential reasons behind the reduction of the fish stocks in the Baltic, and given that the impact of seals on the fisheries is still unknown, researchers need to make the effort to draw sound conclusions that can be used to advice the managers of these issues. And here is when I have to talk about modelling… interesting word, isn’t it? Well, you will have to keep tuned if you want to know what modelling is really about.



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30.06.2016 11:25Time 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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08.05.2016 19:35The arrival of the pioneers David Costalago

A couple of months after his first immersion in the waters of the Baltic, our seal, now nearly a teenager (a juvenile), was resting on a rock hundreds of kilometers away from his birth place when an adult approached him. It was a not very large seal that had a strange look about it. It is not very often that seals come across other individuals, except when they congregate on beaches or rocks, but our young seal knew that there was something really different about that adult seal.

The teen, willing to start a conversation, explained that he was there because he had to take a break.

- It is exhausting to swim all the time, especially after having to chase the fish for a long time-, he said. - I have been swimming to hunt fish during the last few days, and I think I need a break now. The fish I have been eating should be enough for me to obtain, on average, the 5 kg of food per day that my mum said I should consume.

At first, the adult seal didn’t seem very interested in the young one, but once it found a nice spot to lay down, it said to our seal:

- Well, given your size I hope you only ate fish.

- What do you mean? What else could I eat?

- Don’t you read the news or what? There have been a couple of cases, not too far from here, where one of your kind killed and ate one of my species.

- Oh, no, I didn’t know. But now I understand why you look different. You’re another species.

- Yes, I belong to the harbour or common seals, smaller than the seals of your species.

- And how many different species are there?

- In this Sea? Only our two groups here in the south, and then, farther north, you are also likely to encounter the ringed seals, more like my size… but colder-. And the adult harbour seal released a loud laugh, unaware that our little friend did not get the joke.

- How did we all get to this Sea? -, asked the juvenile.

- Hmmm, that’s a very long story-, replied the adult, who began to look interested in the questions dropped by the young one. The young seal’s new companion then started relating the story of when the first seals arrived to the Baltic Sea. - Soon after the big Ice retreated, the Ancylus lake, which was how this region was then known, got filled with so much water that a pass to the Ocean was opened. Then, our ancestors, yours and mine, entered this newly formed sea. Our ancestors liked it here, apparently, because soon after that hundreds more came from around some islands out in the Ocean, and also many of the ringed seals arrived from the North. That happened about 8000 years ago, so no one knows exactly what it was like this place back then, but if you ask your elders they will probably tell you that, for what they know, it wasn’t as hot as it is now.

- Wow, that’s a very long time ago. I wonder how many of us there were around here at that time.

- OK, buddy, I think I am going to take off. My mates must be about to start moulting, and that is one of the few opportunities we have to hang out together. It’s been nice talking to you, but I have to find them before the end of June.

- Moulting?

- Renovating the jacket, that is. Soon your time for that will arrive too. Bye!

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11.04.2016 13:52Pregnant not pregnant David Costalago

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15.03.2016 12:56From birth to weaning David Costalago

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