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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 13:12

Is 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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