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Baltic Diversity Notes - BONUS BIO-C3

Bringing you updates on Baltic Sea biodiversity from the BIO-C3 project and thoughts on life as a young researcher
20.02.2015 14:14

Plenty of fish in the Baltic Sea -no?

Anna Törnroos

Guest post on the diversity of bottom-living fish communities by Laruène Pécuchet

To spice things up a bit in this blog, I´ve invited my colleague Laurène Pécuchet to give a guest post. Lauréne is a PhD student at the Centre for Ocean Life and DTU-Aqua in Copenhagen, and also linked to our BIO-C3 project. She' ll tell you an interesting story about her field studies on the diversity of bottom-living (or demersal) fish in the Baltic Sea , enjoy!    

Tales from a wandering week sampling bottom-living fish communities

Last October I participated in a scientific bottom-trawl fish survey in the eastern Baltic Sea (east of the island Bornholm) with the Danish scientific vessel DANA. The Baltic Sea is every year (in February-March and again in October-November) surveyed to estimate the number of fish and the general state of the bottom fish communities. These investigations help us estimate the fish stocks. Based on these, we, the scientists, give advice on how much (tons of) fish that can be taken from the stocks and still keep a sustainable yield in the future.

Fish trawl
A trawl is brought up to the deck in the early morning with a few fish in its net. This is when the work to sort, count and measure the fish begins for us scientists.

The Baltic Sea fish stocks are assessed at the same stations every year, covering different bottom types for example mud or sand, and bot shallow and deep waters. At each station a trawl is towed after the ship for about 30 minutes. In a typical workday 4 trawls are hauled. This might not sound like much, but all the fish caught by the trawl is brought up on the deck where a team of scientists will sort the species out and weight their biomass like crazy before the next trawl is coming up. The length of the fish is also recorded for the most commercially important species, namely cod, herring, sprat and flatfishes. The otolith, a piece of bone close to the fish ear, is also cut out for these species. The otolith tells us the age of the fish. It works as a tree trunk; each year is marked by a ring. From the knowledge of the age and the length we can calculate the growth of the fish. This is important to know for managing the fish stock.

I once did a similar scientific cruise in the Irish Sea and although I knew the Baltic Sea was poor in species, I naively thought it would be the same work load to sort-out and count the fish now as it was then. I was prepared to get some back pain and get some OCD due to the repetitive length measuring. Oh, how naïve I was. The Baltic Sea is species poor. To give you a comparison: for the same sampling time, a haul in the Irish Sea took 3 to 5 hours to sort out, whereas in the Baltic Sea it took only between 1/2 hour to 1 1/2 hours. The major difference is that the catch in the Baltic Sea was composed of only a few (4 to 6) species, while we had more than 10 species in a catch in the Irish Sea. Sorting only cod, herring and sprat do not take that much time! Did you know that these 3 species make up 90 % of the commercial fisheries catch in the southeastern Baltic Sea? Well, after this survey I, for one, cannot question the truth of this statistic!

The alarming part of the survey was that several hauls in the Bornholm Basin did not contain any fish at all! The low or zero catches of fish were mainly due to the lack of oxygen (hypoxia) in this deep part of the Baltic Sea. The hypoxia in these areas is a consequence of the high eutrophication (excess nutrients) in the Baltic Sea. It is also a result of the strong stratification of water masses with different salinity, which prevent the deep water to mix with the more oxygenated shallow water.

In spite of these surprises, this cruise in the Baltic Sea was a refreshing time away from the computer! To see the numbers on your computer screen become reality in your hands was a really nice feeling. Sometimes when you spend much time analyzing the numbers, you forget that you are dealing with living creatures (or at least they once were)… So, if you ever have the chance, jump on a boat and see what’s beneath this blue surface. It can definitely give you lots of thrills and surprises! 

Laurene 2
PhD-student Lauréne Péuchet studies the diversity of fish in the Baltic Sea.

Thanks for popping by to read my story, I will be back later with some more tales about the Baltic Sea fish diversity!

All the best,


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