Karin's research in the Baltic Sea

While at the University of Stockholm in Sweden (1997-99), I became involved in fisheries research in the Baltic Sea.  For general information about the ecology and environment of the Baltic, you may wish to check out this link.

I’ve had a number of different projects over the intervening years. Early on, I collaborated with Lars Westin (died in 2006), Department of Systems Ecology at U. Stockholm, his student Peter Landergren, and Håkan Wickström and Henrik Svedäng from the Swedish Bureau of Fisheries.  Our project, titled "Revealing Complex Life Histories of  Fishes Through Natural Information Storage Devices: Case Studies of Diadromous Events as Recorded by Otoliths," used chemical information in otoliths (earstones) from fishes to study two problems:

Recruitment of native anadromous brown trout, Salmo trutta, to ephemeral streams on the island of Gotland (arrow on map).  On the Swedish island of Gotland, the many small streams support small populations of brown trout.  The production of spawning fish appears to be exceeded by returning adults about 3-to-1.  We measure the concentrations of strontium (Sr) entrained in the calcium carbonate of the earstones as a proxy for salinity, using a nuclear microprobe coupled with proton-induced x-ray emission analysis (microPIXE) at the Department of Nuclear Physics, University of Lund, Sweden with Per Kristiansson and Mikael Elfman.  The more strontium, the higher the salinity.

We found that brown trout are much more plastic in their use of habitats in early life than previously thought.  Although many individuals behave "according to what is known," spending 2-3 years in fresh water before migrating to sea, we also find many individuals that migrate -- or are flushed out -- as small fry.  Some even appear to use brackish/marine habitats from the egg stage.  The images here show a range of freshwater-seawater use.  Furthermore, the high Sr in the centers of many otoliths is due to Mom!  Brown trout, like many salmonid fishes, take time to develop their eggs.  If the female is a sea-run fish, her egg stock will develop out at sea, and therefore the otoliths (which develop in the embryo) will be imbued with Sr as well.
Strontium maps of otoliths.  Left: a "normal" life-style, where the fish stayed in fresh water for a year and then migrated to sea.  Center: a fish that started off life in slightly brackish water (maybe hatched at the mouth of a creek, but in the Baltic), moved up into fresh water for a few years, then migrated out to sea.  Right: a completely brackish/marine lifestyle...unknown before this study...

  We also used otolith strontium to tell us about European eels (Anguilla anguilla) in the Baltic.  Eels are Sweden's number one fishery in terms of generated revenues, but like so many fisheries, it is in serious decline due to overharvesting problems.  Currently Sweden has a major program to stock eels into lakes and even the Baltic Sea.  Just as with brown trout, we can use the Sr in eel otoliths to assess their life history use of waters of different salinity.  We have also been able to calibrate the Sr:Ca ratios in eel otoliths, and now can hindcast the history of habitat use by eels, in terms of salinity, as recorded in the otoliths.

A third project involves the study of fossil remains of Baltic codfish, Gadus morhua, together with an archaeologist at U. of Stockholm, Carina Olson.  Fossil remains...yep, otoliths once again.  These are 4,500 to 5,000 years old and come from a well preserved site from the Upper Stone Age, again on Gotland.  Working with Yvonne Walther, Bongghi Hong, and Jan Stora, we have been able to reconstruct the age and size structure of the stocks exploited by these pre-Viking people. We have written an article that compares the ancient fishery to the modern, highly over-exploited fishery in the Baltic Sea. We have found evidence of a “shifted baseline” for cod, but it is not quite as extreme as the changes observed in the open Atlantic Ocean. “Why?” is a very good question…

Finally, Carina led a very interesting project where we analyzed a remarkable collection of fish hooks from Ajvide. It’s the largest find of Stone Age bone fish hooks from anywhere in the Baltic! We reconstructed their characteristics, and even made some replica hooks, to see how much weight they could bear. Although they appear to be somewhat over-dimensioned, there is an excellent relationship between the size distributions of hooks and fish (sizes reconstructed from bones and otolith remains), offset by a factor of 10.

(back to KL's fish research)