Herring from different parts of the Baltic Sea belong to distinct populations genetically adapted to local differences in salinity and temperature. However, these populations can also mix with each other, according to a new study by researchers from Uppsala University, Stockholm University and the Swedish University of Agricultural Sciences. These results have important implications for the management of the Baltic herring. The study is published in the Proceedings of the National Academy of Sciences.
Spring- and autumn-spawning herring in the Baltic Sea as well as in the Atlantic Ocean are genetically distinct. This is well known.
"Despite their striking genetic differences, we were able to identify hybrids between the spring- and autumn-spawning populations, thanks to the very large sample size in our present study," says Leif Andersson, Professor at the Department of Medical Biochemistry and Microbiology at Uppsala University, who led the study together with Professor Linda Laikre, Department of Zoology, Stockholm University.
In other words, there are herring that, when they end up in a population spawning at the 'wrong' time of year, have been able to adapt their behaviour and spawn at the same time as the other herring in the surrounding population.
"Our interpretation is that genetics sets an optimal window for spawning, primarily spring or autumn, but water temperature and nutritional status influence when spawning happens. This would imply that there is a communication within the school, possibly due to hormones that set the spawning time for the school", explains Leif Andersson.
The Baltic herring are not only split into spring- and autumn-spawning populations. Thanks to this new study, the researchers discovered that the spring-spawning herring, which are widely distributed in the Baltic Sea, are further subdivided into a Northern, Central and Southern cluster. There are also additional groupings within the major clusters of spring-spawning herring. Linda Laikre points to a striking example from the Stockholm archipelago, the so called 'wild rose herring':
"We noticed that the genetic constitution of this population was more extreme than the populations from the Southern cluster. The explanation was that these herring was spawning in mid-July when the water is much warmer than in the spring. A population like this with adaptation to spawning in warmer waters may harbour gene variants of critical importance for future adaptation to a warming sea", she says.
The locals call the fish 'wild rose herring' because it spawns when the wild roses are in bloom.
The researchers believe their results have very important implications for the management of Baltic herring.
"Our findings showing that herring are subdivided into different clusters and groups are of great importance for management, since herring along Sweden's east coast are currently managed as two large populations, one in the Baltic Proper and one in the Gulf of Bothnia. The current management does not correspond to the genetic groupings we see, says Lovisa Wennerström from the Swedish University of Agricultural Sciences.
"We would like to see a much more restrictive industrial fishing for fish meal production to reduce the risk that important local populations and the genetic diversity they harbour get lost. Further, our results will constitute a basis for the Swedish Agency for Marine and Water Management's monitoring program that aims at tracking genetic changes over time in key species such as herring", says Linda Laikre, Stockholm University.
Herring has a key role in the Baltic Sea ecosystem. They are the most abundant fish in the Baltic Sea acting as a link between plankton production and other organisms, like predatory fish, sea birds, sea mammals, and humans. The Baltic Sea is home to many distinct local herring populations genetically adapted to when and where spawning takes place. These populations also migrate between spawning and feeding grounds.
The present study concerns an extensive characterization of the genetic diversity of Baltic herring by analysing more than 4,500 fish collected during spawning at 150 different locations distributed along the Swedish east coast.
