In order to protect Baltic Sea herring, it is necessary to know where the fish spawn and how the stock is structured. Local fishermen have long possessed this knowledge, but few scientific studies have been conducted to date. However, in the Stockholm archipelago, Monika Winder and her research team at Stockholm University are attempting to fill some of the knowledge gaps in a project funded by BalticWaters.
We meet the researchers early one morning in Saltsjöbaden, just outside Stockholm. The morning sun shines on the research vessel Electra, which slowly rocks in time with the calm waves at the pier. Intensive sampling work is underway during the fall, and the researchers in the project have already spent many long days at sea – and now it’s time again.
Today’s goal is to collect herring larvae samples at several locations around the archipelago. Monika Winder, a researcher at Stockholm University, explains:
– Herring consist of many subpopulations, each with their own spawning and migration patterns, and each exposed to different environmental conditions. By collecting herring larvae, we gain valuable knowledge about how the different spawning grounds function and can help protect the mosaic of spawning grounds and genetic diversity on which the species depends.
The project Where and when do herring spawn in the Stockholm archipelago? builds on a previous project funded by BalticWaters in 2023. At that time, only the southern archipelago was studied, but now sampling is being extended to the northern parts as well to provide a broader understanding of how herring spawning activities vary over time and between different locations.
Mapping of Herring Spawning Grounds in the Stockholm Archipelago
On board the research vessel Electra, scientists are preparing for today’s sampling. Soon, the ship will set sail for another day at sea. One of the project’s goals is to map when and where herring spawn. This involves a large number of sampling sites around the entire archipelago, during spring, summer, and fall.
– We have identified the sites through old fishing reports and tips from fishermen, among other things. Now that the younger generation of fishermen is dwindling, it is especially important to preserve this knowledge before it disappears,” says Monika.
We stop and the researchers expertly lower the small trawl nets below the surface. The herring larvae are only a few millimeters in size and the nets must be very fine-meshed to catch the small larvae.
By analyzing the DNA of the larvae, researchers can also determine whether spring, autumn, or possibly winter spawning herring constitute different sub-populations. At the same time, the development and growth of the larvae are examined. In addition to measuring the larvae, the fish’s otoliths are also studied – hard stuctures that store detailed information about the larva’s life, much like the annual rings in a tree. In this way, researchers can draw conclusions about how quickly different groups grow and which environments provide the best conditions for survival.
Knowledge for the Recovery of Herring
After a long day at sea, and many herring samples later, we slowly steer back towards the mainland. Darkness falls over the water and my thoughts return to today’s sampling. Many questions have been answered, but one remains: why is it so important to understand how herring populations are divided and how they differ from each other?
Many populations are currently under severe pressure from high fishing pressure, and according to Monika, knowledge about the diversity of herring stocks is crucial for designing a management plan that contributes to the recovery of herring.
– Diversity is a cornerstone of ecosystem stability and resilience. Without insight into local variation, we simply do not know what is at stake when populations disappear, or what ecological functions are at risk of being lost, she says.
By mapping both spawning grounds and genetic variation, the project’s results provide a better picture of the local diversity of herring, but also knowledge about which populations are most vulnerable.
– Understanding how different populations grow and cope with environmental changes helps us identify the groups that are most sensitive and need protection in order for the stock as a whole to be strong and productive, she says.
The results can thus provide a better basis for decision-making to protect the coastal spawning grounds on which herring depend and provide better opportunities to make scientifically based decisions on appropriate measures to promote the recovery of herring.
About the project
The project Where and when do herring spawn in the Stockholm archipelago? is being carried out by Stockholm University. Through BalticWaters’ program to fund research projects and pre-studies, the project has been awarded a grant of SEK 999,876. You can read more about the three other projects that have been awarded funding in the article New projects for a living sea.
