Researchers to study Baltic whitefish spawning patterns in Kokemäenjoki river, Finland

Researchers are set to investigate the spawning and movement patterns of Baltic whitefish in the Kokemäenjoki river and its adjacent marine area using new technology. The Natural Resources Institute Finland (Luke) will soon implant small transmitters in the abdominal cavities of 50 whitefish, enabling the study of their movements with underwater acoustic receivers, known as hydrophones.

The process involves making a small incision in the abdomen of the sedated fish to insert the transmitter, which is then sealed with stitches. According to researcher Lari Veneranta, the procedure does not seem to cause suffering or increased mortality among the fish, nor does it affect their movement.

In total, 40 hydrophones will be deployed in the river and marine areas, allowing for comprehensive monitoring of the fish’s movements up to the Harjavalta dam. Each receiver is capable of gathering information from a distance of at least 100 meters, meaning the hydrophone placed in the center of the river can capture data across the entire width of the river. The equipment will be retrieved from the river in the fall and from the sea in the spring.

Veneranta emphasizes the aim is to gather more insights into the behavior of whitefish and determine where the fish spawn in Kokemäenjoki. While most of the local whitefish population is hatchery-reared, there is evidence of natural reproduction.

Whitefish catches have diminished over the years, with peak fishing years occurring in the 1990s. The species faces significant threats from damming rivers, deteriorating water conditions, and increased seal populations.

Whitefish typically spawn in Kokemäenjoki in early November, at depths of 1-4 meters. The preferred spawning habitats consist of gravelly or rocky bottoms with a water flow of 0.2-0.8 meters per second. The eggs sink to the riverbed and ideally attach to favorable spots, with hatchlings emerging as the ice melts.

Veneranta notes that knowledge of whitefish behavior is considerably less comprehensive than that of species like salmon and trout. This research may also lead to surprises for anglers who might catch a fish containing a small black capsule in its abdomen.