Mysid monitoring started on R/V Aranda

In 2021, large herring in the Bothnian Sea experienced starvation and likely starvation-induced deaths, attributed to an exceptionally poor food situation. However, confirming this was challenging due to the lack of time series data on the abundance or biomass of mysids, which are key prey items of large herring and an important part of the Bothnian Sea food web.

To address this, Luke started visually assessing the abundance of mysids in research trawl catches, recording an index value for each haul. However, estimating mysid abundance with a large-mesh trawl designed for catching fish proved unreliable, necessitating additional methods. To obtain more accurate data, Luke and Syke experts collaborated during the annual fish research cruise of the research vessel Aranda, which covers the entire Bothnian Sea. Since mysids are relatively large (about 1–2.5 cm), they can be detected using Aranda’s multi-frequency echo sounding equipment. The team aimed to correlate echo sounder readings with the number of mysids found in trawl catches and samples collected with nets, thereby gaining a comprehensive understanding of the size and distribution of the mysid populations.

The pilot monitoring study was conducted during the autumn 2024 annual ICES BIAS acoustic research cruise. Mysid sampling was performed immediately after trawling in the early night when mysids migrate from the seabed towards the surface. A zooplankton net with a 0.5 mm mesh size was used to capture crustaceans in a vertical haul from the bottom to the surface, and a specialized towed plankton net (MultiNet) was used to sample the specific water layers indicated by the echo sounder. This approach aimed to assess mysid abundance and their distribution across different water layers.

Preliminary results are encouraging, indicating that the ecosounder results and MultiNet sampling from different water layers agreed. The results also align with observations from trawl catches. In 2024, mysid populations and herring condition have significantly improved compared to 2021, when only a few mysids were found during the entire research cruise. In contrast, autumn 2024 trawl catches contained thousands of mysids. The average mysid density in the Bothnian Sea was about one mysid per cubic meter, with denser layers showing up to ten individuals per cubic meter. This suggests a substantial food supply for herring.

For further information, you can contact the researchers involved in the study:

• Jukka Pönni, Researcher at Luke: jukka.ponni@luke.fi
• Jari Raitaniemi, Senior Researcher at Luke: jari.raitaniemi@luke.fi
• Laura Uusitalo, Research Professor at Luke: laura.uusitalo@luke.fi
• Maiju Lehtiniemi, Research Professor at Syke: maiju.lehtiniemi@syke.fi