Calibration between Kristin and Herwig (Blog update after one year)

Almost exactly one year ago we returned from our Sprat Acoustic Survey with the pelagic fishing trawler "Kristin". Over the summer of 2021, we analyzed the data and then presented it to the relevant International Council for the Exploration of the Sea (ICES) working groups in the spring of 2022. Because the Kristin was able to fill in so quickly for the failed "Walther Herwig III," we prevented an interruption in the time series that would have been fatal for the stock calculations of sprat in the Baltic Sea.

But even now, a year later, the work is not yet complete. When an acoustic survey is conducted with a different vessel, the continuity of the time series may be altered. Specifically, this means that different vessels may have different scouring effects on schools of fish, for example, if they are noisy differently. To quantify this and correct for it with a coefficient if necessary, intercalibrations are performed between vessels.

During intercalibration, the two vessels steam in formation over schools of fish and measure them using the echo sounder. All settings are exactly the same as for the classic hydroacoustic survey. Alternately, the two vessels take the lead so that at the end we can compare whether the measurements differ.

So, after we had completed all transects of the Sprat Hydroacoustic Survey 2022 with the "Walther Herwig III" (456th voyage), we met with the "Kristin" in the Bornholm Basin early in the morning of May 22, 2022 to hydroacoustically measure schools of fish in formation for two days. The distance between the two ships was only 400 m and they steamed just far enough to the side so that the rear ship did not sail in the screw water of the leading ship. At the same time, we were in continuous radio communication to indicate maneuvers in time to guarantee a successful measurement. Every four hours, the rear ship then overtook the front ship to take the lead itself.

The cooperation between the two ships was terrific and so we were able to say goodbye to each other on the evening of May 23 with the simultaneous sounding of the ship's horns. The "Kristin" started her journey home and the "Walther Herwig III" steamed towards Rostock to let the science descend there after three weeks of successfully completed survey program. Back on land, the recorded data will now be compared to see if there are any differences. If necessary, the measurements of the last year with the calculated coefficient for the "Kristin" will be adjusted to the long time series of the "Walther Herwig III" accordingly.

You can read the full sea diary at this link.

Contact: Dr. Stefanie Haase