The Dutch Institute for Marine Resources & Ecosystem Studies (IMARES) has been participating in the international North Sea acoustic survey for herring since 1991. Participants in this survey are Scotland, Norway, Germany, Denmark, The Netherlands and Ireland. The survey is part of the EU data collection framework (DCF) and is coordinated by the ICES Working Group for International Pelagic Surveys (WGIPS, formerly PGIPS/PGHERS). The aim of this survey is to provide an abundance estimate of the whole North Sea herring population. This estimate is used as a tuning index by the ICES Herring Assessment Working Group (HAWG) in its assessment of the population size.
The first week of the whole 4 week survey period was used for calibration of the acoustic equipment in a sheltered location in the Orkney Islands. Departure from Scheveningen was on Monday 23 June and Tridens steamed up north-west towards the proposed calibration location in Scapa Flow, UK. Arrival in Scapa Flow was in the morning of Wednesday 25th June. Calibration of the acoustic equipment took place during the following 2 days on 25th/26th June and was completed successfully before the vessel departed again. Conditions at the calibration site were very favourable and good calibration results could be achieved. Tridens then steamed to Aberdeen for an extended weekend break before it left again the following Monday to start conducting the survey.
The survey continued according to the planned transects until Thursday 10th July. Due to bad weather between Wednesday 9th and Thursday 10th July, the towed body housing the echosounder transducers got damaged. In the morning of Thursday 10th July, about 6 hours were lost for repairs (exchanging metal pieces of the towed body, welding on new stabilising weights, and repairing damages to the transducer cable). Therefore, the planned transect coverage had to be compromised in order to reach the end of the transect in time before the agreed weekend stop in Newcastle. After that incident, the survey could be conducted as planned.
During the standard survey protocol, 3 pilot trawls just below the surface aimed at Northeast Atlantic (NEA) mackerel were conducted successfully. The chosen survey setup (1 week calibration/gear trials, followed by 3 weeks of survey transect coverage) proved again to be an optimal solution to complete the transects in good time with adequate possibilities for trawl samples (to guarantee a good survey precision), before the scheduled weekend breaks. Arrival in Scheveningen after the survey was on Thursday 17 July.
Since 2010, cruise leaders keep a weblog during the survey, which can be found at:
Sascha M Fassler (Institute for Marine Resources and Ecosystem Studies, IJmuiden)