Blog post

Approaching the end of the cruise…

Published on
April 12, 2012

We are now approaching Scheveningen, the end of our Blue whiting survey for this year.

 It was indeed a very successful cruise, with more than excellent weather conditions. It is the first time in the history of the blue whiting survey that all participating vessels managed to finish the entire planned track without any major difficulties.


We did a total of 15 hauls (including 2 dedicated deep sea trawls) out of which 11 contained substantial amounts of blue whiting, we caught a total of 70 different species. The table below is a summary of how much blue whiting was found in each of the 15 hauls, values in kg:

haul number species    

1        2        3         4       5       6       7         8        9      11       12       13      14  

Micromesistius poutassou

2750  4350  -       1520   7220   350  3620   6900  -       6450   2900   4050  450

Total  40560

For every catch 100 otolith samples were taken. Otoliths are a stone like structure found in the inner ear of fishes, they are used as gravity, balance, movement and directional indicators and are a kind of hearing organ. Otoliths are collected to age fishes. The technique is very similar to the well known dendrochronology (ageing of trees), just as trees form year rings in their wood, fish do develop year rings on the otoliths. Though stageing those is not always a very easy job, as these otoliths are very small and sometimes the rings are not very clear.The picture below shows our cruise track and the locations of the different trawl stations:

Rplot_thumb.png

Shortly before finishing the last transect, we had an appointment with the Celtic Explorer (Irish research vessel participating in the survey)…We both had agreed  previously to do an inter vessel calibration together. An inter vessel calibration is used to check if the results we are getting are de facto comparable or if we do see, or catch something completely different. To do so, we called the Celtic Explorer, which is the reference vessel of the survey, at the moment we saw a reasonably sized blue whiting school. So the location for our rendez-vous was set. She did not let us wait for long and arrived very quickly at the meeting point. Tridens started off the calibration by moving 10 nm miles towards the North at a constant speed of 10 knots, followed at an approximate distance of 0.5 nm by the Celtic Explorer. After an hour or the travelled distance 10 nm, we did a turn, during which the Celtic Explorer was overtaking us, so she had the leading position now, being followed by Tridens… After we finished the second track, we went for a comparative fishing haul of 20 minutes together, during which we both caught approximately 500 kg of Blue Whiting. Now all we need to do is compare our acoustic recordings and our catch data (length/weight measurements) and find out if both ships do register the same thing or not…

IMGP6588.jpg
IMGP6599.jpg
Untitled.png

After finishing the last transect of the planned cruise track and hence the official data collection period of the survey,we headed towards Stornoway, as we needed some spare parts for the engine and new fresh water. Stornoway on the Isle of Lewis is a small harbour town. The harbour itself was interesting because there were loads of old but still functioning fishing boats, other derelict ships and ships that were in the process of being restored.

Sunday morning we left Stornoway for one last deep sea trawl before finding a loch in northern Scotland to post-calibrate the acoustic equipment. The seas were rough for once, or lets just say we finally got some normal blue whiting weather.

But still we have been blessed this trip that during the past month we have only experienced one day of bad weather! By the time we got to deeper water it was already much calmer and the net was shot at dusk for a one hour trawl at 1000 meters depth.

Again a couple of species were caught that were new to the blue whiting survey, together with a couple of fish and squid species that had been caught before, but were in very good condition.

By the time the deep sea trawl had been sorted and sampled, we were we were close to the Loch Inchard where we initially planned to do our acoustic calibration. On the map this Loch just seemed as the perfect calibration spot, sheltered from every direction, and very deep…As expected it additionally was a beautiful area. Unfortunately, when we got into the bay, it was littered with mussel rope cultures which meant we couldn’t go for anchor and had to do a U-turn.

Consequently we went to Loch Eriboll (aka Loch horrible), where calibrations were successfully executed in previous years, and which is commonly used by our Scottish colleagues for calibration exercises. The calibration was again as almost every calibration more difficult then expected. The conditions seemed nice tough… We ended up spending the entire day calibrating our echosounders…The sphere just seemed not to be very willing to cover the entire beam and we had to do adjustments to the setup a couple of times…The current in the water certainly did not make the job much easier…But in the end we managed to get 2 good calibrations done for our 38kHz and our 120 kHz echosounders…Having this done, it meant we successfully finished the data collection part of the survey, covered the entire planned cruise track, did 2 good calibrations of the acoustic equipment, did an inter-vessel calibration with Celtic Explorer, saw a lot of blue whiting and found 5 new species for the mesopelagic identification book. All in all it was a big success!