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in the North Sea

Expedition

Walther Herwig III, sea blog of the 413th cruise

Duration: January 22 to February23, 2018

Area: North Sea

Purpose: International Bottom Trawl Survey 2018

Objectives and map

Sea area to be investigated (© Thünen-Institut)

The International Bottom Trawl Survey (IBTS) is an internationally coordinated ICES program (International Council for the Exploration of the Sea). The survey aims to provide ICES assessment and science groups with a time series of consistent and standardized data for examining spatial and temporal changes in (a) the distribution and relative abundance of fish and fish assemblages; and (b) of the biological parameters of commercial fish species for stock assessment purposes.

The main objectives are to determine the distribution and relative abundance of pre-recruits of the main commercial species with a view of deriving recruitment indices. This means that the number of juvenile fish caught from one species can give insight on how many individuals of this species are to expect the season to come. This data is used to provide managers with advice for setting the TAC, the total allowable catches of commercial species. Importantly, we monitor the changes in the stocks of commercial fish species independently of commercial fisheries data.

Furthermore, the distribution and relative abundance of all fish species and selected invertebrates are observed. Additionally, we collect hydrographical and environmental information at every station.

At night we work with plankton nets to determine the abundance and distribution of late herring larvae, which is used as a recruitment index in North Sea herring. Also at night, and for the first time on this survey, fish eggs are collected to gain further insight on spawning grounds of different species.

Our research vessel is undertaking research on a 24/7 basis.

Cruise leader: Matthias Kloppmann, Thünen-Institut für Seefischerei

Position of the vessel: sailwx.info

After fishing the “MIK-ing” starts. As soon as the sun has set we start our nightshift “MIK-ing” action to collect some of our tiniest samples. Up to 10 times per night we fish for plankton with a 2m mid-water ring trawl (MIK). It can be trawled down to 100 m depth. We’re keen on catching the larvae of herring. By determining relative abundance and distribution we’re able to estimate next year’s recruitment. With about 20-30 mm of larvae length and a mesh size of 500 µm the young fish easily get caught in our “bucket”. This so called “cod-end bucket” made of PVC forms the end of our MIK-net. The 2m wide MIK-opening collects all what flows through into the back of the net and finally into the bucket. When the net is brought up it is washed thoroughly with sea water to guarantee a complete collection. The bucket is then detached and brought to the lab for sorting. On ice, the different larvae are sorted and conserved in formaldehyde.

Apart from larvae, we also want to collect fish eggs of winter spawning species. That is why we additionally use a MiKey-M net. This 20 cm ring net is attached to the ring of the MIK and collects the small eggs (1-2 mm) floating in the water. The tiny eggs are hard to detect with the bare eye. So sorting is done under a microscope. Anyone who has used a microscope out at sea knows that this should be done as quickly as possible – seasickness is waiting just around the corner.

The CTD is launched. (© Eva Abraham/Thünen-Institut)

But apart from larvae and eggs we are interested in the conditions of their environment. Therefore, we run a CTD at every fishing station – a Conductivity Temperature Depths Sonde. This high tech equipment is conducted by our engineer Andriy Martynenko – and as always by the winch operator. Since the CTD is about 1.5 x 2 m large it needs to be launched via a crane. The side of our ship opens up and the CTD carefully descends to the seafloor. The winch operator and our engineer communicate via intercom when to stop the decent and where to collect what data. Our Sea Bird Probe is able to perform 24 different measurements per second - and that down to a maximum of 6800 m.

We’re after oceanographic data: oxygen saturation, salinity and temperature at different depths of the water column. Our CTD is equipped with several high-sensitivity sensors, which record e.g. conductivity, oxygene and pressure at any given time of the trial. The CTD is connected to the computers on board via a cable to assure permanent data flow throughout the run. Water samples are collected at different depths as well. When desired, one out of 10 probe bottles snaps shut and hereby brings to light waters of a certain stratum. Back on board these samples are carefully extracted and labelled. Some are used for further oxygen saturation tests and some go to the University of Hamburg for additional micro plankton analysis.

Our time at sea is very precious. We ought to collect as much data as possible in the few weeks our cruise has to offer. This way we hope to gain further understanding of the oceans and their inhabitants – the only way to assure a responsible resources management.

After a short pit-stop at home in Bremerhaven, we’re back on track and fishing is on.

But how do we catch our precious data anyway? On this cruise we use a so called GOV net, GOV for “Grande Overture Vertical” – that is French for large vertical opening. It is a typical bottom trawl net for pelagic fish and used mainly on flat, well-known ground. Here, in the southern North Sea it is therefore an ideal pick.

Shooting the net is supervised from the bridge. The winch pilot handles the large six drum winch from his look-out up here. Meanwhile on deck, boatswain Karsten Bosselmann is in charge of his men. He and five other deck-hands (sometimes 6, if the trainee is with them) let the net down into murky depths. First, the end of the net, the so-called codend touches water. It is followed by sort of a net-tunnel, belly, and wings - they are held open by two large otterboards (doors). The sweeps are 50 m long ropes of steel that are responsible for chasing the swarming fish into the “grand overture”. Finally, the trawling wire pulls the fishing gear behind our vessel. In unison with shooting the net, Captain Vandrei has to navigate carefully to assure a proper net-opening and floating throughout the 30 min of our haul. The net sounders tell him how the net is doing under water. Speed and direction of our vessel are adapted depending on waves, wind and drift.

Everybody plays his part. (© Eva Abraham/Thünen-Institut)

Twenty minutes later we hear a “Haul the net 20 minutes!” echoing through our hall ways. The crew gets ready to work on deck and the scientists hurry to the fish lab. So called “stropps” are induced underneath and around the re-boarding net and so form it into a compact tube. This tube is heaved with Gilson winches (very strong winches). At last, the knot at the codend is opened up and via the hopper hatch our catch is dispatched down into the fish lab. Sorting can begin.

Working on deck is tough. Exposed to wind and waves at all times, working at an open stern is challenging and dangerous. Ropes can break and cause major injuries. The winches and the heavy gear being pulled down into the deep bear enormous vigour and are meant to be avoided. It is because of the sedulous work of our decks men, we can start with our data collection. Teamwork!

As expected a first storm hits us and we’re off to calmer waters. Here, in the shelter of Helgoland, scientific fishing is on hold. All items in the fish lab have been safely secured and the scientists are filling their days with data analysis and literature research. The rest of the crew still has plenty of hands-on work to manage.

The heart of the ship, obviously, is the machine and runs under the control of our chief engineer Thomas Koch. Our swimming home is comparable to a micro-village. We produce our own freshwater, using a reverse osmosis filter system. We clean our wastewater, which is divided into grey and black water. Grey is the water coming from showers and washing machines, black is “the rest”. By adding special microorganisms to our sewage and filtering carefully we clean everything to a level, that it can safely be released back into the sea. Of course, we also produce our own electricity. Our electrician Roland Gaebel has a sharp eye on it.

Walther Herwig III is equipped with a main engine of 1200 kW. This asks for a daily gulp of diesel ranging from 4 to 10 m3 (depending on speed, 4000 to 10,000 l). The entire fuel required for the journey is stored in huge tanks, over 350m3 for this trip. If by any case the main engines would break down, one of the two substitute engines will kick in.

We are blessed with quite an old timer engine; this allows direct interaction and repairing. Our highly competent motormen are on point – no matter if a clogged tube, a burnt cable or whatever else unlucky surprise arises, they are off to fix it. We all feel safe and are looking forward to better weather.

Out at sea things don’t always work out as planned. If not, it is crucial to act immediately. A crew member needs to get to land as soon as possible. Medical attention is required. The coastguards assist and take him to Helgoland with the longboat. Fortunately, we can welcome the colleague back on board only a few hours later. And so fishing begins.

Herring and sprat is what we catch the most close the Danish coast. When the haul drops down into the hopper - and runs over the conveyor for sorting – the call “herring and sprat coming through!” echoes over the noise of the machine. We are only to collect what is not Herring or Sprat. The fish-mix runs right through and is collected in huge baskets at the end of the belt. A sample is then taken from these baskets and the two species are sorted (this is not as easy as it sounds – see photo quiz). After measurements the fish heads are being cut open to collect the otoliths. Otoliths are the ear-stones of fish. Like trees they grow a ring each year. This way the age of the individual can be determined. Other information can be read out of it as well – when the fish had grown well, the ring will be of greater diameter as if not.

Everybody is happy that lab-work began. We are looking forward to plenty more days of fishing.

On the afternoon of January 23rd the vessel Walther Herwig III is off to yet another expedition. Our team, 36 people in total, consists of 23 crew members, of whom three are nautical officers, all managed by our captain Jürgen Vandrei, and 12 scientific researchers under the lead of cruise leader Dr. Matthias Kloppmann. Our mission is the annual inventory of the North Sea.

In spite first stormy weather forecasts, the mood aboard is superb. We already underwent substantial safety instructions by our safety officer Franz-Josef Wichmann. Furthermore, the laboratories have been prepared to guarantee a smooth start when fishing begins. Crew members get to know each other in the first days of the cruise. This is very important, since living and working together on limited space, under rough weather conditions for five weeks strait runs better when spirits are high and seamanship is at its finest.

This cruise journal offers the possibility to follow our journey from afar. How does fisheries science work hands-on? Can you handle the bone saw at wind force 8? Where is our vessel at the moment? Let’s see! I’m looking forward to sharing this cruise with you.

Best wishes
Eva Abraham (scientific researcher)