in Greenland seas


Walther Herwig III, sea blog of the 440th cruise

The scientific crew of the 440th expedition of Walther Herwig III (© Thünen-Institut)

Duration: October 5 to November 23, 2020

Survey area: North Atlantic

Purpose: Abundance estimations of demersal fish species (especially cod and redfish) and oceanographic/climatologic investigations in Greenland waters

Objectives and map

Since 1982, the west and east coasts of Greenland have been sampled annually according to a standardised fishing programme. The study area is divided into 14 strata. The first number of each strata indicates the geographical location, the second number the fishing depth (1 = 0-200 metres; 2 = 201-400 metres) (© Thünen-Institut)

The research programme of the 440th cruise incorporates fisheries biological investigations of of the cod and redfish stocks as well as other ecologically important demersal species in Greenland waters. They will serve as a basis for developing improved management and exploitation strategies.

Tasks during the voyage:

  1. Studies on the dynamics of the cod stock: Approximately 80 hauls are planned, half of which will be distributed among the sub-study areas (Strata; see figure).
  2. The remaining hauls will be carried out proportionally according to the average historical abundance of cod in the strata.
  3. Data collection on caught cod, sampling of ear stones, stomach contents.
  4. Studies on the dynamics of the redfish stocks. Studies on the demersal community.
  5. Hydroacoustic studies.
  6. Oceanographic studies.
  7. Plankton surveys.

Cruise leader: Dr. Karl-Michael Werner, Thünen Institute of Sea Fisheries

Golden redfish or deepwater redfish? Polar sculpin or Atlantic hookear sculpin? These are just two of many taxonomic questions we regularly ask ourselves in the laboratory. When the crew hauls up the catch, we do not only find fish species, which are easy to identify, such as Atlantic cod or Atlantic halibut, but also species, where we need a second look for a correct identification.

Although we mainly investigate commercially exploited species, such as cod and redfish, we usually catch a colourful, subpolar mix of many different fish species. In order to provide advice for fishing quotas, we need samples and data of the commercial target species. But why are all species relevant and not just those, which end up on our plates? Because they show us, how ecosystems change.

Climate change affects the distribution of fish species and the composition of fish communities. Many species, which are for example distributed in the North Sea, move northwards, because the rising temperature creates new habitats there. Accordingly the size of the remaining habitats for arctic species decreases and these cold-water specialsts must retreat to the last refuges. Hence, fish species serve as indicators for environmental conditions. That’s why every single species is important to us and can tell us something about the state of the ecosystem and the impact of climate change on our oceans.

Besides science, quite different events can also determine the daily routine. On October 21, the Walther Herwig picked up a message that a Greenlandic tugboat with barge was in distress at sea 140 nautical miles away. We set sail at full speed and reached the damaged ship, which was drifting towards the coast with a crew of three, just in time. Despite a storm, we managed to tow the ship safely to the port of Qarqatoq. Further information can be found in a communication from the BLE.

What sounds like a swimming drop of marmelade at first glance is just the scientific expression for a group of organisms everybody knows and most of us have had undesired contact with before: Jellyfish!

Charlotte Havermans is the leader of a new junior research group at the Alfred-Wegender-Institute, which investigates the role of jellyfish in polar ecosystems. Ayla Murray, a masters student from the University of Bremen, is part of this group and on Board of Walther Herwig to collect samples and look at the function of jellyfish in Greenland ecosystems.

In changing marine ecosystems, which are impacted by climate change, deoxygenation and fishing, jellyfish have for a long time been considered to be an ecological “dead end“: They are one of the few organisms, which are resistent to many environmental changes, but more or less useless in the food chain, because they are not eaten by other animals. However, this opinion is about to change. Now we know that more than 50 fish species as well as sea birds and penguins eat jellyfish. At the same time, jellyfish can directly affect populations of other aquatic organisms, because they are predators.

On board of Walther Herwig, we are this year investigating whether jellyfish are really not eaten by subpolar fish species and what effects jellyfish could have on fish larvae. These are the two central research questions to be investigated as part of the new collaboration between the Thünen Institute and the Alfred-Wegener-Institute.

-> Jellyfish fishing at night

Ayla presents: Subpolar Jellyfish! (© Thünen-Institut)

October 18th, Walther Herwig III, oceanographic laboratory, 2:30 at night: Ayla Murray and Jana Bäger lift the heavy plankton net on board. The so-called „Helgoländer Ringtrawl“ has been towed for 15 minutes behind the ship through upper water layers of the subpolar North Atlantic.

It is a cold and windy night, but Ayla, Jana and the deckhand understand each other already perfectly. In order to operate the net safely on deck, they use their arms and hands to give each other instructions. Like a well-oiled machine! Tension rises, as soon as the net comes on board. We are asking ourselves which organisms came to the surface at night. Vertical migrations are a well-known phenomenon in the world’s oceans: Organisms remain in the deep during the day to escape predation windows of visual predators and come to the surface at night to feed. Here, in surface layers, they prey upon even smaller crustaceans and algae.

The first look at the catch gets us exciting: a colourful mixture of moving, eating and swimming creatues and in the middle of everything, Jellyfish! Ayla picks out every jellyfish individually, takes a photo and puts it in a plastic bag to freeze it later. However, we have not only caught jellyfish but also small crustaceans, such as krill. After the fifth successful plankton haul, Jana and Ayla call it a night and fall exhaustedly into their beds. They now have time to dream of the freshly baked buns in the next morning.

Although we are at sea, Covid19 is of course also a relevant topic for us. All members of the crew were tested prior to the expedition, a hygienic concept is in place and we have a ship doctor, who always takes care of us. We are in good hands!

We began scientific sampling on time on October 11th. The first catches with cod and redfish were made on Dohrn bank in East Greenland. The largest cod individuals were larger than one meter, true giants! This year’s main task is to provide the scientific basis for fishing quota advice. We investigate both, how much fish there is and how fish stocks quantitatively change over time and how the age composition of the stocks changes. It is important for us to find out, how many immature and how many mature fish the stock consists of to draw conclusions about the reproductive potential. Both aspects are part of our annual scientific quota advice. Using our data, which date back to the early 1980s, we can retrospectively calculate, how the stock size has changed and how much can be caught sustainably each year.

However, this is not as easy as it sounds. The spatial structure of a fish stock changes over time, because eggs and fish larvae drift with ocean currents and adult fish move around. That’s why the same spawning ground does not contribute equally to the reproduction. For example, we observed that cod larvae can drift from Iceland to Greenland in particular years, which affects the structure of the local stock in East Greenland. This is just one of the challenges we have to face as fishery biologists, when we try to incorporate these changes into our annual calculations in order to provide scientific quota recommendations.

Cast off in Bremerhaven! We left Bremerhaven with the research vessel Walther Herwig III on the 5th of October in order go to Greenland. “Science for sustainable seas” is again the annual motto of the expedition to East and West Greenland waters, where we monitor the stocks of cod and redfish. These results will contribute to the scientific fishing quota advice necessary for a sustainable use of marine resources.

The first five days of the expedition went well and the weather was fine for most of our path through the North Sea and the North Atlantic. Hence, everybody is optimistic that scientific sampling in East Greenland will begin on time. The scientific crew from the Thünen Institute of Sea Fisheries is complemented by a student from New Zealand this year, who will collect samples for the Alfred-Wegener-Institute. Using these samples she will investigate the role of jelly fish in sub-arctic food webs.

With best wishes
Karl-Michael Werner (chief scientist)