en: Collecting samples on a commercial fishing vessel

Institute of

SF Sea Fisheries

Our colleague Katrine Wilhelmsen Melaa from the Thünen Institute of Sea Fisheries had the opportunity to collect samples for her PhD project on the commercial fishing trawler “Cuxhaven”of the Deutsche Fischfang Union (DFFU). Join Katrine on the journey and her experiences on board!

Arrival at the commercial trawler “Cuxhaven” in Hafnarfjörður (Iceland)

In December 2024, Katrine joined as a PhD student alongside our colleague Sakis Kroupis, who was on board as scientific observer on behalf of the Thünen Institute.

The “Cuxhaven” is a large trawler, approx. 81 meters long and 16 meters wide, operating under the German flag for the “Deutsche Fischfang-Union” (DFFU), based in Cuxhaven, but mainly operating off Greenland and Norway. Sampling this vessel for catch composition and biological parameters is part of the scientific observer programme by the Thünen Institute under the EU-Data Collection Framework (DCF). The “Cuxhaven” mainly targeted cod in the waters off Dohrn Bank - situated between Iceland and Greenland.

The mission of Katrine was to collect stomach and stable isotope samples from two species of redfish (Sebastes mentella and Sebastes norvegicus) on the East Greenland Shelf for her PhD research. This project aims to understand the distribution and ecology of both species in order to enhance our knowledge and improve scientific fishing quota recommendations.

Join Katrine on the journey and her experiences on board:

03.12.2024 - Departure

Our colleague Katrine in front of the commercial trawler “Cuxhaven” in Hafnarfjörður (Iceland) — Arrival at the commercial trawler “Cuxhaven” in Hafnarfjörður (Iceland)

After arriving well in Reykjavík, Iceland, I was excited to see the commercial fishing vessel “Cuxhaven” in the harbour of Hafnarfjörður. Sakis and I boarded the ship around midday as the crew prepared to set off into the cold Icelandic winter. This was the last time we would be in this fjord, as the ship is usually docked in Akureyri (northern Iceland).

Over the course of three weeks, we would make two trips out at sea, stopping in Akureyri once to unload the fresh cod and redfish for further export.

Work on board

A new catch coming up in the trawl net for us to work with — Working on board: a new catch coming up in the trawl net for us to work with

A ne catch of redfish on the conveyor belt for us to work with. — A new catch for us to work with

The day started at 8 a.m. with a warm breakfast and organising for a day of new samples. Often the timing was perfect, with a new catch coming up in the trawl for us to work with! The crew had worked through the night and a new shift would join us in the next few hours.

The experienced captain had knowledge of which areas along the bank would be more likely to hold the target species, cod, and which were the 'redfish hauls', where the golden redfish was more likely to be found. This, combined with the echo sounder, provided some predictability in an otherwise changing marine system. There was a marked reduction in the number of redfish present when we went to the ‘cod-rich’ areas.

The commercial vessel trawls for three to four hours (or more) at a time to fill its net. This should give us time to process the samples, but as the factory has strict hygiene and cleaning requirements, everything has to be done quickly so that the fishers can completely clean the factory after they have finished sorting the catch.

The science behind the samples

My sampling protocol consisted of collecting length and weight measurements, otoliths (ear stones for age determination), stomachs and stable isotope muscle samples for each individual. In contrast to the research surveys, the commercial vessels have a higher selectivity due to larger mesh sizes, so trying to cover all size classes in the samples can be tricky.

The commercially valuable redfish species, beaked redfish (Sebastes mentella) and golden redfish (Sebastes norvegicus), coexist in a mixed fishery, which is a term used to describe a fishery that targets several species within a given ecosystem. As the two species have certain differences in habitat selection and life history traits, they may need to be managed more carefully. For example, there are differences in depth of occurrence and size, with golden redfish tending to occupy shallower areas and grow to larger sizes than beaked redfish. It is therefore important to understand their feeding habits and ecological interactions. Stomach samples provide valuable insights into their diet and ecological role within the food web.

Our colleague Katrine takes stomach samples were taken from both species of redfish

Stoisch content sample with crustaceans that are well intact — Each stomach receives a unique identification number, here with crustaceans that are well intact

The redfish in my PhD setting

In my previous work on the redfish species, I have been modelling their spatial-temporal distribution along the Southeast Greenland shelf. The study on the ecology of the two redfish species build upon a comprehensive study of their interactions in this area.

The stomach samples and muscle tissue samples I took onboard will be analysed when I am back at the institute. Stomach samples are processed using two methods - morphological and metabarcoding. The morphological study involves identifying organisms from the stomachs through binocular inspection. Metabarcoding is another method, which involves extracting DNA to decipher organisms and can be valuable when the contents are very digested. Furthermore, muscle samples are used for stable isotope analysis by determining the ratios of carbon and nitrogen in the tissue, helping to understand the flow of energy and nutrients throughout the food web of the ecosystem.

These samples help to assess the role of redfish species in the marine food chain and their interactions with other organisms in the area.

Redfish on a measuring board: An individual of golden redfish “Sebastes norvegicus”

No two days on sea are alike

Northern lights in the sky seen at the first arrival in Akureyri. — Northern lights seen at the first arrival in Akureyri.

Life on the open sea changes from day to day, and the weather can change from blustery to calm. The scenery can change from steaming calmly through Greenland's ice sheets and past icebergs one day to steaming through waves of 6 metres height the next day.

Some days, when the wind was at its strongest, it was a matter of holding on to what you could, as the ship swayed in all directions. Having a few sea sickness remedies is much appreciated at such times! Fortunately, I found that the sea sickness was not too much of a problem for me, but keeping my balance was sometimes tricky. On other days, the sea was calm, it was easy to glide through and the work was back to being more comfortable.

Apart from icebergs, it was also wonderful with the frequent appearances of Northern lights.

It was very interesting to hear the stories of the fishers, who have experienced so much variety from working at sea throughout the year, with the challenges that can arise when trawling and catching their target species. The community on board was very supportive and welcoming, which boosted the morale when going through hard weather. It was also fascinating to see the commercial side of the fishery, coming from a research perspective, and to see how the results from the research we do in the Thünen Institute influences the quotas that fishing vessels like this follow.

First trip back to Iceland after a week at sea

Steaming through small ice floes off East Greenland, on our way to the next fishing location on the bank

19.12.2024 - Back to the harbour

After a successful voyage, we arrived back in Akureyri for the last time and as the crew unloaded the last of the fresh cod and redfish trays, we prepared to head home. It was a very international crew, with workers from Portugal, Iceland, Romania and England, and it was interesting to hear where the fishermen were calling home for Christmas.

The next step was to prepare my samples for shipment back to Germany. It was a great adventure, with many samples collected to be analysed.

The commercial fishing trawler “Cuxhaven” docked in Akureyri at night — The commercial fishing trawler “Cuxhaven” docked in Akureyri

Final departure of the commercial fishing trawler “Cuxhaven” in Akureyri at night — Final departure of the commercial fishing trawler “Cuxhaven” in Akureyri

Learn more about the project:

You can also find more detailed information about Katrines Reseach project on our project webpage “Ecology of redfish on the East Greenland shelf”.