Guest blog: A week aboard the Tridens – experiences on the Dutch BTS

Zoals gezegd, tijdens de BTS 2026 was er een onderzoeker uit Duitsland mee, als onderdeel van de internationale uitwisseling van kennis over de survey. Hieronder kun je een gastblog lezen van Alexandra Poell, van het Thünen Institut in Bremerhaven, Duitsland. Het is een selectie uit haar hele terugkoppeling, die ook voor ons als onderzoekers van WMR heel waardevol is.

Alexandra nam een pop mee de de hele week heeft meegekeken. Haar ervaringen zijn te vinden op Instagram: https://www.instagram.com/p/Dc03ksBjBOA/?stkn=cTRzMTg2cTR5eWsx

As mentioned, during the BTS 2026 a scientist from Germany (Thünen Institut, Bremerhaven) joined the Dutch survey, as part of international knowledge exchange, Alexandra Poell. She wrote a guest blog on her experiences on board Tridens. It is a selection of her full feedback, that is also highly valuable for us as WMR researchers.

Alexandra brought a doll that watched all activities during the survey. the doll's experiences can be followed on Instagram: https://www.instagram.com/p/Dc03ksBjBOA/?stkn=cTRzMTg2cTR5eWsx

Alexandra's experiences

For about a week, I had the opportunity to take part in the Dutch Beam Trawl Survey (BTS) aboard the Dutch research vessel Tridens. My time at sea was very exciting and, at the same time, gave me an interesting insight into how my Dutch colleagues work.

I found the collaboration on board particularly positive. The ship’s crew were extremely friendly and helpful, and provided us scientists with excellent support in our day-to-day work. Working with the Dutch scientists was also very pleasant and straightforward. Despite differences in language and working methods, communication went very smoothly. There were plenty of opportunities to exchange views on methods, experiences and workflows in both countries.

What I found particularly interesting was the direct comparison with the German BTS, in which I regularly take part. Although both surveys pursue the same overarching aim – to gather data on fish stocks and the composition of fish communities in the North Sea – there are nevertheless some differences in how they are carried out.

All in all, the week on the Tridens was a brilliant experience for me. I found the open and friendly collaboration with the crew and the Dutch scientists particularly positive. At the same time, the differences compared to the German BTS were especially interesting to me from a professional point of view.

In any case, I’m taking away many new insights from the week, as well as a few questions that I’m sure I’ll reflect on differently during future surveys. After all, it is precisely the comparison of different working methods that shows that scientific surveys are not just about collecting as much data as possible, but also about constantly considering how to obtain this data as efficiently as possible whilst minimizing disruption.

Pech // Bad luck

Sommige cliché’s zijn waar. ‘Het venijn zit in de staart’ bijvoorbeeld. Na zes zeer succesvolle surveyweken waarin alles mee zat, hadden we in de laatste week wat pech te verduren. Op maandag voelden we in één van de trekken na zes minuten vissen een grote ruk aan het vistuig; aan stuurboord waren de wekkerkettingen en het kuiltouw kapot. Aangezien er niets spectaculairs in het net zat moet er onder water iets groots, zwaars en scherps (bijvoorbeeld een stuk van een wrak of echt een enorme steen) hebben gelegen. Verrassend genoeg was het net nog heel. 

Some cliche’s are (or become) true. ‘There is a sting in the tail’, for example. After six successful survey weeks we had some issues to deal with this week. In one of Monday’s hauls we felt a tug on the net after six minutes fishing. When we hauled the net, the chains as well as the rope we use to haul in the codend of the net were broken. As there was nothing special in the net, something large, sharp and heavy (like wreck or a boulder) may have been on the seafloor. 

de wekkerkettingen van het tuig zijn kapot (je zou er vier moeten zien) // the chains of the gear are broken (there should be four)

een stuk van het kuiltouw met rafels aan één kant // piece of the codend rope with a cut off end 

Dinsdag verliep ook niet zoals we het van tevoren hadden bedacht: we moesten naar de haven van IJmuiden binnen om een bemanningslid af te zetten. Daarna gingen we weer verder met de bemonstering. Op woensdagochtend waaide het zo hard dat we eerst een paar uur moesten wachten totdat we konden vissen. Na vier minuten vissen was er een grote ruk en zaten beide netten in de bodem verankerd. Met beleid werden ze omhoog getakeld. De pijp van het bakboordtuig was krom, en beide netten waren dusdanig beschadigd. Verrassend genoeg waren alle wekkers nu wel nog heel. Reparatie van de netten op zee is met deze weersomstandigheden niet veilig, we een vrij plotseling einde van de survey hebben. 

Tuesday was also different than we originally planned: we went to the port of IJmuiden to drop off a crew member. After that we continued with the survey. On Wednesday morning we had to wait before we could go fishing due to the weather (too much wind and waves). When we did start fishing, after four minutes we had an enormous tug on the net  and noth nets appeared to be stuck in the bottom. Slowly we hauled them in. When they were above the water line, it was clear that this was the end of the survey: the pipe of the portside gear was curved while it should be straight, and both nets were severely damaged. Interestingly the chains were all still fine. As repairing the nets in this weather cannot safely be done, we will have to return to port, what means the end of the survey. 

kapot net aan stuurboord // damaged net on starboard

kromme pijp (aan de rechterkant op de foto) aan bakboord // bent pipe (on the right side in the picture) on portside

Gelukkig hebben we de meeste stations kunnen bemonsteren en zijn de gebieden die wij nu over moeten slaan al bemonsterd door onze Belgische collega’s.

Luckily we have been able to sample most of the planned stations. In the areas we now have to skip our Belgian colleagues have already fished this year.

Voortplanting // Reproduction

Veel soorten die in zee leven, produceren eieren die buiten het lichaam uitkomen. Bij veel vissoorten vindt de bevruchting plaats in het water en ontwikkelen de (heel kleine) eieren zich in de waterkolom; die eieren vinden we niet in deze survey. Andere soorten die in zee leven zetten hun eieren ergens aan vast, houden ze bij zich, of hebben zulke grote eikapsels dat we ze wel vangen. 

Many marine species produce eggs that hatch outside of the body. In many cases fish eggs are small and are fertilised in the water column; those small eggs we won’t catch with the beam trawl. Other marine species attach their eggs to something, carry their eggs with them, or produce larger egg cases that we may catch in this survey.

Augustinuskrab met eieren // Lithodes maja with eggs (credits: S. Verver)

In de afgelopen weken vingen we onder andere Augustinuskrab (Lithodes maja) met eieren, eieren van wulk (Buccinum undatum) en noordhoorn (Neptunea antiqua), van hondshaai (Scyliorhinus canicula) en lege eikapsels van koekoeksrog (Leucoraja naevus).

Over the past weeks we caught numerous eggs, in this blog a selection: Lithodes maja with eggs, common whelk (Buccinum undatum) and red whelk (Neptunea antiqua) eggs, eggs of lesser spotted dogfish, and empty egg cases of cuckoo ray (Leucoraja naevus).

Eieren van wulk (links) en noordhoorn (rechts) // Common (right) and red (left) whelk eggs (credits: M. Dammers)

Eikapsel hondshaai met embryo // Lesser spotted dogfish egg case with embryo (credits: M. Dammers)

Eikapsels koekoeksrog // Egg cases cuckoo ray (credits: S. Verver)