.TRUST – a result of the Fisherman-Scientist Network, has been working on establishing a sustainable fishery for sandeels in Denmark Photo: Kasper Schaltz
During the spring of 2023, DTU Aqua marked 25,000 pieces of the small fish, sandeel.
The marking, recapture, and stock assessment are carried out in the EMFF project TRUST – a result of the Fisherman-Scientist Network launched in 2016. This year, both TRUST and the Fisherman-Scientist collaboration for sustainable fisheries will conclude.
There was a time when fishermen and scientists were not on the same page, so to speak. For example, back in 2010, researchers at ICES had to inform fishermen that the sandeel stocks needed to be subdivided with lines that better reflected the population structure and biology of sandeel.
The stock assessments for sandeel in the North Sea, obtained by researchers until then, with the division into a northern and a southern stock, were too uncertain.
“Through marking and recapture, we can now see that larger fish tend to move north along the coast of Jutland. They also move north on the Dogger Bank,” explains Henrik Mosegaard, Research Coordinator, DTU Aqua
However, fishermen did not like the method from the beginning – they wanted to be able to fish freely in the North Sea. They did not think it was fair to be constrained by lines. Since then, there has been intense debate between fishermen and scientists about the areas for which ICES advises on quotas every year.
But since then, both fishermen and scientists also realised that they were puzzled by the same drastic decline in pelagic fish production in the North Sea, including the decline in the small shoal fish, sandeel, that occurred in the 2000s. Whether the sandeel was divided into sub-stocks or not, the goal was to identify the best management that ensured a continued economically and ecologically sustainable sandeel fishery.
The researchers and fishermen promised each other that they would investigate the distribution and migration of sandeel in the North Sea using methods that both parties understood and believed in. Since that day, fishermen and scientists began to get on the same page.
Namely, in the collaboration that has become the Fisherman-Scientist Network. Here, researchers, among other things, go out with fishermen’s vessels and from there, they mark fish to investigate what happens to the stocks.
One such project is the large EHFF project TRUST. Here, researchers mark and recapture sandeel. From 2020 until this spring, DTU Aqua has been involved in marking over 25,000 sandeel. Recaptures are continuously registered in the scanners set up at fishmeal factories, where the data is collected.
TRUST – a fisherman-scientist collaboration
TRUST stands for Tobisens Rumlige (bestands)Struktur i Nordsøen (Sandeel Spatial (Stock) Structure in the North Sea) – and the word ‘trust’ means trust in English, which is worth noting in relation to the collaboration between fishermen and scientists that has now demonstrated its strength over several years.
“The sandeel surveys are very well organized,” says Henrik Mosegaard, Research Coordinator and one of the coordinators of the Fisherman-Scientist Network from the beginning.
Henrik Mosegaard further explains that the marking of sandeel is done in a way that a fishing boat catches live sandeel either with a small scraper or in a short trawl.
The sandeel are brought on board the fishing vessel, and the researchers stand in the hold and mark them. The fish are marked with a syringe that injects a small glass tube with a tiny radio transmitter into the abdominal cavity of the sandeel. The registration occurs when the marked sandeel passes a scanner. It works roughly the same way as an anti-theft alarm in a store.
The fish are collected in containers with seawater, and when the researchers have marked a batch of between 100 and 1,500 sandeel, they are released back into the sea over the side of the ship using a small net.
New benchmark in 2023
In the TRUST project, researchers investigate the migration of sandeel in the North Sea – do they move between the sandeel areas established by ICES in 2010 and later modified in 2016? Is there any reason to maintain these lines? These are questions that fishermen naturally want answers to.
The last time a sandeel benchmark was conducted by ICES was in 2016. Researchers particularly focused on the movement of sandeel larvae – where they come from and where they end up. The results formed the basis for how the current boundaries have been drawn.
From autumn 2022 and in 2023, ICES is conducting a new benchmark for sandeel, and for that benchmark, researchers wanted results from the latest marking experiment of the 25,000 sandeel.
The central North Sea with the fout most important bivalve areas marked with red lines. The yellow fields are sandeel banks with names in black. The arrows mark possible important migration routes identified through tag-recapture.
The project involves both physically marking the sandeel and recording their recapture, as well as studying the natural markers that the fish possess in the form of genetic markers and otoliths embedded in the sandeel’s ear.
When an otolith is formed, it collects chemical signals from the sea and the seabed where the sandeel buries itself. These signals tell us where sandeel have been at a given time, i.e., how much they move around from being larvae to being caught as adult sandeel.
They don’t swim very far
When the sandeel larva becomes a small fish and begins to grow, the results from the marking so far show that they swim around but not particularly far:
“We have no indications that they have swum very far, i.e., over 250 km. But with the help of marking and recapture, we can now also see that larger fish tend to move north along the coast of Jutland. They also move north on the Dogger Bank,” says Henrik Mosegaard.
When asked about what the current results from TRUST indicate regarding the need to change ICES’ current lines concerning fishing, Henrik Mosegaard says:
“Based on the marking results, there is no good reason to lift the lines since there are no indications that adult sandeel move so much between the areas. Let’s say we had seen that they constantly swim from one area to another – then there would have been a reason to remove a boundary. But that is not the case.”
Today, there are four main areas for sandeel, as outlined by ICES. The sandeel areas are not defined ICES areas, but the boundaries follow ICES statistical rectangles, as shown in the illustration of the areas above.
The TRUST project has been initiated by the industry and will conclude in October 2023.