Press release Sole getting smaller due to ocean warming
Sole become smaller as the oceans warm. They also grow faster and spawn at a younger age. This is shown by calculations and a large-scale analysis of the otoliths of sole (Solea solea) from the North Sea, the Irish Sea and the Bay of Biscay, carried out as part of doctoral research performed at ILVO, Ghent University and Wageningen University. The research confirms that sole follow the temperature-size rule (TSR) for cold-blooded animals, and that smaller fish, like shifts in habitat and adaptations in the life cycle, are a consequence of climate warming. Sole is the most valuable species for the Belgian fleet, and further warming of the seas may put fisheries yields under pressure.
“We urgently need a better understanding of this phenomenon, because climate change is starting to affect the Belgian fleet directly. These new insights can also improve scientific advice for commercial fisheries.”
At higher ambient temperatures, cold-blooded animals accelerate their growth and sexual maturation while reaching a smaller size. This is known as the temperature-size rule (TSR). The exact biological mechanisms behind it are not yet known, but it was suspected that this rule also applies to sole. The doctoral research of Tuan Anh Bui has now confirmed this. He studied the individual growth patterns of sole populations over a period of more than 60 years (1958–2019) in the North Sea, the Irish Sea and the Bay of Biscay.
These areas were chosen for their extensive long-term data, including sea temperature and sole otoliths. Otoliths are small ear stones in the inner ear of fish that form new growth layers each year like a tree trunk. By measuring the distance between the annual rings — an indicator of fish size at different ages — scientists can determine the fishes’ annual growth. Bui collected no fewer than 2,154 sole otoliths from the three areas, carefully preserved in the archives of ILVO (Belgium), WUR (the Netherlands), and IFREMER (France).
Young sole now grow faster than older sole
The otolith analyses show that sole growth did indeed change between 1970 and 2020, in all three seas studied. As expected under the TSR, this change depended on age: young sole now grow more quickly than those born even a few years earlier. The greatest change occurred among young sole: one-year-old sole reach 14% larger size when sea temperature rises by 1 °C. A sole measuring 8 cm at one year of age at 10 °C measures just over 9 cm at 11 °C. A five-year-old sole, by contrast, grows 5% more slowly.
Smaller sole that reach sexual maturity sooner
Otoliths reveal a great deal, but not everything. For example, they do not show how large a fish would ultimately have become, or at what age it would first have spawned. To investigate the temperature effect on these aspects, Bui used a mathematical model. This model indicated a scientifically significant effect only in the North Sea, where the rise in seawater temperature was greatest (+0.9°C from 1970 to 2020), not in the Irish Sea and Bay of Biscay (+0.5°C and +0.4°C, respectively).
Among the generations from 1975 to 2012, Bui observed that the simulated maximum size decreased and the simulated spawning age became younger. For every 1 °C increase in seawater temperature, the largest sole were on average 5.5% smaller and spawned 5.5% earlier. For example, when comparing the 1980 and 2000 generations of sole, the average sea temperature during their lifetime rose from 9.1 °C to 10.2 °C, while according to the model the sole remained on average 40 mm smaller and spawned three months earlier.
Fisheries data also confirm smaller sole
The trend towards smaller sole is also confirmed by fisheries data from ICES (International Council for the Exploration of the Sea). In all age classes, the weight of sole has been declining since the 1980s, with older fish showing the strongest decrease. This is very clear for the North Sea, the eastern English Channel and the Celtic Sea, and somewhat less pronounced for the Irish Sea and the western English Channel.
The effects of climate may possibly have been reinforced by historically high fishing pressure. By removing primarily the fastest-growing and largest individuals over decades, fisheries may also have contributed to selection towards smaller sole.
Potentially negative impact on Belgian sole fisheries
With a landings value of €35.7 million in 2025, an increase of 5.6% compared with 2024, sole is and remains the most important and most valuable species for the Belgian fleet. In terms of landing volume, sole also increased by 7% compared with 2024: at 2,000 tons, it now ranks second after squid (3,553 tons) and ahead of plaice (1,564 tons). The share of sole in total landings value remained stable at 40%.
Jochen Depestele, ILVO researcher: "This doctoral research shows that sole adapt very rapidly to a warmer sea: they grow faster, reach sexual maturity earlier, and remain smaller. In concrete terms, this means that fishermen now have to catch more sole to achieve the same yield, but also that fewer large mother fish are present to produce strong new generations. These insights can help align fisheries advice more closely with the changing climate.”
The doctoral research by Tuan Anh Bui, entitled “Warm and Wanted: Effects of Climate Change and Fisheries on Fish Growth”, was funded by the Research Foundation – Flanders (FWO). The supervisors are Prof. Dr. Marleen De Troch (Ghent University, Marine Biology research group), Dr. Ir. Jochen Depestele (ILVO) and Prof. Dr. Jan Jaap Poos (Wageningen University & Research).
