The coastal zone is no longer an ideal nursery for plaice

The temperature of the North Sea has risen by almost half a degree in the past 20 years – twice the global average, according to the European Environment Agency in 2022. This has various consequences for fish species. Theoretical ecologist Karen van de Wolfshaar of Wageningen Marine Research investigated the impact of the rising seawater temperature on the reproduction and growth of plaice, an economically important flatfish species. “At Wageningen Marine Research, we can draw on data from many years of surveys and extensive ecological knowledge”, she says. “Much of the research on fish is coordinated at European level. Since the Netherlands has the largest fisheries for flatfish, our country is coordinating the research for these species.”

Pressing questions

Researchers from Wageningen Marine Research made a number of striking discoveries. "Plaice have started spawning earlier and the the juveniles are also reaching the nurseries earlier, a month earlier than at the end of last century," Van de Wolfshaar says. In addition, she says, it seems plaice are spawning in other places. “Traditionally, the adults spawn along the British coast, in the Channel, after which the currents carry the fertilised eggs towards the Dutch coast. One of the pressing questions is: if the fish spawn earlier in the season or in other places, for example further north, will the eggs still end up in our shallow coastal zone, the nursery areas? Or will they end up in the deeper parts of the North Sea, or even in the deep troughs off the Norwegian coast, where they have no chance of survival?”

Plaice is a cold-water species, unlike its relative the sole, which prefers warmer water. This is important in light of climate change. “Our warmer coastal zone is gradually losing its function as a nursery for plaice. At first all the two-year-old fish disappeared to deeper, colder water, and more recently also the one-year-old fish. Maybe even the zero-year-olds may, in due time, head for deeper water earlier in the year – but we don’t know that yet.”

Impact on fisheries

It may be disadvantageous if young plaice spend less time in the shallow Dutch coastal waters, according to Van de Wolfshaar: "In deeper water there are more predators, thus there’s a higher mortality among young plaice."

There is also another problem: “The sex of the young fish may be determined in the nursery”, says the ecologist. “We know that this is at least partly temperature-driven. In other flatfish, research has shown that relatively more males develop in warmer water. Males are generally smaller as adults, so this change has direct implications for the fishing yield.”