Decades-old fish stomach contents reveal: who eats whom in the Wadden Sea?

It is actually quite bizarre. Since 1960, NIOZ researchers have been sailing to the Marsdiep between Texel and Den Helder every day in the spring and autumn to empty the same fish trap day after day. The institute has been conducting such research since 1946, albeit in slightly less detail. The size and weight of all fish caught were recorded and the contents of their stomachs were examined. This provided a wealth of data. And yet it is only now that someone has mapped the complete fish food web of the Wadden Sea.

That person is Suzanne Poiesz, who knew she wanted to be a marine biologist from the age of four. “Many researchers looked at individual species, for example how plaice, which was becoming increasingly scarce, relates to other species. For the first time, I have mapped out who eats whom on a broad scale.”

Stage in the food pyramid

She classified species according to their “trophic value”, in fact their position in the food pyramid. These values ranged from 2 for mainly herbivores to 5 for top predators, such as adult spiny dogfish, which eat all other fish and are not eaten by fish themselves. In addition to the fish, the prey in their stomachs were also given such a value. “Suppose a sea bass has eaten a herring, a shrimp and a crab, then this individual is given a trophic position of 3.8.”

Over the years, Poiesz averaged the trophic value of individuals of eighteen species for which a lot of data was available. Conclusion: no major changes in the trophic value of a specific species. If the garfish used to be a high predator, it remained so. Even “lower” trophic species, such as plaice and flounder, remained on average the same trophic value.

Most striking: the decline in catch

The most striking change between the past and the present is the decline in catches in fish traps. “In photographs from the 1980s, you can see how the researchers emptied overflowing fish traps; the boat was completely full. Now they only catch 10 per cent of those quantities of fish,” says Poiesz. The largest decline took place between 1980 and 2010.

Human activities and global warming

Poiesz suspects that human activities are a major cause. Fishing, ecotourism, and possible pollution have all increased. “As a result, there is almost no seagrass left, which used to provide a lot of shelter.” In addition, the increasing average temperature in the Wadden Sea is also likely to play a role. “Since 2000, the temperature has risen by half a degree Celsius almost every year, and there are more and more heat waves during which the water can reach 25 degrees. Two years ago, the water reached 27 degrees and there was a massive cockle die-off.” Every animal species has an optimum temperature; that of sea bass, for example, is higher than that of plaice. “Large specimens of plaice in particular are no longer seen, while southern species such as sea bass are becoming more common.”

Digging into stomach contents remains necessary

The PhD student also tried out a method that could potentially eliminate the need for time-consuming and therefore expensive analysis of stomach contents. She investigated stable isotopes. Elements such as carbon and nitrogen exist in different variants, with one or more extra neutrons in the atomic nucleus. “Some variants are more stable than others. Stable isotopes accumulate in predators.” Poiesz determined the ratio between the isotopes in the fresh fish from the fish trap. "And indeed: the higher the trophic value, the greater the proportion of stable isotopes." Mission accomplished? "In a sense, yes, but stomach contents tell us more than just a hard value. They allow us to accurately determine the type of prey."

Sheltered nursery

The Wadden Sea is known as a nursery: you will find many young, small fish there. Poiesz: “It has always been a productive and sheltered system, where you will find many young herring and flatfish such as plaice, flounder and sole, for example.” Because the tide empties and refills this sea every day, the many fertilizers that flowed into the water in the 1970s had relatively little effect. “There are almost no dead zones, as in the North Sea.”

There used to be sharks and rays

In the very early data, the researcher came across information about sharks and rays, species that are now hardly found in the area. Now, sea bass, which can tolerate higher temperatures, is one of the top predators, alongside garfish and mackerel, for example. Is it a bad thing that there are now fewer fish and other fish species in the Wadden Sea? "Ecosystems are always changing, so change is not a bad thing. Rapid change is, however, as it makes the ecosystem vulnerable. If the cockles suddenly disappear, this could have a major impact on migratory birds, for example."

Text and images: Royal Netherlands Institute for Sea Research (NIOZ)