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Since the late Stone Age, farmers have brought their livestock to the Wadden Sea, in northern Europe, to graze on the vegetated salt marshes. This intensive and prolonged grazing has transformed the salt marshes, affecting everything from plant diversity to invertebrate populations to bird breeding habitats. This is nothing new: the environmental effects of salt marsh grazing have been well studied. But researchers have now discovered an overlooked consequence of this degradation in the intertidal Wadden Sea: its effects on fish.

A team led by University of Hamburg ecologist Julia Friese, who has spent the past four years studying the salt marsh ecosystem at this UNESCO World Heritage Site, discovered that grazing by sheep reduces the availability of food for fish.

In the Wadden Sea along the Dutch, German, and Danish coastlines, some fish species use the intertidal zone as a nursery, some are full-time residents, and others pass through as they migrate between marine and freshwater habitats. During the daily high tides, fish move into the creeks that run through the salt marshes to find shelter and feed. In winter, storms drive larger tides inland, flooding the elevated marshlands and giving fish access to otherwise unreachable vegetated terrain.

Friese discovered that when they find themselves in the flooded salt marshes, some fish species, particularly the three-spined sticklebacks, stop eating their primary prey—tiny marine bristle worms—and gorge themselves on a species of terrestrial amphipod that gets picked up by the floodwater. The terrestrial amphipods are high in energy and are an important source of food for the fish. But as Friese and her colleagues found, the fish’s ability to catch the amphipods depends on a range of factors, including changes to the salt marshes caused by sheep grazing.

To demonstrate the relationship between sheep grazing and prey availability for fish, Friese and her team analyzed four creeks in northern Germany that had been affected by varying degrees of grazing.

For a few long days and nights each month for a year, the team studied changes in the number and types of fish and prey species in water near the flooded banks of the salt marsh creeks, along with creek water volume. In the lab, they dissected their catch to identify what the fish were eating and in what amounts.

Analyses of the fish’s stomach contents showed that the three-spined sticklebacks ate the most amphipods compared to other fish species, particularly in creeks where there is no grazing on the surrounding salt marshes. Friese says this finding aligns with what researchers know about the effects grazing has on amphipod abundance. The terrestrial amphipods feed on organic detritus and prefer to live in vegetated areas with large amounts of it. But the organic material the amphipods need to thrive only builds up if hungry sheep are kept at bay.

Ingrid Tulp, an ecologist at Wageningen University and Research in the Netherlands who was not involved in the research, says the study adds another layer to an already complicated conservation challenge.

“Managers want salt marshes to be good bird breeding habitats, have nice plant diversity, food for fish, et cetera,” she says. But after centuries of change, some species have adapted to the grazed marshland, and now those goals don’t necessarily align. “Sheep sometimes step on eggs, and now it looks like they impact fish, too. But grazed areas are also important for some bird species and can assist plant diversity. So, there are choices to make.”