Wetlands are important ecosystems, harbouring biodiversity, capturing

nutrients, and providing recruitment habitats for several fish species. The Baltic Sea, among the

largest brackish, semi-enclosed seas in the world, is exposed to large amounts of nutrients as well as

the overexploitation of several fish species. One measure to counteract eutrophication is to restore or

build wetlands for nutrient catchment (phosphorus) or removal (nitrogen). To function optimally for

nutrient removal, these wetlands should cover large areas, be shallow, and be covered by vegetation,

creating zones of high microbial denitrification. Wetland environments are also suitable spawning and

nursery areas for several fish species inhabiting the coastal zone of the Baltic Sea. Pike (Esox lucius) a

predatory species that has decreased in abundance in recent decades, use wetlands as recruitment areas

during their anadromous life cycle. After restoring wetlands near the coast and opening the waterways

towards the sea, pike larval emigration increased from a few thousand to over a hundred thousand

individuals. The habitat and food choice, growth and migration of larvae and juveniles were followed

over time, revealing that 80–95% left the wetlands within one month (at a size <6 cm). An optimal

wetland enriches nutrients and creates a high primary production base for zooplankton that are used

as food for pike larvae. Wetlands near the sea can function both as nutrient traps and as recruitment

areas for fish.