Why fish need many and “messy” nurseries

“Unfortunately, it has often been the case that fish communities do not increase after the implementation of river restoration measures. Our results stress that river fish need interconnected floodplain habitats as nurseries; larval and juvenile fish in particular are dependent on different habitats as they grow, and these need to be accessible”, said lead researcher Twan Stoffers, who began the study at Wageningen University and completed it at IGB.

Restoration measures: mosaic of different habitats allows fish to thrive

In the lower river Rhine study, the researchers identified five main types of habitat for young river fish: (1) exposed, fast-flowing habitats with coarse substrate; (2) turbid, nonflowing areas with high turbidity and chlorophyll – indicating algae as a food ressource; (3) shallow, vegetated habitats with macrophytes and shoreline vegetation; (4) deeper, sheltered habitats with structural complexity; and (5) shallow, slow-flowing areas.

Habitat use shifted significantly with ontogeny across species. Larvae generally preferred shallow habitats (< 50-cm depth), either in slow-flowing areas (e.g., asp, ide, monkey goby, nase, and whitefin gudgeon) or vegetated zones with macrophytes (e.g., bleak, bitterling, bream, round goby, and zander). Juveniles increasingly used deeper habitats (> 50-cm depth), favoring fast-flowing areas (e.g., asp, barbel, ide), or deeper, nonflowing habitats (e.g., bream, zander).  Particularly relevant for restoration measures: More than 60 per cent of the fish species studied switched between all five habitat types during their development. Roach and zander, for example, begin their lives in shallow water areas rich in plants before migrating to deeper areas with ample of hiding spots. In constrast, endangered species such as nase and barbel rely heavily on specific nurseries - shallow, flowing waters with coarse substratum. “Restoration measures especially designed to increase fish diversity should therefore create a mosaic of habitats that are interconnected”, said Twan Stoffers.

River sections with interconnected floodplains are richer in species

The study also highlights the importance of the connection between floodplains and the river, as well as between different habitats. Floodplains that remain connected to the main channel throughout the year show higher biodiversity and fish diversity, are home to more fish and many different species. “However, many restored floodplains lose this connection during low water levels in Summer, which results in a bottleneck in the development of young fishes. Juvenile fish need access to the main river as soon as they are strong enough to migrate”, said Leo Nagelkerke, co-leader of the study from Wageningen University. The loss of this connection could largely negate the effects of restoration measures on fish communities.

Adaptation to climate change

The study also shows how important it is to consider restoration strategies and climate adaptation measures together. Rising temperatures and more frequent extreme fluctuations in water and discharge levels threaten the role of floodplains as spawning and nursery grounds for fish, but at the same time they also need deeper passages as a refuge when water becomes scarce. Near-natural, diverse floodplains that are connected to the river are therefore important precautionary measures for the animals and at the same time serve as natural water retention in the landscape – an added value for nature, climate and people alike.