The successful re-introduction of North Sea houting to the River Rhine System in Europe

Summary

North Sea houting (Coregonus oxyrinchus), an anadromous salmonid considered extinct in the River Rhine since the 1940s, was targeted for reintroduction by the University of Cologne starting in 1996 using juveniles sourced from a German hatchery descended from the last known reproducing wild population in Denmark's River Vida. A 2000 feasibility study identified two key life-cycle bottlenecks for the anthropogenically altered Rhine: whether juveniles could cope with increased river current from canalization, and whether fish could pass dams blocking natural sea migration routes. From 2000 to 2006, an average of 280,000 juveniles were stocked annually at two Lower Rhine sites, and catches in the Netherlands' Lake IJsselmeer rose steadily from 1996 through 2007, with the first Lower Rhine catch recorded in 2005. Chemical scale analysis of fish caught in Lake IJsselmeer revealed that houting sometimes pass the Wadden Sea-IJsselmeer migration barrier and, notably, do not need to migrate to sea to reach maturity, with 70% of the IJsselmeer population never having been to sea at all. In 2006, only 3.6% of juveniles sampled bore an artificial marking dye applied to that year's hatchery stock, indicating the large majority of juveniles present originated from natural reproduction, and in March 2010 roughly 200 freshly hatched wild larvae were captured by drift net in the River IJssel, confirming natural spawning in the Rhine delta. The main remaining obstacle is large flood-control dams that still impede movement between freshwater and sea. The project rates itself Highly Successful, citing the rapid establishment of a self-sustaining, adapting population.