Colossal swarm of giant alien catfish

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An enormous aggregation of Wels catfish (Silurus glanis) in the Rhône River in France may represent one of the the highest biogeochemical hotspots reported in freshwater ecosystems.

This is according to a study by French scientists published in a recent issue of the online journal PLoS ONE.  

Stéphanie Boulêtreau and colleagues report the consistent and previously undocumented occurrences of a huge swarm of Wels catfish in the Rhône River downstream of Lyon. The fish were visually surveyed by snorkelers on 17 occasions from May 2009 to August 2011. The aggregations consisted of 15 to 44 adult Wels catfish, estimated to have a biomass of up to 1132 kg and a biomass density of 14–40 kilograms per square metre.

The catfish in the aggregations did not behave like fish in a traditional school, as they faced different directions and sometimes came into physical contact with their neighbours (see video below).

The authors were not able to determine the reason for this behaviour, although they ruled out reproduction, feeding, and protection from predators.

The Wels catfish is not native to the Rhône River and because of their very large size (about five times larger than native fish species), there is a very high potential for the production of biogeochemical hotspots, ie. places where nutrient release by animals through excretion exceeds the need of primary producers.

Based on their calculations, the catfish aggregations release 83–286 times the highest amount of phosphorus and 17–56 times the corresponding value for nitrogen excreted by fishes reported in the literature. These aggregations can thus have a potentially strong effect on ecosystem functioning since these fish may move nutrients from their feeding areas, concentrate them locally in the aggregation area and subsequently affect primary production and nutrient cycling.

Therefore, this phenomenon represents another example of the unexpected potential ecological impacts of alien species.

For more information, see the paper: Boulêtreau S, J Cucherousset, S Villéger, R Masson and F Santoul (2011) Colossal aggregations of giant alien freshwater fish as a potential biogeochemical hotspot. PLoS ONE 6, e25732.

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