Colossal Aggregations of Giant Alien Freshwater Fish as a Potential Biogeochemical Hotspot

Abstract

The ubiquity and fascinating nature of animal aggregations are widely recognised. We report here consistent and previously undocumented occurences of aggregations of a giant alien freshwater fish, the Wels catfish (Silurus glanis). Aggregative groups were on average composed of 25 (±10 SD, ranging from 15 to 44) adults with estimated average total biomass of 651 kg (386 – 1132) and biomass density of 23 kg m−2 (14 – 40). Aggregations always occurred within the same location. No foraging, reproductive or anti-predator behaviour were observed during the aggregations. A mass-balance model estimated that these colossal aggregations of an alien species can locally release, through excretion only, up to 70 mg P m−2 h−1 and 400 mg N m−2 h−1, potentially representing the highest biogeochemical hotspots reported in freshwater ecosystems and another unexpected ecological effect of alien species.

“Freshwater Killer Whales”: Beaching Behavior of an Alien Fish to Hunt Land Birds

“…In conclusion, these findings suggest that this new predation behavior might represent an extreme example of the ability of introduced species to adapt to a new environment that could have unexpected implications for consumer-resources dynamics and ecosystem functioning [27], [28] that deserve further investigations.…”

Move or die: change in European catfish (Silurus glanisL.) behaviour caused by oxygen deficiency

“…This behaviour was forced by DO conditions in the beginning, but the fish stayed aggregated even after the DO deficiency disappeared and DO rose up to 4.0 mg·L −1 . A large aggregation of fish brings many additional aspects, including the formation of biogeochemical hotspots (places where nutrient release by animals exceeds the need of primary producers) (Boulêtreau et al, 2011) or facilitation of spread of disease (Arneberg et al, 1998;Ogut et al, 2005). We hypothesise that the reason why the fish stayed aggregated and why they did not move back to their previous HR is connected to low temperature, which could inhibit the activity of fish.…”

“…Nevertheless, some information regarding the aggregation of adult European catfish in a very restricted area does exist. Boulêtreau et al (2011) observed such a phenomenon in the Rhone River, but the cause of their behaviour was unclear, and no juveniles participated. Recently, some temporary catfish aggregations connected to their predatory behaviour have also been observed (Cucherousset et al, 2012).…”

Non-native fish in aquaculture and sport fishing in Brazil: economic benefits versus risks to fish diversity in the upper River Paraná Basin

“…Similarly, the creation and enhancement of sport fishing using non-native fishes is a well-established management technique (Hickley and Chare 2004). In France and Spain, introductions of European catfish Silurus glanis (Siluridae) now support fisheries attracting anglers from across Europe, with colossal aggregations of large individuals ([1 m in length) in some rivers (Bouletreau et al 2011). In Florida, USA, Cichla species have also been introduced for sport fishing purposes (Shafland 1995).…”