Colossal Catfish!

In a French river, mysterious schools of big fish create record nutrient “hotspots”

To scientists snorkeling in France’s Rhone River, it was a remarkable sight: “colossal” aggregations of hefty, 25- to 140-pound catfish swirling around each other in a mesmerizing dance. But the big schools also have a potentially significant ecological impact, they note in a new study: as the megafish poop and pee, they create nutrient-rich “hotspots” that may be the largest ever recorded in freshwater.

It’s not clear why the fish – an invasive predator called the Wels catfish (Silurus glanis) – gather into groups, a French research team reports in PLoS ONE. But the aggregations sure are noticeable, because “this species is the world’s third largest and Europe’s largest freshwater fish,” they write. During 17 surveys completed between May 2009 and August 2011, divers saw groups of 15 to 44 adults – together weighing, on average, a total of 1,432 pounds (651 kilograms).

“Because of the very large size of these alien individuals (approximately five times heavier than native fish species), their aggregation can potentially lead to important functional consequences in recipient ecosystems,” the researchers note. Defecation and excretion by the fish, for example, can pump nitrogen, phosphorous and other nutrients into the water. And, “in some cases, fish can translocate nutrients within the ecosystem by feeding in one location while defecating in another.”

To figure out just how much fertilizing the fish were doing, the team used a model common in fisheries biology to estimate nitrogen and phosphorous production. For phosphorous, the results were 83 to 286 times greater than anything previously reported, and the nitrogen totals were 17 to 56 times greater. The figures “potentially represent the highest biogeochemical hotspots ever reported for freshwater ecosystems,” they note.

The “phenomenon represents another example of unexpected potential ecological impacts of alien species,” the researchers conclude. And it is a reminder, they add, of “the ubiquity and fascinating nature of animal aggregations.” – David Malakoff | October 11, 2011

Source: Bouletreau S, Cucherousset J, Ville´ger S, Masson R, Santoul F (2011). Colossal Aggregations of Giant Alien Freshwater Fish as a Potential Biogeochemical Hotspot. PLoS ONE 6(10): e25732. doi:10.1371/journal.pone.0025732

Image: © 2011 Bouletreau et al.