Saturday 20 November 2010


Subadult Cormorant (Phalacrocorax carbo) catching a Black Bullhead Catfish (Ameiurus melas). Galisteo, Cáceres. 02-11-2010 (J. Prieta).

The above sequence of photos shows a Black Bullhead Catfish (Ameiurus melas) being caught and eaten by a Cormorant (Phalacrocorax carbo). The photos would obviously win no prizes; neither is the observation anything out of the ordinary, since Cormorants are skilful fishers. The surprising factor is the place, a recently opened and shallow gravel pit where you wouldn't expect to find fish of this size yet. And the worrying aspect is that this gravel pit should already be occupied by an invasive species that is quickly spreading in native waters. In every single visit to this gravel pit we have seen successful Cormorant captures of Black Bullhead Catfish, which is also a prey species of Little Grebe and several heron species (Grey Heron, Little Egret, Great White Egret).

The Black Bullhead Catfish, native to North America belongs to the siluriformes order, which does not exist naturally on the Iberian Peninsula. Its biggest Extremadura populations are in the catchment areas of the rivers Tiétar and Alagón. As in so many other cases it has been introduced deliberately and also as the accidental result of escaped livebait. There are also records of another introduced North American catfish in Extremadura, the Channel Catfish (Ictalurus punctatus) dating from the 1990s in the Badajoz reservoirs of Zújar and La Serena. Luckily there have been no more recent records. Lastly a third siluriforme, the European Catfish, also called Wels Catfish or Sheatfish (Silurus glanis), has also found its way into some Extremadura reservoirs. In Cedillo there was a one-off record from the 1990s and another was caught in Alcántara reservoir in 2008, this time within the Monfragüe National Park. In this case we are not talking about any old fish but a species that can grow 2.5 metres long and weigh over 100 kg! Native to the large rivers of Central Europe, it is a rapacious predator that has become highly prized by some fishermen, who insist on introducing it illegally wherever they can. Something akin to releasing lions or tigers in our dehesas.

The boom in introduced species contrasts sharply with the lamentable situation of our native species. Largely overlooked, these small denizens of our rivers often turn out to be surprisingly diverse. The latest studies continue to differentiate new species with tiny ranges; endemics have been discovered for Salamanca, Guadalajara and Málaga. In Extremadura the most similar case is the ray-finned species Cobitis vettonica, called Vettonian Spined-loach (Cobitis vettonica) in Spanish. It is exclusive to the catchment area of the river Alagón from which it takes its name, straddling the provinces of Salamanca and Cáceres, with its biggest populations in the rivers Alagón, Jerte and Ambroz.

References: - Pérez-Bote, J. L. 2006. Peces introducidos en Extremadura. Análisis histórico y tendencias de futuro. Revista de Estudios Extremeños 1:485-494 [PDF] - Pérez-Bote, J. L. & Roso, R. 2009. First record of the European catfish Silurus glanis Linnaeus, 1758 (Siluriformes, Siluridae) in the Alcántara reservoir (Tagus basin, Spain). Anales de Biología, 31:59-60. [PDF] - Leunda, P. M. et al. 2009. International Standardization of Common Names for Iberian Endemic Freshwater Fishes. Limnetica, 28:189-202. [PDF]