Killer Catfish

Wels Catfish (Silurus glanis) is an introduced species, found in many areas of Europe including southwestern France, Italy and Spain. It is the largest freshwater fish in Europe, and the third largest fish in the world (Copp et al. 2009). Preying usually on small crustaceans, other fish, the occasional human remains and even cannibalism, the opportunistic catfish have found a new and more unusual idea of a meal: pigeons.

The catfish have evolved to hunt in low visibility waters, which has significantly reduced the size of their eyes and their eyesight capabilities. Instead, they have taste organs on their barbels, lips, fins and skin of their head and body. In addition to this, their barbels have an electroreceptive system to pick up movement in the water, and highly sensitive hearing, which is specialized for the water. Their hearing organ is attached to their swim bladder, which amplifies any sounds in the water (Copp et al. 2009).

Image: Brian Coad - www.briancoad.com

In an area of the Tarn River, a study was conducted on the behavior and hunting of Wels Catfish. This particular area is protected, and angling is prohibited. A gravel island was observed, a popular place where pigeons land to clean themselves and drink from the river. The catfish were observed waiting by the edge of the water, supposedly attracted by the sound of movement in the water produced by the birds. 

Similar to marine mammals such as dolphins and killer whales, the catfish were seen beaching themselves and successfully capturing pigeons from the waters edge.

This is considered highly unusual, even for an opportunistic animal such as the catfish. The strategy involves the hunting of prey that is outside the predator’s ecosystem boundaries (Cucherousset et. al, 2012).

This risky behavior may be associated with a lack of prey abundance, as a study conducted in 2005 found that prey abundance is influenced by water temperature. Stomach contents of Wels Catfish were analyzed, and it was found that areas with higher water temperatures had catfish with fuller stomachs (Wysujack & Mehner, 2005). This may suggest a more desperate hunting tactic in areas with less prey availability, or simply the surprising ability of the catfish to adapt to its new surroundings and a wider range of prey. 

See the video below, thanks to Animal Wire!

References

Copp, G. H., Robert Britton, J., Cucherousset, J., García-Berthou, E., Kirk, R., Peeler, E. and Stakėnas, S. (2009), Voracious invader or benign feline? A review of the environmental biology of European catfish Silurus glanis in its native and introduced ranges. Fish and Fisheries, 10: 252–282. doi: 10.1111/j.1467-2979.2008.00321.x

Julien Cucherousset, Stéphanie Boulêtreau, Frédéric Azémar, Arthur Compin, Mathieu Guillaume, Frédéric Santoul (2012) “Freshwater Killer Whales”: Beaching Behavior of an Alien Fish to Hunt Land Birds. PLoS ONE 7(12): e50840. doi:10.1371/journal.pone.0050840

Wysujack, K., Mehner, T. (2005) Can feeding of European catfish prevent cyprinids from reaching a size refuge? Ecology of Freshwater Fish. 14; 87 - 95

Diverse Bottlenose Dolphins

Tursiops truncatus, the Bottlenose Dolphin, is found worldwide in temperate and tropical areas, ranging from inshore to oceanic zones. They have been known to participate in many types of group hunting, and prey on mostly squid and fish (Hammond et. al, 2008). Two particularly interesting hunting techniques have been documented, one involving role specialization in dolphins within their social groups, and the other involving cooperation with humans for mutual benefits.

Image: Photogapher "FOTOLEN" sourced from Flickr.com [Accessed 12 April]

Role specialization is unusual in the animal kingdom, and even considered rare. It involves individuals within a group dividing up roles in the hunting technique, and specializing in that role whenever the hunt is initiated. It was suggested that individual foraging specialization techniques are more common in marine mammals than terrestrial mammals (Gazda 2005). 

Dolphins observed off Cedar Key in Florida were seen to have a specific ‘driver’ in each group. The driver herded the fish in tight circles, which acted as a barrier for the fish. Other dolphins in the group (named the ‘barrier’ or ‘non-driver’ dolphins in the study) would lift their heads out of the water and wait for the confused fish to leap out of the water, and straight into their mouths. Two separate groups in Cedar key were seen to have specialised roles of ‘driver’ dolphins, demonstrating this rare example of role specialisation (Gazda 2005). Other dolphins have also been observed using this hunting technique, however with the additional use of stirring up "mud rings" with their tails, that act as barriers to channel fish towards the rest of dolphins group waiting. (See video at the end of the post!)

Image: Photographer Todd Pusser [Accessed 12 April 2014]

In Laguna, Brazil, local fishermen catch mullet on a small scale fishing level, providing mostly for self-consumption, but sometimes selling to the nearby markets. One of the main fish that are sought is mullet, which are fished all year around but more intensively during their migration months, which are April to June. This time of year is strongly anticipated by the locals, who cooperatively fish with groups of resident bottlenose dolphins. The residents are a population of approximately 51 individuals, around 20 of which are involved in the fishing activities with humans. Knowledge of this technique is passed down from the elders to the young in the humans, and the female dolphins are thought to pass it to their calves (Peterson 2008).

Image: Photographer "Alberto" sourced from Flickr.com [Accessed 12 April]

The dolphins herd schools of fish towards the fishermen’s boats and nets, and signal to the men by doing a distinctive dive, letting them know when to throw their nets. The dolphins then collect any stray fish, to eat for themselves. This unusual friendship is strongly valued by the fishermen, who have affectionately named each dolphin, and are able to recognize them individually. A video filmed by the BBC shows the bond between the men and the dolphins, and provides an insight into a truly unique method of hunting.

Mud Net Fishing in Florida

Dolphin Assisted Fishing in Laguna, Brazil

References

Gazda, S. K., Connor, R.C., Edgar, R.K., Cox, F. (2005). "A division of labour with role specialization in group-hunting bottlenose dolphins (Tursiops truncatus) off Cedar Key, Florida." Proceedings of the Royal Society B: Biological Sciences 272(1559): 135-140. Available at: http://www.ncbi.nlm.nih.gov/pmc/articles/PMC1634948/?tool=pmcentrez&report=abstract 

[Accessed 12 April 2014]

Hammond, P.S., Bearzi, G., Bjørge, A., Forney, K.A., Karkzmarski, L., Kasuya, T., Perrin, W.F., Scott, M.D., Wang, J.Y. , Wells, R.S. & Wilson, B. 2012. Tursiops truncatus. In: IUCN 2013. IUCN Red List of Threatened Species. Version 2013.2. <www.iucnredlist.org>. Downloaded on 12 April 2014.

           

Peterson, D., Hanazaki, N., Simões-Lopes P.C. (2008). "Natural resource appropriation in cooperative artisana between fishermen and dolphins (Tursiops truncatus) in Laguna, Brazil." Ocean & Coastal Management 51(6): 469-475. Available at: http://dx.doi.org.elibrary.jcu.edu.au/10.1016/j.ocecoaman.2008.04.003 [Accessed 12 April 2014]