Sunday, 1 June 2014

African Tigerfish observed snatching Barn Swallows from the air.

A variety of Fish are known to occasionally take Birds as prey. Sharks and other large carnivorous marine Fish will frequently feed on Seabirds on the water surface, and in freshwater aquatic environments Bass, Eels, Piranhas, Pike and Catfish have all been observed to take avian prey, either from the water surface, from land close to the water, or from overhanging vegetation. Uniquely, the African Tigerfish (Hydrocynus spp.) has a reputation for taking Birds on the wing; this has been reported by numerous authors from 1945 onwards, but these reports have all been based upon anecdotal evidence, never witnessed by the authors themselves as part of an organized study.

In a paper published in the Journal of Fish Biology on 19 December 2013, Gordon O’Brien, Francois Jacobs, S Evans and Nico Smit of the Water Research Group (Ecology) at the Unit of Environmental Sciences and Management at North-West University report direct observation of feeding on Birds in flight by a population of the African Tigerfish (Hydrocynus vittaltus) at Schroda Dam, an artificial lake in the Mapungubwe National Park in Limpopo Province, South Africa.

The Shroda Dam was constructed in 1993 as a storage lake, and covers 4 100 000 m³. The African Tigerfish (Hydrocynus vittaltus) was deliberately introduced to the lake in 2003. The lake is considered to be nutrient poor compared to typical Hydrocynus vittaltus environments, and the Fish spend a higher proportion of their time foraging than has been reported in other populations. However they typically avoid open waters during the hours of daylight, probably due to the presence of the African Fish Eagle (Haliaeetus vocifer).

In February 2011 a number of Fish that had been radio-tagged as part of a study into their behaviour were observed to be breaking this taboo, and becoming highly active in open waters during the mid-morning. Direct observation of the lake revealed that the fish were striking at Barn Swallows (Hirundo rustica), a migratory Bird which visits the lake seasonally, both drinking on the wing and hunting small Insects close to the water surface. None of the radio-tagged Fish was observed capturing a Barn Swallow, though other Fish were on numerous occasions, and some instances were filmed with a hand-held camcorder.

The Tigerfish employed two different strategies when hunting Birds. Some Fish chased the Birds at the surface prior to striking, while others remained deeper in the water and suddenly lunged upwards at Birds. This second hunting behaviour was more successful, capturing a Bird on roughly one-in-three attempts (as opposed to about one-in-seven for the chase and lunge technique), but involved compensating for light refraction at the water surface, something not all the fish appeared able to do.

Avivorous behavioural strategies adopted by Hydrocynus vittatus in the Schroda Dam man-made lake. (a) Flight path of the prey Hirundo rustica, (b) surface pursuit strategy of Hydrocynus vittatus to overcome surface image shift due to light refraction (angle θ) and (c) direct aerial strikes by adult Hydrocynus vittatus that compensate for the image shift (not drawn to scale). O’Bien et al. (2013).

Since the Shroda Dam is a new environment for the Tigerfish, and fast-moving Barn Swallows are a fairly difficult Bird to target from the water, O’Brien et al. suggest that this behaviour is likely to be quite widespread in Tigerfish, and that it would be worth investigating at other localities.

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