Volume 20 Issue 2 Pages 58 - 90 (October 2003)

Citation: Ruiz-Olmo, J., Marsol, R. and Asensio, V. (2003) More on Predation of Fish Eating Birds by Eurasian Otters (Lutra lutra) in Captivity .  IUCN Otter Spec. Group Bull. 20(1): 73

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More on Predation of Fish Eating Birds by Eurasian Otters (Lutra lutra) in Captivity

Jordi Ruiz-Olmo, Rosa Marsol and Victoria Asensio

Servei de Protecció i Gestió de la Fauna, Direcció general del Patrimoni Natural i del Medi Físic, Dr. Roux 80, 08017 Barcelona, Spain. e-mail: ajruiol@gencat.net

In a previous issue of the IUCN OSG Bulletin (no.19/1 2002), we presented a report on evidence of predation on fish eating birds by the Eurasian otter (Lutra lutra) in northeast Spain. However, whilst this work was still in press we collected new interesting data. A further four adult grey herons (Ardea cinerea) were captured and killed by the captive otters living in the enclosures of the Pont de Suert Otter Centre (Pyrenean mountains, Spain). A female otter (6 kg and 5 years old) killed two of the herons on April 10th and 12th of 2003; and a second female (7 kg and 7 years old), with one sub-adult cub (14 to 21 months old), killed one heron on April 16th 2003 and another on January 20th 2004. First three episodes occurred during a six-day period, coinciding with a cold week when minimum air temperatures ranged between -0.6 to 7.2 °C and minimum water temperatures between 5 and 8 °C. The fourth episode also occurred during a cold week: minimum air temperature at -5.2 °C, and water temperature at 4 °C.

There is a trout (Salmo trutta) fish farm close to the otter enclosures, where herons and other birds regularly go to scavenge fish (as they also do at the otter centre); however, since the existence of the centre (1995), there have been no reports of these captive otters catching herons at this facility. The practically simultaneous capture of first three herons could be due to a special situation, such as low temperatures (as happened again in 2004) or food shortages in the River Noguera Ribagorçana and the Escales reservoir; though the heron's body condition was not bad, the cold weather may have forced them to take greater risks than normal in an effort to gain food. Only in 2004 the heron was consumed by otters. Herons have shown wounds to the skull and the neck (between wings), the later being fractured in three of the four cases.

All these cases at the Pont de Suert Otter Centre involved captive otters with wild herons, as we have found in the wild for 1990 (see the previous report). However, as regards wild otters, heron and otter "concentrations" can occur, particularly when environmental conditions are bad (e.g. at reservoir tails, fish farms, holes in ice cover, ponds during droughts, etc.) and such interactions could occur more frequently than presently recognised.

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