Is the decline of birds and amphibians in a steppe lake of northern Patagonia a consequence of limnological changes following fish introduction?

ORTUBAY SILVIA, CUSSAC VICTOR, BATTINI MIGUEL, BARRIGA JUAN, AIGO JUANA, ALONSO MIGUEL, MACCHI PATRICIO, REISSIG MARIANA, YOSHIOKA JOY & FOX STANLEY

AQUATIC CONSERVATION-MARINE AND FRESHWATER ECOSYSTEMS

Wiley InterScience

Lugar: Hoboken, New Jersey - USA; Año: 2006 vol. 16 p. 93 - 105

1052-7613

1. Laguna Blanca, in Laguna Blanca National Park, is a lake in Patagonia which has beendesignated as a Ramsar site since 1971 because of bird diversity and abundance and importance for nesting, particularly for the black neck swan, Cygnus melanocoryphus. It is also valued for its populations of endemic amphibians, Atelognathus patagonicus and Atelognathus praebasalticus.2. Avian and amphibian populations have decreased dramatically in recent years. Percichthyscolhuapiensis, Percichthys trucha (Pisces, Percichthyidae), Salmo trutta and Oncorhynchus mykiss (Pisces, Salmonidae) were introduced into Laguna Blanca in 1965. Since 1986, no Atelognathus frogs have been found. The abundance of swans and coots, which are strongly associated with macrophytes for nesting and feeding, has diminished drastically, whereas piscivorous birds have increased.3. The fishless condition of some neighbouring small lakes with abundant pond weeds, aquaticbirds and endemic amphibians, was assessed in order to compare the physical and chemicalcharacteristics and the quantitative composition of the benthos among lakes. Fish presence atLaguna Blanca and its absence at El Burro, Antin˜ ir and Jabo´ n lakes, were confirmed. Compared with previous results, it seems that the abundance of Amphipoda (Hyalella), Copepoda and Cladocera at Laguna Blanca has decreased, while Acari has increased and Notostraca has disappeared. Water transparency has diminished in Laguna Blanca and now is lower than that of fishless lakes.4. P. colhuapiensis were captured only in Laguna Blanca, with the highest captures in the littoralzone. The population shows rapid individual growth in the early years and an absence of fish older than 6 years. The preponderance of benthos and the presence of macrophytes in the gut contents of adult P. colhuapiensis appear to indicate that they are important consumers of these resources.5. This paper concludes that fish introduction in Laguna Blanca led to a complex trophic cascadeeffect (fish predation on tadpoles, fish competition for avian food, bottom disturbance, zooplankton reduction) producing deleterious effects on the amphibian and bird populations.