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

ORTUBAY, S,; V. CUSSAC,; M. BATTINI,; J. BARRIGA; J. AIGO; M. ALONSO; P. MACCHI; M. REISSIG,; J. YOSHIOKA; S. FOX

AQUATIC CONSERVATION-MARINE AND FRESHWATER ECOSYSTEMS

John Wiley & Sons Ltd

Lugar: Chichester; Año: 2006 vol. 16 p. 93 - 105

1052-7613

1.      Laguna Blanca, in Laguna Blanca National Park, is a lake in Patagonia which has been considered a RAMSAR site since 1971 because of avian 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 A. praebasalticus. 2.      Avian and amphibian populations have decreased dramatically in recent years. Percichthys colhuapiensis, 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.      We assessed the fishless condition of some neighbouring small lakes with abundant pond weeds, aquatic birds and endemic amphibians, in order to compare the physicochemical characteristics and the quantitative composition of the benthos among lakes. Fish presence at Laguna Blanca and its absence at El Burro, Antiñir and Jabón 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.      Only in Laguna Blanca were P. colhuapiensis captured, with the highest captures in the littoral zone. The population shows fast individual growth in the first years and an absence of fish older than six 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.      We presume that a complex trophic cascade effect (fish predation on tadpoles, fish competition for avian food, bottom disturbance, zooplankton reduction) acted on the amphibian and bird populations as the main mechanisms of deleterious effects of fish introduction in Laguna Blanca.