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

Abstract

ABSTRACT1. Laguna Blanca, in Laguna Blanca National Park, is a lake in Patagonia which has been designated 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. 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. The fishless condition of some neighbouring small lakes with abundant pond weeds, aquatic birds and endemic amphibians, was assessed in order to compare the physical and chemical characteristics and the quantitative composition of the benthos among lakes. Fish presence at Laguna 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 littoral zone. 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.

Feeding habits and their implications for the conservation of the endangered semiaquatic frog Atelognathus patagonicus (Anura, Neobatrachia) in a northwestern Patagonian pond

“…The extinction of A. patagonicus in Laguna Blanca may have occurred due to the establishment of fish in the environment (Administración de Parques Nacionales 1993, Cuello 2002, Fox et al 2005, Ortubay et al 2006. Several decades after the introduction of P. colhuapiensis, significant changes were reported in the aquatic biota: alteration of plankton and littoral benthos, reduction of macrophytia and decline of aquatic birds populations, along with the disappearance of A. patagonicus (Ortubay et al 2006).…”

“…Several decades after the introduction of P. colhuapiensis, significant changes were reported in the aquatic biota: alteration of plankton and littoral benthos, reduction of macrophytia and decline of aquatic birds populations, along with the disappearance of A. patagonicus (Ortubay et al 2006).…”

“…In the light of current knowledge, the following factors may have contributed to the local extinction of the frog in Laguna Blanca after the introduction of fishes: disturbance of their habitat (Ortubay et al 2006), predation (Cuello and Perotti 2006), susceptibility to disease (Fox et al 2005) and alteration of their diet. This study provides evidence supporting the idea that might have occurred a trophic niche overlap between native frogs and introduced fishes in Laguna Blanca.…”

“…belongs to a system of endorheic ponds in basalt basins, where the arid climate determines vegetation made up of low, thorny shrubby steppe. The pond has a surface area of 14 ha, a perimeter of 1.8 km (Cuello 2002) and a maximum depth of 2 m (Ortubay et al 2006). The surface freezes during cold winters, while in summer the surface temperature may reach 23°C.…”

Diet of the exotic Rainbow Trout in the critical habitat of the threatened Hooded Grebe

“…In the Patagonian steppe the introduction of exotic fishes into lakes has been responsible for local extinctions of cladoceran species, the reduction of size in zooplankton and the homogenization of communities (Modenutti & Balseiro, 1994;Reissig et al, 2006). The translocation of native fish and the introduction of Rainbow Trout into the shallow lake Laguna Blanca (70 • 22 W, 39 • 02 S) led to the local extinction of amphibians, and the loss of habitat for waterbirds (Ortubay et al, 2006) . Available information suggests that the diet composition of trout and Hooded Grebes -both visual predators -could overlap.…”