Fish as local stressors of Pyrenean high mountain lakes: Arrival process and impact on amphibians and other organisms

Abstract

Pyrenean high mountain lakes are naturally fishless due to natural barriers that have prevented the natural colonisation of fish species from lower streams. However, there have been numerous trout introductions to such ecosystems, both in historical and recent periods. Differing from other high mountain regions, some high mountain lakes of the Pyrenees were exploited traditionally for trout cultivation. This activity started at least centuries ago and lasted until 1950s, affecting approximately 25% of the lakes. Since 1950, and similar to other high mountain regions, a wave of modern introductions with exotic species has affected among 35% and 85% of the lakes depending on the valley. In recent decades, minnows have also been introduced, being present in 2000 in 27% of Pyrenean high mountain lakes, as a result of recreational fishing with livebait.

The first part of the thesis was intended to provide an objective description of the fish introduction process in the Pyrenees. For that purpose we collected data on trout and minnow occurrence from 520 high mountain lakes >0.5 ha of the southern Pyrenees and quantified, by generalized additive models, which particular factors either environmental or anthropogenic, best explained their present distribution and as a result their lake conservation status.

The distribution of Salmo trutta in the southern slope of the Pyrenees was best explained by both, anthropogenic factors and lake characteristics, while only anthropogenic factors linked to recreational fishing were associated with the distribution of the exotic trout Salvelinus fontinalis and Oncorhynchus mykiss. For the case of minnow occurrence, previous presence of trout in the lake was the most explanative variable, confirming its association with recreational fishing using them as livebait.

Trout and minnow introductions can cause large ecological problems and ecosystem alterations in high mountain lakes and ponds, since both trout and minnows occupy the top of a lake’s food chain. In the second part of the thesis, we sampled 1736 Pyrenean high mountain lakes and ponds at different levels of intensity and investigated the effects of introduced fish on indicator faunal groups such as amphibians and conspicuous macroinvertebrates in the littoral, and crustaceans from the plankton. We also studied if there was a trophic cascade from fish to the littoral epilithic community of these ecosystems.

Our results showed that fish presence was linked with the disappearance of most amphibian species and of most conspicuous macroinvertebrates. Minnows also showed sizeable impact on the pelagic habitat reducing the abundance of some herbivorous species of zooplankton that appeared to be unaffected by trout. In the case of amphibian species, we found that, although introduced fish had a high local impact, western-eastern patterns of some environmental variables are the main drivers of amphibian species distribution at Pyrenean range scale. In addition, we confirmed the presence of a littoral trophic cascade that define most of the characteristics of the littoral epilithon of Pyrenean high mountain lakes and ponds, through fish predation of tadpoles and hence, by a drastic reduction of grazing activity.