Historical use, fishing management and lake characteristics explain the presence of non-native trout in Pyrenean lakes: Implications for conservation

High mountain lakes are naturally fishless, although many have had introductions of non-native fish species, predominantly trout. Predation on native fauna by introduced trout involves profound ecological changes. The objective of this study was to reconstruct the historical process of trout introduction in 520 high mountain lakes >0.5 ha of the southern Pyrenees and quantify which particular factors either environmental or anthropogenic, best explained their present distribution and lake conservation status. The first written evidence of trout introductions dated back to 1371 AD. By 1900, trout had been introduced to 26.5% of lakes during preceding centuries. A subsequent wave of introductions began in 1960 when stocking led to trout becoming established in 52.5% of lakes. From 1900 to 1950, walking distance from nearby urban centres was the dominant factor explaining 29–60% of the variation in trout distribution, indicating that trout introductions were in lakes closer to human settlements. In contrast, with the onset of modern fish management that took place during the period 1960–2000, the most significant factors were both the management practices and lake characteristics which are likely related with the probability of survival of the fish populations. After 2000, the remaining fishless lakes were those of highest altitudes, and the shallowest and those with the lowest surface area. The recent fishing ban in protected areas of National Parks has resulted in a stabilization of the number of lakes with fish, suggesting that this is the only reliable management policy to avoid new introductions and preserve the conservation status of high mountain lakes.