A new tool to counter the fish invasion in high-mountain lakes

If in some regions, we are currently observing a slowdown of the invasion of fish in originally fishless high-mountain lakes, it is only because fish are now widespread almost everywhere. This gives an idea of the large extent of the problem, which is now far from being a simple local threat. The introduction of game fish and of their live baits (i.e., small minnows) is dramatically affecting native amphibians and aquatic biota and probably represent the most serious threat to the conservation of mountain lake ecosystems.

Given the ridiculous number of mountain lakes on our planet, this scenario was probably unimaginable when, at the beginning of the XX^th century, the modern fishery started to promote recreational angling in many mountain areas of the developed countries. However, during the last century and decades the invasion was sustained by a myriad of institutional and illegal introductions, which made the current scenario a reality. Even if fish distribution data are unavailable for many mountain regions, introduced fish are probably becoming widespread also in developing countries. Even today, despite its threat to biodiversity is now very clear, fish introductions continue all over the world, even in many protected areas. In addition, in the last very few decades, illegal release of small fish used as live baits is increasing the number of introduced species in mountain lakes, exacerbating their impacts, and, ironically, affecting many game fish populations, which may eventually be extirpated and lose their recreational value.

Who studies biological invasions is used to dealing with an irreversible global process, but in the case of introduced fish in mountain lakes, things may be less frustrating, because we already know the way to reverse the invasion. Despite the popular appeal of fish stocking, officials and conservation-minded anglers have therefore begun to take action to mitigate the damage. When they did it, conservation measures and actions worked! Where implemented, stocking and fishing bans stop or reverse (i.e., not self-sustaining populations got extinct) the invasion, usually without significant opposition by anglers and their associations.

The most advanced conservation experiences were completed in some protected areas of North America and Europe, where several mountain lakes were actively returned to their original fishless state by mechanical eradication actions (by gillnetting, electrofishing, etc.). Compared to their main alternative, i.e., eradication using chemical piscicides, mechanical methods present both advantages and disadvantages: they are safe for non-target species and less conflictive, but their effectiveness is constrained by several factors (i.e., lake size, environmental complexity, and presence of minnows) and they require larger human and economic resources. Nevertheless, mechanical methods may represent the only possible choice because chemical methods often concern both conservation authorities and public.

In such a context, resource availability is a major issue for the restoration of mountain lakes. In order to foster the replicability of eradication actions, reducing the efforts -and connected costs- is a necessary step. Getting an estimate of the size of residual populations can reduce the eradication costs, by stopping the efforts when such estimates go to zero and the probability of eradication success is sufficiently high. However, getting reliable estimates of the population size is a challenging task due to the problem of imperfect detection and resulting uncertainty as to whether an invasive alien species has been completely eradicated or not. For example, an underestimation of the needed efforts can lead to species re-invasion from residual individuals.

Most methods to assess residual abundance and eradication probability rely on the often unrealistic assumption that a population is closed to mortality and recruitment processes during the eradication actions. We addressed this issue in a paper recently published in the Journal of Applied Ecology and titled “Alien fish eradication from high mountain lakes by multiple removal methods: estimating residual abundance and eradication probability in open populations”. The paper has been led by Rocco Tiberti (Cnr-IRSA) e Simone Tenan (Cnr-ISMAR), in collaboration with CEAB-CSIC (Blanes, Spain), Sequoia and Kings Canyon National Parks, University of California, Universitat de Girona (Spain) and IMEDEA (CSIC-UIB, Spain). We used the eradication data from 20 fish eradication actions from as many mountain lakes to estimate, with a novel analytical approach, the residual population size, and derive eradication probability. The main novelties of this model are considering that population can be open to mortality and recruitment and that their eradication may require the use of concomitant removal methods (i.e., electrofishing, gillnetting, fyke-netting). Compared to empirical data from the field, the model provides reliable estimates of the residual population sizes and eradication probabilities, as well as the method specific capture probabilities, which can help practitioners during the fish eradication actions.

Rocco Tiberti, Teresa Buchaca, Daniel Boiano, Roland A. Knapp, Quim Pou Rovira, Giacomo Tavecchia, Marc Ventura, Simone Tenan,
Alien fish eradication from high mountain lakes by multiple removal methods: Estimating residual abundance and eradication probability in open populations,
Journal of Applied Ecology, First published: 02 March 2021; https://doi.org/10.1111