Many of you might be surprised when I tell you that high mountain lakes are naturally fishless. Is it so surprising? Many mountains are too steep for fish to simply swim up brooks. Cascades and strong currents will block their way. Fish eggs could be brought up there by birds and then hatch there. Possible. However, in many mountain lakes you will see only few large species of waterbirds, if at all. It is unlikely that those birds bring fish eggs to all the different mountain lakes we observe fish in, not to say near to impossible. Actually, fish stocking of mountain lakes has a long history in European mountain regions and can be dated back to at least 1371 Anno Domini. During these early fish introductions, the main motivation was to create another source of animal protein for local populations. More recent introductions in the Mid 20th century were motivated primarily by increasing aquatic biodiversity in those fishless lakes and touristic fishing activities. Recent fish introductions started to include also relatively small lakes and adjacent wetlands, which increased the ecological impact yet further. In Europe, touristic fishing activities are the main motivation for the continuous introductions of fish to mountain lakes. In countries with a larger proportion of poor people, such as e.g. Ecuador, introduction of trout into fishless lakes is still motivated by providing an additional animal protein source to local communities.
Fish are a detrimental force in mountain freshwater ecosystems. Fish are voracious predators in an ecosystem low in nutrients. Fish feed on and thus reduce biodiversity of biofilms, microorganisms, tiny plankton species, insects, and amphibians, some of which are protected species. Fish stocking also leads to a large introduction of biomass with the ability to alter both ecological and geochemical processes as e.g. carcass from fish will contribute nutrients and organic carbon to the aquatic cycle. Fish introductions have therefore the force to change lakes and ponds profoundly and threaten mountain biodiversity. This is particularly important to know, given that mountain ecosystems are fragile and already heavily impacted by climate change. The synergistic effects of climate change and fish stocking lead to an increasing eutrophication – an enrichment of nutrients. E.g., increasing temperatures lead to a higher survival rate of fish during winter, during which their impact on microorganisms and plankton is the strongest. Increased temperatures also then let algae proliferate, leading to algal blooms of cyanobacteria. Those can produce toxins – cyanotoxins. These can even kill animals and humans, if occurring in high enough concentrations. If that happens, mountain lakes turn into a sink of life. Their water will not be the source of life it was before.
Fish introductions are also polluting mountain ecosystems more directly. This was shown by analysing the mercury found in fish and lakes. Actually, in lakes of the French Pyrenees, we can find mercury from the southern hemisphere… Astonishing, no? The fish released in mountains come from fish farms, in which fish powder is used for feeding. That fish powder comes from the southern hemisphere, where environmental mercury concentrations are higher. As mercury cannot be biologically reduced, it remains in the system and may even enrich in a fish farm to high levels. Mercury is most notable for its neurological effects to animals and humans.
Fish stocking in mountain ecosystems, despite some traditional history, poisons our water resources and the effects will increase with increasing climate change impacts. There are societal issues to be considered, but most importantly, we need to understand that fish introductions harm our life-support system. They are not sustainable in fragile mountain aquatic ecosystems.