Mountain Lake Fish: Stocking, Eggs, Geology

This audio overview details how fish arrived in isolated mountain lakes often without river outlets, with the primary method being human stocking for over 100 years to introduce sport fish like trout for recreational fishing and as a food source. Historically, methods ranged from hiking with fish in milk cans or teapots to modern aerial drops by airplanes, which are considered less stressful for tiny trout with a high 95% survival rate. Some unauthorized stocking by military personnel using cargo planes also occurred. The practice is often driven by federal land managers to attract anglers and their dollars, which, while sometimes ecologically catastrophic, contributes to conservation efforts. Historically, aboriginal people in western North America also stocked high mountain lakes over thousands of years, carrying live trout and performing rituals for propagation.

These introduced non-native invasive species, such as brown trout and minnows, cause significant negative ecological impacts by predation on native fauna like aquatic salamanders and frogs, altering the trophic food web and balance of the entire ecosystem, affecting both littoral and pelagic zones. Some agencies now actively work to remove these stocked fish to protect local amphibians. Mountain lakes are typically fed by snowmelt and their cold, clear (oligotrophic) waters naturally hold more oxygen, supporting fish populations.

Natural dispersal theories include geological changes, where lakes were once part of larger river systems that became isolated over time due to changing water levels or erosion, trapping fish populations, exemplified by pupfish. Flooding can also temporarily connect water bodies, allowing fish to swim upstream to previously isolated areas, as seen with bull sharks in an Australian golf course pond. The theory that sticky fish eggs could attach to water birds' feet and be transported between lakes was proposed by natural scientists like Charles Darwin. However, scientific studies indicate this is an unproven hypothesis for fish eggs, despite being confirmed for aquatic invertebrates, and notably, trout do not have sticky eggs. The overwhelming evidence points to human intervention as the primary explanation for fish in many isolated mountain lakes.