Experimental mixing of a north-temperate lake: Testing the thermal limits of a cold-water invasive fish

Species’ thermal limits play a key role in determining spatial distributions and understanding their response to changing environments. Manipulation of thermal habitat is a potential avenue of exploration for management of invasive species such as the cold-water rainbow smelt (Osmerus mordax), which has adverse effects on native fish communities in central North American inland lakes. In an effort to test the thermal limits and selectively eradicate rainbow smelt, we experimentally mixed Crystal Lake, Wisconsin, USA, during summer of 2012 and 2013 to warm the hypolimnion and eliminate cold-water habitat. This whole-ecosystem manipulation allowed for field testing of published thermal thresholds reported for rainbow smelt. The rainbow smelt population responded to the thermal manipulation by exhibiting unexpected shifts in behavior, intrapopulation divergence in body condition, and significant population declines. Small individuals of each adult age-class tended to survive the manipulation, and the population persisted despite high mortality rates. Our results indicate a high degree of size-based intrapopulation variation in thermal sensitivity for this species. Our findings also raise questions regarding applicability of lab- and model-derived thermal limits to field scenarios, highlighting a need for further field evaluations of species’ thermal limits.