Towards using thermal stress thresholds to predict salmonid invasion potential

Abstract

The salmonids rainbow trout Oncorhynchus mykiss and brown trout Salmo trutta are amongst the world’s worst invaders with devastating impacts where introduced. Information on their invasive potential is key to predicting range expansion in recipient environments. Salmonids were introduced into the Keiskamma River system, South Africa in the late 1890s, established and spread throughout its headwaters. This provided an opportunity to document their distribution in relation to stream thermal regimes. Salmonids conformed to documented thermal tolerances predominantly occupying cool-water sites (13.7 ± 3.5 °C). Salmonid probability of occurrence was assessed using seven-day moving average of daily mean water temperatures (MWAT) and exceedances of 100 and 59 days reduce the probability of occurrence for S. trutta and O. mykiss to 0.5 respectively, with probability of occurrence falling to zero at 150 days for both species. Greater tolerance of MWAT exceedance was observed in impoundment (233 days) rather than riverine (146 days) habitats. Epilimnetic releases from impoundments increased temporal heat loads and average temperatures were 3 °C higher below than immediately above impoundments. Salmonids did not occupy all sites within their thermal preferendum, indicating invasion debt and further potential to spread. These models highlight subtle ecological niche differences between both salmonids with O. mykiss more likely to survive warmer water temperatures than S. trutta, but at the expense of a lower altitudinal range and with a greater susceptibility to extinction with an increase in water temperatures. Aquaculture and fisheries enhancements using salmonids are increasing, and these results may contribute to risk assessments for salmonid invasions globally.