Daily, seasonal, and annual variability of temperature in streams inhabited by the endemic San Pedro Martir trout (Oncorhynchus mykiss nelsoni), in Baja California, Mexico, and the predicted temperature for the years 2025 and 2050

The present study measured the daily, seasonal, and annual variability of the water temperature of streams in which the endemic rainbow trout, Oncorhynchus mykiss nelsoni, is distributed on the western slope of the Sierra San Pedro Mártir, Baja California, Mexico, between 1996 and 2019. The seasonal thermal interval and daily duration of summer temperatures above the thermal threshold for this trout subspecies (≥28°C) were determined in streams at different elevations (553, 1,220, and 2,080 m asl, or meters above sea level). Temperatures 28°C were recorded at the study site on the stream with the lowest elevation (San Antonio de Murillos Creek) over an accumulated 365 h between June and September 2014, with the maximum temperature recorded there, 30.66 °C, making it the site most vulnerable to global warming. At the San Antonio de Murillos Creek site, the average water temperature predicted by three models (GFDL R30, HadCM3, and Mote) for the year 2025 would be a non-lethal temperature, <28°C, for trout at a minimum elevation of 491-511 masl, while this was predicted to be 545-701 masl for the year 2050. Predicted hourly water temperatures of 28°C (non-lethal) may occur at minimum elevations of 868-898 masl in 2025 and at 908-1028 masl in 2050, reducing a 21-23% and 23-31% its current altitudinal distribution range, respectively, thus avoiding its presence at the type locality (San Antonio de Murillos). Corresponding author: gruiz@uabc.edu.mx