Bonneville Cutthroat Trout

The Bear Lake strain of the Bonneville Cutthroat Trout (hereafter referred to as cutthroat) is the only natural adfluvial population of Bonneville Cutthroat Trout in Idaho and Utah, are one of the main sportfish of the lake, and the natural apex predator (Lentsch et al. 2000, Teuscher and Capurso 2007). Cutthroat begin maturing at age 3, but may not spawn until age 10 (Kershner 1995, Neilson and Lentsch 1988, Teuscher and Capurso 2007). Cutthroat spawn in tributaries of Bear Lake for 11 weeks in the spring (approximately Apr 15 – June 30, Scott Tolentino, personal communication 2015). Little is known about the population’s population vital rates or demographics despite its ecological and economical importance.

Water diverted from Bear Lake for irrigation and hydropower can influence lake levels. The Bear River Compact allocates water in Bear Lake from 1805.52 to 1802.26 meters above sea level for hydropower use (Bear River Compact 1980). Any water below that elevation is reserved exclusively for irrigation purposes (Bear River Compact 1980). Human water use for agricultural irrigation, reduced precipitation, and increased evapotranspiration can exacerbate lake level reductions during times of drought, and these factors are projected to increase with climate change (Mortsch and Quinn 1996, Schindler 2009). Bear Lake contains a population of adfluvial Bonneville Cutthroat Trout that rely on tributary-lake connectivity for adult and juvenile migration between the tributaries and the lake (Teuscher and Capurso 2007). Yet, Bear Lake’s main tributaries can lose connectivity to the lake itself at lake levels below 1802.00 meters above sea level, becoming impassible to spawning fish (Teuscher and Capurso 2007).