DETERMINATION OF UPPER SALINITY TOLERANCE IN AGE 0 AND ADULT WOUNDFIN (Plagopterus argentissimus)

Endangered woundfin (Plagopterus argentissimus Cyprinidae) are native to the Virgin River downstream of Pah Tempe Mineral Hot Springs near Hurricane, Utah.  Pah Tempe Springs alters river salinity with measured salinity ranging from 9 parts per thousand (ppt) immediately below the springs to 2.2 ppt at 2100 m downstream.  Salinity tolerance has been shown to influence fish community composition over large geographic scales, but can vary by species and age class.  Determining the salinity tolerance of woundfin in two age classes would improve distribution modeling of the species throughout the Virgin River, and provide essential baseline salinity tolerance for future physiological assessments.

To assess woundfin salinity tolerance, a 96-hr (LC50) was conducted. Three hundred age 0 (23 mm length) and 146 mature (75 mm length) fish from a captively bred population were stocked into 1000 mL and 9.5 L replicated static water tanks, respectively. Salinities tested ranged from 0 to 15 ppt and 0 to 12 ppt, respectively for the age 0 and mature woundfin. Thirty percent water exchanges were conducted daily.  Water temperature was maintained at 21.8-22.2°C, while average nitrite, ammonia, pH, and dissolved oxygen levels were 0.0008 mg/L, 0.4 mg/L, 8.4, and 7.7 mg/L, respectively.  Fish were exposed to salinity treatments for 96 hr and mortality was recorded every 24 hours.

Mortality occurred in age-0 fish at 12 ppt, and the 96- hr LC50 was 13 ppt.  Mortality occurred in adult fish at 8 ppt and the LC50 was 11 ppt.  These results suggest woundfin can tolerate salinities up to 8 ppt, and that younger fish may be more tolerant of higher salinities.  The identification of an acceptable salinity range allows for improvements in assessment of woundfin prevalence in the Virgin River. These results are particularly pertinent in consideration of proposals brought forth that suggest removal of Pah Tempe Springs in order to reduce salinity loading for irrigation users downstream.