EFFECT OF SALINITY ON SURVIVAL OF AN ENGANGERED MINNOW EXPOSED TO Ichthyophthirius multifiliis

Endangered woundfin (Plagopterus argentissimus Cyprinidae) are native to the Virgin River downstream of Pah Tempe Mineral Hot Springs near Hurricane, Utah.  Removal of Pah Tempe Springs or diversions of its outflow are proposed to reduce river salinity loading to improve the quality of water used for irrigation.  However, there are concerns because Pah Tempe Springs is a natural feature and natural salinity loading has been hypothesized to reduce the negative impacts of Ichthyophthirius multifiliis infections on fish survival by conferring anti-parasite properties.

We established an experimental design to test this hypothesis using a captive population of woundfin.  Juvenile (1.8 g weight and 55 mm total length) and adult (6.7 g weight and 89 mm total length) fish were randomly stocked into a recirculating water system with five replicate 94- L tanks/size class at a rate of 10 fish/tank.  Two control tanks equipped with in-line ultraviolet water treatment were stocked with fish in each size class to validate that observed mortality was due to Ichthyophthirius multifiliis.  Water temperature was maintained at 21.8-22.2°C, and average nitrite and ammonia were 0.04 and 0.03 mg/L, respectively.  Following an acclimation period of five days, fish were exposed to Ichthyophthirius multifiliis by introducing three infected fish in the upstream water source.  When total mortality reached 15%, water salinity was raised to 3.5 ppt. Mortality was then monitored for an additional 21 days.

Trophonts were first observed on test fish at 7 days post-exposure (DPE).  Mortality began at 19 DPE and reached 15% at 21 DPE.  Mortality ceased at 28 DPE (7 days post-salinity).  Diagnostic evaluation at 44 DPE validated a clearance of the parasite in all surviving fish.  Cumulative mortality was similar in juvenile and adult fish (32 vs 48%, respectively; P=0.271).

These results indicate that salinity treatment confers anti-parasite properties in woundfin and provide preliminary evidence the natural salinity loading in the Virgin River may mitigate Ichthyophthirius multifiliis epizootics.