Sensitivity of Guadalupe Bass Swim‐up Fry to Hyperoxia

Abstract

AbstractHatchery‐reared fingerling Guadalupe Bass Micropterus treculii are stocked into central Texas rivers to restore threatened populations and combat hybridization with Smallmouth Bass M. dolomieu. Fry survival in hatchery rearing ponds has declined by more than 50% in recent years, and pilot investigations indicated that exposure to high dissolved oxygen (DO) concentrations (>20 mg/L; hyperoxia) during tempering might have contributed to fry mortality. To test this hypothesis, Guadalupe Bass swim‐up fry (11 d posthatch; 8.45 ± 0.25 mm TL [mean ± SD]) were exposed to normbaric DO concentrations ranging from 7.9 to 33.3 mg/L at 18°C, mimicking the hatchery tempering protocols followed during 2014 and 2015. After a 30‐min exposure to the DO treatment (flow rate = 18 L/h), jars were flushed with nonoxygenated water for 1 min to rapidly lower the DO, mimicking the release of fry into ponds after tempering. Jars were then supplied with freshwater at a rate of 36 L/h and were monitored for 72 h. One control (A) was subjected to the same protocol but without oxygen supplementation; the other control (B) was not subjected to the 30‐min exposure or flushing. Dead fry among treatments were recorded and removed every 24 h. Mortalities were observed within 6 h. Surviving fry were counted and measured after 72 h. Fry survival at 72 h was negatively correlated with increasing DO concentrations (Pearson's product‐moment correlation coefficient = –0.79) and declined significantly above 20 mg/L. Fry populations exposed to the highest oxygen concentration visually appeared more lethargic than fry in other treatments. The results of our study suggest that hyperoxia contributed to the low survival of fry in recent years. We recommend DO concentrations below 15 mg/L and avoiding DO levels above 20 mg/L (212% saturation at 18°C) when transporting and tempering Guadalupe Bass fry. We also recommend verifying the oxygen limits of other black bass Micropterus spp.