THE SENSITIVITY OF SWIM-UP GUADALUPE BASS Micropterus treculii FRY TO HYPEROXIA

Fingerling Guadalupe Bass Micropterus treculii are produced at the A. E. Wood Fish Hatchery, San Marcos, Texas and stocked into central Texas rivers to restore threatened populations and combat hybridization with Smallmouth Bass Micropterus dolomieui. Fry survival in fingerling rearing ponds has fallen from 57-70% in 2011-13 to 11-25% in 2014-15 for unknown reasons and meeting the fingerling stocking request is challenging. We hypothesized that exposure to high oxygen concentrations (>20 mg/L, hyperoxia) from pure oxygen supplementation during transport and acclimation to ponds may have contributed to fry mortality in 2014-15.    

To test this hypothesis, we exposed swim-up Guadalupe Bass fry (11 days-post-hatch, 8.45±0.25 mm total length) to normbaric dissolved oxygen (DO) concentrations of 7.9-8.6 mg/L (controls with no supplemental oxygen), 11.5-12.8 mg/L, 17.3-17.8 mg/L, 20.3-22.8 mg/L, 26.4-28.2 mg/L, and 32.1-33.3 mg/L for 30 min at 18^oC, mimicking transport practices during 2014-2015. Pure oxygen was supplied to water in a 227-L fiberglass trough to maintain the target oxygen concentration for each treatment which was continuously siphoned into five replicate 6-L McDonald jars each stocked with 1 g of fry. After the 30-min exposure, jars were flushed with non-oxygenated water (DO 8.0-8.5 mg/L) for 1 min to rapidly lower the DO, mimicking fry release into ponds after transport. Jars were then moved to an incubation rack, supplied with 36 L/h fresh water (DO 7.6-8.8 mg/L), and monitored for 72 h. One control (A) was subjected to the same protocol without supplemental oxygen, while for the other control (B), fry were stocked directly into jars on the incubation rack and were not exposed to flushing. Dead fry were removed and recorded every 24 h. Fry behavior was observed throughout the study. After 72 h, surviving fry were counted and measured.

There were no significant differences (P>0.05) in temperature, pH, ammonia, or alkalinity between treatments. Fry exposed to the highest oxygen concentration appeared more lethargic than in other treatments after 6 h. Fry survival after 72 h was strongly negatively correlated with increasing DO concentrations (Pearson: -0.79) and declined significantly (P<0.05) above 20.3-22.8 mg/L (Table 1). The results of this study suggest that hyperoxia may lower fry survival. We recommend avoiding DO above 20 mg/L when transporting Guadalupe Bass fry.