Effect of oxygenation and location on survival and growth of endangered Lost River Suckers in net pens

Acclimation of captively reared fishes to their release environment through soft releases and cage culture can improve survival. Recovery strategies for imperiled Lost River Suckers Deltistes luxatus includes soft releases of captive reared juveniles in net pens in Upper Klamath Lake, Oregon. However, intermittent and sometimes extreme hypoxia in the lake can reduce juvenile fish survival in net pets. To ensure juvenile fish receive adequate oxygen for growth and survival, net pens can be placed in areas of higher oxygen concentration or oxygen can be supplemented. We experimented with a low-cost oxygenation system in Upper Klamath Lake net pens and its effects on survival and growth of juvenile Lost River Suckers. Pairs of net pens, one with supplemental oxygen and one without, were located at a deep offshore site with historically higher oxygen concentration called Mid North and a shallow near shore site with historically lower oxygen concentration called Fish Banks. Survival of passive integrated transponder tagged juvenile Lost River Suckers was monitored and evaluated with Kaplan-Meier survival analyses. We measured two full water column hypoxia events at Fish Banks during the study period, but none at the Mid North site. Oxygenation decreased the number of hours oxygen concentration was less than and pH was greater than known lethal and sublethal thresholds for juvenile Lost River Suckers. For juvenile Lost River Suckers that survived the entire July to September study period, growth was faster at Mid North than Fish Banks. Additionally, there was faster growth at the oxygenated than non-oxygenated Fish Bank’s site, whereas growth was faster at the non-oxygenated than oxygenated Mid North site. Survival did not differ between Mid North net pens, and the Fish Banks oxygenated net pen had greater survival compared to the non-oxygenated Fish Banks net pen during our study. Lost River Sucker growth and survival are dependent on net pen site selection and a low-cost oxygenation system can prevent mass mortality because of summertime hypoxia in shallow freshwater ecosystems.