Endangered Yellowcheek Darter ( Nothonotus moorei ) and special concern Yoke Darter ( Nothonotus juliae ) responses to simulated drying and PIT tags

Drying conditions are impacting aquatic systems at greater frequency and intensity across the globe, and these trends are expected to be exacerbated with climate projections and higher anthropogenic water demands. Riverine systems often lose critical habitat during drying, which can reduce or eliminate species. For many species, there is a knowledge gap regarding how they respond to changes in abiotic and biotic interactions that occur during drying. Yellowcheek Darter (Nothonotus moorei) and Yoke Darter (Nothonotus juliae) are two species facing challenges from increased stream-drying that would benefit from further evaluation. Our study uses stream-mesocosms to evaluate the effects of stream-drying on survival, growth, and behavior, while evaluating 8 mm passive integrated transponder (PIT) tags. Tag-mortality, tag-detection, habitat selection, survival, and growth were analyzed for surviving individuals. Yellowcheek Darters had lower tag-mortality (16.7%) than Yoke Darters (31.1%). Tag-detection was high for Yoke Darters (77%) and Yellowcheek Darters (73%). The control fish of both species used all three habitats (riffle, run and pool). However, Yellowcheek Darters in the drying treatments used run and pool habitats exclusively, whereas Yoke Darters in the drying treatments were observed in the riffle habitats until day 30. Survival was low and did not differ between treatments (Yoke control 22.5%, drying 28.6%; Yellowcheek control 33.3% and drying 53.3%). Yoke Darters had an 80.4% increase in mass in the control treatments (p = 0.029), but there was no significant difference in length change. Yellowcheek Darters did not differ between treatments for mass or length changes. Our study provides laboratory drying information for Yellowcheek Darters and Yoke Darters while evaluating PIT tags for small-bodied stream fishes. Our study is informative for fishes in stream settings experiencing drying by providing behavioral, physiological, detection, and mortality information. Overall, our study provides information to improve conservation prioritization for freshwater fishes in drying systems.