Effect of Incubation Temperature on Survival of Embryos of Native Colorado River Fishes

Abstract

Razorback sucker (Xyrauchen texanus (Abbott)), bonytail (Gila elegans Baird and Girard) and humpback (G. cypha Miller) chubs, and Colorado squawfish (Ptychocheilus lucius) Girard) comprise the endemic big river fish fauna of the Colorado River, western North Amer- ica. All are near extirpation in the wild. Effects of incubation temperature upon embryo hatch success were experimentally examined to evaluate potential impacts of cold hypolimnetic reservoir releases upon species reproductive success. Eggs were spawned and fertilized at 18?C, and embryos incubated at 5, 10, 15, 20, 25, and 30?C. Total mortality of embryos of all species occurred in 12-96 hours at 5, 10, and 30?C; and in 48-60 hours at 15?C for P. lucius only. Survival and percentage hatch were highest at 15 and 20?C (G. elegans) or 20?C (all others). Hatched prolarvae were 0.2-1.3 mm TL longer at 20?C than at 15 or 25?C. Spinal or other anomalies were more frequent at 15 and 25?C than at 20?C. Development rates (V) were similar for all species, 4.4-6.1 (15?C), 8.4-9.9 (20?C), and 12.4-18.8 (25?C). Because of lower survival and higher incidence of anomalies at 15 and 25?C, optimal temperatures for development and hatching are probably near 20?C. Reproduc- tion may thus be limited in the wild by low water temperatures, although successful hatching is certainly not precluded. Other hypotheses to explain the demise of these fishes in thermally suita- ble portions of the Colorado River, such as predation upon young, should be examined.