Managed spring runoff to improve nursery floodplain habitat for endangered Rio Grande silvery minnow

Abstract

AbstractWater managers in New Mexico, USA, stored water in El Vado Reservoir and coordinated releases into the Chama River that augmented the runoff of the Rio Grande, resulting in a discharge >1,500 ft3/s (42.5 m3/s) for 35 days (May 17 to June 20, 2016) at Albuquerque. The managed runoff inundated over 400 ha of previously restored floodplains in the Middle Rio Grande, thereby providing spawning and nursery habitat for the endangered Rio Grande silvery minnow (Hybognathus amarus, RGSM). Spawning began April 9 at annual cumulative degree‐days of 717, during daily increases in discharge of 200–300 ft3/s (5.7–8.5 m3/s), and hatch dates were normally distributed over 53 days (April 11 to June 3). RGSM were 73% of larvae collected in six restored floodplain sites and found in shallow water (mean = 19.6 cm), low velocity (mean = 3.9 cm/s), near vegetative cover, and with 75% within 1 m of the water's edge. Declining proportions of early to late larval phases and a near absence of juveniles indicate a gradual departure from floodplains as postflexion mesolarvae and metalarvae 14–22 days post hatch (dph), with most leaving by the juvenile stage 40 dph. The annual RGSM October census showed an increase of 0.16 to 7.20 fish/100 m2 from 2015 to 2016, indicating that the managed runoff resulted in a positive population response. This study showed that constructing floodplains and managing river and reservoir operations to inundate those floodplains during and after RGSM spawning can provide nursery habitat that improves reproductive success and recruitment.