Simulated Transport and Retention of Pelagic Fish Eggs during an Irrigation Release in the Pecos River, New Mexico

ABSTRACT

The Pecos River between Sumner Dam and Brantley Reservoir (320 km), New Mexico, contains populations of several pelagic-spawning cyprinid fish species, including the federally threatened Pecos bluntnose shiner (Notropis simus pecosensis). Females of this reproductive guild release semi-buoyant eggs that are fertilized and drift, incubate, and hatch on increased flows associated with spring runoff, summer rainstorms, or irrigation releases from Sumner Reservoir. We used artificial eggs (beads) to estimate transport and retention of natural eggs during an irrigation release in five reaches of the Pecos River occupied by bluntnose shiner. Bead retention was highest (1.7–4.5%/km) in the broad complex reaches of upper critical habitat and the quality section (upper 200 km), and bead retention was lowest (0.6%/km) in the narrow, deep, channelized reach of lower critical habitat (60 km). Only 10% of beads were transported more than 50 km from their release location in the reach with highest retention. Modeled spawning events in all five reaches showed highest egg retention of 35% in the reach that also had the highest density of adult Pecos bluntnose shiner (37 km from Cortez Gasline to Acme). The model predicted that during an irrigation release, about 90% of bluntnose shiner eggs produced during a natural spawning event are retained in the upper 250 km of occupied habitat, with only 10% passing downstream into unsuitable habitat or into Brantley Reservoir. We conclude that retention of drifting eggs is greatest during high flows when flood wave attenuation and channel storage are greatest and in broad complex channel reaches, which coincide with existing population centers.