Technical Report
Rio Grande Silvery Minnow Reproductive Monitoring During 2021
Date: 2021/10/25
Author(s): Dudley R.K., Robbins T.O., Platania S.P., White G.C.
Publication: Prepared for U.S. Bureau of Reclamation, 96 p.
Abstract:
The primary objective of the Rio Grande Silvery Minnow Reproductive Monitoring Program is to characterize the timing, duration, frequency, and magnitude of spawning for this species in the Angostura, Isleta, and San Acacia reaches of the Middle Rio Grande. Additional objectives include characterizing reach-specific spawning patterns over time; examining the relationships between flow, temperature, and spawning; and assessing linkages between egg passage rates and seasonal flows across years. This long-term monitoring study provides insight into key environmental factors affecting trends in the temporal and spatial spawning patterns of Rio Grande Silvery Minnow, which can assist managers in developing successful strategies for its long-term recovery.
Systematic reproductive monitoring of Rio Grande Silvery Minnow has been conducted annually since 2001. Previous studies demonstrated mid-April to mid-June as the primary period of spawning activity. The 2021 study was a continuation of the long-term monitoring effort in the lower portion of the San Acacia Reach (San Marcial), just upstream of Elephant Butte Reservoir. Two additional sites (one in the Angostura Reach [Albuquerque] and one in the Isleta Reach [Sevilleta]), which had been sampled periodically from 2006 to 2011, were also sampled from 2017 to 2021.
In 2021, we collected drifting eggs from three fish species. Most of the eggs were identified as Rio Grande Silvery Minnow (n = 26,868), but a few were identified as Common Carp (n = 72) or Flathead Chub (n = 33). We caught the most Rio Grande Silvery Minnow eggs at Albuquerque (n = 12,627), followed by San Marcial (n = 9,813), and Sevilleta (n = 4,428).
Reproductive monitoring of Rio Grande Silvery Minnow was reinitiated at the Albuquerque and Sevilleta sites in 2017, which allowed for spatial comparisons of estimated egg-passage rates (E(x); eggs per second) across years (2006–2011, 2017–2021). Overall, the annual passage rates at Sevilleta and San Marcial were consistently higher than at Albuquerque, with the notable exception of the near absence of eggs at San Marcial in 2018 and 2020. In 2021, the passage rate was highest at Albuquerque (2.21·101 ), intermediate at Sevilleta (3.77·100 ), and lowest at San Marcial (3.47·100 ). We roughly estimated that about 9.54·107 eggs, 1.63·107 eggs, and 1.50·107 eggs were transported downstream of Albuquerque, Sevilleta, and San Marcial, respectively, during 2021.
Long-term spawning patterns and trends were based on data from all sites (2003–2021). Logistic regression modeling of daily egg presence-absence data revealed strong associations with the percentage change in mean daily discharge (i.e., independent of flow magnitude) just prior to egg collection. The probability of collecting eggs (i.e., daily egg-occurrence probability) was highest when river flows increased substantially across consecutive days. The occurrence probability during a 100% increase in flow was 0.84, whereas the occurrence probability was 0.98 during a 200% increase in flow. In contrast to the robust discharge relationship, daily egg presence-absence data revealed a very weak and nonsignificant association with mean daily water temperature.
Annual egg-passage rates, using data from all sites (2003–2021), were lowest in 2004 (1.65·10-3 ) and highest in 2011 (2.24·101 ). There was a steady decline in passage rates from 2011 to 2013, followed by an increase in 2014. Passage rates declined again from 2014 (7.62·100 ) to 2016 (1.43·10-1 ). The 2021 passage rate (8.26·100 ) was higher (P < 0.05) than the 2019 rate (4.58·10-2 ).
Changes in annual egg-occurrence probabilities and annual egg-passage rates, using data from all sites, were moderately predicted by differences in seasonal river flows across years (2003–2021). Out of 224 models considered, we found that the top three models, which represented high flows during spring, were most informative (ca. 40% of cumulative model weight) in explaining why some years had lower passage rates (i.e., reduced downstream transport) than others. In summary, we found that occurrence probabilities were slightly higher during years with reduced and truncated spring flows, and that annual egg-passage rates were slightly lower during years with elevated and extended spring flows.
Despite the seemingly large number of eggs, and presumably larvae, transported downstream into the southern reaches of the Middle Rio Grande each year, some portion of this reproductive effort remains upstream. It is likely that the proportion of individuals retained and successfully recruited upstream is positively related to the complexity of instream habitat conditions and the availability of nursery habitats. The availability of floodplain habitat could be particularly important, as these areas are likely locations for the increased retention of drifting fish eggs and larvae. As newly hatched fish require about one month to progress through the early larval phases, the persistence of these habitats is essential during this initial period. The future conservation status and long-term recovery of Rio Grande Silvery Minnow appear strongly dependent on reliably ensuring sufficient seasonal flow, habitat conditions, and river connectivity that will promote the successful spawning and early recruitment of this imperiled species.