Technical Report

Rio Grande Silvery Minnow Reproductive Monitoring During 2023


Date: 2023/10/27

Author(s): Dudley R.K., Platania S.P., White G.C.

Publication: Prepared for U.S. Bureau of Reclamation, 96 p.


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 imperiled 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. Our ongoing research 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 2023 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 2023.

In 2023, we collected drifting eggs from three fish species. All eggs (n = 5) were immediately identified in the field as Common Carp (n = 1), Rio Grande Silvery Minnow (n = 1), or Flathead Chub (n = 3). We caught one Rio Grande Silvery Minnow egg at San Marcial but no eggs at either Albuquerque or Sevilleta in 2023.

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, 2017–2023). The passage rates at all three sampling sites were lower in 2022 than in 2021. The passage rates at Albuquerque, Sevilleta, and San Marcial could not be estimated in 2023, however, as only a single nonzero value was recorded.

Long-term spawning patterns and trends were based on all available data across sites (Albuquerque, Sevilleta, San Marcial) and years (2003–2023). 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 0.78, whereas the occurrence probability was 0.96 during a 200% increase in flow. In contrast to the robust discharge relationship, daily egg presence-absence data revealed a weak and nonsignificant association with mean daily water temperature.

Annual egg-passage rates, using data from all sites (2003–2023), were lowest in 2004 (1.66·10-3) and highest in 2011 (2.32·101). There was a steady decline in passage rates from 2011 to 2013, followed by an increase in 2014. Passage rates declined again from 2014 (7.64·100) to 2016 (1.42·10-1). The 2022 passage rate (6.89·10-2) was lower than in 2021 (8.21·100), and only one egg was collected in 2023.

Changes in egg-occurrence probabilities and egg-passage rates, using data from all sites, were moderately predicted by differences in seasonal river flows across years (2003–2023). Out of 224 models considered, we found that the top two models, which represented elevated flows during spring, were most informative (ca. 49% 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 egg-occurrence probabilities were higher during years with low, truncated, and fluctuating spring flows, whereas egg-passage rates were lower during years with high, prolonged, and stable 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 is 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 their early larval phases, the stability and persistence of these productive nursery habitats is essential during this initial period (ca. May–June). The conservation status and long- term recovery of Rio Grande Silvery Minnow appear strongly dependent on reliably ensuring sufficient seasonal flow and habitat conditions that will promote the successful spawning, recruitment, and survival of this imperiled species.

Related Information
  • Species: Rio Grande Silvery Minnow
  • Organization: U.S. Bureau of Reclamation
  • Keywords: Population Monitoring
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