Annual Report

Rio Grande Silvery Minnow Population Monitoring During 2023

URL: https://webapps.usgs.gov/mrgescp/documents/2023-RGSM-Population-Monitoring-Final-Report.pdf

Date: 2024/05/17

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

Publication: Report by 1) American Southwest Ichthyological Researchers, 2) Museum of Southwestern Biology (Fishes), UNM, and 3) Department of Fish, Wildlife, and Conservation Biology, CSU submitted to US Bureau of Reclamation

Abstract:

As part of the Rio Grande Silvery Minnow Population Monitoring Program, the status of this imperiled species and the associated Middle Rio Grande ichthyofaunal community has been systematically monitored since 1993. This effort is unique among ichthyofaunal research studies in the Middle Rio Grande, as it provides consistent sampling of fishes over an extended duration. Long-term sampling studies, like this one, also provide the data necessary to test and compare different ecological hypotheses. Our primary research objective is to evaluate how seasonal and annual changes in river flows affect the distribution and abundance of Rio Grande Silvery Minnow throughout its current range over time (1993–2023).

The annual occurrence and density of Rio Grande Silvery Minnow, during October, has fluctuated widely over the past three decades (1993–2023). Estimated densities (E(x); fish per 100 m2) were notably higher in 2016 and 2017, as compared with 2015, but then decreased dramatically from 2017 to 2018. Recent monitoring efforts revealed a 96.1% decrease in its density from 2019 (2.10) to 2022 (0.08), but its density again increased by 2023 (3.37). While Rio Grande Silvery Minnow represented only 0.68% of the fish community in 2022, it had increased to 8.09% by 2023.

Changes in the occurrence and density of Rio Grande Silvery Minnow were reliably predicted by seasonal differences in river flows across years (1993–2023). Further, our findings were consistent regardless of whether dry sites or additional sites were or were not included in the analyses. Out of 440 models considered, we found that the top three models, which represented extended high flows during spring, were crucial (i.e., 92.3% of cumulative model weight) in explaining why some years had dramatically elevated densities of Rio Grande Silvery Minnow. In contrast, we found that extended low flows during summer were key to explaining reductions in the occurrence of this species across years. Thus, prolonged low flows during summer were most predictive of decreased occurrence and prolonged high flows during spring were most predictive of increased density of Rio Grande Silvery Minnow over time.

Additional analyses revealed that population trends in different mesohabitats (October [2002–2023]), or on different days during repeated sampling (November [2005–2023]), were comparable to population trends obtained from the long-term dataset (October [1993–2023]). These results indicate that the current sampling protocols are resulting in a reliable level of sampling precision and population trend consistency, especially when considering the substantial changes in the occurrence and density of Rio Grande Silvery Minnow over time. Additionally, these population metrics were far more closely related to seasonal flow conditions across years than to local/regional sampling conditions (i.e., sampling occasions, mesohabitats, or reaches).

Site occupancy models, based on repeated sampling (November [2005–2023]), further revealed that Rio Grande Silvery Minnow occupancy probabilities decreased from 2019 (1.00) to 2022 (0.49) before improving (0.75) in 2023. While estimated extinction probabilities were elevated during an extended drought (i.e., 2012–2014), they decreased substantially from 2014 to 2017, as seasonal river flows progressively improved. Likewise, estimated colonization probabilities increased considerably from 2013 to 2017, as this species gradually repatriated multiple sites that had been previously unoccupied. However, these positive trends reversed in 2018 and 2020 following poor spring and summer flow conditions. While the long-term conservation status of Rio Grande Silvery Minnow declined from 2019 to 2022, its status again improved somewhat in 2023.

Pronounced changes in the occurrence and density of Rio Grande Silvery Minnow over the past three decades were closely related to the timing, duration, and magnitude of river flows during spring and summer. Prolonged and elevated spring flows result in extensive overbank flooding of vegetated areas and the formation of inundated habitats within the river channel (e.g., shoreline pools and backwaters). The unique early life history of this species ensures that its propagules (drifting eggs and larvae) are rapidly dispersed throughout these low-velocity and productive habitats when spring flows begin to rise. These conditions, combined with a protracted spring runoff, help ensure the inundation of nursery habitats for larval fish. As growth from the egg phase through the vulnerable early larval phases (i.e., protolarvae and mesolarvae) requires about one month, the stability and persistence of these habitats is essential for ensuring the successful recruitment and survival of young to later life phases (i.e., metalarvae and juveniles).

Rio Grande Silvery Minnow was consistently most abundant in downstream reaches (i.e., Isleta and San Acacia) of the Middle Rio Grande. This long-term pattern could be explained by the cumulative displacement of propagules past instream barriers. Also, river incision, habitat degradation, abandonment of the floodplain, and reductions in suspended sediments downstream of Cochiti Dam are likely limiting the number and size of appropriate habitats available for the successful retention and recruitment of early life phases, especially in the Cochiti and Angostura reaches. While it is evident that seasonally elevated flows, combined with habitat restoration, should lead to increased recruitment success, the long-term efficacy of those efforts will also depend on ensuring their utility and permanence by restoring a more dynamic flow regime and reestablishing river connectivity across fragmented reaches.

While extensive and diverse conservation-management efforts over the past three decades have provided protection against the extinction of Rio Grande Silvery Minnow, ongoing and planned efforts (e.g., restoring dynamic river flows, reconnecting fragmented reaches, and reestablishing a functional floodplain) should help to support resilient and self-sustaining populations of this imperiled species in the future. Fortunately, both the occurrence and density of Rio Grande Silvery Minnow increased in 2023, following high and prolonged flows during spring runoff. Continued efforts to provide reasonable spring spawning and summer survival conditions will be essential for securing a self-sustaining wild population of this species in the Middle Rio Grande. Additionally, reestablishing resilient populations at other locations within its historical range would substantially help to further ensure its long-term persistence in the wild. Finally, future study of the relationships among aquatic species (i.e., from plankton to fish), instream habitats, and river flows should continue to elucidate key factors that regulate this complex ecosystem, which will be essential for developing and implementing successful strategies that support the long-term recovery of Rio Grande Silvery Minnow.