Collaborative Seminar: 2021 RGSM Population Monitoring


Date: 2022/08/23

Author(s): Dudley R.K.

Publication: Video Link


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 in that it has been providing consistent sampling of fishes over a very long 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 (1993–2021).

The annual occurrence and density of Rio Grande Silvery Minnow, using October data (i.e., as required by USFWS’s Biological Opinion), has fluctuated widely over the past three decades (1993–2021). Its estimated density (E(x); fish per 100 m2) was notably higher in 2016 and 2017, as compared with 2015, but then decreased dramatically from 2017 to 2018. Recent monitoring efforts revealed an 88.8% decrease in its density from 2019 (2.10) to 2020 (0.23), and its density remained low in 2021 (0.27). While Rio Grande Silvery Minnow represented 8.61% of the fish community in 2019, it had decreased to only 0.67% by 2021.

Changes in the occurrence and density of Rio Grande Silvery Minnow were reliably predicted by seasonal differences in river flows across years (1993–2021). 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 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 [2002–2021]), or on different days during repeated sampling (November [2005–2021]), were comparable to population trends obtained from the long-term dataset (October [1993–2021]). 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, Rio Grande Silvery Minnow 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 [2005–2021]), further revealed that Rio Grande Silvery Minnow occupancy probabilities increased from 2018 (0.77) to 2019 (1.00), decreased markedly in 2020 (0.65), but increased again in 2021 (0.81). While estimated extinction probabilities were elevated during recent drought years (i.e., 2012–2014), they decreased substantially from 2014 to 2017, as seasonal river flows progressively improved. Likewise, estimated colonization probabilities increased considerably in recent years (2013–2017), as this species gradually repatriated multiple sites that had been previously unoccupied. However, these trends reversed in 2018 following poor spring and summer flow conditions. While the overall conservation status of Rio Grande Silvery Minnow declined from 2019 to 2020, its status again improved in 2021.

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, warm, and productive habitats when spring flows begin to rise. These conditions, combined with a protracted spring runoff, help ensure the inundation and persistence of these nursery habitats, which are required for the successful growth, survival, and recruitment of newly spawned Rio Grande Silvery Minnow. As growth from the egg phase through the vulnerable early larval phases (i.e., protolarvae and mesolarvae) requires about one month, the persistence of these habitats seems essential for ensuring the successful recruitment of young to later life phases (i.e., metalarvae and juveniles).

Further, Rio Grande Silvery Minnow was consistently most abundant in downstream reaches (i.e., Isleta and San Acacia) of the Middle Rio Grande. This pattern has persisted over time (1993–2021) even though upstream reaches have been regularly augmented with large numbers of hatchery-reared fish. One explanation for this pattern is the cumulative downstream transport of propagules (drifting eggs and larvae) past instream barriers over time. Also, river channelization, habitat degradation, abandonment of the floodplain, and reductions in suspended sediments downstream of Cochiti Dam are likely limiting the number 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 assuring 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 slightly in 2021, following improved spring and summer flow conditions. 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 phytoplankton 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 for the long-term recovery of Rio Grande Silvery Minnow.

Related Information
  • Species: Rio Grande Silvery Minnow
  • Groups: Collaborative Seminar
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