2022 RGSM Population Monitoring

Robert K. Dudley, American Southwest Ichthyological Researchers and Museum of Southwestern Biology (Fishes), UNM

August 15, 2023


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 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 (1993–2022).

The annual occurrence and density of Rio Grande Silvery Minnow, during October, has fluctuated widely over the past three decades (1993–2022). 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 an 88.8% decrease in its density from 2019 (2.10) to 2020 (0.23), and its density had declined further by 2022 (0.08). While Rio Grande Silvery Minnow represented 8.61% of the fish community in 2019, it had decreased to only 0.68% by 2022.

Changes in the occurrence and density of Rio Grande Silvery Minnow were reliably predicted by seasonal differences in river flows across years (1993–2022). 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 [2002–2022]), or on different days during repeated sampling (November [2005–2022]), were comparable to population trends obtained from the long-term dataset (October [1993–2022]). 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 [2005–2022]), further revealed that Rio Grande Silvery Minnow occupancy probabilities decreased from 2019 (1.00) to 2020 (0.65), increased modestly in 2021 (0.81), but decreased markedly in 2022 (0.49). 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 in recent years (2013–2017), as this species gradually repatriated multiple sites that had been previously unoccupied. However, these trends reversed in 2018 and 2020 following poor spring and summer flow conditions. While the long-term conservation status of Rio Grande Silvery Minnow improved from 2020 to 2021, its status again declined in 2022.