2021 RGSM Population Monitoring

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 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...