Exploring movement patterns of an endangered minnow in a fragmented desert river

Authors: Martinique Chavez, Phaedra Budy, Casey Pennock

Year: 2022

Abstract

Most large rivers in North America are fragmented by dams that modify natural flow regimes, reduce connectivity, and imperil freshwater fishes by restricting movement among patchily distributed resources. Coincident with widespread river fragmentation are declines of numerous endemic desert fishes. The Rio Grande Silvery Minnow (Hybognathus amarus, RGSM) has experienced a 95% reduction in its historical range. The overall goal of this study is to better understand the movement ecology of RGSM, and identify possible drivers of movement patterns. We used Passive Integrated Transponder (PIT) tags in stocked RGSM with stationary and mobile PIT-tag antenna systems to detect and track movement patterns across time and space. We detected 16,036 PIT-tagged RGSM of the 36,644 released, an extremely high resight rate (43%). The mean distance moved by PIT-tagged RGSM was 10.6 km, and the maximum distance moved was 142.9 km. Tag movements were generally directed downstream, but 26% of movements were directed upstream. We observed a bimodal distribution of home range sizes with a mean home range of 2.4 km and a maximum home range of 78.2 km. The strongest predictor variables for an individual having a larger home range size were number of days at large (R2 = 0.22) and number of days above 1,000 cfs (R2 = 0.19), indicating RGSM movement might be associated with higher flows. Surprisingly, a total of 104 individuals passed upstream through the San Acacia Dam. Our data suggest RGSM are highly mobile, with the predilection to make long-distance movements both upstream (to the extent possible) and downstream. Direct evidence of RGSM movement patterns may provide crucial insights that could help target recovery efforts and highlight the need to restore connectivity through the removal or redesign of diversion dams to include fishways. A greater understanding of how river fragmentation impacts the movement patterns of imperiled fishes can help guide and better inform the management of desert riverscapes to ultimately achieve self-sustaining populations of native fishes.