Failure to achieve recommended environmental flows coincides with declining fish populations: Long‐term trends in regulated and unregulated rivers

Abstract

Dams can be operated to mimic components of the natural flow regime to minimise impacts on downstream ecosystems. However, infrastructure, societal needs, water management, and catchment runoff constrain which and when flow regime attributes can be mimicked. We compared fish assemblage responses, including native and non‐native species, over 2 decades of managed environmental flows to those in a river retaining a relatively unaltered flow regime. Both of these arid‐land rivers are within the overallocated Colorado River basin and have experienced declines in catchment runoff over the past 20 years. We predicted that fish–flow relationships would be conserved across time and between managed and unmanaged rivers. Declines in flow in both rivers coincided with declines in some native fishes, and more native and non‐native fish species exhibited declines in the managed river than in the unmanaged river. Our ability to detect previously documented native fish–flow relationships diminished in the managed river system because established environmental flow targets were not met due to water management, but we detected these fish–flow relationships in the unmanaged river. Our results suggest declining catchment runoff and increased consumptive water use could reduce the effectiveness of environmental flows that have lower priority in most years.

Changes to the trophic structure of a desert fish community following river regulation and species turnover: implications for an endangered top predator

“…In the 1990s, river ecologists began highlighting the importance of a river's annual peak discharge in creating fish habitat as high flows can create and maintain fish habitat through increased channel heterogeneity (Poff, 1997;Van Steeter and Pitlick, 1998). This research supported the establishment of Navajo Dam flow recommendations developed to mimic the timing, frequency, and duration of the San Juan River's natural hydrology albeit at reduced magnitudes (Holden, 1999;Pennock et al ., 2022). Although Navajo Dam operators attempted to meet these flow recommendations, continued significant deviations from the natural hydrograph and the establishment of nonnative vegetation likely continued to simplify and narrow the river's channel which precluded inundation of the floodplain (Gidoet al ., 2013;Bassett, 2015;Franssen et al ., 2015).…”

“…While changes to the San Juan River invertebrate community are unknown, if invertebrate numbers are insufficient, competition for this prey could reduce overall fish abundances (Van Poorten et al ., 2018). Indeed, the densities of invertivorous Flannelmouth Sucker, Bluehead Sucker, Speckled Dace, and Red Shiner have declined in the San Juan River over the last two decades following ongoing deviations from the natural flow regime (Pennock et al ., 2022). Given the surprising and significant increase in niche overlap by Colorado Pikeminnow into all other species, this may also be indicative of a paucity of fish prey given their presumed piscivorous nature (Vanicek and Kramer, 1969;Franssen et al ., 2007;Franssen et al ., 2019).…”

“…A significant contribution to these low survival rates seems unlikely to be driven by nonnative fish predation as neither Colorado Pikeminnow nor more common fishes, responded positively to intensive nonnative fish removal (Franssen et al ., 2014); possibly because the most abundant large-bodied nonnative fish, Channel Catfish (Ictalurus punctatus ), is not highly piscivorous (Hedden et al ., 2021;Pennock et al ., 2021;Heddenet al ., 2022). Additionally, attempts to mimic a more-natural hydrograph have been hampered by increased aridity in the San Juan River basin (Pennock et al ., 2022).…”

Fisheries Volume 48 Number 5 May 2023

“…Base fl ows were consistently >11.3 m 3 /s over the past decade, despite low rain and snowfall in 2018, but the duration and frequency of high fl ows during spring runoff and monsoon seasons declined. The absence of high fl ows coincides with a gradual decline in densities of native and nonnative fi shes (Pennock et al 2022 ), possibly caused by armoring of substrates, but densities of nonnative fi shes increased in recent years following record lows in 2016.…”

“…Water storage crises are occurring in reservoir and groundwater-regulated systems, and sustained drought creates a positive feedback where water scarcity increases demands on stored water. Flow recommendations on the San Juan River were established to benefi t native fi shes, but water scarcity has limited the ability to implement the recommended magnitude and duration of high fl ows necessary to move bed sediments and freshen spawning habitat of native fi shes (Pennock et al 2022 ). The legacies of megadroughts, particularly in water-limited regions where large agricultural and urban demands exist, will necessitate a rethinking of both the design and implementation of environmental fl ows to ensure favorable ecological outcomes (Stein et al 2021 ).…”