Tributary streams provide migratory fish with access to floodplain habitats in a regulated river: evidence from alligator gar, Atractosteus spatula

Abstract

Migratory fishes with periodic life history strategies are sensitive to river regulation. Populations of these fishes may persist in highly regulated rivers by using tributaries that provide access to intermittently connected spawning and early life stage habitat in floodplains. We analyzed system hydrology and associated movement of a long-lived periodic strategist, the alligator gar (Atractosteus spatula), in the Brazos River, Texas. We hypothesized that: 1) flow regulation on the mainstem has resulted in the reduction of flood pulses, but tributaries have been less altered; and 2) alligator gar migrate into tributaries during high flows and temperatures. Our analysis revealed that flood pulses were reduced in the mainstem but not in an adjacent, less regulated tributary where floodplain-inundating pulses now outnumber those in the mainstem. Using data from tagged fish, we derived statistical models predicting greater occurrence of alligator gar in tributaries when water temperature exceeded 25° C and mainstem discharge exceeded 400 cms. These results emphasize that movement into less regulated tributaries can mitigate impacts of river regulation on mainstem river populations of alligator gar.

Diverse portfolios: Investing in tributaries for restoration of large river fishes in the Anthropocene

“…Tributaries also can provide habitat conditions that have been lost in contemporary mainstem rivers (i.e., vestige large river habitats) due to regulation or channelization. For example, alligator gar (Atractosteus spatula) in the regulated Brazos and Arkansas rivers make predictable seasonal movements into freeflowing tributaries to access inundated floodplains as discharge and water temperatures rise (Naus and Adams, 2018;Roberts et al, 2022). In the lower Missouri River, channelization and levees preclude low velocity, shallow habitat conditions within the mainstem; however, the lower reaches of tributaries often exhibit reduced velocities and shallower depths compared to the mainstem (Brown and Coon, 1994).…”

“…Thus, the value of a tributary likely depends on the ability of tributaries to provide habitats that fulfill specific life-history functions of mainstem fishes. Small tributaries within floodplain rivers can provide offchannel, low-velocity habitats, Brown and Coon, 1994;Bottcher et al, 2013;Naus and Adams, 2018;Phillis et al, 2018) and vital conduits to backwater habitats (Roberts et al, 2022). Further, small tributaries may contribute to invertebrate and allochthonous inputs to mainstem regulated rivers (Whiting et al, 2014;Sabo et al, 2018;Milner et al, 2019), which can be food limited given extensive alteration (Cross et al, 2013).…”

Applying Floodplain Inundation Modeling to Estimate Suitable Spawning Habitat and Recruitment Success for Alligator Gar in the Guadalupe River, Texas

“…Numerous studies on Alligator Gar movement provide evidence of their preferential use of different habitats relative to seasonal patterns and flow conditions [8,10,15,[18][19][20][21][22][23]. Allen et al [15], Buckmeier et al [20], and Kluender et al [22] reported the least amount of movement during the cooler winter season and increased movement relative to warmer temperatures and higher water levels associated with their spawning season.…”

“…Allen et al [15], Buckmeier et al [20], and Kluender et al [22] reported the least amount of movement during the cooler winter season and increased movement relative to warmer temperatures and higher water levels associated with their spawning season. In riverine settings, reproductive migrations of Alligator Gar included two distinct macrohabitats off the main channel, inundated floodplains [10,15,20,22,23] and tributaries [18,22,23]. Data from these studies, and others, help constrain known factors that influence floodplain spawning behavior.…”

“…Allen et al [10] advanced knowledge in a much larger area of the Mississippi River spanning 93 km length of floodplain valley from Kentucky to Louisiana with a more focused case study also near Natchez, Mississippi on the floodplain of the protected St. Catherine Creek National Wildlife Refuge (SCCNWR), a site studied extensively by Allen et al [15] via fieldsurveyed Alligator Gar movement and water quality patterns. Within Texas, Hoeinghaus et al [29] focused on a 120 km reach of the lower Trinity River, Robertson et al [3] examined a 334 km reach of the middle Trinity River, and Roberts et al [23] assessed the movement to spawning habitats on a 200 km reach of the lower to middle Brazos River and 147 km of large tributaries (Little River, Navasota River, and Yegua Creek). While the use of tributaries is previously documented [18,22], Roberts et al [23] made an important contribution by documenting the movement of Alligator Gar into unregulated tributaries of the regulated Trinity Rivers in response to flood pulses and floodplain access in these less flow-altered networks of the river system.…”