Alligator gar (Atractosteus spatula) populations are declining throughout their range in the United States, even where considered stable. Similar declines in other riverine species are occurring worldwide due to alteration of habitat in river-floodplain systems. Most rivers in the world are highly regulated, resulting in departures from historic hydrology that provides connection to the floodplain habitats used by riverine fishes. Knowledge of the seasonal habitat requirements of alligator gar and similar riverine fishes at a watershed scale is limited, hindering management practices. We used radiotelemetry to monitor movements and habitat use of adult alligator gar (N = 32; 25.0-84.5 kg; 149-224 cm) at macro-and microhabitat scales during 2 years of varying hydrology. Fish showed seasonal differences in their use of main channel and floodplain macrohabitats. Floodplain tributaries and their upper reaches were especially important to reproductive ecology. Minimum distance travelled per day varied seasonally. While using the main channel Fourche LaFave River, gar selected low velocity, shallow depth and complex structure along channel margins. The Fourche LaFave River is unique in its connection to floodplain habitats on a relatively predictable basis, with frequent flooding during the spring and summer reproductive period. Our study emphasises the importance of connectivity between the main river channel and floodplain habitats to a floodplain-obligate riverine species. An intact, heterogeneous riparian zone creates essential microhabitat for the species. Understanding habitat requirements of alligator gar at multiple spatial scales in a river-floodplain system is crucial to management of many other species and river systems.
Connection events between rivers and their adjacent floodplains can alter availability of resources. Riverine fishes opportunistically exploit seasonally available resources during periods of high water. Gars are known to utilize floodplain habitats throughout different life stages for spawning and feeding. Food habits of gars have been the focus of many studies; however, less is known regarding the partitioning of food resources between syntopic species overlapping in body size. Further, little information exists on food resources of shortnose gar, Lepisosteus platostomus. We report results of diet analysis of spotted, Lepisosteus oculatus, and shortnose gar from the Fourche LaFave River in central Arkansas. Stomachs were examined from 74 adult spotted gar (46–81 cm TL) and 91 adult shortnose gar (49–76 cm TL) collected between May and July 2007 during flooding. Forty‐seven (64%) spotted and 54 (59%) shortnose gar contained identifiable prey items. Spotted and shortnose gars had low diet overlap (33.0%), indicating these two species partition food resources. Considering percent composition by weight (%Cw), spotted and shortnose gar appear to partition available food resources during summer flooding. Percent composition by weight of fishes and crustaceans was significantly greater in the diet of spotted gar than shortnose gar (Table 2). Shortnose gar had significantly greater %Cw of amphibians and terrestrial invertebrates compared to spotted gar. It is likely that the seasonal and consistent annual availability of terrestrial subsidies to the system holds a unique importance for the shortnose gar population.
Riverine habitat mosaics, including tributaries, are an important reason the Green River subbasin supports the largest remaining population of federally endangered Colorado Pikeminnow Ptychocheilus lucius in the Colorado River Basin. Upstream Colorado Pikeminnow distribution is limited by Flaming Gorge Dam and few typically occurred in the reach immediately downstream of the dam, which is most affected by thermally and hydrologically altered dam releases. However, fish captures and passive integrated transponder (PIT) antenna sampling of previously tagged individuals from 2011-2021 revealed seasonal congregations of up to 75 Colorado Pikeminnow annually in the mouth of Vermillion Creek, a small tributary in the regulated reach. Approximately 11% of the entire 2017-2018 Green River basin population (N=93 individuals) were encountered in Vermillion Creek over the 11-year study, an underestimate of use considering untagged fish were not detected by antennas. Colorado Pikeminnow used Vermillion Creek primarily when Green River spring flows from Flaming Gorge Dam were high and cold in May through mid-June when the confluence was a large, deep backwater that was warmer than the main channel and supported forage fishes. Intra-annual encounters revealed seasonal residence times for individual Colorado Pikeminnow up to 91 days, and multiple inter-annual encounters indicated site fidelity. Frequent detections of individual Colorado Pikeminnow in a Yampa River spawning area soon after their detections in Vermillion Creek indicate this tributary may be an important resource for reproductive adults. The intensive and basin-wide PIT tagging and detection program for Colorado Pikeminnow enhanced our understanding of the importance of small habitat nodes such as Vermillion Creek in the Green River drainage network. Understanding and protecting these seasonally available riverine habitat mosaics used for prespawning conditioning may assist with recovery of Colorado Pikeminnow.
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