Seeing Is Believing: Alligator Gar Spawning Event Confirms Model Predictions

Kayla Kimmel, Yvonne Allen & Glenn Constant

US Fish and Wildlife Service

Baton Rouge Fish and Wildlife Conservation Ofce &

Gulf Coastal Plains and Ozarks Landscape Conservation Cooperative

Background The floodplain is an integral part of large river ecosystems where high flows provide connectivity to inundated lands that ofer elevated primary production, moderate environmental conditions and vegetation for refuge. Like many other floodplain dependent aquatic species, the alligator gar (Atractosteus spatula) requires access to the seasonally inundated floodplain for spawning and nursery habitat (Welcomme 1979, Bayley 1988, Kwak 1988, Agostinho et al. 2004, Balcombe et al. 2005). The alligator gar is a large, long-lived, apex predator and serves as an important recreational and commercial fishery (Inebnit 2009; Kluender 2011, Buckmeier 2013). Historically, alligator gar were distributed throughout the central U.S., ranging from Oklahoma southward to the Gulf of Mexico but more recently, abundances have declined and alligator gar are now considered vulnerable to localized extirpation (Ferrara 2001). Hydrologic alterations including levee construction, dam building, and channel training over the past century have disconnected over 90% of the historical floodplain from the main Mississippi River (Baker et al. 1991). This reduction in habitat is particularly detrimental for riverine species like alligator gar that depend on a much larger floodplain for reproductive success (Simmon and Wallus 1989; Etnier and Starnes 1993; Boschung and Mayden 2004). Due to concern for decreased population sizes, alligator gar have been identified by the American Fisheries Society (AFS), the US Fish and Wildlife Service, many state fisheries programs, and the Gulf Coastal Plain Ozarks Landscape Conservation Cooperative as a species of concern in the Mississippi Alluvial Valley.

Alligator gar have frequently been observed using the floodplain on St. Catherine Creek National Wildlife Refuge near Natchez, Mississippi. The refuge lies on a narrow corridor between the Mississippi River to the west and high elevations to the east in which levees were not constructed. As a result, rise and fall of the Mississippi River directly influences inundation extent and duration on the floodplain. This is one of the few areas remaining that is still directly connected to the lower Mississippi River. Because of the strong numbers of gator gar here, the Service’s Refuge and Fisheries Programs (SCC and BRFWCO) have been engaged for many years in a partnership to collect alligator gar from St. Catherine Creek for propagation at the Private John Allen National Fish Hatchery. The ofspring of these fish are then stocked within historic ranges that have been depleted or extirpated. In an efort to better understand the importance of St. Catherine Creek NWR to alligator gar and other floodplain-dependent species this partnership began a tagging and telemetry efort to document movement and habitat use throughout the flood cycle. In addition to this study, members of the Alligator Gar Technical Committee have identified the variables most critically limiting to alligator gar populations in the lower Mississippi River as inundation extent and frequency, connectivity, temperature, and availability of suitable physical structure as related to spawning habitat. With the telemetry efort and this research need in place the Baton Rouge Fish and Wildlife Conservation Ofce requested the assistance of the Gulf Coastal Plains and Ozarks Landscape Conservation Cooperative to find landscape level geospatial data that would adequately capture these variables at a landscape scale and determine similar locations throughout the lower Mississippi river that may ofer suitable spawning habitat for alligator gar.

A multi-temporal analytical approach based on Landsat imagery was used to develop novel landscape scale spatial information related to inundation frequency and thermal conditions on the floodplain. These data products were combined with an existing geospatial assessment of physical structure (land cover mapping) to determine a habitat suitability measure for spawning by alligator gar on the lower Mississippi River. Landsat satellites have provided an invaluable library of images documenting changing landscape and environmental conditions. This library of imagery can assist scientists, managers and the public in understanding not only dynamics of land conditions, such as urbanization, drought, storms and fires, but it can also capture the dynamics and impacts of changing water conditions. This information was coupled with an intensive telemetry array and continuous water quality monitoring program which resulted in the Alligator Gar Habitat Suitability Index and a greatly improved understanding of floodplain dynamics that would not have been possible otherwise. Seeing Tuesday April 22, 2014 Baton Rouge Fish and Wildlife Conservation Ofce and St. Catherine Creek National Wildlife Refuge were able to document alligator gar spawning in a shallow floodplain area of the refuge. The splashing of the gar was an indicator and sign the fish were displaying spawning behavior. The alligator gar were in a combination of habitat that consisted of a low depression ditch lined with button bush, shrubs and herbaceous vegetation at the edge of an overgrown ag field with woody vegetation. There was little canopy cover. The distance from the spawning location to the closest permanent water holding area (the Bluehole) is roughly 4 miles. The next closest holding area, Butler Lake, is 9 miles away, and the Mississippi River is roughly 12 miles distant. The water temperature was 23.3°C (74°F) and water depth ranged from 12 – 48 inches. There was little flow at the spawning location but the water was slightly pushing in. It was estimated that there were roughly two hundred gar in a ditch that was less than a half a mile long. The gar were visible and swam in large groups of up to 10 fish. Usually a large female would lead followed by several males that would nudge and bump her. The small size (~36 inches) of a few of the alligator gar partaking in spawning was surprising. The water was not turbid in most areas and the green eggs of the alligator gar adhered to the herbaceous and woody vegetation. There were thousands of eggs in the small amount of water. None of the eggs had hatched yet indicating the fish started spawning within the past 2 days. Two fish with transmitters were detected with the mobile acoustic receiver. They were both male alligator gar that were tagged March 3, 2013 and are part of the telemetry study. This spawning event and location was combined with their detailed movement data. Believing Documenting this event allowed the alligator gar spawning habitat suitability model to be tested. That was very important considering the model combines information related to floodplain inundation frequency, thermal characteristics and physical structure derived from satellite imagery and many other sources. The model (figure below, right) identified the best possible spawning habitat in the color red, and even at this small scale and fine resolution it is accurate for this location (marked with black cross). Inundation extent (figure below, left) was also checked on the ground level and was accurate as well. While the thresholds developed for this application are specific to alligator gar spawning, the landscape level data products of inundation frequency and thermal habitat character combined with various evaluations of land cover are potentially applicable to a wide variety of both terrestrial and aquatic species that occupy the floodplain habitat of the St. Catherine Creek NWR.