Federal Status: Threatened
The Concho Water Snake is small compared to most other water snakes, with adults rarely exceeding 3 feet in total length. This nonvenomous snake has four rows of alternating dark brown spots or blotches on its back, two rows on each side. The coloration on its back is similar to a checkerboard of dark brown spots on a gray, brown, or reddish-brown color. The Concho Water Snake has a light pinkish or orange belly that is unmarked or has somewhat indistinct spots along the sides.
Two other water snakes occur within the range of the Concho Water Snake. Both the Diamondbacked Water Snake (Nerodia rhombifer rhombifer) and the Blotched Water Snake (Nerodia erythrogaster transversa) have dark markings on the back. However, adult Diamondbacked and Blotched Water Snakes may be distinguished from adult Concho Water Snakes by their larger size. The Diamondbacked Water Snake has a distinct black cha in-like pattern on its back. The Blotched Water Snake has three rows of large squarish blotches on the back, which are especially prominent in juveniles. As Blotched Water Snakes age, they become darker in color and may appea r to lack markings.
The cottonmouth is another large aquatic snake that may be confused with the Concho Water Snake. The cottonmouth, a venomous snake, is usually uniformly black or dark brown with little or no trace of a pattern, although both neonates and juveniles are banded; neonates have a yellow or gold-colored tail tip, juveniles retain it but not as distinct. Cottonmouths often vibrate their tails when excited, whereas water snakes usually do not. Also, an aroused cottonmouth will sometimes stand its ground, throw its head upward and backward, and hold its mouth wide open, revealing a
white “cotton-like” interior.
Historically, the Concho Water Snake occurred over about 276 river miles of the Colorado and Concho Rivers in central Texas. The snake was first collected from the South Concho River and Dove Creek, which are tributaries to the Concho River west of San Angelo, Texas. When the sub species was described in 1961, these records and one other on the Colorado River south of Robert Lee in Coke County were the only known localities for this snake. The Concho Water Snake is endemic to Texas, which means it lives nowhere else in the world. It has one of the smallest distributions of any North American snake.
The Concho Water Snake may once have been more widely distributed, but the E.V. Spence Reservoir upstream and Lake Buchanan downstream have inundated many miles of river habitat at both ends of the current range. Scientists have estimated the historic range of the subspecies based on museum records and unpublished records supported by specimens. The probable historic range of this snake is estimated to include, at a minimum, the Colorado River from Spence Reservoir down stream to the vicinity of Lake Buchanan, Elm, Bluff, and Coyote Creeks (Runnels County), and the entire Concho River (Tom Green and Concho Counties) and its headwater tributaries, Dove Creek, Spring Creek, and the South Concho River (Irionand Tom Green Counties).
Today, the Concho Water Snake occupies a restricted geographic range in the Concho and Colorado River Basins. The current distribution includes relatively continuous occupation of riverine habitat of the Col orado River below the town of Bronte (Coke County), of Elm, Coyote, and Bluff creeks below Winters (Runnels County), and of the Concho River from San Angelo (Tom Green Country) downstream to its confluence with the Colorado River, and then downstream to the FM 45 bridge over the Colorado River (Mills and San Saba Counties). This is a distance of about 233 river-miles.
Apparently isolated lake populations have been found in E.V. Spence Reservoir and Ballinger Municipal Lake (formerly Lake Moonen). Concho Water Snakes have also been found at a number of sites in O.H. Ivie Reservoir, and there are indications that this population is reproducing. Scattered river populations occur along the Colorado River above Lake Buchanan, near the towns of Regency, Harmony Ridge, Adams, and Bend. Recently, Concho Water Snakes have been found at all six artificial riffles (fast-flowing, shallow water over a rocky bottom) constructed in 1989 in the 17-mile stretch of the Colorado River between the Robert Lee Dam and Bronte.
Although the Concho River has been dammed and channelized within the City of San A ngelo, a population of Concho Water Snakes persists just below the Bell Street Bridge. They have also been found about 4 river miles downstream from Bell Street Dam. From this point they are pre sent in all suitable habitats to the confluence with Ivie Reservoir, a distance of about 43 rivermiles.
Optimal habitat for the Concho Water Snake consists of free-f lowing streams over rocky substrates, abundant rock debris and crevices for shelter, and shallow riffles. Periodic scouring by floods is important in providing relatively sediment free rock rubble and open banks.
Riffles, considered critical to juvenile survival, are characterized by shallow, fast-flowing water connecting deeper areas of quiet water. Riffles begin when the upper pool overflows at a change in slope and forms rapids. They end when the rapids enter the next downstream pool. Riffles often contain bars, shoals, or islands separated by flowing water. Limestone bedrock shelves in and along the stream channel seem to support the largest snake populations. The snakes forage and seek cover among the numerous splits, crevices, cracks, and jumbled stream cobble of shelf rock. Other rock, such as limestone boulders, can also provide suitable habitat.
Juvenile snakes are generally restricted to rocky riffles. Neonates (newborn snakes) are most often found in gravel bars or along the shoreline in areas where rocks range in size from small cobbles to small boulders. However, some habitats with thriving populations lack typical gravel bars. In these areas, juveniles use boulders and shelf rock for cover. During their second year, snakes begin to use larger rocks, usually medium to large boulders.
Mature snakes use a much wider range of habitats than juveniles. Although adults forage in riffles, they are known to use a variety of cover sites for resting, including exposed bedrock, thick herbaceous vegetation, debris piles, and crayfish burrows. During the latter stages of gestation, gravid females occupy dense patches of vegetation and brush piles.
In lake habitats, Concho Water Snakes occupy areas of broken rock along the shoreline. Although they seem to prefer the shallower areas, they are occasionally found on steeper shorelines where rock is present. As in river habitats, first-year snakes use smaller rocks for cover, while mature snakes use medium to large rocks. When available, dead shrubs and trees killed by fluctuating water levels are used as basking sites by juveniles and adults. At Spence Reservoir, where there are almost no dead trees or shrubs, snakes bask on the ground among the protection of rocks.
Bank and shoreline vegetation is important in providing cover and basking sites for Concho Water Snakes. A lthough the type of vegetation does not appear to be important, its use depends on vegetation density and orientation. For example, pregnant females seek basking sites protected by thick vegetation. Larger trees and shrubs, such as pecan, cedar elm, and willow, with limbs that hang over water, provide basking sites for juveniles and adults. Common bank and shoreline vegetation used for cover and basking sites include switchgrass, devil-weed aster, greenbrier, poison ivy, willow, salt cedar, button bush, hackberry, pecan, cedar elm, and mesquite.
Concho Water Snakes hibernate during the winter, either singly or in small groups. Adults use a variety of sites for hibernation, including cray fish burrows, rock ledges, debris piles, and concrete low water crossings. These sites are usually within 20 feet of the water. Newborn snakes have been found hibernating in areas of loose rock and moist soil.
Concho Water Snakes are active primarily from March through October. Adult activity gradually decreases during June and remains low until mid-September. Activity levels increase again during late September and October. The snakes enter the hibernation site (hibernacula) during late October, although they can occasionally be seen on warm winter days basking in the open. Newborn Concho Water Snakes, born in August and September, are commonly found under rocks in late summer and early fall. In the heat of the summer, Concho Water Snakes are active primarily
in the early morning and evening until about 9 p.m.
Research indicates that adult males move an average of 141 to 325 ft/day. Pregnant females moveless, averaging 62 to 131 ft/day, with distances decreasing as parturition approaches, and increasing again after the young are born. Linear distances of river habitat occupied by individual snakes range from 689 to1,542 feet.
Long range movements of 3.1 and 4.5 miles have been recorded for juvenile snakes dispersing from their birth place. In one instance, a snake moved 12 river-miles over a four-year period.
The diet of the Concho Water Snake is composed almost entirely of fish. In river habitats, minnows are most often consumed. Neonates (newborn snakes) feed almost exclusively on minnows, particularly the red shiner and bullhead minnow. Their diet becomes more varied as their body size increases. In addition to minnows, large snakes consume mosquitofish, channel catfish, f lat head catfish, gizzard shad, and several species of sunfish. The bullhead minnow, sheepshead minnow, and bigscale logperch were found to be the dominant prey of snakes in Ballinger Municipal Lake.
Concho Water Snakes catch prey by remaining stationary near fish concentrations or by actively searching under and around rocks in riff les. Juveniles are most often seen using the “sit-and-wait” strategy.
Mating occurs predominantly in April and early May, and sometimes again in October. Litter sizes average 10 embryos per female, and births occur from late July through September. As females increase in size during their lifetimes, their litter size also increases.
Concho Water Snakes grow rapidly and mature early, at about 11 to 12 months of age. Females produce their first litters at 2 or 3 years of age. Females grow more rapidly and mature at larger sizes than males, with adult females reaching a length on average 30% greater than adult males. Differences in growth rates and mature sizes have been observed between populations, suggesting differences in food availability.
Survivorship of Concho Water Snakes is directly related to age. Only about 20% of Concho Water Snakes survive their first year. The adult survival rate is estimated to be about 50 percent. Population studies have shown that most adults are less than 4 years old, and only one snake in 100 exceeds 5 years of age. Predation is considered to be a significant source of mortality. Major natural predators include kingsnakes, Coach whip Snakes, racers, Raccoons, Great Blue Herons, and various hawks and owls.
To be continued…