Overview
Even as far as minnows go, the sharpnose shiner (Notropis oxyrhynchus) is considered to be small and slender. Sharpnose shiners are currently restricted almost entirely to the contiguous river segments of the upper Brazos River basin in north-central Texas. The Brazos River and its major tributaries, the Double Mountain and Salt Forks, provide fairly shallow, flowing water with sandy substrates that comprise the general habitat for the sharpnose shiner. Historically, the shiner was found in most of the Brazo River, Wichita River, and Colorado River in Texas; however major impoundments (reservoirs) that began in 1940’s, have now restricted the shiner to the upper Brazos basin, upstream of Possum Kingdom Lake. This represents an approximate 50 percent reduction in its historical range.
Throughout much of its historical range, the decline of the sharpnose shiner is attributed primarily to habitat loss and modification due to fragmentation and decreased river flow resulting from major water impoundments, drought, and groundwater withdrawals. Water quality degradation, invasive salt cedar, and other factors may have also contributed to its decline. The sharpnose shiner has been of conservation concern since 1982 and was determined to be an endangered species in 2014.
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Characteristics
Food
The sharpnose shiner is a generalist feeder, relying on a variety of food items to sustain growth and reproduction. Averaged over one year, the gut contents (by weight) of sharpnose shiners consisted primarily of invertebrates (71 percent), sand-silt (18 percent), plant material (7 percent), and detritus (4 percent). However, feeding habits vary by season with most of the sand-silt gut contents occurring mid-summer, plant contents during spring and summer, and detritus contents during spring and fall. Invertebrate consumption, primarily insects, make up a majority of the diet of the sharpnose shiner except during midsummer when pools become isolated and the gut contents shifts primarily to sand-silt and plant material. The prevalence of sand-silt in the digestive tract of the sharpnose shiner suggests that this species forages among sediments on the river bottom. The proportion of terrestrial insects in the diet of the sharpnose shiner also suggests that during periods of prey availability this species frequently feeds in the water column.
Physical Characteristics
The sharpnose shiner is a small, slender minnow. Adult sharpnose shiners are approximately 3 to 5 centimeters (1.2 to 2.0 inches) long, have a strongly curved underside, and a pointy, slightly upturned mouth.
The sharpnose shiner is typically an olive color on top, silver-white below, and has a faint silver stripe running down the length that is more noticeable on the back half of the fish.
Life Cycle
Sharpnose shiners broadcast-spawn eggs and sperm into open water asynchronously (fish not spawning at the same time) from April through September during periods of low flow, and synchronously (many fish spawning at the same time) during periods of elevated streamflow. Sharpnose shiners spawn continuously during their reproductive season, a strategy that is adaptive to stochastic environments and ensures that at least some offspring are potentially produced. Given the limited survival and longevity of these shiners, most individuals have only one reproductive season during their less than 3-year lifetime. Their eggs are semi-buoyant and remain suspended one or two days in flowing water as they develop into larvae. Larval fish remain suspended in the flowing water column an additional two to three days as they develop into free-swimming juvenile fish. Once capable of horizontal swimming, juvenile sharpnose shiners likely move to the margins of the main channel, to eddies, and to water near tributary mouths where flow velocity is reduced and food sources are more abundant.
Geography
Despite a historical distribution including the Brazos River basin, the Wichita River system of the Red River basin, and the Colorado River basin, since 1993 the sharpnose shiner has been primarily restricted to the Brazos River and its major tributaries upstream of Possum Kingdom Lake with very few specimens collected in the lower Brazos River downstream of the City of Marlin, Falls County, Texas. Several survey efforts have failed to collect sharpnose shiners from locations downstream of Possum Kingdom Lake where they were historically present. The sharpnose shiner has not been collected from the Brazos River downstream of Possum Kingdom Lake since 2006, when one specimen was collected from the confluence with the Navasota River in Washington County. The most recent collections prior to the 2006 collection were: in 2001 from the Brazos River at its confluence with Allens Creek, Austin County, where three individuals were collected; and in 2004, when two fish were collected from the Brazos River near Hempstead, Washington County; and another six individuals from the Brazos River near Bryan, Brazos County. Sharpnose shiner currently occupies the following stream segments in the upper Brazos River basin: White River, North Fork Double Mountain Fork Brazos River, South Fork Double Mountain Fork Brazos River, Salt Fork Brazos River, Double Mountain Fork Brazos River, and Brazos River. With a short life span allowing only one or two breeding seasons and the need for long, unobstructed flowing river reaches, the species is at a high risk of extirpation when rivers are fragmented by fish barriers and flows are reduced. These conditions have already resulted in substantial range reduction, isolating the one remaining population of the sharpnose shiner to the upper Brazos River basin.
Timeline
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