White Sturgeon

Acipenser transmontanus

Description

The white sturgeon is a large, slow-maturing, long-lived freshwater fish. It has a flattened, shovel-like head and a row of sensory barbels in front of its ventral, toothless mouth.

All sturgeon are distinguished from other fish in that they have a cartilaginous skeleton with a persistent notochord, and a protractile, tube-like mouth and sensory barbels ventrally on the snout. The white sturgeon is distinguished from other Acipenser by the specific arrangement and number of scutes (bony plates) along its body.

Behavior

For the white sturgeon in general, the size or age of first maturity in the wild is quite variable. Females normally require a longer period to mature than males, with females from most sturgeon species spawning between 15-25 years of age. Only a portion of adult white sturgeon are reproductive or spawn each year. The spawning frequency for females is estimated at two to 11 years.

Spawning occurs when the physical environment permits vitellogenesis (egg development) and cues ovulation. White sturgeon are broadcast spawners, releasing their eggs and sperm in fast water. In the lower Columbia River below McNary Dam, land-locked populations of white sturgeon normally spawn during the period of peak flows from April through July. Spawning at peak flows with high water velocities disperses and prevents clumping of the adhesive eggs. Following fertilization, eggs adhere to the river substrate and hatch after a relatively brief incubation period of eight to 15 days, depending on water temperature. Recently hatched yolk-sac larvae swim or drift in the current for a period of several hours and settle into interstitial spaces in the substrate. Larval white sturgeon require 20-30 days to metamorphose into juveniles with a full complement of fin rays and scutes.

Habitat

The Kootenai River population of white sturgeon is restricted to approximately 168 river mi (270 river km) in the Kootenai River basin. This reach extends from Kootenai Falls, Montana, located 31 river mi (50 river km) below Libby Dam, downstream through Kootenay Lake to Cora Linn Dam at the outflow from Kootenay Lake, British Columbia, Canada. Historically, Kootenai Falls represented an impassible natural barrier to the upstream migration of the white sturgeon. A natural barrier at Bonnington Falls downstream of Kootenay Lake has isolated the Kootenai River white sturgeon from other white sturgeon populations in the Columbia River basin since the last glacial age (approximately 10,000 years).

Based on tagging studies, Kootenai River white sturgeon are relatively sedentary during the summer and inhabit the deepest holes of the Kootenai River and Kootenay Lake. Kootenai River locations used by white sturgeon were generally sites over 20 ft (6 m) deep with column velocities less than 0.77 fps (less than 0.24 mps) and water temperature of 57-68°F (14-20°C), while depths utilized in Kootenay Lake ranged from 30-300 ft (10-100.5 m). Compared with other waters containing white sturgeon, the Kootenai River is a relatively cool river with summer high temperatures of 68-72°F (20-22°C).

White sturgeon in the Kootenai River are considered opportunistic feeders, although white sturgeon more than 28 in (80 cm) in length may feed on a variety of prey items, including chironomids, clams, snails, aquatic insects, and fish. Kokanee salmon (Oncorhynchus nerka ) in Kootenay Lake, prior to a dramatic population crash beginning in the mid 1970's, were once considered an important prey item for adult white sturgeon.

Distribution

White sturgeon historically occurred on the Pacific Coast from the Aleutian Islands to central California.

Little was known regarding the status and life history of the white sturgeon population in the Kootenai River basin prior to studies initiated during the late 1970s. The Kootenai River population of white sturgeon is one of 18 landlocked populations of white sturgeon known to occur in western North America. The Kootenai River originates in Kootenay National Park in British Columbia, Canada. The river flows south into Montana, turns northwest into Idaho, and north through the Kootenai Valley back into British Columbia, where it flows through Kootenay Lake and eventually joins the Columbia River at Castlegar, British Columbia.

Estimates show a decline in the white sturgeon population from an estimated 1,194 fish (range 907-1503) in 1982 to 880 (range 638-1,211) in 1990, although these are not directly comparable because the 1990 survey occurred in a river sampling reach almost 31 river mi (50 river km) longer. However, FWS believes recent population trends and population estimates accurately reflect the current status of the fish. Trends in population demographics reveal an aging population with no known recruitment of age one sturgeon since 1978. Additionally, although mark-recapture studies reveal that white sturgeon move freely between the Kootenai River and Kootenay Lake, there is no evidence that white sturgeon reside or spawn in other tributaries entering Kootenay Lake, British Columbia.

In general, individual white sturgeon in the Kootenai River are broadly distributed, migrating freely between the Kootenai River and the deep, oligotrophic Kootenay Lake. In 1980, it was thought that only one to five adult white sturgeon resided in Montana, found in the river reach immediately downstream of Kootenai Falls. Although white sturgeon use the main channel of the Kootenai River upstream to Kootenai Falls, few individuals have been reported from tributaries to the Kootenai River in Idaho and Montana.

Threats

Significant modifications to the natural hydro-graph in the Kootenai River, caused by flow regulation at Libby Dam, is considered the primary reason for the Kootenai River white sturgeon's continuing lack of recruitment and declining numbers. Since 1972, when Libby Dam began regulating flows (though not fully operational until 1975), spring flows in the Kootenai River have been reduced an average 50%, and winter flows have increased by 300% over normal. As a consequence, natural high spring flows required by white sturgeon for reproduction rarely occur during the May to July spawning season when suitable temperature, water velocity, and photoperiod conditions exist.

Another contributing factor to the white sturgeon decline is the elimination of side channel slough habitat in the Kootenai River floodplain due to diking and bank stabilization to protect agricultural lands from flooding. Much of the Kootenai River has been channelized and stabilized from Bonners Ferry downstream to Kootenay Lake, resulting in reduced aquatic habitat diversity, altering flow conditions at potential remaining spawning and nursery areas, and altering remaining substrates and conditions necessary for survival.

Although not fully understood, there is evidence that the overall biological productivity of the Kootenai River downstream of Libby Dam has been altered, resulting in modifications to the quality of water now entering the lake by removing nutrients, by permitting the stripping of nutrients from the water in the river downstream from the dam, and altering the time at which the nutrients are supplied to the lake. Potential threats to the Kootenai River white sturgeon from declining biological productivity include: (1) decreased prey abundance and limited food availability for all life stages of sturgeon downstream of Libby Dam, (2) reduced condition factor in adult white sturgeon, possibly impacting fecundity and reproduction, and (3) a possible reduction in the overall capacity for the Kootenai River and Kootenay Lake systems to sustain substantial populations of white sturgeon and other native fishes.

Conservation and Recovery

Water flows in the river are the most significant factor affecting the survival of the sturgeon. The Army Corps of Engineers and BPA have committed to experimental flow releases from Libby Dam for Kootenai River white sturgeon in possibly three out of the next ten years. However, providing these flows is contingent upon meeting other project priority uses. The proposed action increases discharge and sustains flows in the Kootenai River at only 57% of the discharge the Service believes is necessary to maximize sturgeon spawning and maintain suitable larval rearing habitats. Existing regulatory mechanisms are not sufficient to ensure the survival and recovery of this species. The British Columbia Ministry of Environment, Lands and Parks, is currently experimenting with fertilization of Kootenay Lake to increase biological productivity and enhance native fisheries. Beginning in 1993, BPA funded IDFG and Idaho State University to study primary productivity, community respiration, and nutrient cycling in the Kootenai River from Libby Dam downstream to Kootenay Lake. It will be several years before results from these studies explain to what extent, if any, reduced biological productivity has been a contributing factor to the Kootenai River white sturgeon's population decline.

Contacts

U.S. Fish and Wildlife Service
Division of Endangered Species
Eastside Federal Complex
911 N.E. 11th Avenue
Portland, Oregon 97232
http://pacific.fws.gov/

U.S. Fish and Wildlife Service
Division of Endangered Species
Denver Federal Center
P.O. Box 25486
Denver, Colorado 80225
http://www.r6.fws.gov/

Reference

U.S. Fish and Wildlife Service. 6 September 1994. "Determination of Endangered Status for the Kootenai River Population of the White Sturgeon." Federal Register 59(171).