Fish Slough Milk-vetch

Astragalus lentiginosus var. piscinensis

Description

The Fish Slough milk-vetch (Astragalus lentiginosus var. piscinensis ) is a prostrate perennial with few-branching stems that are up to 3 ft (0.9 m) long and are covered with stiff appressed hairs. The leaflets are reduced to only one to two pairs laterally, with a greatly elongated terminal leaflet. The lavender flowers are arranged in loose but short five-to-twelve-flowered racemes. The fruits are papery, strongly inflated with a complete septum, and covered with appressed hairs.

Habitat

The Fish Slough milk-vetch is restricted to a 6-mi (9.7-km) stretch of alkaline flats paralleling Fish Slough, a desert wetland ecosystem in Inyo and Mono counties, California. It grows in seasonally moist alkaline flats that support a cordgrass-dropseed association and is absent from nearby lower areas that are seasonally flooded. Appropriate alkali habitat covers less than 540 acres (218.5 hectares) of the slough, and, for unknown reasons, portions of this area do not currently support Fish Slough milk-vetch.

Distribution

In 1992, during intensive surveys of all potential habitat of Fish Slough milk-vetch within Fish Slough, about 3,200 individuals were found widely scattered or grouped over approximately 530 acres (214.5 hectares). This first complete and intensive survey for the species was conducted over several days and covered all suitable alkali habitat at Fish Slough. During the survey, several of the previously monitored sites were found to be much greater in extent than had been previously known. However, one site that had supported six plants in earlier visits failed to support any, and another previously recorded site showed a substantial decline—44 plants in 1983, 29 in 1985, and just eight in 1992. The fourfold increase in the total number of plants encountered in the 1992 survey does not suggest an increase or decrease in population size but provides the first comprehensive data on the species-wide abundance of Fish Slough milk-vetch. More than 60% of this population is located in the northern portion of the slough on land owned by the Los Angeles Department of Water and Power, and approximately 35% of known Fish Slough milk-vetch plants grow in the central zone of the slough on lands owned and managed by both the Bureau of Land Management (BLM) and the Los Angeles Department of Water and Power. About 5% are in scattered patches downstream as far as McNally Canal, but Fish Slough is narrow at its southern end, with little suitable habitat.

In 1991 the Los Angeles Department of Water and Power constructed an 80-acre (32.4-hectare) cattle enclosure at the northern end of the slough. The next year more than 95% of the Fish Slough milk-vetch plants in the northern zone were within the enclosure. Other than the area encompassed by the enclosure at the north end of Fish Slough, lands under Los Angeles Department of Water and Power management that support this taxon are grazed. Grazing is not permitted in the habitat of Fish Slough milk-vetch on lands managed by the BLM (in the central zone of the slough).

Threats

Current threats to Fish Slough milk-vetch include a lack of recruitment in the central zone population of Fish Slough, trampling and grazing by cattle, modification of wetlands, and alteration of slough hydrology. A long-term threat may be the expansion of Fish Slough Lake, which could be due to natural geologic processes or the existence of Red Willow Dam, resulting in increased inundation of soils and loss of suitable alkali habitat for this taxon. Historical alterations of the Fish Slough ecosystem to enhance fisheries appear to have caused similar increases in seasonally flooded habitats, which are less suitable for Fish Slough milk-vetch. Modifications include the creation of dams and weirs in the main slough channel, construction of a dirt road through milk-vetch habitat, and soil compaction and trail creation by cattle. These activities have altered the slough hydrology by causing an increase in permanently flooded habitats, artificial ponding, alteration in drainage patterns, and changes in seasonal flooding of milk-vetch habitat. Such changes have resulted in the expansion of emergent wetland vegetation and the conversion of alkali flat habitats—habitats that support Fish Slough milk-vetch—to other vegetation types. Trampling and grazing by cattle and associated ecological changes also potentially threaten this taxon.

Conservation and Recovery

The Fish Slough milk-vetch occurs on public lands managed by the BLM and on lands owned by the Los Angeles Department of Water and Power. A joint management committee composed of representatives of the Los Angeles Department of Water and Power, the BLM, the U.S. Fish and Wildlife Service, the California Department of Fish and Game, and the University of California Natural Reserve System provide guidance on management issues. Although the management committee is making progress in addressing the needs of the sensitive plants and animals in the Fish Slough ecosystem, the changes in slough hydrology resulting from existing dams and, potentially, from natural causes are complex and will not be easily resolved.

Contact

U. S. Fish and Wildlife Service
Regional Office, Division of Endangered Species
Eastside Federal Complex
911 N.E. 11th Ave.
Portland, Oregon 97232-4181
Telephone: (503) 231-6121
http://pacific.fws.gov/

Reference

U. S. Fish and Wildlife Service. 6 October 1998. "Determination of Endangered or Threatened Status for Five Desert Milk-Vetch Taxa from California." Federal Register 61 (193): 53596-53615.