Etheostoma microperca Jordan and Gilbert, 1888
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Basis for Listing
Little information exists about the population status of the least darter in Minnesota or throughout North America. As of 2008, it had been documented at a limited number of sites across the southern two-thirds of Minnesota, but most of its populations occured in the west-central portion of the state in the Otter Tail River and Upper Mississippi River drainages. Minnesota populations represent the northwestern limit of the species' range, and are disjunct from those of the Ozark and eastern Great Lakes regions (Becker 1983). In addition to being a major center of abundance for this species, the Otter Tail River supports a population that exhibits life history characteristics substantially different from those that occur near the center of the species' range (Johnson and Hatch 1991). For these reasons, the least darter was designated as special concern in Minnesota in 1996.
The least darter is the smallest vertebrate fish species native to Minnesota, averaging 37.5 mm (1.5 in.) in total length. It has no lateral line, and its body is stout and laterally compressed. Least darters are a light creamy color with an olive green back and they have 6-9 irregular blotches on their midline. Females are slightly larger than males, and breeding females have yellow fins (Eddy and Underhill 1974). Breeding males have a large, orange pelvic fin, orange spots on the membranes between each dorsal fin spine, and tubercles (small, hardened breeding growths) on the pelvic and anal fins.
Least darters prefer crystal clear, freshwater streams and lakes with cool to warm waters. They are strongly associated with pools of water that have dense, submerged aquatic vegetation, such as eelgrass (Vallisnaria americana), Canadian elodea (Elodea canadensis), pondweed (Potamogeton spp.) in streams, and muskgrass (Chara spp.) in lakes (Becker 1983; Johnson and Hatch 1991). In Minnesota, least darters are usually found in low velocity streams that are part of a connected lake or stream system. They typically use weedy, shallow pools during the spawning season and deeper pools outside of the spawning season (Johnson and Hatch 1991).
Biology / Life History
Least darters migrate from the deeper pools of streams to shallow, weedy habitats from March to May. During spawning, males develop three-dimensional territories of about 30 cm (11.8 in.) in diameter and defend them from other males. Females enter a male's territory and spawn on pieces of aquatic vegetation. Females will travel through many male's territories and can lay about 30 eggs per day (Petravitz 1936). Least darters are a short-lived species. In Minnesota, they live 2-3 years. The least darter's diet includes aquatic invertebrate larvae, particularly midge larvae, and microcrustaceans (Johnson and Hatch 1991).
Conservation / Management
Habitat loss and degradation are the greatest threats to least darter populations. They are vulnerable to pollution, pesticides, agricultural and urban runoff, eutrophication, and loss of habitat elements such as low velocity waters and aquatic vegetation. Other potential threats to this species include loss of forested habitats around streams, stream reclamation, and the introduction of non-native and predatory fish species. Maintenance of high quality water systems is recommended for this species. Water systems with large populations of least darters should be protected from human disturbances and development.
Conservation Efforts in Minnesota
Prior to 1990, the least darter was known from 31 sites in 10 streams and 9 lakes scattered across the southern three-fourths of the state. Surveys from 1990-2002 added 47 sites in 9 new streams and 41 new lakes, mostly in the Otter Tail River system and the Upper Mississippi River drainage. A collection from Long Lake on the border of Itasca and St. Louis counties was the first record of the least darter in the Lake Superior drainage (Hatch et al. 2003). Surveys completed by the Minnesota Biological Survey from 1997-2008 found new sites for this species, but also found that some historical sites had been lost in highly developed areas. More thorough sampling is needed to obtain accurate population estimates. Additionally, the identification of specific environmental stressors is needed. Genetic analysis may also aid in the conservation of this rare species. Currently, there are no specific management plans for the least darter in Minnesota.