From July 1994 through August 1995, attempts were made to elucidate the life history of crystal darters (Crystallaria asprella ) in Pools 4, 5, and 5a of the Upper Mississippi River and in the Zumbro River of southeastern Minnesota. Thirty-three collections from 14 sites on 18 dates produced a total of 27 crystal darters, all but two of which were taken from the navigation channel side of Island #36 in Pool 4. Another 9 specimens collected by state biologists from the Mississippi and Zumbro rivers were included in the analyses. One Zumbro River specimen was in its third year of growth and one Pool 4 specimen was in its second growth year when captured; all remaining specimens were young-of-the-year. The full spawning season could not be determined but included at least May and June in the Mississippi River. Growth was rapid with young-of-the-year reaching 80-100 mm total length (65-80 mm standard length) and 3-5 g total body mass by the end of their first growing season. Little energy was used for gonadal development during this time, leaving fall females with gonadosomatic indices of 0.03-0.04 and males with 0.005 or less. All Mississippi River specimens came from channel or channel-margin portions of the river in close proximity to islands where substrata were coarse sand and gravel with 30-40% embedded cobble and boulder and large amounts of dark silt particles. Most individuals occurred in water ? 1.8 m deep with near-bottom current velocities of 16-22 cm/sec. Two adults and 17 juveniles utilized 11 taxa of insects, 2 taxa of microcrustaceans and 1 taxon of water mites as food. The overall average number of items per stomach was 16.0 but was highly variable (0-80). Only small juveniles utilized microcrustaceans, otherwise both adults and juveniles primarily eat chironomid and hydropsychid larvae. Two hypotheses were developed to explain the extreme rarity of this species in the Upper Mississippi River: 1) crystal darters occur in deeper and swifter water than can be sampled by current techniques and, therefore, only appear to be rare or 2) they are, in fact, rare because navigation controls have reduced current velocities and caused excessive infusion of sand and gravel substrata with silt. These hypotheses need to be tested in the near future.
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Minnesota Water Research Digital Library