Ecology of the shorthead redhorse (Moxostoma macrolepidotum) (Leseur) 1817 in Dauphin Lake, Manitoba

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Harbicht, Stephen
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Shorthead redhorse, Moxostoma macrolepidotum, from Dauphin Lake were studied during the open water period from 1983 to 1987. Spawning habits, age, length frequency and counts (meristic characters) were evaluated for the shorthead redhorse migrating into the Ochre River, from Dauphin Lake. Shorthead redhorse spawning migrations varied in numbers of fish (857 - 6568) and timing (May 6 - April 26) within the three years of this study. Spawning migrations began when mean daily stream temperatures reached 10 C. Upstream and downstream spawning movements were most intense from mid-afternoon until two hours after darkness. Fish moved up to 32 km upstream and the overall spawning period was 3 to 4 weeks. The mean fork length for spawning females and males ranged from 353.5 to 375.6 mm and 323.0 to 338.2 mm respectively. Both males and females reached sexual maturity at 5 years however, a few males matured at age 4. Spring caught females produced 12,660 to 44,329 eggs for fork lengths 310 to 418 mm. Both fecundity and egg diameters increased with fork length. Spawning occurred over a gravel/sand/rock substrate on the downstream side of riffles, at water velocities 0.3 to 0.7 m/sec, and depths of 20 to 90 cm. Shorthead redhorse tagged in Ochre River returned to spawn in the stream in successive years. Sections of left pectoral fin rays were used for age determinations, with validation accomplished by aging of the right pectoral fin of recaptured tagged fish. Ages up to 18 years were observed. Females grew faster than males. Juvenile shorthead redhorse (less than or equal to 100 mm fork length) preferred planktonic organisms including cladocerans, copepods and ostracods while adult fish (greater than 100 mm fork length) consumed a wider range of food items with chironomids, Ephemeroptera, Mollusca and Diptera other than chironomids being predominant. Shorthead redhorse generally were free from external parasites; acanthocephalans, trematodes and a nematode were the major internal parasites found in the digestive tract. Juvenile shorthead redhorse represented 4% of the diet of juvenile walleye from Dauphin Lake.