O'Brien Lake

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O'Brien Reservoir, also known as Blue Lake, is located about two miles east of Nashwauk, MN. The reservoir, created in 1978 by an earthen dam at the south end of the watershed, engulfed O'Brien and Little O'Brien lakes. The intent was to create a tailings basin, which was never used. An outlet structure on the south end maintains the water at current levels. The shoreline consists of disturbed mine land and flooded timber. There is a county owned gravel access on the north shore, while the remaining shoreline is within an active mining area and closed to public access.

Northern pike were the most numerous fish observed in gill nets, being sampled at an expected rate for lakes similar to O'Brien and the previous assessment. Lengths ranged from 13.2-30.7 inches with an average length of 21.5 inches. Size structure was moderately poor with 56% of the sample less than 22 inches. Neascus was observed in 84% of the 38 fish examined. Scale and bone analysis identified ages 1-7 in the sample. Growth was similar to the statewide average for all ages. Northern pike averaged 22.7 inches by age 5.

A fishable population of walleye was present in the 2013 sample, being sampled near the expected range. Walleye were not sampled in the previous assessment, but present in the initial 1985 survey. Records indicate walleye have never been stocked and are likely of natural origin. The sample size was too small to accurately assess size structure, but length frequency indicated a quality distribution of lengths. Lengths ranged from 15.7-27.2 inches and an average length of 20.0 inches. Scale and bone analysis identified ages 2-4, 7, and 8, suggesting relatively consistent recruitment. Growth was fast early, but slowed to be near the statewide average by age 4. Neascus was observed all seven of the examined fish.

Yellow perch were sampled at a rate of 3.0/gill net, similar to the previous assessment (5.6/net). The trap net catch of 1.9/net was also similar to the previous assessment (3.0/net). Lengths from the gill net sample ranged from 5.6-9.7 inches but had a mean length of 6.4 inches.

Black crappie were sampled at a rate of 0.8/gill net and 0.9/trap net. Both catches were similar to the previous assessment. The gill net sample had a mean length of 6.8 inches while the trap net sample had a mean length of 5.9 inches. Ages 1-3 were present in the sample with age-3 comprising 50% of the sample. Growth was similar to the statewide average.

Bluegill were sampled at a rate of 3.4/gill net and 3.0/trap net. The gill net sample exceeded the previous assessment (1.5/net) while the trap net sample was considerably lower than the previous assessment (20.6/net). Lengths ranged from 3.8-9.1 inches with a mean length of 5.9 inches in the gill net sample, similar to the trap net sample with a mean length of 4.7 inches. Ages 2-7 were present in the sample with growth similar to ecological lake class 25 average.

Other species sampled included bowfin, hybrid sunfish, largemouth bass, pumpkinseed sunfish, and yellow bullhead.

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