Rock Lake

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INTRODUCTION
Rock Lake is a 439-acre lake located approximately 3 miles south of the City of Russell in Lyon County. Rock Lake has a relatively small watershed to lake ratio of 8 to 1, with much of the watershed being comprised of agricultural lands. Rock Lake is managed primarily for walleye and secondarily for yellow perch and black crappie. The yellow perch and black crappie populations in Rock Lake are self-sustaining as they have not been stocked in recent history. The walleye population is maintained primarily through fry stocking 2 out of 3 years (2010, 2011, 2013, 2014, 2016), with limited natural reproduction contributing occasionally. A population assessment was conducted the week of June 23, 2014 to monitor fish populations using three gill nets and nine trap nets.

WALLEYE
Historically, Rock Lake has had an abundant walleye population, with catch rates often exceeding the expected range of catch rates for similar lakes (2.3 to 18.1 per gill net). More recently, catch rates have declined to historic lows, as walleye were captured at a rate of 6.3 per gill net in 2010 and 6.0 per gill net in 2014. The 2014 catch of 6.0 per gill net is still within the expected range of catch rates for similar lakes (2.3 to 18.1 per gill net), but is well below the long term average of 17.9 per gill net for Rock Lake. The walleye stocking strategy and quota have remained stable since 1992, so it is difficult to identify why catch rates have declined in the last two surveys in 2010 and 2014. A wide range of lengths were present, as walleyes ranged from 12.5 to 26.1 inches (mean=16.0 inches) in gill nets and from 6.8 to 13.4 inches (mean=8.0 inches) in trap nets. Six year classes of walleye were present including fish from 2004 (age-10), 2005 (age-9), 2008 (age-6), 2010 (age-4), 2011 (age-3), and 2013 (age-1), all of which corresponded to years that walleye fry were stocked. Age-3 walleye were the most prevalent, accounting for 52 percent of the fish aged. When compared to growth of walleye from similar lakes, growth was slow for Rock Lake walleye, as back calculated mean length at age-1 was 8.0 inches, age-2 was 11.4 inches, and age-3 was 13.4 inches. Growth of Rock Lake walleye beyond age-3 was similar to growth for similar lakes as mean lengths were 17.5 inches at age-4, 18.7 inches at age-5, 20.4 inches at age-6, and 22.5 inches at age-7. Slow growth and skinny fish may indicate that prey resources for walleye may be limited in Rock Lake. Rock Lake's walleye population is stable with many year classes of walleye present. The current stocking strategy (fry, two out of three years) will be continued because it has proven to be successful in previous years, and is effective at maintaining multiple year classes of walleye in the population.

YELLOW PERCH
Rock Lake's yellow perch population has historically been stable with catch rates ranging from 2.5 per gill net in 1985 to 33.7 per gill net in 2010. Since 1987, catch rates have been above the median catch rate for similar lakes (8.0 per gill net), and this trend continued in 2014 with a catch rate of 12.7 per gill net. Yellow perch have not been stocked in Rock Lake, indicating that this population sustains itself through natural reproduction. Yellow perch ranged in length from 5.2 to 10.5 inches and averaged 6.6 inches. Most of the yellow perch were small with 81 percent being between 5.2 and 6.5 inches in length. Yellow perch were plump, indicating that yellow perch are successfully foraging in Rock Lake.

BLACK CRAPPIE
Black crappie have historically occurred in low abundance in Rock Lake with catch rates ranging from 0.0 per trap net in 2014 to 5.6 per trap net in 1998. Since 1998, trap net catches of black crappie have steadily declined from 5.6 per trap net in 1998 to 4.7 per trap net in 2002 to 2.9 per trap net in 2006 to 1.4 per trap net in 2010 to 0.0 per trap net in 2014. The 2014 catch rate of 0.0 per trap net is the lowest observed catch rate for Rock Lake since surveys began, and is below the expected range of catch rates for similar lakes (1.2 to 20.5 per trap net). Although not sampled in the survey, black crappie are present in Rock Lake. A near shore survey conducted on July 10, 2014, captured 1,151 young-of-the-year (YOY) black crappie, suggesting that a strong year class was produced in 2014. Black crappie recruitment tends to be highly variable and is often characterized as cyclic, with strong year classes reoccurring every three to five years. This variable and cyclic recruitment often results in a "boom-or-bust" crappie fishery, where years of increased angler catch rates are followed by years of decreased angler catch rates. If the strong 2014 year class survives through the winter, the crappie population should be on the upswing in Rock Lake.

CHANNEL CATFISH
Five channel catfish were sampled in 2014, marking the first time that this species has been captured during a survey in Rock Lake. Channel catfish ranged in length from 13.4 to 19.2 inches and averaged 15.5 inches. Channel catfish were not stocked by MNDNR, therefore it is likely that they were illegally stocked by an angler. To avoid unintended introductions, anglers should be diligent about disposing of unused bait in the trash, and should report unpermitted movement of fish. More information on how channel catfish respond to this new environment will be available after the next scheduled population assessment in 2018.

OTHER SPECIES
Black bullhead abundance remained below the expected range of catch rates for similar lakes (30.3 to 150.6 per gill net) at 27.7 per trap net. The majority (87 percent) of black bullheads were less than 7.5 inches in length. Black bullhead lengths ranged from 5.0 to 10.9 inches and averaged 6.4 inches.

The catch rate of white sucker in 2014 was 3.3 per gill net. This is the highest observed catch rate for this species, but is within the expected range of catch rates for similar lakes (0.8 to 6.5 per gill net). White suckers ranged in length from 8.5 to 19.1 inches and averaged 17.0 inches.

Common carp abundance has historically been low in Rock Lake. The 2014 catch rate of common carp (3.7 per gill net) was below the expected range of catch rates for similar lakes (1.0 to 13.8 per gill net). Common carp ranged in length from 9.1 to 27.6 inches and averaged 19.5 inches.

Northern pike occur in low abundance in Rock Lake. Two northern pike were sampled in the 2014 survey and measured 15.9 and 17.0 inches in length.

Other species sampled included orangespotted sunfish.

Plants in the water and at the water's edge provide habitat, prevent erosion, and absorb excess nutrients. Shrubs, trees, and woody debris such as fallen trees or limbs provide good habitat both above and below the water and should be left in place. By leaving a buffer strip of natural vegetation along the shoreline, property owners can reduce erosion, help maintain or improve water quality, and provide habitat and travel corridors for wildlife.

Best management practices within the watershed (no-till farming, cover crops, buffer strips, targeted fertilizer application, reduced or metered tiling) would help reduce nutrients entering the lake. High nutrient and sediment input can cause algae blooms and reduce overall water quality. Any improvements in the watershed are likely to have positive impacts on the fishery.

Prepared by Jonah Dagel

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