CENTRARCHID LATE FALL MOVEMENTS OBSERVED THROUGH ELECTROFISHING SURVEYS IN 
MISSISSIPPI RIVER POOLS 8, 10, AND 11

James Webster and Jeffrey Janvrin 
Wisconsin Department of Natural Resources, 3550 Mormon Coulee Road, La 
Crosse, Wisconsin, 54601
 
Backwater fisheries habitat has been included as one of the objectives for 
many habitat restoration projects on the Upper Mississippi River.   Design of 
projects with backwater fisheries habitat objectives includes an evaluation 
of various project and pool scale factors that may be influencing the 
fisheries use in the area (i.e. physical and chemical factors affecting 
spawning, juvenile, and adult survival throughout the year).   One of the 
seasonal habitat needs that has been identified as being degraded, or 
lacking, for many of the backwater fisheries projects has been centrarchid 
overwintering (OW) habitat, defined as aquatic areas that meet the following 
winter conditions:  water depths > 1.2 m, velocities < 0.01 m/sec, water 
temperature > 1 degree C, dissolved oxygen > 5.0 mg/l.  Past radio telemetry 
studies have indicated bluegills Lepomis macrochirus and largemouth bass 
Micropterus salmoides may utilize large sections of the pools before 
“migrating” to OW sites.  Therefore, summertime and early fall surveys most 
likely provide a poor representation of utilization of OW habitats.  
Quantifying wintertime fisheries utilization of the aquatic habitats by 
active sampling is difficult to impossible due to ice coverage.  Therefore, 
managers have used fall electrofishing as an indicator of centrarchid OW 
habitat use and quantification of that usage.  The objective of our pilot 
study was to determine if fall catch per unit effort (CPUE) of centrarchids 
changes as fish move into OW habitat. Electrofishing surveys were repeated 
weekly throughout the fall of 2007 to determine centrarchid movement into OW 
areas in navigation pools 8, 10, and 11 of the Upper Mississippi River. Ten 
known centrarchid OW areas of varying quality were surveyed; four in Pool 8, 
four in Pool 10, and two in Pool 11. Four weekly electrofishing runs were 
conducted at all sites except one that was visited three times, resulting in 
a total of 39 runs varying in length from 8 min to 35 min (average = 24 min) 
for a total effort of 15.43 hours. Centrarchids were measured to the nearest 
0.1 inch T.L. in Pools 8 and 10.  In Pool 11, the bluegill T.L. was measured 
into 0.5 in bins and other centrarchid T.L. measured into 1.0 in bins. Other 
fish species encountered during the surveys were captured and documented. The 
surveys were conducted between 10/11/2007 and 11/27/2007 intending to 
encompass the autumnal migration into the overwintering areas occurring 
before ice formation. The data shows an inverse relationship between weekly 
average bluegill CPUE in OW areas and October-November main channel water 
temperature (R2 = 0.97).   The median CPUE of bluegills was significantly 
lower at warmer water temperatures (>10°C) than at colder water temperatures 
(<5.5°C) (P<0.05).  However, the exact temperature threshold eliciting 
movement into OW areas is not discernable from our data. 

Keywords:  Centrarchids, Mississippi River, overwintering habitat, movement, 
bluegill

Jeff Janvrin, 3550 Mormon Coulee Road, La Crosse, WI  54601 608-785-9005, Fax 
608-785-9990,  Jeff.Janvrin@Wisconsin.gov, Platform power point, Poster No, 
Professional