cm.) mesh, extension measure. Series of tows (one 

 haul at each of 14 depths) were made from the 

 R/V KaJio in mid-February and mid-March, and 

 from the E/V Cisco at about 3-week intervals from 

 April 11 to August 21, in mid-October, and in early 

 November. A "standard" series of tows consisted 

 of bottom tows along the contour at 3, 5, 7, 10, 12, 

 15, and 17 fathoms (5.5, 9.1, 12.8, 18.3, 21.9, 27.4, 

 and 31.1 m.) and at 5-fathom (9.1 m.) intervals 

 from 20 to 50 fathoms (36.6 to 91.5 m.). Distances 

 from shore of the samjiling depths ranged from 

 0.4 to 16.7 nautical miles (0.7-31.0 km.) (table 1). 

 Ice prevented the collection of samples at 3 fath- 

 oms (5.5 m.) in Febiiiary and March. All tows 

 lasted 10 minutes except those at 3 fathoms (5.5 

 m.) which lasted only 5 uiinutes, due to limited 

 areas of suitable trawling bottom. The catches at 3 

 fathoms (5.5 m.) were adjusted to 10 minutes in 

 all tabulations. All 14 tows of each standard se- 

 ries were made on the same day, with two excep- 

 tions (February 13 and 14; July 7 and 9), when 

 tows at 45 and 50 fathoms (82.3 and 91.5 m.) were 

 made 1 or 2 days after shallower tows. The dates 

 "February 13" and "July 7" are used here in all 

 further references to these series. 



Engine speed was the same for all tows, and the 

 trawling speed was about 2.6 knots. Variation in 

 speed of tows during periotls of significant wind 

 was minimized by making all tows (which, to fol- 

 low the bottom contours, were always either north 



or south) with the wind on a stern quadrant. Cur- 

 rents along the shore in the study area are highly 

 responsive to wind and ordinarily flow north or 

 south according to the wind. We did not trawl dur- 

 ing i^eriods of high winds. 



Bathythennograph casts were made at the be- 

 ginning of each tow, and the surface water tem- 

 perature was recorded continuously with a 

 thermograph. 



The catch was sorted by siaecies and by age 

 groups when possible. TTsually total catches of all 

 species were retained, but occasionally large 

 catches of individual si^ecies were subsampled. 

 Discarded fish of a species were weighed as a group 

 and counted or, rarely, their number was estimated 

 on the basis of their average weight in the sub- 

 sample. Data were taken from some fish on the 

 date of collection, but most specimens were stored 

 in plastic bags in an ice chest and examined on 

 the following day. The data for each tow included 

 counts and total weights of each species and indi- 

 vidual total lengths and weights of certain species. 



Although this study is based primarily on 

 catches of the "standard" series of bottom trawl 

 tows, information has been used from other bottom 

 trawling, midwater trawls, gill nets, and under- 

 water television. These supplementary data, which 

 were gathered in Lake Michigan (mostly by the 

 R/V Ohro) in 1954, 1955, and 1960-67, serve pri- 

 marily to extend observations into midlevels, into 



Table 1. — Bottom temperatures at all sampling depths during all standard series of trawl tows in southeastern Lake Michigan 



U.S. FISH AND WILDLIFE SERVICE 



