128 



FISHERY BULLETIN OF THE FISH AND WILDLIFE SERVICE 



hypolimnion, lake herring are forced to inhabit 

 the area of the thermocline. In years of such 

 extreme stagnation lake herring must choose be- 

 tween the epUimnion with adequate oxygen and 

 unsuitable temperatures, or the hypolimnion with 

 inadequate oxygen and suitable temperatures. 

 Cahn (1927) reported that in situations of this 

 type heavy mortalities occur in southern Wiscon- 

 sin lakes. During a period of extreme stagnation 

 in Snow Lake, Indiana, Scott observed ciscoes 

 coming to the surface, apparently in a state of 

 asphyxiation. These fish recovered quickly, how- 

 ever, and returned to deeper water. According 

 to Koelz (1929) the lake herring of Lakes Erie 

 and Huron follow the normal pattern by descend- 

 ing into deep water during the midsummer months. 

 Koelz reported that lake herring were taken in 

 Lake Superior 1 mOe off Grand Marais, Minn., 

 in floating gill nets all year except late July and 

 early August. Surface temperatures in this region 

 are always relatively low. 



Among the authors who have reported on the 

 vertical distribution of the lake herring, only a 

 few have given limnological records or experi- 

 mental data from which judgment can be formed 

 as to limiting values of the controlling factors. 

 Most detailed consideration of the problem was 

 given by Hile (1936) and Fry (1937). From Hile's 

 data on the vertical distribution of ciscoes and on 

 temperature and oxygen conditions during the 

 general period of his fishing operations (he had no 

 limnological records on the actual dates of lifting 

 gill nets) in Muskellunge and Silver Lakes, it 

 may be seen that ciscoes were taken only rarely 

 at temperatures above 17° to 18° C, which marked 

 the upper limit of their distribution, or at oxygen 

 concentrations below 3 or 4 parts per million at 

 the deeper limit of their distribution. Fry com- 

 mented that he seldom took ciscoes in Lake 

 Nipissing in water 20° C. or warmer. He men- 

 tioned oxygen depletion as a possible limiting 

 factor for the lower limit of distribution, but con- 

 sidered carbon dioxide concentration to be of 

 greater importance in making the hypolimnion 

 uninhabitable. Hile did not mention carbon diox- 

 ide as a factor in the distribution of the cisco, but 

 in a later publication on the bathymetric distribu- 

 tion of fish in several lakes of northeastern Wiscon- 

 sin (Hile and Juday, 1941) skepticism was ex- 

 pressed as to the influence of both carbon dioxide 

 and pH concentrations on the distribution of 



various species in those waters. Cahn's (1927) 

 aquarium experiments indicated that ciscoes 

 avoided temperatures above 17° C. 



Of the several possible limiting factors men- 

 tioned by earlier investigators only temperature 

 can be held important in Green Bay. Oxygen 

 concentrations in the deeper waters of the bay 

 during the summer of 1952 were always above 

 7 p. p. m. and the pH fell within the range 7.8 to 

 8.2. Although determinations were not made of 

 carbon dioxide concentrations in deep water 

 during the summer, the values for oxygen and pH 

 constitute prima-facie evidence that carbon di- 

 oxide was not present in excessive amounts. 



VERTICAL DISTRIBUTION IN GREEN BAY 



The occurrence of lake herring in commercial 

 nets in Green Bay gives some information about 

 the vertical distribution of the herring. In months 

 of cool weather (September or October to May 

 or June) lake herring are commonly taken in 

 pound nets fished in shallow areas and in gill nets 

 fished at all depths. During other months, how- 

 ever, nets set in shallow water make only oc- 

 casional catches, usually following a storm, and 

 gill nets fished on the bottom in deeper water take 

 few lake herring. 



In 1952, a study was undertaken to determine 

 the distribution of the lake herring before, during, 

 and after the summer period when, according to 

 the fishermen, the lake herring "disappear." 

 Oblique gUl-net sets,'" similar to those used by 

 Fry (1937), were employed to determine the depth 

 at which lake herring were located. In these 

 sets 140 linear feet of gill netting were fished in 

 every 15-foot stratum. At station B, where the 

 water was 40 feet deep, 140 feet of gill netting 

 were fished in each 20-foot stratum. One end of 

 the gang of nets was tied to an anchor and the 

 other end to a 15-gallon-drum float. The depths 

 at which segments of the nets fished were con- 

 trolled by gallon-jug floats attached to the nets 

 with lines whose lengths were multiples of 15 

 feet. To hold the nets tight, an anchor rope 

 about equal in length to the gang was tied to the 

 15-gallon drum, puUed against the first anchor, 

 and set with a long buoy line (see figure 14 for a 

 diagram of an oblique set in 60 feet of water). 



" These experimental nylon gill-nets were 280 feet long and 6 feet deep, and 

 had mesh sizes of 1, H, and 2 inches, eitension measure. The 2-inch-mesh 

 nets were used most eitensively. 



