Electrofishing survey 30 May 1991 Page 12 



The relative weights of white bass, smallmouth bass, and largemouth bass 

 increased to values greater than 1.0 between 1982 and 1985 (Figure 3). The greatest 

 increase occurred in smallmouth bass, whose Wr values went from approximately 0.8 in 

 1983 to 1.3 in 1985. No electrofishing was done from 1986 through 1988, so we do not 

 know what happened to the Wr values in this interval. However, the values for all three 

 species were lower in 1989-1990 than in 1985. Changes in weight-length relationships, 

 including Wr, can reflect changes in the food supply, water quality, habitat quality, or in 

 the length range of the fish. One might speculate that these three species may show the 

 same pattern because they have similar feeding habits: they consume invertebrates when 

 young and graduate to larger prey, primarily fish, as adults. 



The hypothesis that the quality or abundance of forage for adult sport fish might 

 explain the common downward trend does not explain why the common carp, a bottom- 

 feeding omnivore that is capable of consuming detritus, shows the same pattern. Also, 

 the channel catfish shows a unique pattern: its Wr value peaked in 1983, declined 

 through 1985, and is at an intermediate level in 1989-1990. The channel catfish is an 

 omnivore that consumes benthic macroinvertebrates, but will also take small fish. With- 

 out information on populations of forage organisms, including benthic macroinverte- 

 brates, it is virtually impossible to test the food deficiency hypothesis. 



There is a suggestive negative correlation between Wr values and ammonia 

 concentrations in the Upper Illinois River (personal communication, 9 April 1991, Mr. 

 William Bertrand, Streams Program Administrator, Illinois Department of Conservation). 

 The un-ionized form of ammonia is toxic to fish and other aquatic animals. The 

 concentration of municipalities and industries in the Chicago-Joliet discharge ammonia, 

 as well as nitrogen-containing organic matter that accumulates in sediment deposits and 



