240 



Aquatic Ecosystems — Our Living Resources 



Fig. 2. Number of individual fish 

 obtained per hour of eieclrofishing 

 from the upper Illinois Waterway 

 for fish species identified as pollu- 

 tion indicators. Dashed lines con- 

 nect data points for years between 

 which electrofishing was not con- 

 ducted. 



fish obtained per hour of sampling. Fish were 

 stunned in an electric field, gathered with a net. 

 measured, checked for externally visible abnor- 

 malities (sores, eroded fins, etc.), and returned 

 to the water. The same methods and similar 

 equipment have been used for all years of the 

 survey to allow comparability of data among 

 years. 



Fishes of the family Centrarchidae (e.g.. 

 largemouth bass [Micropterus salmoldes], 

 bluegill \Lcpoiiiis niacmchiriis]) were treated as 

 a group to simplify data analysis because they 

 have very similar habitat requirements and are 

 generally considered intolerant of polluted con- 

 ditions. (The green sunfish \Leponus cyaneUiis]. 

 howe\'er, is usually indicative of a stressed envi- 

 ronment |Karr et al. 1986].) Also, because many 

 of these fishes are piscivorous, their presence or 

 absence will have a direct impact on overall fish 

 community composition. Catches of common 

 caip iCypriiuis ciirpio) and goldfish (Canissiiis 

 aiinitus). both non-natives to North America, 

 and their hydrids were analyzed separately. 

 These two species are omnivorous habitat gen- 

 eralists that are tolerant of polluted waters. 



Centrarchids 



No data 



T 



Carp 



"^ 



■§ 0- 



Sediments of the upper Illinois River contain 

 varying amounts of toxic substances (lEPA 

 1992). which are thought to contribute to the 

 incidence of abnormalities on fishes that forage 

 in sediments while minimally affecting fishes 

 that forage in the water column. To test this 

 hypothesis, all fishes were assigned to one of 

 two groups: benthic species that frequently for- 

 age in bottom sediments (e.g.. common caip) 

 and pelagic species that usually inhabit the 

 water column (e.g.. bluegill). 



Substantial changes have occuned in catch 

 rates from the seven upper Illinois Waterway 

 stations between 1962 and 1993 (Fig. 2). 

 Catches of centrarchids have increased (D = 

 327, P < 0.001 ) since the early I960"s, peaking 

 at 52/lir in 1983. Catches of centrarchids appear 

 to have stabilized during the last 5 years (Fig. 

 2). indicating their populations may have 

 reached carrying capacity. Catches have 

 decreased for carp (P < 0.001). goldfish (P < 

 0.001 ). and caip x goldfish hybrids (f < 0.001 ) 

 since the early I960"s. Carp were able to main- 

 tain their numbers unfil the mid-1980"s (Fig. 2), 

 as larger, older individuals probably died off 

 and smaller, younger individuals were more 

 vulnerable to predation by piscivores. Catches 

 of goldfish declined rather precipitously from 

 1963 to 1966 for an unknown reason before 

 substantial increases in centrarchids. 



Data from 1963 and 1992 were chosen for 

 more detailed examination, those years being 

 representative of catches from early and recent 

 years of the electrofishing survey. In 1963. 

 goldfish accounted for almost one-third of all 

 fish collected per hour, followed by carp, emer- 

 ald shiner (Notwpis atherinoides), and gizzard 

 shad {Dorosomo cepediamim): together these 

 four species dominated the catch, accounting 

 for 95.8% of all individuals collected per hour 

 (Fig. 3). In 1992. 13 species accounted for 

 95.4% of all fish collected per hour: emerald 

 shiners were most abundant followed by cen- 

 trarchids: carp and goldfish were reduced to a 

 minor component (Fig. 3). The increase in cen- 

 trarchids and decrease in carp and goldfish 

 since the early 1960's (Fig. 2) reflect a more 

 diverse fish community in recent years (Fig. 3). 



For all years when data were collected from 

 1959 to 1993. the percentages offish with exter- 

 nal abnormalities were higher on benthic fishes 

 than on pelagic fishes, suggesting that sedi- 

 ments may contain significant amounts of con- 

 taminants. In fact, the lEPA (1992) identified 

 several locations near our electrofishing stations 

 on the upper Illinois and Des Plaines rivers as 

 having sediments that contained elevated levels 

 of toxicants, including mercury, lead, and 

 PCBs. Brown et al. (1973) reported, however, 

 that benthic fishes had a higher frequency of 



