Knothead Carp (continued)
Table 14: Distribution and Percentage of Knothead Carp in the Illinois River 1926-1927 (Thompson 1928)
| Stations Going Downstream | Number of Carp Examined | Observed Percentages of Knotheads | Information and estimates of local fishermen |
| Utica | None | - | About 50% are knotheads |
| La Salle-Peru | None | - | About 50% are knotheads |
| Depue | 23 | 91 | 60-75% is more accurate |
| Henry | 316 | 75 | 60% is more accurate as a year-round average |
| Chillicothe | 117 | 64 | Correct estimate |
| Spring Bay | 302 | 46 | Usually more are found |
| Averyville | 149 | 57 | Correct estimate |
| Peoria | 256 | 78 | Average is 50-60% |
| Pekin | 500 | 30 | Correct estimate |
| Havana | 539 | None | Rare, believed to have come from upstream |
| Beardstown | 3 | None | None |
| Meredosia | 42 | 2 | None found; the ones identified were pot-bellied |
A second condition occurs, referred to as the “pot-bellied knothead carp” (Figure 30). These carp have the same anatomical malformations but are pot-bellied rather than slim. Carp with this condition are found between Pekin and Copperas Creek. Approximately 30% of the catch in this stretch of the river is pot-bellied carp. This condition has several potential causes: first, the fish were exposed to conditions inducing malformation for a short period; and second, these fish migrated from the Peoria Lake region after their growth had been altered by knothead inducing conditions.
Figure 30: Pot-bellied Knothead Carp from the Lower Illinois River (Thompson 1928)
In addition to the anatomical differences, the knothead carp behave differently from normal carp. These behavioral changes are attributed to an inefficient respiratory system, a softer body implying a lack of muscular tone, and low sensitivity to stimuli. These malformed, emaciated carp can often be picked up by hand in the vegetated shorelines. This suggests that their respiratory function is compromised so that they move to areas with highest oxygen concentrations. Further, their breathing is so labored that they neither feed nor react to stimuli, allowing them to be picked by hand.
These changes to the carp are of concern for two major reasons: first, knothead carp often have a disagreeable “gassy” taste or odor that reduces their palatability and therefore, their market value. Second, knothead carp weigh less, and this also decreases their market value. Despite these changes, Thompson does not believe their value as a food source for humans is impaired.
