Demographic Responses of Asian Carp to Harvest in the Illinois River

Numerous strategies have been devised to reduce the spread of Asian carp toward the Great Lakes through the Chicago Area Waterway System (CAWS), yet the last line of defense remains a series of electric dispersal barriers located on the Chicago Sanitary and Ship Canal. Harvest could be a viable option to push the leading edge of the Asian carp population downstream to reduce pressure on the electric barriers, yet it is unclear how to best allocate fishing effort along the Illinois River waterway. Commercial fishing is legal only in the downstream reaches where reproducing Asian carp are in high abundance; fish processing plants have been expanded in this area to support the removal of Asian carp yet there is currently no mechanism in place for subsidizing these fishing efforts. The upper Illinois waterway is not conducive for Asian carp spawning, but it does harbor large reproductively capable individuals that are kept at bay via state-contracted commercial fishermen. To jump-start the free-market commercial fishery downstream, we initiated an Asian Carp Incentives Pilot Program in spring 2012 that resulted in the removal of nearly 3 million pounds of Asian carp from the lower Illinois River for conversion into fish meal. Significant changes in the size structure, relative abundance, and sex ratios of Asian carp were observed in 2012 relative to 2011. In the upper Illinois waterway, a mark-recapture study conducted in 2012 indicated that while contracted fish exploitation was as high as 76% over a 20-week period, the population did not begin declining until fall when immigration of Asian carp into the upper Illinois waterway carp declined. As such, late-year harvest could be essential at keeping Asian carp densities low to curtail upstream movement toward the CAWS barrier. Moreover, limiting immigration to upstream areas via harvest of Asian carp from the downstream source will likely be imperative for controlling their further expansion toward the CAWS. Optimal harvest strategies along the Illinois River will have to weigh the benefits of removing reproductively viable individuals from the downstream areas against the potential risks of not focusing harvest toward non-reproducing, albeit capable, individuals that pose an immediate threat to the Great Lakes due to proximity.