| Summary: | Current levels of Asian carp harvest lead to measurable increases in zooplankton abundance and biomass meaning zooplankton are an indicator of ecosystem response to carp suppression. However, diversity especially of large bodied cladocerans and copepods does not. This suggests that the most beneficial plankton food resources, cladocerans and copepods, are not recovering as quickly as the microzooplankton. Additionally, while there were differences in zooplankton community structure and population densities between years and among river reaches, these did not obscure the positive influence of harvest that zooplankton showed. What was known about Asian carp and plankton before this project: - The distribution of Asian carp has been expanding up the Illinois River towards Lake Michigan since at least the late 1990’s (Chick and Pegg 2001). - Where they establish and remain unmanaged, Asian carp densities have increased dramatically over time (DeBoer et al. 2018). - The upstream expansion of these carp appears to have stalled near Starved Rock Lock and Dam at roughly the same time as management actions (commercial fishing) were ramping up in the same area (Coulter et al. 2018). - Ambiguous response by primary production: chlorophyll is highly variable from year to year and depending on habitat (DeBoer et al. 2018) - Main channel zooplankton have declined and composition has shifted as carp have increased (Sass et al. 2014; DeBoer et al. 2018) - Native planktivorous fish body condition is strongly affected but the response of population size is more variable (Love et al. 2018; DeBoer et al. 2018; Pendleton et al. 2017) - Fish assemblage diversity and composition have both shifted measurably since 2000, but these responses have not been either quick or large (Solomon et al. 2017) What was not known about Asian carp and plankton before this project: Zooplankton are a basal food resource affecting all fish at some point in their life cycle. We have clear evidence that as the Asian carp presence increases the zooplankton abundance and biomass decrease leading to negative impacts on the ecosystem. However, as of 2018 we do not have clear evidence of whether suppression of the carp through commercial harvest mitigates those negative impacts on the plankton or planktivorous fish.Thus the goal of this project assesses whether zooplankton can tell us whether suppression of carp through commercial harvest is working and, ultimately, benefiting the ecosystem? More specifically; a) What is the lag time between management event/effort and a measurable response? b) Is there a harvest threshold to cross before there is a measurable response? What we can conclude because of this project: Section 1: Across the river the total species richness of all zooplankton has declined steadily as Asian carp have increased between 2010 and 2015. However, zooplankton density and biomass 3 which oscillated during the same period. Specifically, microzooplankton (Rotifers) dominated plankton numerically in all years, regions, and habitats. In contrast, macrozooplankton (Crustacean Copepods and Cladocerans) never dominate numerically, but are a disproportionately large part of the zooplankton biomass. In the upper river, where harvest pressure is strongest and carp abundance is lowest, interannual changes in zooplankton density and abundance are explained mainly by river hydrology (stage height and velocity) while biodiversity is explained mainly by water quality (temperature and turbidity). In contrast, temperature is the main factor affecting diversity, abundance, and biomass of zooplankton in the lower river where harvest is lower and carp abundance is higher. We believe the take-away message is that zooplankton community composition can be used as an indicator of ecosystem response to the arrival and expansion of invasive carp, if there is pre-existing info on what the plankton looks like without carp present in the river reach of concern. Section 2: Effect of standard single crew carp harvest by individual commercial fishing crews on is measurable and there is a relatively short lag time (i.e. weeks to months within the same summer). However, the response to harvest differs depending on type of zooplankton: microzooplankton (rotifers) responded to all tested levels of harvest but macrozooplankton (Cladocera, adult and juvenile Copepods) only responded to highest levels of removal (~10,000 kg per month). We believe a main take-away message should be that higher levels of harvest will lead to a greater benefit. Section 3: Effect of intense multi-crew harvest shows that greater harvest rate leads to greater positive ecosystem response. While the effect of harvest is significant, it can also complex and dependent on season and which category of zooplankton is considered (> 40,000 kg during a March 21 to April 1, 2016 event and an additional > 30,000 kg during a February 27 to March 10, 2017 event). Lag time to the initial positive response when the intense harvest took place in the early spring as in 2016 was as short as 4 to 5 weeks for rotifers and nauplii and 10+ weeks for Cladocera and Copepods. If the harvest event occurred late spring as in 2017, then the lag time decreased 3 – 4 weeks for rotifers, nauplii, Cladocera and Copepods. For instance, by day 89 of 2017 rotifers responded positively in both treatments suggesting seasonal succession was more important than harvest. However, by day 145 of 2017 rotifer density without harvest was lower than with harvest whereas the inverse was true for nauplii, and there was no difference for Cladocera and Copepods. We believe the take-away message is that while more intense harvest benefits the ecosystem, the level of benefit is dependent on the type of zooplankton as well as the season and year of harvest. Illinois Department of Natural Resources, Division of Fisheries unpublished not peer reviewed Open
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