Incorporating Chlorophyll-a Levels into an Integral Projection Model of Gizzard Shad (Dorosoma cepedianum) in the Upper Mississippi River

Background/Question/Methods

A novel approach to acquiring economical and time-saving management strategies for wild populations is with data-driven models such as Integral Projection Models (IPM). IPMs models over a continuous variable that best describe an invasive or threatened species life history. At the Upper Mississippi River, the establishment of invasive carp in 2000 threaten native fish populations, leading to lower body conditions and a decline in maximum adult fish length. Among the native species is the Gizzard Shad (Dorosoma cepedianum), a planktivore with an overlapping diet with the invasive carp; consuming plankton as larvae, and shifting to a majority detritus diet as juveniles. Given the limitations of plankton community data, we used chlorophyll-a as an estimation of plankton density (energy flow from chlorophyll-a to phytoplankton to zooplankton to juvenile Gizzard Shad). To research the influence of chlorophyll-a levels on Gizzard Shad populations in four distinct pools in the Upper Mississippi River, we aimed to (1) incorporate the consumption of plankton, using chlorophyll-a as a proxy, into the juvenile stage of the Gizzard Shad IPM, and (2) determine how varying juvenile length distributions influence the adult stage of the Gizzard Shad.

Results/Conclusions

We parameterized the IPM with data from previous papers on Gizzard Shad, including adult survival, adult growth, egg viability, number of eggs laid, and juvenile survival. Creating a linear relationship between chlorophyll-a levels and mean juvenile length per pool, we established a changing juvenile distribution dependent on chlorophyll-a levels. The model produced two scenarios with a high chlorophyll-a pool and a low chlorophyll-a pool. The location with a low chlorophyll-a pool has a smaller but longer juvenile population with a larger adult population. Hence, a high chlorophyll-a pool has a larger but shorter juvenile population with a smaller adult population. The model suggests a trade off between length and density of the juvenile population dependent on chlorophyll-a levels. This supports the ecological cascade of a low chlorophyll-a pool that will have a high plankton population consuming chlorophyll, driving the chlorophyll levels low, and creating a longer juvenile population.