Assessing the Potential Impacts of Microcystis on Threadfin Shad (Dorosoma pentenese) In the San Francisco Estuary

In the San Francisco Estuary (SFE), toxic algal bloom Microcystis has been suggested as a link to the Pelagic Organism Decline (POD) by the Interagency Ecology Program (IEP). The bloom predominantly produces the hepatotoxin microcystin-LR (MC-LR), which may impact the survival of fish. The purpose of this study is to evaluate the application of biomarkers of exposure and effects for determining the toxic effects of local Microcystis to threadfin shad (TFS), Dorosoma petenense. To fulfill this objective, TFS were collected during Microcystis blooms in the SFE and were assessed on their potential consumption of Microcystis and localization of MC-LR within their tissue. In the laboratory, juvenile TFS were fed Microcystis diets containing 5 (D5) and 10 (D10) μg/g MC-LR for 57 days. Fish fed the treated diets were compared to fish receiving the control diet (D0) lacking MC-LR (0 μg/g). A significant inverse relationship was observed between condition factor and MC-LR. Gross examination revealed severe cachexia in fish exposed to MC-LR. Liver histopathology showed greater prevalence in sinusoidal congestion, hemorrhage, and glycogen depletion with increasing MC-LR concentrations. In females, there was a signficant increase in severe ovarian atresia with increasing MC-LR concentration. Laboratory resutls were reflective of field samples. Immunohistochemistry of field samples confirmed the consumption of Microcystis among wild TFS that also demonstrated similar liver lesions characteristic of MC-LR exposure. In summary, results indicate that chronic dietary exposure to Microcystis significantly impairs the health and reproductive potential of threadfin shad . This study established the adverse effects of the bloom on threadfin shad in the SFE and will therefore aid in determining management protocols to address impacts of exposure to Microcystis blooms in the SFE.