Invasive Species as Sentinels: Measuring Health Outcomes in Silver Carp (Hypophthalmichthys molitrix) during Removal

Invasive species threaten ecosystems with destruction of native habitat, introduction of novel diseases, and enhanced competition with native wildlife subsequent to reduced predator control, leading, in many cases, towards efforts to actively remove individuals. While these effects are frequently studied, minimal research has investigated the individual or population health of the invasive species themselves. In this study, we describe multiple health outcomes of Silver Carp (Hypophthalmichthys molitrix), an invasive species in the Illinois River, as a component of a long-term monitoring and removal program using electrofishing, gillnets, and trammel nets. Between April-October 2018, Silver Carp were collected from two reaches of the Illinois River, examined, blood was collected for hematologic measurements, and cranial kidneys collected for histologic examination of melanomacrophage centers. Collection location impacted body condition, as Silver Carp closer to the leading edge of invasion were heavier than those from more established populations. Silver Carp caught by nets had lower packed cell volumes than those caught by electrofishing. The Health Assessment Index (HAI) showed that 52% of livers and 53% of kidneys were grossly abnormal. The HAI comes with a caveat that validation protocols are required to implement this technique effectively. Hematology and histology are more likely to be useful in species for which reference ranges exist. Overall, invasive species contain a wealth of information on health outcomes that could be used to monitor ecosystem health, but techniques used for monitoring must be adapted to the species, management needs, and removal methods.