HISTOLOGICAL EVALUATION OF PARASITE PREVALENCE IN YELLOWTAIL FLOUNDER Limanda ferruginea ON GEORGES BANK  

The diminishing stock of yellowtail flounder on Georges Bank is a growing concern for the sea scallop industry since the species is recognized as a choke stock. The parasitic protozoan, Ichthyophonus sp., has been identified by Coonamessett Farm Foundation (CFF) as a causative agent of mortality in this stock. Since 2012, CFF has been performing systematic sampling on yellowtail flounder as part of ongoing seasonal dredge surveys on Georges Bank to investigate yellowtail disease and understand seasonal fish distribution. Samples of yellowtail flounder caught during each survey were selected for at-sea examination. Peritoneal and pericardial cavities were macroscopically inspected for abnormalities. Observations of parasitic infections were documented and tissues were biopsied and preserved in 10% neutral buffered formalin for histological evaluation. Hematoxylin and eosin-stained paraffin embedded tissues were examined microscopically for intensity of parasitic infection and parasite morphology. Intensity was rated on a scale of zero to five and microscopic images were collected for further analysis. Histological evaluation revealed that the majority of yellowtail flounder hosted various parasites including Ichthyophonus sp., cestodes and ascarids. Yellowtail flounder severely infected by Ichthyophonus sp. exhibited granulomas in the heart, liver, stomach, intestine, kidney, spleen and mesentery tissue. Further yellowtail sampling on Georges Bank is underway to gain more information on the distribution of Ichthyophonus sp. within the flounder population. Increased knowledge of the prevalence and severity of parasitic infection will help us to better evaluate how disease is affecting mortality of the Georges Bank stock.