The occurrence of fin rot in mullet (Mugil cephalus) associated with crude oil contamination of an estuarine pond-ecosystem

Four estuarine pond‐ecosystems equipped with tidal simulation were established and stocked with mullet, shrimp, and oysters. A 6 month pre‐spill sampling period established a consistently low incidence of disease in the mullet, Mugil cephalus, from all ponds. Following the preliminary study period, Empire Mix crude oil (4.0‐5.0 p.p.m.) was spilled on each of two of the ponds in mid‐July, 1974. Six to eight days following the oil spill all the mullet examined from the treated ponds had varying degrees of fin rot on one or more of their fins. The fin erosion involved primarily the caudal, pectoral, and pelvic fins with the caudal fin the most severely damaged. The degree of damage to the fins varied from a slight discoloration with no visible fraying to complete erosion of all of the fin elements. A gram negative rod tentatively identified as Vibrio sp. is considered the primary pathogen responsible for the fin erosion. Although the condition of the infected fish indicated the possibility of a systemic infection, only a low percentage of the kidney cultures taken from the diseased fish were positive indicating the infection was mostly external. The course of the infection was documented over a 56‐day period following the oil spill. Significantly, 96% of the mullet observed from the treated ponds had some degree of fin damage while only 6 % of those observed from the control ponds had eroded fins. The high incidence of fin rot which occurred in the estuarine‐pond ecosystems did not occur during numerous acute exposures of mullet to crude oil in the laboratory.