Bacterial Numbers from Landlocked Fall Chinook Salmon Eyed Eggs Subjected to Various Formalin Treatments as Determined by Scanning Electron Microscopy and Bacteriological Culture Methods

Abstract

AbstractThis study compared two methods of enumerating bacteria adhered to the external membrane of eggs of landlocked fall Chinook salmon Oncorhynchus tshawytscha that were subjected to different formalin treatment regimes from egg eye‐up to fry hatch. Bacterial numbers were recorded by either directly counting bacteria via a scanning electron microscope (SEM) or via established bacterial culture methods that provided the number of colony‐forming units. Treatment regimes consisted of a daily 15‐min exposure to formalin at either 500 mg/L or 1,667 mg/ L or no formalin initially followed by exposure to a 1,667‐mg/L dose starting 7 d after eye‐up. A control group did not receive any formalin treatments throughout the experiment. No significant correlation was observed between the number of colony‐forming units (CFUs) per square millimeter of egg membrane determined by plate culture and the number of bacteria per square millimeter counted with the SEM. Bacterial numbers determined by the SEM were 40–120 times greater than those obtained by plate culture. The SEM numbers were correlated to embryo survival, but CFU numbers were not. For both counting methods, bacterial numbers among the untreated controls increased over time. Of the three species of bacteria identified in cultures (Flavobacterium columnare, Sphingobacterium spp., and Pseudomonas spp.), F. columnare was predominant. Embryo survival in the trays that did not receive any formalin was significantly less than that in all of the other treatments, but there was no significant difference in survival among any of the three formalin treatment regimes.