Bacterial Analysis of Fertilized Eggs of Atlantic Salmon from the Penobscot,Naraguagus, and Machias Rivers,Maine

Serious losses have occurred at the U.S. Fish and Wildlife Service, Craig Brook National Fish Hatchery, East Orland, Maine, among eggs that were taken from Atlantic Salmon Salmo salar, which were held as captive broodfish during their returns to the Penobscot River, Naraguagus River, and Machias River to spawn. Bacterial isolations were attempted from external surfaces and the internal contents of individual eggs. Externally and in all cases, Pseudomonas fluorescens was the predominant bacterium associated with the surface of all eggs. These bacteria were resistant to a surface treatment of 1,667 ppm formalin for 15 min and, therefore, the monoclonal nature of P. fluorescens on egg surfaces was considered to result from its ability to resist the germicidal activity of formalin administered for antifungal treatments. Flavobacterium psychrophilum, the cause of bacterial coldwater disease, was isolated from the interior of 23.6, 18.1, and 29.2% of the dead Atlantic Salmon eggs from Penobscot River egg lots A-98, A-100, and A-101, respectively, and concentrations of this pathogen ranged from 1.0 × 10³ to >5 × 10⁸ CFU per gram of dead egg. Flavobacterium psychrophilum was also isolated from 8.3, 26.7, and 10.0% of the dead eggs from Naraguagus River egg lots N-158, N-161, and N-163, respectively, in which concentrations of this organism ranged from 1.0 × 10³ to 7.5 × 10⁸ CFU per gram of egg. This bacterium was also isolated from within 18.3% and 3.3% of the dead eggs from Machias River egg lots M-128 and M-142, respectively, and its concentrations ranged from 1.0 × 10³ to 1.5 × 10⁸ CFU per gram of egg. The finding of F. psychrophilum from within these eggs is indicative of this pathogen's widespread and persistent prevalence in Atlantic Salmon in New England.

Received December 19, 2014; accepted May 4, 2015