Commensal bacteria and their possible relationship to the mortahty of incubating salmonid eggs

Abstract. Eggs of female rainbow trout, Oncorhynchus mykiss (Walbaum), were fertilized and subsequently exposed to high eoncentrations of Pseudomonas fluorescens or Cytophaga sp. (10⁶ cells/ml) for 45 min during water hardening. Eggs were placed in horizontal hatchery trays alongside sterile water-treated eggs and mortality rates were observed daily throughout incubation. Numbers and species of bacteria present on egg surfaces and within egg contents were monitored at the green stage (unfertilized), after hardening, and at 12h, 7 days, 14 days, 21 days and 28 days incubation. Surfaces of the incubating eggs of all groups, irrespective of treatment at water hardening, were eventually colonized by considerable numbers of Cytophaga sp., P. fluorescens and to a lesser extent Pseudomonas sp. and Aeromonas hydrophila. Few bacteria were recovered from inside eggs. Overall, no correlation was found between bacteria on the egg surface and egg death. However, a significantly higher 'rate' of egg death during early stages of incubation was found amongst eggs exposed to P. fluorescens at water hardening. Thus, high numbers of P. fluorescens on egg surfaces during initial stages of incubation might pose a potential threat to egg survival. Therefore, it is suggested that strenuous efforts are made at time of hardening to obtain the purest available source of water.