Effects of accessory factors on survival of newly fertilized salmonid eggs treated with an antibiotic

A trial use of erythromycin phosphate was conducted in a salmonid hatchery to determine its efficacy, when used as a rinse of newly fertilized coho ( Oncorhynchus kisutch) eggs, as a prophylatic measure against vertical transmission of bacterial kidney disease (BKD). Substantial egg mortality occurring in this trial resulted in further investigation of the rinse technique used. An aqueous solution of 2 mg/l of erythromycin phosphate was found to have no deleterious effect on fertilization or activation of eggs. Egg viability was not affected by (a) delay between taking of gametes and egg fertilization (10 or 40 min), (b) contact time between gametes at fertilization (1 or 10 min), and (c) the number of rinses employed (1 or 2). Mortality of the type seen in the initial trial was concluded to have occurred through shock sensitivity of the eggs, arising from (d) delay between egg activation and transfer of treated eggs to incubation trays, and (e) the height from which treated eggs were poured into incubation trays. Recommendations are given for the use of prophylactic egg rinses at fertilization, while avoiding egg damage resulting from shock sensitivity. Although not conclusive, results of the initial trial indicated that the survivors essentially were free of BKD up to the time of release 18 months after their treatment, as eggs, with erythromycin phosphate.