Decrease in the Bactericidal Activity of Normal Serum during the Spawning Period of Rainbow Trout

Abstract

The effects of seasonal changes and sexual maturation on the serum bactericidal activity in rainbow trout Salmo gairdneri were investigated. Two fish populations separated in spawning season were studied. One population reared under natural environmental conditions had spawning period between September and November, while the other reared under controlled day length had spawning period between April and June. The sera collected from trout were pooled together and mixed with an Escherichia coli suspension. This mixture was incubated at 25°C for 6 h. After incubation the number of viable bacteria were counted by the plate counting method and the survival rate was calculated. Among the heated (45°C for 30 min) sera, the bacteria multiplied about 10 times during the 6 h incubation. In contrast, the bacterial count in the normal sera decreased in number to about one tenth of the initial count after 6 h. However, the normal sera sampled during spawning period showed an increase of bacteria. This change of the serum bactericidal activity level was observed in both fish populations. In addition, the bactericidal activity level of the sera from fish before being stripped was lower than that from fish after being stripped. From these results, it is probable that the decrease of the serum bactericidal activity during the spawning period increase the susceptibility to disease in fish. © 1989, The Japanese Society of Fisheries Science. All rights reserved.