INFLUENCE OF FEEDING ON THE DEVELOPMENT OF BACTERIAL GILL DISEASE IN RAINBOW-TROUT ONCORHYNCHUS-MYKISS

Three trials were conducted to determine if feeding influenced the dev elopment of bacterial gill disease (BGD) in fingerling rainbow trout O ncorhynchus mykiss following bath exposure to Flavobacterium branchiop hilum (LAB 4a). In each trial (Trial 1, n = 70 x 3; Trial 2, n = 140 x 3; Trial 3, n = 250 x 3), the comparison groups (fed, unfed, negative controls) were held in similar conditions and (where applicable) fed prior to exposure to the pathogen. Severe clinical signs and high rate s of mortality developed within 24 h in fish which were provided feed following the bath exposure. Cumulative mortalities ranged from 43 to 63% by 12 d post-exposure. Unfed fish demonstrated only mild and trans ient clinical signs of BGD, and cumulative mortalities of 0 to 2% 12 d post-exposure. In one trial, resumption of feeding 6 d post-exposure resulted in 26% cumulative mortality within 7 d. Estimation of the qua ntity of F. branchiophilum antigen by enzyme-linked immunosorbent assa y (ELISA) indicated that feeding facilitated colonization of the gill. The evaluation of water quality and direct observations suggest that the influence of feeding on BGD is linked to the consumption of feed b y the fish, rather than to environmental changes arising from feeding. Alterations in the unstirred layer on the gill, secondary to feed con sumption and waste excretion, may aid bacterial colonization.