Combined effects of UV irradiation, ozonation, and the probiotic Bacillus spp. on growth, survival, and general fitness in European lobster (Homarus gammarus)

Bacterial pathogens are a leading cause of disease in hatchery aquaculture systems and preventative methods such as use of probiotics as feed supplements and water additives are well documented. However, comparisons between the effectiveness of using probiotic water additives over traditional biocontrol methods are less understood. This study assessed the combined effects of ultraviolet (UV) irradiation, O3 (ozone) and Bacillus spp. as a water additive (probiotic), in the culture of European lobster (Homarus gammarus) in a semi-closed recirculation system. Larvae were categorised as zoea stages I-III, megalopa (stage IV) and juvenile (stage V) onwards. Stage I larvae were assigned to one of six treatment groups consisting of 1) O3, 2) probiotic, 3) probiotic + O3, 4) probiotic + O3+ UV, 5) O3 + UV, or 6) probiotic + UV, for 18 days. During stages I-V, growth was measured on 1, 6, 11, 18, 24, and 31 dph (days post hatch), and survival was measured on 1, 18, 24 and 31 dph. Bacterial counts of pathogenic Vibrio spp. in culture water were measured at 1, 4, 9, 14, and 18 dph. Lobsters were also exposed to a physiological fitness test (low salinity challenge) at stage IV, 7 days post treatment. Results showed that O3 is comparatively more beneficial than probiotic with increased LWG (live weight gain) in the O3 treatment over probiotic between stage IV and V (> 5 mg). Survival rates were ~ 10% higher in the O3 treatment group than probiotic treatment group on day 18, then ~ 5% and ~ 4% higher on days 24 and 31. Lobster biomass on day 18 was ~ 60% higher in the O3 treatment than probiotic treatment and 116% higher on day 31. Total Vibrio spp. present in the O3 treatment was 0.05% of the total in the probiotic treated culture water (day 18). Results between UV treatment groups showed significantly lower numbers of Vibrio spp. present in probiotic + O3 + UV culture water 4 dph than O3 + UV (~ 10 fold higher) or UV + probiotic (~ 15 fold higher) and by day 18 probiotic + O3 + UV was significantly higher than O3 + UV (~ 8 fold higher). Osmoregulatory challenge test resulted in no significant differences in physiological fitness between any treatment groups. The present study shows the effectiveness of O3 in aquaculture facilities for control of pathogens in the rearing of European lobster over either a probiotic water additive (at 3.75 × 107 CFUs L− 1) or UV irradiation.