Physiological responses of cobia rachycentron canadum following exposure to low water and air exposure stress challenges

Prevention is the most viable disease management strategy in aquaculture, and prevention is primarily driven by strategies to avoid or minimize the effects of stress. Unfortunately, there is little information available regarding the stress physiology of emerging aquaculture species or appropriate experimental stressing protocols for these fishes, and thus very little context in which to evaluate mitigation strategies. Accordingly, the stress response of cobia was evaluated following exposure to 2 experimental stressors: low water and air exposure. Juveniles were exposed to air for 1 min (AIR EXPOSURE), held for 15 min in water lowered to the fish's lateral midline (LOW WATER), or unchallenged (CONTROL) prior to the collection of blood samples at 0 (pre-challenge), 0.5, 1, 2, 6, 12, 24, 48, and 72 h post-challenge. Both stressors elicited classical haematological changes indicative of the generalized stress response, however, the magnitude of the response was consistently greater in the AIR EXPOSURE group. Plasma cortisol, glucose, and lactate concentrations increased rapidly in the AIR EXPOSURE and LOW WATER groups, peaking within 1 h of challenge. Cortisol returned to basal levels rapidly, whereas glucose and lactate remained elevated for a longer period of time. Regardless of the stressor used, fish recovered within 12 h of the challenge. The primary and secondary responses of juvenile cobia challenged with low water and air exposure appear to respond in a similar fashion to other species exposed to these experimental stressors. Both low water and air exposure are suitable experimental stressors for use in cobia based on their ability to induce a classical stress response and ease of implementation

Acute toxicity and sublethal effects of ammonia and nitrite for juvenile cobia rachycentron canadum

Nitrogenous compounds can be toxic to aquatic animals especially when they are reared at high stocking densities. Cobia (Rachycentron canadum) is a fast growing fish currently reared in cages, but with expanding production in intensive recirculating aquaculture systems (RAS). Therefore, the objective of this study was to evaluate the acute toxicity of ammonia and nitrite to juvenile cobia. Juveniles (1.74±0.11 g for ammonia and 0.88±0.06 g for nitrite toxicity evaluation) were acclimated to test conditions(temperature26°Candsalinity22‰)andacutelyexposedtoammonia(0.25–1.30ppmNH 3-N)andnitrite(30–210ppm NO2-N) at 0.2 fish L − 1. Tests were run in 50 L semi-static tanks, experimental water was fully renewed daily, and all test concentrations plus the controls were run in triplicate. Mortality, feeding and swimming behavior were observed during 96 h, toxic concentrations for 50% the population and the respective 95% confidence intervals for these three end points were estimated using the Trimmed Spearman Karber Method. Cobia ceased to eat at 0.62 (0.56–0.70) ppm NH3-N and 76.1 (73.2–79.0) ppm NO2-N. Swimming behavior was affected at higher concentrations: 0.80 (0.74–0.85) ppm NH3-N and 88.8 (82.6–95.5) ppm NO 2-N. Even higherconcentrationswerenecessarytokilljuvenilecobia,LC50–96hforammoniawasestimatedat1.13(1.06–1.19)ppmNH 3-N, andwithintherangeofconcentrationstestedfornitriteitwasnotpossibletoestimatetheLC50–96h,asonly30%oftheindividuals died at the highest concentration after 96 h (210 ppm NO2-N). The results of the present experiments demonstrate that ammonia couldbeproblematicatrelativelylowlevelsfortheintensiverearingofjuvenilecobia;however,itisunlikelythatthehighlevelsof nitrite needed to harm juvenile cobia would be reached in a well designed and properly operating RAS

Probiotic effects on cobia Rachycentron canadum larvae reared in a recirculating aquaculture system

Cobia (Rachycentron canadum) is a marine finfish with good potential for mariculture. This study analyzes the effects of probiotic Bacillus spp. on the performance of cobia larvae reared in a recirculating aquaculture system (RAS). Larvae were stocked into two independent RAS for 26 days after hatching. One of the systems (Probiotic treatment) received the addition of a commercial probiotic consisting of B. subtilis, B. licheniformis and B. pumilus directly into the water and by live feed. Survival, final weight and water quality were not affected by probiotics. Results showed larvae of the probiotic treatment demonstrated a greater resistance to salinity stress. Immunohistochemical analysis showed a higher expression of CD4 in probiotic treatment. These results suggest that Bacillus spp. probiotics used in RAS have a potential stimulating impact on immune system differentiation and increases salinity stress resistance of cobia larvae

Acute exposure of juvenile cobia Rachycentron canadum to nitrate induces gill, esophageal and brain damage

Cobia Rachycentron canadum is a fast growing fish with world-wide potential for aquaculture, and has been considered for rearing in recirculating aquaculture systems (RAS). Nitrate is considered the least toxic nitrogenous product in the ammonia nitrification process, but as it may accumulate in RAS, toxic levels can be reached. The objective of this study was to evaluate the acute toxicity and the histopathological effects of nitrate on juvenile cobia. Juveniles (6.87±0.36 g; 11.8±0.19 cm) were acutely exposed to six concentrations of nitrate (500–3000 ppm NO3 −-N) plus a control during 96 h. At the end of this period of exposure, juvenile cobia were sampled for histopathological evaluation. The estimated LC50 of nitrate to juvenile cobia was equal to 2407 and 1829 mg/L NO3 −-N at 24 and 96 h, respectively. Cobia exposed to sub-lethal nitrate concentrations showed histopathologic alterations in the gills, esophagus and brain. The gills revealed epithelial hyperplasia with complete lamellar fusion, telangiectasia, and lamellar shorting induced by necrosis, and the esophagus presented hyperplasia of epithelium and mucus cells. In the brain, glial cells proliferation, satellitosis (microglial cells surrounding neurons with swollen and prenecrotic neurons), and Virchow-Robin spaces (enlarged perivascular spaces, EPVS) were observed. The results of the present study indicate that juvenile cobia have a high tolerance to acute exposure of nitrate. However, assorted histopathological responses were observed for cobia at sub-lethal nitrate concentrations. Therefore, further studies are needed to estimate safe chronic nitrate levels for juvenile cobia culture

Feeding rate and frequency affect growth of juvenile atlantic spadefish

The Atlantic spadefish Chaetodipterus faber is an excellent candidate for aquaculture development, but success will depend on the identification of proper feeds and feeding regimens for this species. Accordingly, we evaluated the growth performance of juvenile Atlantic spadefish (3.60 ± 0.03 g [mean ± SE]) fed at rates of 3, 5, or 7% of body weight (BW) per day, either in a single feeding (1×) or in three equal feedings (3×). Weight gain, specific growth rate, feed conversion ratio, and feed intake were significantly affected by both feeding rate and frequency. Weight gain and the specific growth rate increased significantly with feeding rate, and growth was generally greater and more efficient in the 3× groups than in the 1× groups. Fish fed at higher feeding rates accumulated significantly more lipid within the body and had associated decreases in moisture, protein, and ash content, but carcass composition was unaffected by feeding frequency. We suggest that the growth of juvenile Atlantic spadefish can be optimized when they are fed at 5–7% BW/d in three daily feedings, with 7% BW/d yielding the greatest, albeit slightly less efficient, growth