Use of avoidance response by rainbow trout to carbon dioxide for fish self-transfer between tanks

Convenient, economical, and reduced labor fish harvest and transfer systems are required to realize operating cost savings that can be achieved with the use of much larger and deeper circular culture tanks. To achieve these goals, we developed a new technology for transferring fish based on their avoidance behavior to elevated concentrations of dissolved carbon dioxide (CO₂). We observed this behavioral response during controlled, replicated experiments that showed dissolved CO₂ concentrations of 60–120 mg/L induced rainbow trout (Oncorhynchus mykiss) to swim out of their 11 m³ “growout” tank, through a transfer pipe carrying a flow with ≤23 mg/L dissolved CO₂, into a second 11 m³ “harvest” tank. The research was conducted using separate groups of rainbow trout held at commercially relevant densities (40–60 kg/m³). The average weight of fish ranged from 0.15 to 1.3 kg during the various trials. In all trials that used a constant flow of low CO₂ water (≤23 mg/L) entering the growout tank from the harvest tank, approximately 80–90% of the fish swam from the growout tank, through the transfer pipe, and into the harvest tank after the CO₂ concentration in the growout tank had exceeded 60 mg/L. The fish that remained in the growout tank stayed within the area of relatively low CO₂ water at the entrance of the transfer pipe. However, the rate of fish transfer from the growout tank to the harvest tank was more than doubled when the diameter of the transfer pipe was increased from 203 to 406 mm. To consistently achieve fish transfer efficiencies of 99%, water flow rate through the fish transfer pipe had to be reduced to 10–20% of the original flow just before the conclusion of each trial. Reducing the flow of relatively low CO₂ water near the end of each fish transfer event, restricted the zone of relatively low CO₂ water about the entrance of the fish transfer pipe, and provided the stimulus for all but a few remaining fish to swim out of the growout tank. Results indicate that the CO₂ avoidance technique can provide a convenient, efficient, more economical, and reduced labor approach for fish transfer, especially in applications using large and well mixed circular culture tanks.     

Modulation of the response of rainbow trout (Oncorhynchus mykiss Walbaum) to confinement, by an ectoparasitic (Argulus foliaceus L.) infestation and cortisol feeding

The effects of a 21 day infestation with the ectoparasite Argulus foliaceus on the response of rainbow trout Oncorhynchus mykiss to an acute stressor (confinement) were investigated. Fish were divided into a control and two parasitized groups, with one parasitized group having received cortisol feeding 72 and 24 h before infection. Prior to confinement only control fish showed a significant increase in body mass and condition factor. Following 4 h confinement, plasma glucose levels in the normal- fed/infested group were significantly higher than in the controls, while the cortisol-fed/infested group had significantly lower plasma levels of adrenocorticotropic hormone (ACTH) and glucose. Both infested groups, but not controls, had higher plasma ACTH and cortisol levels 48 h after confinement when compared with their pre-confinement levels. Stressor-induced immunomodulation was more evident in the parasitized fish after confinement, as was concluded from decreased lysozyme activity, alternative complement activity and oxygen radical production either immediately after, or 48 h post 4 h confinement. Circulating lymphocyte numbers were significantly reduced in the control and normal-fed/infested fish. Results indicate that low ectoparasitic infestations can, for a prolonged period, lead to a stress experience with effects that impair the response to subsequent stressors. Such effects may be further confounded by a previous elevation of plasma cortisol levels.     

Effects of Panax ginseng extract in practical diets for rainbow trout (Oncorhynchus mykiss) on growth performance,immune response and resistance to Yersinia ruckeri

In a preliminary in vitro study, a Panax ginseng extract exhibited an evident antibacterial activity against Yersinia ruckeri and Lactococcus garvieae and affected the respiratory burst and proliferation of rainbow trout Oncorhynchus mykiss leukocytes. Subsequently, the effects of a dietary ginseng extract supplementation on growth, blood biochemical profile, innate immune response and resistance against Y. ruckeri infection were investigated in vivo in rainbow trout juveniles. Four experimental diets were obtained by adding 0.0%, 0.01%, 0.02%, 0.03% of ginseng ethanolic extract to a commercial feed. Each diet was fed to triplicate groups of fish (mean body weight 30.5 ± 0.15 g) at 1% of body weight day^?1 for 10 weeks. The dietary supplementation with ginseng extract did not affect growth performance, feed utilization, biometric traits and fish whole body composition (P > 0.05). No major changes due to graded levels of ginseng extract in the diet were observed in blood biochemical parameters except for increasing plasma triglycerides and non‐esterified fatty acids in fish fed diets including 0.01% and 0.02% of extract (P < 0.05). The innate immune response was barely modulated by the dietary addition of ginseng extract. Serum lysozyme and leukocytes respiratory burst activities were just slightly increased in fish fed all the ginseng extract‐supplemented diets compared with controls, whereas serum antiproteases and leukocyte MPO were not affected (P > 0.05). The dietary administration of ginseng extract induced a reduction in mortality of rainbow trout infected with Y. ruckeri, although no significant differences between treated and control groups were observed (P > 0.05).