We evaluated three alternative procedures for tagging juvenile Nile tilapia Oreochromis niloticus (1.9–13.7 g) with passive integrated transponder (PIT) tags: injection with hypodermic syringe injectors (N = 40) and abdominal surgery with or without suturing (N = 55 and 40, respectively). The survival of Nile tilapia tagged with injectors was low (10–50% at 10 d) and proportional to fish size because of the difficulty of controlling the penetration of the syringe into the intraperitoneal cavity following the piercing of the body wall. Surgically implanted fish had much higher survival rates (78–100% at 10 d). Suturing reduced the risks of tag expulsion and protrusion of the viscera through the open incision within the first 3 d following surgery (10% risk in the nonsutured fish). Over the first 3 d after tagging, tagged fish showed depressed growth rates (to a greater extent in smaller fish and proportionally greater in sutured fish) but showed normal growth later on. These results demonstrate that surgery is suitable for PIT‐tagging small juvenile Nile tilapia, which offers opportunities for studies of individual performance.
SummaryRecent developments in Eurasian perch (Percafluuiatilis) larval rearing and weaning techniques allow large-scale intensive rearing tests, both with juveniles and adults. This review presents the results of experiments conducted in a commercial scale production environment (0.5-1.6m3 flow-through tanks) and focuses on the most relevant factors influencing growth and production in intensive rearing systems. Combined effects of food ration and temperature showed that the maximum growth rates (0.06-1.80 g fish-t day-l for 1-300 g fish) were observed at 23"C. The thermal range for intensive rearing of perch extends at least down to ll'C (growth rate equivalent to 29o of that observed at 23"C). The maximum daily food ration (94.5% lipid;5744o/o crude protein) at23"C resulted in feed conversion rates from 1.1 to 2.0 in fish ranging from I to 1509 and was modelled as follows (R' :0.89): R-u* (7o duy-') : 7.60 w (weight, g)'". Juvenile (1-l5g) perch reared at 10000 fish m-3 showed a growth rate of 0.2 g fish I day ', thus 670 higher than at 400 fish m-3. In the most intensive rearing trials (I24kgm 3), the maximum production rate observed was 2.6 kg -' day '.Growth heterogeneity constitutes a major constraint in perch culture. Size grading experiments resulted in the emergence of fast-growing fish in each sorted group. In addition, the sorting process caused the productivity to be slightly lower (5-6%) than in non-sorted populations of the same origin and body weight. Mass mortality may take place at any stage of perch rearing within the 1-200 g body weight range. In general, an overall 500% survival rate was recorded after 14 months. Parasites, bacteria and fungi were the most frequently encountered causes of mortality. The occurrence and impact of diseases were largely reduced in recirculating systems where the survival rate is 70o/o higher than the values observed in flow-through rearing systems. In intensive rearing systems at 23"C and at maximum stocking biomass of 50 kg --', 44-day-old weaned perch juveniles (ranging from 0.5 to 1g initial body weight) reached an average body weight of 130-150 g after 1 year of rearing. Still, this marketable size was reached about 3 months earlier by fast growing individuals within the population.The intensive culture of perch at a constant23"C water temperature allowed much higher growth rates than usually observed in conventional, extensive systems under natural thermal regimes. Constant temperatures also inhibited the sexual maturation in females. Further improvements in growth and productivity in intensive perch rearing will closely rely on techniques such as the selection of fast-growing strains, monosex female or sterile triploïd fish production.
Cyprinid herpesvirus 3 (CyHV-3) is the aetiological agent of a mortal and highly contagious disease in common and koi carp. The skin is the major portal of entry of CyHV-3 in carp after immersion in water containing the virus. In the present study, we used in vivo bioluminescence imaging to investigate the effect of skin mucus removal and skin epidermis lesion on CyHV-3 entry. Physical treatments inducing removal of the mucus up to complete erosion of the epidermis were applied on a defined area of carp skin just before inoculation by immersion in infectious water. CyHV-3 entry in carp was drastically enhanced on the area of the skin where the mucus was removed with or without associated epidermal lesion. To investigate whether skin mucus inhibits CyHV-3 binding to epidermal cells, tail fins with an intact mucus layer or without mucus were inoculated ex vivo. While electron microscopy examination revealed numerous viral particles bound on the fins inoculated after mucus removal, no particle could be detected after infection of mucus-covered fins. Finally, anti-CyHV-3 neutralising activity of mucus extract was tested in vitro. Incubation of CyHV-3 with mucus extract reduced its infectivity in a dose dependent manner. The present study demonstrates that skin mucus removal and epidermal lesions enhance CyHV-3 entry in carp. It highlights the role of fish skin mucus as an innate immune protection against viral epidermal entry.
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