A herpesvirus was isolated from adult koi, a strain of common carp Cyprinus carpio, suffering mass mortality in two outbreaks-one in the mid-Atlantic region of the United States and the second in Israel. The principal external signs of dying fish were pale and irregularly colored gills. There were few consistent internal signs in either outbreak. The most prominent microscopic lesions were in the gills, where hyperplasia and necrosis of the epithelium were severe. Other lesions included interstitial nephritis, splenitis, and enteritis. Affected cells often contained nuclei with marginated chromatin and faint intranuclear inclusions. Typical herpesvirus particles were present in branchial epithelial cells, hepatocytes, and among circulating leukocytes. Inoculations of the koi fin (KF-1) cell line with tissue extracts from the gill and kidney-spleen resulted in cytopathic effects characterized by severe vacuolation first detected after 7 d incubation at 20°C. Exposures of adult koi to the herpesvirus as propagated in KF-1 cells by bath or intraperitoneal injections resulted in 80-100% mortality during a 26-d period, and the virus was reisolated from the gill, kidney, liver, spleen, intestine, and brain of dead fish. The viral agents from koi in Israel and the United States appear to be similar if not identical; both could be distinguished from Herpesvirus cyprini by indirect fluorescent antibody tests with rabbit anti-H. cyprini serum. Other factors should be examined but we strongly suspect that this newly recognized koi herpesvirus (KHV) has the potential to be a significant cause of mortality among koi and presumably common carp.

One-year-old rainbow trout Oncorhynchus mykiss were reared for 8 weeks at a density of either 56 or 267 g fish/L (based on the volume of net-pens), equivalent to density indexes of 2.3 and I l.l g fish/L-cm total fish length), respectively. The fish were held in 0.6 x 0.3-m netpens submerged to a depth of 0.15 m. Two net-pens for each fish density were suspended in each of three 3,040-L circular tanks provided with sufficient flow to maintain loading rates in the tanks at less than 800 g/(L-min). The fish were then subjected to an acute handling stress by being removed from the water for 60 s. No differences in the time course of changes in serum cortisol levels or hematocrits were observed over a 12-h period between fish in the two density groups. There were also no differences between the two groups of fish in weight, length, body condition factor (weight/length 3 ), interrenal nuclear diameter, or the percentage of the anterior stomach that was mucosa. These results indicate that if high water quality is maintained, 1-year-old rainbow trout can be reared at a density index as high as 11.1 g/(L'Cm) without impairing their growth or causing overt chronic stress.

No abstract

No abstract

A commercially available heterogeneous, solid‐phase tube enzyme‐linked immunoassay (ELISA) was modified and validated for the measurement of serum cortisol in rainbow trout Oncorhynchus mykiss. The assay is accurate and precise. Resting and stress‐elevated serum cortisol concentrations were measured in rainbow trout with a sensitivity of 1.5 ng/ml. Fish held in net‐pens at a density of 0.4 kg/m3/cm had a resting cortisol level of 16.5 ± 3.8 ng/ml (mean ± SE). At 3 h postdisturbance, serum cortisol levels were not affected by the removal of fish from adjacent net‐pens with dip nets or by the use of 200 mg/L tricaine methanesulfonate (MS‐222) as an anesthetic for obtaining samples. However, an acute stress (60 s removal from water) elevated serum cortisol levels to 73.7 ± 9.4 ng/ml.