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Collection of fish milt by stripping risks the danger of milt contamination by urine. This may seriously influence milt characteristics and quality, including usefulness for cryopreservation. Urine contamination of milt may be avoided by using a catheter for sperm collection. The objectives of this study were to provide basic characteristics of milt collected with a catheter, to test the usefulness of this milt for cryopreservation, and to correlate characteristics of fresh and cryopreserved semen with sperm fertility rates. Milt from 25 rainbow trout Oncorhynchus mykiss (Walbaum) males were used. All samples were cryopreserved using the pellet method within 1 h of collection, using 0.6 m sucrose and 10% dimethyl sulphoxide (DMSO) as an extender. Catheterization resulted in semen of very good motility (> 90% motile spermatozoa) and high fertilization rates after cryopreservation (mean fertilization rate 81.8 ± 13.3% of control, at a sperm/egg ratio of 2.4 ± 0.3 × 106). Osmolality of seminal plasma and concentrations of sodium, potassium and magnesium ions had low variability, which suggests that they are important for creating a stable environment for sperm storage in the sperm duct. Higher variability of certain seminal plasma characteristics, such as protein concentration and antiproteinase activity, suggests that these characteristics are related to individual semen features of particular males. A strong correlation of seminal plasma zinc concentration with protein concentration may reflect an importance of zinc in semen biology. Cryopreservation caused a significant release of protein and acid phosphatase from spermatozoa. Our results did not reveal any single characteristic of semen collected by catheter that could be used as a powerful predictor of cryopreservation success, presumably because all samples were of high quality.

Summary Previous studies have shown that leucine metabolite β‐hydroxy‐β‐methylbutyrate (HMB) increases the immune function in animals as measured by cellular and humoral immune responses. In the present study the influence of feeding HMB on the nonspecific cellular and humoral defence mechanisms and protection against furunculosis in rainbow trout (Oncorhynchus mykiss) was examined. HMB was fed in a pelleted ration at either 0 (control), 10, 25 or 50 mg  kg−1 bw day−1 for 8 weeks. Blood and pronephros cells were taken at random from 10 fish in each group for the analyses. The respiratory burst activity (RBA) and potential killing activity (PKA) of phagocytes, lymphocyte proliferation stimulated by either concanavalin (ConA) or lipopolysaccharide (LPS), lysozyme activities and total immunoglobulin (Ig) levels in plasma were analysed at 0, 1, 2, 3, 4, 6 and 8 weeks. After 4 weeks of feeding HMB, a challenge test was performed by injection of Aeromonas salmonicida into the fish. HMB approximately doubled the respiratory burst activity and potential killing activity ability of polymorphonuclear (PMN) and morphonuclear (MN) cells (P < 0.01) and increased the mitogen‐stimulated lymphocyte proliferation (P < 0.01) when compared with the control group. HMB feeding also increased (P < 0.01) the lysozyme activity in plasma and total Ig levels in serum. During the 14‐day challenge test, mortality was decreased (P < 0.01) by up to 62% in HMB‐fed fish compared with the control group mortality. In conclusion, the non‐specific immune enhancement by HMB resulted in protection against furunculosis in the rainbow trout.

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