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The study was designed to test the hypothesis that male aging is associated with a change in reproductive function in the zebrafish. Young (290 ± 37 d) and older (911 ± 48 d) males were combined with females (604 ± 24 d) to test the effect of male age on the number and fertility of eggs laid by their mates. 48% of breeding trials with young males and 25% of the trails with older males resulted in egg deposition. Although young males were associated with significantly more successful breeding attempts than older males, number of eggs laid per clutch, number and percent of fertilized eggs and the number and percent living embryos were not statistically different between young and older males. These data suggest that male aging is associated with altered reproductive behavior and/or female response but not in sperm quality per se. Consistent with this interpretation were the findings that percent motility and sperm motility characteristics did not differ between sperm from young and older males as assessed by computer-assisted sperm analysis. However, older males contained higher quantities of extractable sperm than did young males, perhaps associated with fewer successful breeding attempts. Age-related effects on male reproductive in the zebrafish may therefore be a consequence of behavioral or morphological features that play a role in female mate choice and/ or male sexual response.

The effects of temperature (11, 15, and 20°C) on clove‐oil‐induced anesthesia in fry of steelhead Oncorhynchus mykiss (0.18 ± 0.01 g [mean ± SE]) were evaluated at 25, 50, and 100 mg clove oil/L. Induction time decreased significantly with increasing temperature and dose. Recovery time after removal from anesthesia decreased significantly with increasing temperature. Mortality at 24 h postanesthesia increased significantly with increasing temperature and dose. Although clove oil appears to be an effective general anesthetic for salmonid fry, the data indicate that care must be taken in determining the appropriate dose to minimize temperature‐associated mortality.

Steelhead trout Oncorhynchus mykiss sperm held in seminal plasma or sperm‐immobilizing buffer (pH 8·6) at 10° C consumed O2 at the rate of c. 2 nmol O2 min−1 10−9 sperm; the rate of O2 consumption was not different in sperm held for 4 or 24 h. Decreasing the extracellular pH from 8·5 to 7·5 either by diluting semen with buffer titrated with HCl or by increasing the partial pressure of CO2 in the incubation atmosphere resulted in c. a 40% decrease in the rate of sperm respiration. The data did not, however, support the hypothesis that the precipitous reduction in the capacity for sperm motility that occurs as external pH is reduced is a result of a decrease in cellular metabolism. The rate of O2 consumption of freshly collected semen from different males was not correlated to cellular ATP content or to the proportion of sperm that were motile upon activation; the initial ATP content and sperm motility were positively correlated. The rate of O2 consumption was not significantly increased following sperm activation or by the addition of an uncoupler of oxidative phosphorylation, carbonyl cyanide p‐trifluoromethoxyphenylhydrazone, suggesting that these sperm have little, if any, capacity for increased oxidative metabolism.