We developed methods for cryopreserving sperm of channel catnsh Ictalurus punctatus and evaluated the use of cryopreserved sperm for reproduction. Five Cryoprotectants were evaluated: methanol, glycerol, dimethyl sulfoxide (DMSO), sucrose, and polyvinylpyrrolidone. We measured the motility of sperm that had been stored at 4°C in three concentrations of Cryoprotectants (5%, 10%, 15%) dissolved in a modified Hank's balanced salt solution. All Cryoprotectants reduced motility within 6 h; 5% methanol and 5% DMSO caused the smallest reduction. After sperm were frozen at -80°C and stored for 2 d at -196°C, motility was highest (5-10%) in samples cryopreserved with 5% and 10% solutions of methanol. Sperm cells cryopreserved in methanol solutions (5%, 10%, and 15%) were used to fertilize channel catfish eggs from three females. Fertilization ranged from 24% to 97%, and no difference in fertilization success was found between cryopreserved sperm and untreated sperm from the same males. Growth of channel catfish produced with cryopreserved sperm was not different from the growth of siblings produced with untreated sperm. Sperm cryopreservation offers utility as a routine method for gamete storage and genetic improvement of catnsh.

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The twenty-four-hour median lethal concentrations (24-hour LC50) of total ammonia nitrogen (TA-N) to channel catfish (lctalurus punctatus) at pH 7, 8, and 9 (total hardness, 40 rag/liter; temperature, 21-25 C) were 263.6 -+ 11.3 (SE), 38.8 -+ 1.8, and 4.5 -+ 0.2 rag/liter, respectively. The 24-hour LC50 of un-ionized ammonia nitrogen (UIA-N) concentration at pH 8 was significantly higher (1.82 -+ 0.06 mg/liter) than at pH 7 or 9 (1.39 -+ 0.06 and 1.49 -+ 0.12 mg/liter). Enrichment of the water to 440 mg/liter total hardness at pH 7 significantly increased the 24hour LC50 of TA-N and UIA-N (356.3 -+ 16.4 and 1.79 -+ 0.07). Fish exposed to 25 mg/liter TA-N for 12 hours at pH 7 and 8 showed no differences from control fish in hematocrit, percent total plasma protein• or plasma and muscle chloride. Plasma sodium showed no difference between control and experimental groups at pH 7; however, a significant decrease occurred in fish exposed to 25 mg/liter TA-N at pH 8. No differences in blood pH were found between the control groups and fish exposed to 100 and 200 mg/liter TA-N at pH 7, and to 10 and 25 mg/ liter TA-N at pH 8. Plasma sodium depletion is suggested as a contributing mechanism of ammonia toxicity.The principal nitrogenous compound excreted by fish is ammonia, which may reach concentrations that are detrimental to fish and reduce productivity during high density fish culture. Ammonia toxicity has been primarily attributed to the un-ionized form, with the ion-

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