INFLUENCE OF RAINBOW-TROUT THERMAL-ACCLIMATION ON SPERM CRYOPRESERVATION - RELATION TO CHANGE IN THE LIPID-COMPOSITION OF THE PLASMA-MEMBRANE

The ability of rainbow trout (Oncorhynchus mykiss) spermatozoa to with stand cryopreservation was tested after the fish and sperm were expose d to various temperature conditions, In the first experiment, trout we re acclimated at either 8 degrees C or 18 degrees C during gametogenes is, and transferred to 13 degrees C at the beginning of the spawning s eason, Following this 13 degrees C transfer, the fertilizing ability o f frozen-thawed sperm from the fish reared at 18 degrees C was found t o be twice as better (60-80% fertilization rate) than that of the fish reared at 8 degrees C (20-30% fertilization rate) up to 42 days after the transfer. A low membrane cholesterol/phospholipid ratio was stron gly correlated (P < 0.002) with the better freezing resistance of the corresponding sperm. The relevance of this parameter in predicting spe rm ability to survive cryopreservation is discussed, In another experi ment, fish with the same thermal background during gametogenesis were transferred to 13 degrees C or to 5 degrees C before the beginning of the spawning season, The same fertilization rates after freeze-thawing were observed between the two groups (30-35%), Weekly or bi-weekly st ripping frequencies did not alter the fertilization rates. Thus, no da mage can be attributed to a long in vivo storage of sperm at a relativ ely high temperature, In an in vitro experiment, the storage temperatu re (0 degrees C to 25 degrees C) of sperm before freezing was shown to have little influence on the fertilizing results after thawing althou gh membrane fluidity was higher in sperm stored at 24 degrees C compar ed with those stored at 4 degrees C. It is concluded from this study t hat the quality of sperm cryopreservation can be improved by appropria te broodstock handling. A high rearing temperature during gametogenesi s followed by transfer to colder water improves sperm resistance durin g cryopreservation. The stripping frequency of the fish does not influ ence sperm quality at the temperature tested. Storing sperm in vitro b elow 5 degrees C is recommended, although higher temperatures are not markedly deleterious.