Principles of Genetics and Breeding

9.1.12 Importance of cryopreservation

• Asynchrony in spawning state between sexes can often result in a shortfall of gametes of the required quality and quantity. Depending on species and biogeography, male and female fish can mature at different times in a spawning season. Under these asynchronous conditions, especially during the beginning and end of the spawning season, gametes can be collected and held under appropriate storage conditions for later use. Similarly, if milt or ova of special interest are acquired for hybridization, they may need to be stored until fertilization can be carried out.
• Few male broodstock can be maintained and effective population size can be increased.
• Cryobanks can play a crucial role in the genetic management and conservation of aquatic resources. Through frozen sperm banks, genes from valuable individuals or stocks which have desirable characteristics, e.g., for use in inbred lines and selection programmes, can be preserved for future use and development. In such case representative genes in the form of sperm can be selected for cryopreservation.
• Sperm cryobanks can also be used to manage the genetic integrity of farmed stocks. For fish species, especially those which have a short life cycle, such banks are of particular importance to protect against inbreeding, founder effects, and random drift of captive stocks. To minimize these detrimental effects milt from the representative stocks can be cryopreserved and genetic material periodically reintroduced to increase the heterozygosity in stocks or to increase the contribution of desired genes in the offspring.
• In selection programmes, milt from original stocks can be cryopreserved and used to make half-sibling comparisons at a later date.
• During disease outbreaks entire stocks can be affected. Therefore, periodic freezing of milt from disease free males can help to buffer farms against such catastrophes. The use of post-thawed milt to fertilize ova from similar species or strains can aid the rate of recovery of farms.
• In instances of scarcity of males, e.g., protoandrous hermaphrodites, species of common interest, such as groupers, milt can be stored for later use.
• The value of cryopreserved milt is even greater in species such as bass, Epinephelus malabaricus which can take up to 8-10 years to mature.