**1. Introduction**

Cryopreservation biotechnology has important roles for aquaculture industry and also for conservation of aquatic genetic resources. In this field, sperm cryopreservation has been used

© 2015 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. © 2016 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

for transporting of genetic material between facilities, optimal using of gametes in aquaculture, reducing risk of spreading infections, performing of hybridization studies, conserving of protecting endangered species, and also for conserving of biodiversity [1, 2].

Cryopreservation technique involves addition of cryoprotectants to the extender and freezing and thawing of sperm samples, which may result in some damage to the spermatozoa and may decrease egg fertilization rate. Therefore, before cryopreservation of spermatozoa, a thorough evaluation of different extender solutions, cryoprotectants, and cooling and thawing rates are essential to develop optimum cryopreservation protocol for various species [3–5].

During the cryopreservation process, some factors may change the physiological status of sperm. The success of cryopreservation depends not only on preserving the motility of the spermatozoa but also on maintaining their metabolic functions [6]. Extender composition and cryoprotectant concentration are the main factors affecting cryopreservation success [7]. Extenders are required for dilution of fish sperm prior to cryopreservation and are generally designed to be compatible with the physiochemical composition of the fish seminal plasma. Most important function of the extenders is to maintain the spermatozoa in immotile state until required [8].

Cryoprotectants are added to the extenders to protect the cells against ice crystal formation during freezing and thawing [9]. Although cryoprotectants help to the prevention of cryoinjuries during freezing and thawing, they may become toxic to the cells when exposure time and concentration are increased [10, 11]. Thus, one of the most critical steps in successful cryopreservation of fish semen is the choice of the cryoprotectant and its ratio in the extender during the process.

Another important problem is the handling of sperm produced in small volumes by some fish species such as tilapia. In spite of packaging of sperm in traditional 0.25-mL and 0.5-mL straws has been successfully applied to freeze semen of the most fish species and to fertilize small egg batches [9], there is a lack of information regarding their usage in cryopreservation of Nile tilapia (*Oreochromis niloticus*) semen.

The Nile tilapia is one of the most cultivated freshwater fish species in the world aquaculture [12]. This species has great breeding potential due to its hardiness against worse environmental conditions, fast growth rate, adaptation to different environmental conditions (e.g. salinity, temperature), and also good organoleptic characteristics of its flesh [13, 14]. On the other hand, most of studies related with fish sperm cryopreservation have focused on some freshwater species, such as cyprinids [15, 16], salmonids [17, 18], catfishes [19, 20], and loach [21].

Even though many successes have been achieved in fish semen cryopreservation, the technique remains as a method that is difficult to standardize and use in all types of fishes. This is due to the fact that cryopreservation of sperm from different fish species required different conditions, where the protocol needs to be established individually [22]. To the best of our knowledge, there is limited information regarding cryopreservation of Nile tilapia sperm. In this concept, the effect of cryoprotectants and packaging methods on freezability and also on post-thaw quality of Nile tilapia sperm still remains unclear. Thus, standardization and simplification of cryopreservation procedure for Nile tilapia sperm are needed for commercial and gene bank applications.

The main aim of this study was to establish an efficient method for cryopreservation of Nile tilapia sperm that can be applied to aquaculture of this species. The present experiment was designed to study the straw volume (0.25 vs 0.5 mL) on Nile tilapia sperm quality after cryopreservation using glucose-Tris–based solution containing 10% dimethylacetamide.
