Mature spermatozoa of sturgeon fish in the testes and seminal ducts are immotile as long as they contact tissue fluids or seminal plasma [14; 15]. During spawning, spermatozoa in contact with freshwater are activated and become motile. For most sturgeon fish, the active motility time does not exceed 2-5 minutes, and the total motility time does not exceed 10-15 minutes [15]. Together with concentration, motility is one of the main characteristics of sperm quality and its suitability for artificial insemination [11]. Despite this, the relationship between motility duration and sperm fertility is not evident because all whole eggs capable of fertilization are inseminated within the first 30-60 seconds [11]. N.V. Barulin et al. [1], using CASA (computer assisted sperm analyses) to study motility and other characteristics of Lena sturgeon sperm, proved the inexpediency of artificial insemination for more than 90 seconds.
The motility of sturgeon sperm depends on many factors [16]. But in the case of short-term or long-term sperm storage technologies, such as hypothermic storage or cryopreservation, sperm is additionally exposed to non-specific factors (cryoprotectants, antibiotics, etc.) or environmental factors beyond the physiological ranges (low temperatures, pH, differences in osmotic pressure, etc.) that affect not only sperm viability, but also motility patterns [26-28].
Despite a large number of developments, cryopreservation of sturgeon sperm is not used as a standard technology in sturgeon breeding [7]. Much more often, short-term hypothermic storage without freezing, at near-zero temperatures, is used for transport or delayed insemination of eggs. The method of hypothermic storage of sturgeon sperm recommended by FAO [11] is the Bijar-DiLauro method: in plastic (polyethylene) bags or containers filled with atmospheric air or pure oxygen [20; 23; 24].
Hypothermic storage decreases sperm motility as a result of progressive death of spermatozoa or loss of their ability to activate. The main causes are depletion of ATP reserves, lipid peroxidation, and, consequently, destruction of cell membranes [19], spontaneous premature activation and subsequent death of spermatozoa; microbial overgrowth due to inevitable contamination is also possible [5]. To improve the preservation of sterlet sperm during hypothermic storage in 2012-2016, we developed a salt-free medium ISGT-80 based on glucose, trehalose, and bovine serum albumin with osmolality similar to that of sterlet seminal plasma. After storage in ISGT-80 at +2-4 °C, at least 50% of sterlet sperm became motile after activation for 6 days. The fraction of spermatozoa with damaged DNA after 6 days of storage did not exceed 1.5%, and spermatozoa with damaged membranes did not exceed 10% [8]. When sterlet eggs were inseminated with sperm stored for 10 days in ISGT-80, the fertilization rate was at least 90% [10].