A Strategy for Sperm Cryopreservation of Atlantic Salmon, Salmo salar, for Remote Commercial-scale High-throughput Processing

Sperm cryopreservation is an essential tool for long-term storage of genetic resources for aquaculture fishes. The goal of this study was to develop an efficient and streamlined protocol for high-throughput processing for sperm cryopreservation in Atlantic salmon, Salmo salar. The objectives were to evaluate: 1) osmolality of blood serum for determining extender osmolality; 2) effects of extenders for fresh sperm dilution and refrigerated storage; 3) effects of methanol and dimethyl sulfoxide (DMSO) on fresh sperm motility, and 4) motility and fertility after thawing. In this study, sperm samples were collected at a hatchery site in Canada, and shipped to a freezing site located 2200 miles (3550 km) away in the United States. Evaluation of three extenders indicated that Mounib solution was suitable for diluting dry sperm for sample processing. Ten percent of methanol or DMSO was less toxic to sperm cells than was 15% within 30 min. Further testing with methanol at 5, 10, and 15%, and sperm solution:extender dilutions (v:v) of 1:1, 1:3, 1:19 (at concentrations of ~5×10⁷; 3×10⁸, and 1×10⁹ cells/mL) indicated that methanol at 5% and 10% showed less toxicity to fresh sperm within 1 hr at sperm: extender dilutions of 1:1 and 1:3. Post-thaw motility of sperm cryopreserved with 10% methanol was significantly higher than that with 10% DMSO, and fertility reflected those results (0-1% in DMSO vs. 38-55% in methanol). Further evaluation of sperm cryopreservation with 10 and 15% methanol at sperm dilution ratios of 1:1, 1:3, 1:19 indicated post-thaw motility in 10% methanol was significantly higher than that in 15% methanol, and post-thaw fertility in 10% methanol at 1:1 and 1:3 dilution ratios had fertilization rates similar to that of fresh sperm controls. Sperm samples from 12 males cryopreserved with 10% methanol showed male-to-male variation in post-thaw motility (0-36%). Overall, a simplified standard protocol was established for cryopreservation of shipped sperm of Atlantic salmon using extender without egg yolk and yielded satisfactory post-thaw motility and fertilization rates. This procedure can be readily adopted by aquaculture facilities to take advantage of high-throughput cryopreservation capabilities at remote service centers. Most importantly, this approach lays the groundwork for an alternative commercial model for commercial-scale production, quality control and development of industrial standards. Control of male variability and sperm quality remain important considerations for future work.