Seminal Quality Variation in Chirostoma humboldtianum During an Annual Cycle and Cryopreservation Effect

Chirostoma humboldtianum is an endemic atherinopsid from Mexico that has been of high socio-cultural and economic importance since pre-Hispanic times. It was the first ictic species from Mexico described for science, and it is considered the basal species that gave rise to a nominal group known as white fish or silversides. The aim of this research was to analyze semen quality in relation to breeding fish size and its effect on sperm cryopreservation during an annual cycle. Sexually mature males were collected from January to December 2023, in San Felix dam, Tiacaque, Mexico State, Mexico. The water temperature was measured, and the photoperiod was obtained from the Instituto Nacional de Geografía y Estadística (INEGI). Males were classified into two groups of total length (TL) after analyzing the variation in TL through a size histogram: (G1) 9–13 cm and (G2) longer than 13 cm. Semen volume (µL), sperm concentration (cells µL−1), and motility percentage (%) were determined in all individuals of each group. Likewise, eight straws were cryopreserved per month per group, each one with 10 µL of semen from a mixture of three randomly selected individuals and cryopreserved at −196 °C for 72 h. The post-thawing motility percentage was subsequently verified. Males produce semen continuously all year round, with two periods of higher volume, March and June–August, defining two reproductive periods. The beginning of the first one coincides with the increase in water temperature, from 13 ± 2 to 18 °C. Males with a length more than 13 cm had a higher semen production compared to smaller males (17.33 ± 7.34 and 12.52 ± 4.41 µL, respectively, p < 0.05). The largest semen volumes were registered in March and from June to August in both groups. However, G2 males presented with a larger semen volume. Both groups had a marked decrease in a similar manner in April–May and September to January (p < 0.05). Sperm concentration was similar throughout the year in both groups (p > 0.05) (G1 = 1.35 ± 0.36 × 106 µL−1) (G2 = 1.31 ± 0.35 × 106 µL−1). In addition, fresh sperm motility was high in both groups (G1 = 96 ± 3%, G2 = 97 ± 4%) (p > 0.05). The highest sperm concentrations were observed in March to July, through to October for both groups (p > 0.05), while post-thaw sperm motility decreased by about 50% (G1 = 45 ± 4%) (G2 = 46 ± 6%) during the annual cycle (p > 0.05). The largest post-thaw motility was observed in March and from July to September in G1 and in March and from June to October in G2. Analysis of semen quality throughout the annual cycle reveals aspects of the reproductive biology of C. humboldtianum, including two reproductive peaks and continuous semen production.