Barramundi (Lates calcarifer) has a mass-spawning reproductive strategy whereby adults synchronously release their gametes into the water column. In captivity, this spawning strategy usually results in highly skewed family sizes, hindering the effectiveness of selective breeding programs. As high inter-male variation in sperm quality was observed in previous studies, we investigated the relationship between sperm quality and fertilization success and early larval development to provide a better understanding of broodstock contribution during mass-spawning events.
To do so, we assessed the fertility of male barramundi (n = 22) from three different breeding cohorts. The physical condition of broodstock was recorded, including body weight, total length, and condition factor (K). Milt samples were collected through testicular cannulation, and semen volume, and the concentration and total count of spermatozoa determined. Sperm quality was assessed using computer-assisted sperm analysis for motility and flow cytometry for sperm integrity (i.e. Hoechst/PI viability assay and TUNEL DNA fragmentation assay). Mass-spawning events were induced using intramuscular injection of luteinizing hormone-releasing analog. Broodstock spawned on two consecutive nights after the injection. Eggs at 2.5 h and 12 h post-fertilization (hpf), and larvae at 24 h and 48 h post-hatch (hph) were collected to assess spawning success and survival. Offspring collected at 2.5 hpf and 24 hph were also genotyped using microsatellites for parentage analysis.
In this study, we observed highly skewed paternal and maternal contributions in all spawns resulting in a loss of genetic diversity and increased inbreeding rates in offspring in agreement with previous studies. However, three key insights were found. Firstly, males with higher physical conditions and higher sperm motility also had the highest level of sperm DNA damage. Secondly, sperm DNA damage correlated negatively with offspring survival rates (r(22) = -0.67, P < 0.001) for the first, but not the second, spawning night. Lastly, it was determined that male fertility accounted for up to 33% of the total variation in mass-spawning events. This study highlights the importance of male fertility to spawning outcomes and the need for advanced sperm quality assessment during the selection of broodstock. However, these results also raise further questions about male-female spawning behaviour and gamete quality. Notably, further study is required to unravel the causes of sperm DNA damage. These results also support the need to further develop advanced reproductive technology in barramundi to precisely control the breeding process for selective breeding programs.