Sperm of many aquaculture species are often inadequate in quality/quantity and result in low fertility and offspring viability. This is particularly true for fish that do not release milt via manual-stripping, such as with the commercially important blue catfish (Ictalurus furcatus), which, is mated with female channel catfish (I. punctatus ) to produce a hybrid that accounts for more than 50% of US catfish production. Pond aquaculture systems are prevalent in the catfish industry, where temperatures can reach upwards of 36o C in warmer months, and daily fluctuations of 3-6o C are common . These extreme conditions will likely be intensified by the effects of global climate change. In fishes, most studies have focused on how environmental factors influence sires before mating. However, sperm themselves experience fluctuating environments, and potential environmentally and genetically induced changes to sperm have implications on offspring traits. T his study investigated how thermal stress impacts sperm quality and whether thermal stress-induced changes in sperm negatively impact offspring performance.
Blue catfish sperm from 8 males were placed into temperature blocks at 4oC (control), 32o C (pond temperature towards the end of spawning), and 36oC (pond heatwave). After 0- 90 min of temperature exposure, sperm cell health (viability) was assessed. Sperm were activated and kinematics analyzed using Computer Assisted Sperm Analyses software. Sperm exposed to the thermal environments were then used for in vitro fertilization trials, where eggs from three females were fertilized with 9 males (3 females × 3 males each nested in female × 3 temperature exposures × 3 replicates = 81 experimental units). Hatching success was determined. Additionally, fry deformity rate, body weight, and survival were recorded at 30 days post-hatch.
Sperm viability decreased over the thermal gradient, with the lowest viability at 36oC after 30, 60, and 90 min of exposure (Fig. 1A). Sperm motility also declined over the thermal gradient (Fig. 1B). Offspring performance data from fertilization trials are currently being analyzed. Data will help predict paternity outcomes in response to environmental thermal stressors.
Fig 1. Impact of acute thermal stress on sperm viability and motility in blue catfish .