The effects of salinity on reproductive development, plasma steroid levels, fertilisation and egg survival in black bream Acanthopagrus butcheri

Abstract

The effects of salinity of holding water of 5‰, 20‰ or 35‰ on seasonal reproductive development, plasma steroid levels, the efficacy of luteinizing hormone releasing hormone ethylamide (LHRHa) to stimulate ovulation, sperm motility, and egg fertility and development to hatching were investigated. Fish were captured from the wild from December to February, placed into salinity regimes in May and held until the normal times of spawning the following November. Blood samples were taken in August, September and November. Female fish were injected with saline or LHRHa (50 μg kg −1) in November and bled and checked for ovulation for 5 days. Gonadal maturation was unaffected by salinity in both sexes. In females, seasonal plasma steroid levels were unaffected by salinity, whereas in males, plasma levels of 17,20β-dihydroxy-4-pregnen-3-one (17,20βP) and 11-ketotestosterone were higher in fish held at 35‰ than in fish held at 5‰ in September, and in fish held at 5‰ and 20‰ in November, respectively. Plasma estradiol (E 2) and testosterone (T) levels in saline-injected fish, either remained low or were significantly suppressed. LHRHa treatment resulted in the short-term elevation of plasma E 2 and T levels at all salinities, whereas plasma 17,20βP levels were elevated over controls on days 1 and 2 post-injection in fish held in 20‰, but remained unchanged in fish held in 35‰ or 5‰. All but one fish ovulated in response to LHRHa, however, the number of ovulations and egg volumes was lowest in fish held at 5‰. Both fertilisation and sperm motility were significantly reduced at 5‰. Naturally fertilised eggs (35‰; 2–8 cell stage) were incubated to hatching at salinities of 0‰, 5‰, 10‰, 15‰, 20‰, 25‰, 30‰ or 35‰. Eggs hatched in all salinities except 0‰, but with lowered survival at 5‰ and 10‰. Larvae showed high levels of deformity at salinities below 15‰.