Fertility and motility of
sperm from Atlantic halibut (Hippoglossus hippoglossus) in relation
to dose and timing of gonadotrophin-releasing hormone agonist implant
E.L.M. Vermeirssen, C. Mazorra de Quero, R.J.
Shields, B. Norberg, D.E. Kime, A.P. Scott-2004
Aquaculture, 230(1-4): 547-567
Abstract
In broodstocks of Atlantic halibut, Hippoglossus
hippoglossus, male and female gamete production often becomes
unsynchronised towards the end of the spawning season––milt becomes very
viscous and difficult to express while the females are still producing
batches of good quality eggs. Gonadotrophin-releasing hormone agonist
(GnRHa) has been shown to stimulate spermiation in a number of fish species.
Therefore, we conducted two experiments where male halibut were implanted
intramuscularly with pellets containing GnRHa. The effect of the pellets was
tested at three periods: before, at the height of and at the end of
spermiation. In the middle period, GnRHa was tested at two doses (5 and 25
µg/kg bodyweight). Measurements were made of milt hydration, sperm motility
and fertilisation rate. Implanted males began spermiation at least 4 weeks
before control males. Both doses of GnRHa increased the fluidity of the
milt. This effect lasted for at least 20 days in the low dose group and for
40 days in the high dose group. When applied at the end of the season, GnRHa
reversed the normal trend for the milt to become more viscous. GnRHa
treatments did not affect fertilisation rates obtained with the sperm.
However, towards the end of the spawning season, sperm motility was enhanced
in males treated with the high dose of GnRHa (25 µg/kg) compared to
controls. As described previously, plasma concentrations of the gonadal
steroids, 5β-pregnane-3β,17,20β-triol 20-sulphate and 17,20α-dihydroxy-4-pregnen-3-one,
were significantly enhanced by GnRHa treatment. Concentrations of
testosterone on the other hand decreased when spermiating males were treated
with GnRHa. Our data suggest that 17,20β-dihydroxy-4-pregnen-3-one or
its metabolites are involved in milt hydration, possibly through affecting
ion transport.