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Abstract

Stress inhibits reproduction in most fish and can be exerted through normal hatchery
practices in aquaculture. Previous research with wild New Zealand snapper, has shown
inhibitory effects of stress on reproduction, however, there is no information on the effect of
stress in captive or domesticated stocks. This study examines the role of stress in the
reproduction of snapper, by determining the effect of capture and confinement on plasma
steroid levels, gonadal condition, the ovulatory response to exogenous hormone treatment
and in vitro ovarian steroidogenesis.
The effects of stress were investigated in wild fish caught by trap, 5-year-old fish, caught as
juveniles and on-grown in captivity and 2- and 3-year-old hatchery-reared snapper. Fish
were held post capture for up to 168h. Blood was sampled at 0h and after the confinement
period, the fish were sacrificed and the gonads preserved for histology. Plasma levels of
cortisol and the gonadal steroids, 17β-estradiol (`E_2`), testosterone (T), 11-ketotestosterone
(11KT) and 17,20β dihydroxy-4-pregnen-3-one (17,20βP) were determined by
radioimmunoassay (RIA). Stress resulted in an increase in plasma cortisol and concomitant
decreases in `E_2`, and T in females, and plasma T and 11 KT in males. In addition there was
an increase in the incidence of ovarian atresia in females, and in the proportion of
spermatozoa in the testes of males. These results confirm that snapper are highly
susceptible to stress-induced impairment of reproduction, and this response is still present in
hatchery-reared fish.
Induced ovulation is a common requirement in aquaculture, but stress effects associated with
handling may affect the efficacy of exogenous hormone treatments. This was tested by
treating hatchery-reared female snapper, Oh (unstressed) or 24h after capture (stressed),
with either luteinising hormone releasing hormone analogue (LHRHa), human chorionic
gonadotropin (hCG), or 17α-hydroxyprogesterone (17P). Blood was sampled prior to
treatment and again after 168 h and fish were checked periodically for ovulation. In
unstressed fish, hCG gave the best ovulatory response, followed by LHRHa, in terms of
numbers of ovulators, egg volumes, egg quality and percent fertilisation. A delay in injection
resulted in significantly lower `E_2` and T at injection, smaller egg volumes, and poorer egg
quality, confirming that treatment at first capture, yields a better ovulatory response than
treatment after exposure to capture and handling stress.
Studies on plasma levels of gonadal steroids suggest that stress effects of the type found in
snapper might result from impairment of ovarian steroidogenesis. Accordingly 3-year-old
female hatchery-reared snapper were stressed for up to 168h and isolated ovarian follicles
were incubated with hCG, 17P, or hCG plus 17P. 17P was most effective at stimulating
production of `E_2` and T, and there was no further benefit in co-stimulation with hCG. Stress
markedly reduced the capacity of ovarian follicles to convert 17P to T. `E_2` production was
unaffected, suggesting that aromatase-mediated conversion of T to `E_2` is not affected by
stress. This indicates that decreases in E2 concentrations evident in the plasma following
stress are possibly the result of stress-induced reduction in substrate availability and not a
reduction in aromatase activity.

Item Type:	Thesis - PhD
Authors/Creators:	Cleary, JJ
Keywords:	Pagrosomus auratus, Fishes
Copyright Information:	Copyright 1997 the author - The University is continuing to endeavour to trace the copyright owner(s) and in the meantime this item has been reproduced here in good faith. We would be pleased to hear from the copyright owner(s).
Additional Information:	Thesis (Ph.D)--University of Tasmania, 1998. Includes bibliographical references
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