PRELIMINARY EVIDENCE THAT CHRONIC CONFINEMENT STRESS REDUCES THE QUALITY OF GAMETES PRODUCED BY BROWN AND RAINBOW-TROUT

This study presents preliminary evidence that periods of chronic confinement stress experienced during the final stages of reproductive development not only disrupt the reproductive endocrinology of trout, but also result in reduced egg size in rainbow trout (Oncorhynchus mykiss) and, most importantly, significantly lower survival rates for progeny from both stressed brown (Salmo trutta) and rainbow trout compared to progeny from unstressed controls. Groups of male and female brown and rainbow trout were subjected to one or two episodes of confinement stress in the months immediately prior to spawning. Plasma levels of cortisol, testosterone, 17beta-estradiol and vitellogenin were determined following the 2-week period of confinement. Plasma levels of cortisol were significantly elevated in all groups of stressed fish, whereas plasma levels of testosterone were significantly reduced in some, but not all, groups of trout. Plasma levels of 17beta-estradiol were unaffected by confinement stress, whereas the plasma vitellogenin levels of the stressed female rainbow trout were significantly reduced. Female rainbow trout subjected to two episodes of confinement stress, experienced 1 and 3 months prior to spawning, produced eggs which were significantly smaller than eggs from control fish. Eggs from ovulated brown and rainbow trout females were fertilised with milt from males subjected to the corresponding treatment regime. Subsequent development of the fertilised eggs was then monitored through the stages of eying, hatch and swim-up. The significantly reduced survival rates observed in the progeny from the stressed crosses compared to the controls indicates that stress reduces the quality of gametes produced by trout.