IMPACT OF SEA-WATER REARING OR FRESH-WATE R TRANSFER ON FINAL MATURATION AND ON GAMETE QUALITY IN ATLANTIC SALMON SALMO-SALAR

Breeding management of Atlantic salmon generally requires transfer of spawners to fresh water prior to final sexual maturation, in order to avoid poor gamete quality. To study the impact of sea water environmen t, pre-spawners of Atlantic salmon Salmo salar (2(+)) were either rear ed in sea cages until final sexual maturation or transferred to fresh water in June or October. The beginning of spermiation or ovulation wa s recorded by weekly checking using hand-stripping. The quality of spa wners was tested by fertilizing batches of 200 eggs, using DIA 532(R) insemination extender with sperm dilution of 10(-3) and assessing surv ival at eyeing stage. Males transferred to fresh water (ED) presented normal spermiation cumulative frequency curves in both fresh water gro ups. Females transferred to fresh water ovulated from November to Dece mber while ovulation of females reared in sea water was delayed and th e period lengthened. Furthermore, only 67% of the EM females had ovula ted during the experiment up to April. Relative fecundity of females t ransferred to ED in spring was significantly lower than that of female s maintained in EM during summer. In spite of delayed and lengthened o vulation and of significantly higher osmolality of coelomic plasma (34 8 +/- 13 mOsmol/l, in EM and 270 +/- 23 mOsmoyl in ED) eggs of EM fema les were of a similar high quality to those of ED females, when fertil ized less that one week after ovulation. Males reared in sea water pre sented similar but slightly earlier spermiation curves. Heterogeneous and lower mean semen quality were recorded in EM males as compared wit h the ED males. Semen plasma osmolality was significantly higher in EM males (347 +/- 27 mOsmol/l in EM and 265 +/- 30 mOsmol/l in ED). EM s perm that show low fertilizing capacity gave similarly poor results wi th insemination diluents at varying osmolalities (150 to 450 mOsmol/l) . Furthermore, no relationship could be established between seminal pl asma osmolality and low fertility for each individual. In terms of bre eding management, seminal plasma osmolality cannot be retained as a cr iterion for selecting ''good'' EM males. Oocytes ageing in the abdomin al cavity should be strictly avoided for females reared in sea water.