The involvement of sex steroid hormones in downstream and upstream migratory behavior of masu salmon

From May through July when masu salmon, Oncorhynchus masou, commence downst ream migration under natural conditions, yearling precocious male masu salm on (resident form) showed higher GSI and plasma levels of testosterone (T) and 11-ketotestosterone (11-KT) in contrast to immature smelts (migratory f orm). From March through September coinciding with the upstream migration p eriod, 2-year-old male and female adults also showed higher GSI and plasma levels of T, estradiol-17 beta (E-2) 11-KT, 17 alpha -hydroxyprogesterone a nd 17 alpha ,20 beta -dihydroxy-4-pregnene-3-one (DHP). In order to test th e effects of steroid hormones on migratory behaviors, silascone tube capsul es containing 500 mug of T, E-2, 11-KT, DHP, or a vehicle was implanted int o smelts, castrated precocious males, or immature parr, and downstream and upstream behavior were observed in artificial raceways in spring and autumn . Downstream behavior of smelts was inhibited significantly by T, E-2 and 1 1-KT. Upstream behavior was stimulated by T and 11-KT in castrated precocio us males and stimulated by T, E-2 and 11-KT in immature parr. These results indicate that T, E-2 and 11-KT are the factors regulating downstream and u pstream migratory behavior. In particular, because of its changing patterns in plasma and significant effects, T, the common precursor hormone of E-2 (female) and 11-KT (male), is considered to play central roles in both type s of behavior. (C) 2001 Elsevier Science Inc. All rights reserved.