title: Evaluation of seasonal and daily changes of plasma thyroxine and cortisol levels in wild masu salmon Oncorhynchus masou, sampled by a Japanese fishing method.
abstract: A new fish sampling method was developed using a Japanese bait fishing rod (8-9 m carbon rod and a nylon line with a small fine wire single hook), which is considered to catch wild salmonid juveniles with low sampling stress. Using this method, seasonal and daily changes of plasma thyroxine (T4 ) and cortisol levels were examined in wild parr, pre-smolts and smolts of masu salmon Oncorhynchus masou in contiguous locations in a coastal river (Kesen River; 44 km) in northern Honshu Island, Japan, overlapping the period of smoltification and seaward migration from August to March. Plasma T4 and cortisol were low in 0+ and 1+ year parr caught in August and September. In March, some yearling (1+ year) fish, which were judged as pre-smolts, and smolts appeared mainly in mid and lower reaches, while parr (0+ and 1+ year parr) continued to appear in the upper and mid reaches. In March, 1+ year pre-smolts and smolts showed high plasma T4 levels while the levels of 1+ year parr were low. During March 2008-2010, plasma T4 levels of 1+ year pre-smolts and smolts had high levels from early to mid-March, whereas plasma cortisol levels of 1+ year smolts were low in early March and increased towards mid-March. Based on these data, plasma cortisol increases probably occur following the increases of plasma T4 levels to lead the 1+ year O. masou to the completion of smoltification and initiation of seaward migration.
authors.affiliation: Department of Biology, Miyagi University of Education, Sendai, Miyagi 980-0845, Japan.
authors.forename: A
authors.initials: A
authors.lastname: Munakata
authors.forename: G
authors.initials: G
authors.lastname: Miura
authors.forename: H
authors.initials: H
authors.lastname: Matsuda
mesh_terms: D025041:Animal Migration; D000818:Animals; D049628:Body Size; D006854:Hydrocortisone / Q000097:blood*; D007564:Japan; D017683:Oncorhynchus / Q000502:physiology*; D017440:Photoperiod; D045483:Rivers; D012621:Seasons; D013312:Stress, Physiological; D013974:Thyroxine / Q000097:blood*
chemical_list: D013974:Thyroxine; D006854:Hydrocortisone
keywords: behaviour; hormone; migration; parr; rod; smolt