 2004 by the American Society of Ichthyologists and Herpetologists Copeia, 2004(2), pp. 235–244 Seasonal Variation in Androgen Levels in the Oyster Toadfish MICHAEL L. FINE,MARY S. JOHNSON, AND DENNIS W. MATT Male Oyster Toadfish Opsanus tau produce an advertisement call, the boatwhistle, using sexually dimorphic sonic muscles attached to the swimbladder. The funda- mental frequency and duration of the boatwhistle change seasonally suggesting hor- monal modulation of the output of pattern generators in the brain. The toadfish has an unusual protracted reproductive cycle in which testes contain mature sperm throughout the year, and females develop large eggs during late summer and fall for spawning the following spring although some may mate in the fall. This study quantified gonad development and plasma androgens in males and females through- out a seasonal cycle to relate them to the prolonged reproductive cycle and to quan- titative changes in boatwhistle parameters. Median levels of testosterone (T) and 11- ketotestosterone (11KT) in males peak in May during the early part of the spawning season (461 pg/mL for T and 3746 for 11KT) and decline to 153 and 43 pg/mL, respectively, in June although spawning continues into July. A minor increase in gonosomatic index (GSI) and levels of both androgens (180 and 94 pg/mL, respec- tively) occurs in October. Median levels of T (328 pg/mL) and GSI in females also peak in May. In June, T levels drop in spawned females but remain elevated in those still gravid. Ovaries start to develop in late summer, and T levels increase above levels of individuals spawned in June. A spring peak in T in unspawned females and increasing levels in the fall correlate with estradiol (E) levels. Androgen levels do not correlate with the seasonal cycle in boatwhistle parameters suggesting that some other factor is responsible. T HE Oyster Toadfish, Opsanus tau, has a pro- tracted spawning season in the Chesa- peake Bay, ranging from May to the end of July, with a possible second spawning in the fall (Gudger, 1910; Gray and Winn, 1961; Fine, 1978). As water temperature increases in the spring, toadfish migrate into shallow water for spawning. Males enter mating shelters (under boards, tin cans, terra cotta drainage tiles, eel grass, etc.) and produce their advertisement call, the boatwhistle, to attract females (Winn, 1964, 1967, 1972), who lay a single clutch for the season containing several hundred 5.0–5.5 mm eggs (Gray and Winn, 1961; Able and Fa- hay, 1998). Males are polygynous, continue to call and will accept additional females while they guard developing eggs and larvae (cling young) for six to seven weeks (Gray and Winn, 1961; Galeo et al., 1987). Schwartz (1974) found June movements in male toadfish, and it is likely that, after raising one or more broods, males will leave their nests to forage, providing nesting opportunities for later breeding males. Hilde- brand and Schroeder (1972) suggested a con- tinuous spawning season into the fall based on mature-appearing eggs in the ovary. However, boatwhistles cease toward the end of July as wa- ter temperature approaches a seasonal high (Fine, 1978), and females that failed to spawn resorb their eggs, which become vascularized, dull in color and shrink (pers. obs.). Boatwhis- tle calls are produced again in the fall in much smaller numbers as water temperatures drop be- low summer highs (Fine, 1978). In late summer and early fall, follicles start to grow, enlarging into 5-mm, bright yellow eggs. Ovaries appear identical macroscopically to those of prespawn- ing females in the spring. Unlike most teleosts in which final maturation involves taking up wa- ter and swelling, toadfish eggs actually increase in specific gravity and lose water as they mature (Fine et al., 1995). Michael Costagna, a former Director of the Eastern Shore Branch of the Vir- ginia Institute of Marine Science (personal com- munication), has occasionally seen developing eggs in nests on the Eastern Shore in Virginia in the fall. Therefore, there is a possibility of a second spawning although most females devel- op and over winter with mature eggs. Fine (1978) found regional differences (dia- lects) and a seasonal cycle in boatwhistle dura- tion and fundamental frequency. Fundamental frequency is equivalent to the driving frequency of the paired sonic muscles (Skoglund, 1961; Fine et al., 2001). In the short term, fundamen- tal frequency increases linearly with tempera- ture, whereas duration is temperature indepen- dent (Fine, 1978). Seasonally however, funda- mental frequency increases faster than predict- ed by temperature, and both fundamental