Environmental Biology of Fishes 51: 97–105, 1998.  1998 Kluwer A cademic Publishers. Printed in the Netherlands. Plasma concentrations of steroid hormones in male yellow perch, Perca flavescens: the effect of age and photothermal manipulation Renata E. Ciereszko 1,2,3 , Konrad Dabrowski 12 *, Andrzej Ciereszko 1,4 & Joseph S. Ottobre 2 1 School of Natural Resources, 2 Department of A nimal Sciences, Ohio State University, Columbus, Ohio, U.S.A . 3 Institute of A nimal Physiology, University of A griculture and Technology, Olsztyn, Poland 4 Department of Molecular A ndrology, Polish A cademy of Sciences, Olsztyn, Poland * Corresponding author (e-mail: dabrowskiosu.edu) R eceived 17.10.1995 Accepted 4.6.1997 Key words: fish, 11-ketotestosterone, testosterone, estradiol, sperm Synopsis Plasma steroid concentrations in two and three year-old male yellow perch maintained under two different photothermal regimes were investigated. Initially, all fish kept indoors were exposed to the same water tem- perature (22 °C) and photoperiod (15L:9D). By the end of August, following the first sampling, fish were exposed to different photothermal regimes. Groups A 2 (2 year old) and A 3 (3 year old) were maintained under photothermal conditions similar to those of southern Ohio. Groups B 2 (2 year old) and B 3 (3 year old) were exposed to a condensed light/temperature regime designed to accelerate maturation. Testosterone (T) was the major circulating androgen in all groups. In regime A fish, plasma concentrations of 11-ketotestosterone (11KT) and T were very low in August, increased in October and remained elevated until March. In regime B plasma androgens were high until February and then dropped abruptly in March. The elevated circulating levels of 11KT and T were associated with production of sperm. The highest sperm concentration in the groups A 3 and B 3 was observed in February and December, respectively. There were no major differences in profiles and levels of plasma steroids between two age categories within each photothermal regime. These data in- dicate that the compression of the photothermal cycle accelerated both the occurrence of the low postspawn- ing levels of circulating steroids and the completion of milt production. Higher sperm concentration observed in B 3 group earlier in the season compared to A 3 group also support the notion that the condensed photother- mal cycle accelerated gonadal maturation. It appears that modification of the environmental cues may be a useful tool for manipulation of reproductive processes in male yellow perch. Introduction The annual reproductive cycle in the male yellow perch, a freshwater perciform fish, is characterized by a fast growth of the testes during the autumn. The weight of the perch testes begins to increase in September and reaches maximum size in Novem- ber. Maximal size of gonads is observed until spring. The changes in size are accompanied by differentia- tion of germ cells. The first spermatozoa are observ- ed in September. Testicular weight declines from March to May as spermatozoa are expelled during