Effects of preventing a temperature decrease during winter on reproduction of male small-egged Kajika, Cottus pollux SE.

To investigate the effects of preventing temperature decrease on the reproductive activity of the male cold-water teleost, Cottus pollux SE, testicular development, serum 11-ketotestosterone (11-KT) levels, and physiological responses associated with nesting behavior (i.e., elevation of serum 11-KT levels and accumulation of urine in the urinary bladder) were observed from November to January. Specifically, males were exposed to three different cooling regimes (control, 16 to 6 °C; H1, 16 to 11 °C; H2, 16 to 14 °C), and the results were compared. In addition, the effects of temperature on male reproductive behavior were also clarified. At higher water temperature regimes, the rate of testicular development and serum 11-KT levels were both higher from November to mid-December than from mid-December to January. However, the results showed that high water temperature regimes in the coldest period of winter did not suppress spermatogenesis completely. Conversely, the physiological responses to nesting were affected by high water temperatures, with serum 11-KT levels increasing and urine accumulation in the urinary bladder being suppressed. Furthermore, frequencies of two behaviors associated with nesting, i.e., body undulation and face displays, were also suppressed under high water temperatures (~ 14 °C) compared with normal temperatures (~ 7 °C) during the breeding season. Based on the physiological and behavioral responses to nesting, findings showed that preventing a water temperature decrease during winter suppresses reproductive activity in Cottus pollux SE.