The effects of fluctuating seasonal and constant water temperatures on the photoperiodic advancement of reproduction in female rainbow trout Oncorhynchus mykiss

A series of experiments were performed in order to determine the effects of varying water temperature on the maturation of female rainbow trout subjected to photoperiodic manipulation. Long-short photoperiod regimes, i.e. long days of LD18:6 followed by an abrupt change to short days of LD6:18, were used to advance or delay maturation to summer months, producing commercially desirable out-of-season eggs. At the same time, these fish were exposed to two different seasonal water temperatures supplied by river (range 0-20.5degreesC) or borehole (range 7.0-10.5degreesC) sources. The photoperiod regime was seen to have the primary effect on altering the timing of maturation and appeared similar, irrespective of the prevailing water temperature. However, water temperature had a modulating effect on time of maturation and ovulation; fish showing an ability to delay the timing of final maturation and ovulation, when temperatures were at extremes, high or low. Extreme temperatures, also, had a major effect on the later stages of ovary development and subsequent egg quality. When the major time of spawning was advanced to July-August, when river water temperatures regularly approach 20degreesC, a dysfunction in ovarian development occurred, and no viable eggs were obtainable in these conditions. When the maximum water temperature was reduced to 16degreesC, fish stripped in a normal fashion, but egg quality was significantly reduced. Possible alterations in fanning practice in order to improve egg survival in such situations are discussed, (C) 2002 Elsevier Science B.V. All rights reserved.