Changes in reproductive strategy in the ruffe during a period of establishment in a new habitat

The pattern of maturation, body size and fecundity was examined in a popula tion of ruffe (Gymnocephalus cernuus L.) three times during a period of rap id growth, and eventual stabilization, following its introduction to a new habitat. When the ruffe were less common, maturing ruffe were relatively la rge and immature ruffe relatively small, compared with when the ruffe were abundant. Intermediate ruffe population size showed a maturation pattern in termediate between these two extremes. It is suggested that this pattern of maturation is a response of the ruffe population to changing growth opport unity induced by changing intraspecific competition. This fluctuating matur ation pattern is interpreted in terms of a threshold-dependent maturation t rigger, operating on the rate of accumulation of energy and a trade-off bet ween somatic growth and gonad development. When the ruffe population was la rge, high intraspecific competition resulted in low opportunity for growth; only fish with the highest rate of food acquisition were able to mature in a given year-the investment in gonadal tissue reducing somatic growth. Whe n the ruffe population was low, the high rate of energy acquisition in the population resulted in the triggering of maturation, even at small size, on ly the very smallest fish remaining immature. High growth opportunity allow ed maturing fish to develop gonad and maintain somatic growth. The pattern of size related fecundity also changed over the three periods. When growth opportunity was low, size related fecundity was greater than when opportuni ty for growth was high. This suggests that maturing females faced with poor growth conditions compensated by increasing egg number for a given body si ze either by decreasing egg size or by increasing total investment in ovari an tissue. (C) 2000 The Fisheries Society of the British Isles.