Reproductive strategy and growth of the Australian smelt, Retropinna semoni (Weber) (Pisces : Retropinnidae), and the olive perchlet, Ambassis nigripinnis (De Vis) (Pisces : Ambassidae), in Brisbane, south-eastern Queensland

Abstract

The reproductive biology and growth of R. semoni and A. nigripinnis were studied in Brisbane, south- eastern Queensland, over a 20-month period (1981-1982). R. semoni began to breed in winter at water temperatures exceeding 15ºC, reflecting its salmoniform affinities and temperate distribution. A. nigripinnis, a subtropical species of Indo-Pacific origin, required a period of rising temperature before breeding began in spring at 22ºC. Both species concentrated their breeding activity into the months that precede summer storms and high, variable stream discharges, which can generate fluctuating water levels, destruction of weedbeds and instability of the stream substrate. Pre-flood spawning also occurs in three other small-bodied native species in the region, and appears to have adaptive value in the seasonally unstable environments of coastal streams. Dispersal of juvenile fish may be facilitated by rising water levels during summer months following spawning. R. semoni grew faster and to larger sizes than A. nigripinnis and growth of both species was adequately described by the von Bertalanffy equation. Male and female R. semoni and male A. nigripinnis live and breed for 2 years but female A. nigripinnis survive and may breed into their third year. These patterns of survivorship and reproduction are consistent with the contention that natural mortality is higher in open-water foragers such as R. semoni than in species like A. nigripinnis, which tends to forage amongst littoral vegetation and in mid-water. Differences in the sizes of eggs and larvae of the two species are also consistent with the probability that they experience different relative exposure to predation. These and other attributes, coupled with pre-flood spawning, enable both species to achieve large populations in suitable habitats within coastal streams, in spite of their relatively low fecundities.