Biology of the Nightfish, Bostockia porosa Castelnau, in a South-western Australian River

Abstract

The biology of the nightfish, Bostockia porosa, is described on the basis of data obtained from monthly samples collected from the southern branch of the Collie River in south-western Australia during 1984, 1985 and 1988. It is estimated that most of the 2463 fish caught during those years belonged to either the 0+ (81.8%) or 1 + (12.5%) age classes. Representatives of the 2+ and 3+ age classes contributed 4.2 and 1.0% respectively, and one fish as old as 6+ was found. The parameters of the von Bertalanffy growth curves (La_∞ K and t_o) were, respectively, 92, 0.818 and -0.151 for males and 104, 0.669 and -0.176 for females. The lengths attained at the end of the 1st, 2nd and 3rd years of life were, respectively, 56 mm (- 1.8 g), 76 mm (-4.5 g) and 85 mm (-6.3 g) for males and 56 mm (-1.8 g), 79 mm (-5.1 g) and 91 mm (-7.8 g) for females. Sexual maturity was attained by most males (61.4%) at the end of the 1st year of life but not by females until the end of the 2nd year of life. The great majority of fish migrated into tributary creeks when these started to flow in the winter. Spawning occurred in the creeks during late August and early September, when freshwater discharge was still high and temperatures and daylight length had just begun to increase. Fecundity ranged from 230 to 1138, with a mean (±1 s.e.) of 608 ± 49.1. Bostockia porosa is carnivorous, feeding on a wide range of benthic organisms, particularly ostracods and dipteran larvae. In contrast to Galaxias occidentalis, another abundant endemic species in the Collie River, B. porosa is more active during the night than during the day, spawns 1 month later, and feeds mainly on benthic fauna rather than terrestrial and planktonic fauna.