Reproductive biology of the threatened golden galaxias Galaxias auratus Johnston and the influence of lake hydrology

Abstract

Golden galaxias Galaxias auratus (31-235 mm fork length, LF) were collected monthly from littoral habitats in Lakes Crescent and Sorell, Tasmania, Australia, between July 2000 and December 2002. Spawning habitats were identified and monitored in both lakes, and surveyed in Lake Crescent. Trends in gonado-somatic indices and reproductive stages of development indicated that gonad development in both sexes begins in midsummer and peaks in late autumn to early winter. Males mature at smaller sizes (50% at 52 mm LF) than females (50% at 76 mm LF), larger individuals are predominately females (95% of fish ≥138 mm LF), and overall male to female ratios are female biased (c. 1:2). Spawning occurs late autumn to early spring (water temperatures = 1.4-9.7°C) with peaks in spawning activity in winter (mean water temperatures <5°C). Demersal adhesive eggs (c. 1.5 mm diameter) were found on cobble substrata (c. 20-250 mm diameter) in littoral areas (c. 0.2-0.6 m deep) and fecundity of fish 71-181 mm LF ranged from 619 to 14 478 eggs. The rate of change in water level over the 20 days prior to monthly sampling was important in explaining the occurrence of spent fish and this accounted for temporal differences in spawning between the populations. Lake hydrology influences the reproductive cycle of G. auratus by possibly providing a stimulus for spawning and it controls the availability of spawning habitat in Lake Crescent. Seasonal hydrological cycles (i.e. rises during late autumn to winter) and a minimum water level of 802.20 m Australian Height Datum in Lake Crescent during autumn (above which littoral areas of cobble substratum are inundated) are critical to G. auratus populations.