Of Life and Death in a Lowland River: Investigating Mortality during the Early Life Stages of Murray-Darling Fishes

The early life history is a crucial period for fish. Mortality rates are highest during this time, and because of the high fecundity of many fishes, slight variation in mortality rates can result in major fluctuations in recruitment strength. Knowledge of the magnitude and variation of mortality rates of fish species at different ages and developmental stages, and the biotic and abiotic conditions responsible for these, is the key to understanding the nature of recruitment variability in fish populations. The aim of this thesis was to investigate sources and severity of mortality in native Murray-Darling fishes during the larval period. Cohort-specific mortality schedules were estimated for carp gudgeon, Hypseletoris spp. (Eleotridae), and unspecked hardyhead, Craterocephalus stercusmuscarum fulvus (Atherinidae) as they developed from yolksac larvae through to juveniles. Multiple cohorts were tracked at two-day intervals, from four reaches, as they appeared over the four month 2005-06 spawning period in the Lindsay River, a lowland river in the Murray-Darling Basin, south-eastern Australia. Comparisons of age/stage specific mortality rates revealed that there were particular times during the larval period when fish experienced greater rates of mortality than at other times, and that the degree of mortality which occurred during this time was species-specific. Mortality rates of carp gudgeon larvae were greatest when larvae were 3-6 days of age, and this coincided with the transition from endogenous to exogenous feeding. In contrast, changes in mortality rates during larval development for unspecked hardyhead were less pronounced. The findings of this thesis provide strong evidence in support of the critical period hypothesis for carp gudgeon, and more equivocal results for unspecked hardyhead.