Understanding the biological and environmental processes that regulate populations is a central goal of ecology and is fundamental to predicting population responses to environmental change and harvest. However, disentangling the relative influence of biotic versus abiotic forces is challenging due to the myriad of processes which may potentially affect population productivity and demography. Long-term datasets provide invaluable tools in this regard, as they capture infrequent events, and variation in population size and demography across biologically relevant timescales. Here, I will discuss the results of a historical depletion sampling dataset from Corroboree Billabong, a seasonally connected reach of the Mary River, located in the wet/ dry tropics of the Northern Territory. Barramundi (Lates calcarifer), a catadromous, predatory fish, were sampled using gillnets in an isolated section of the billabong annually across a ~20-year period. Ages of captured individuals were estimated by counting annual checks on scales, and annual, age-specific population estimates were derived using depletion methods. To explore the biotic (e.g., density-dependence) and abiotic (e.g., rainfall) drivers of recruitment and size-at-age, a Piecewise Structural Equation Model was developed. Our results show that density-dependence, and, to a lesser extent, rainfall, are linked to the abundance and size of recruits in the billabong. The abundance of young-of-year individuals was negatively correlated with abundance of the 1-year-old cohort, suggesting that cannibalism may be an important factor regulating recruitment. Young-of-year abundance was also negatively correlated with mean length, suggesting that intraspecific competition within cohorts may be an important factor limiting juvenile growth rates. I will discuss how these important aspects of barramundi population dynamics may ultimately manifest at the population level.