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Understanding how and why the size of populations varies is critical knowledge for conservation and management. While considerable work has explored how different demographic parameters affect population growth, less is known the drivers of variability in these parameters. Long-term time series tracking population size that are coupled with empirical data to examine the relative importance of different drivers are rare, especially in freshwater systems. Even rarer are studies that collect this information concurrently from multiple species with contrasting life history strategies in the same system to assess whether population size and the relative importance of drivers also vary. We studied changes in the abundance and size structure of four native freshwater fish species in the Murray River, southeastern Australia, over a continuous 19-yr period. Two species with traits typical of "equilibrium" species (Murray cod Maccullochella peelii and trout cod Maccullochella macquariensis) and two with traits of "periodic" species (golden perch Macquaria ambigua and silver perch Bidyanus bidyanus) were sampled annually and capture-mark-recapture modeling was used to ask (1) how did population size change during this period, (2) how were changes in population size related to variability in hydrology, and (3) how were changes in population size driven by different processes (local recruitment or migration events)? Populations of all four species varied throughout the study, and our results are consistent with the notion that local recruitment is an important driver of this variability for Murray cod and trout cod, whereas immigration is more important for the two other species. Increases in spring river discharge strongly influenced these responses for trout cod and golden perch. Our study provides fundamental insights into the population dynamics of these valued species, and how management strategies might differ based on their life histories. Management should focus on allowing connectivity for golden and silver perch, and on promoting local scale recruitment and survival for Murray cod and trout cod. More generally, our study highlights the importance of understanding the processes underpinning population persistence, how these processes may vary for different species, and ultimately how this knowledge can inform targeted management actions.

Globally, river degradation has decimated freshwater fish populations. To help reverse this trend in a southeastern Australia river, we used multiple restoration actions, including reintroduction of instream woody habitat, riparian revegetation, removal of a weir hindering fish movement, fencing out livestock, and controlling riparian weeds. We monitored the responses of native fish at the segment scale (20 km) and reach scale (0.3 km) over 7 years to assess the effectiveness of the different restoration strategies. Two closely related species, Murray cod Maccullochella peeli and trout cod Maccullochella macquariensis, increased at the restored segment compared with the control segment. However, inherent differences between river segments and low sample size hampered assessment of the mechanisms responsible for segment‐scale changes in fish abundance. In contrast, at the reach scale, only M. peeli abundance significantly increased in reaches supplemented with wood. These differential responses by 2 closely related fish species likely reflect species‐specific responses to increased habitat availability and enhanced longitudinal connectivity when the weir improved passage around a fishway. Changes in M. peeli abundance in segments supplemented with and without wood suggest an increase in carrying capacity and not simply a redistribution of individuals within the segment, facilitated the observed expansion. Our findings confirm the need to consider individual fish species' habitat preferences carefully when designing restoration interventions. Further, species‐specific responses to restoration actions provide waterway managers with precise strategies to target fish species for recovery and the potential to predict fish outcomes based on ecological preferences.