Does life history mediate discharge as a driver of multi-decadal changes in populations of freshwater fish?

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.

Automated summary

This study tracked changes in abundance and size structure of four native freshwater fish species in the Murray River over 19 years, finding that local recruitment and immigration play different roles in driving population changes for different species. Local recruitment was an important driver for Murray cod and trout cod, while immigration was more important for golden perch and silver perch.Increases in spring river discharge strongly influenced responses for certain species.