Risk v. reward for responsive movements by a highly mobile fish species in a flow-constrained and barrier-laden river

Context Understanding the movement behaviour of flow-dependent fish species is a foundational principle underlying the effective management of highly modified riverscapes. Aims To determine how variations in river discharge and instream barriers affect the residency, survival and movement of golden perch (Macquaria ambigua) in the degraded Gwydir River system within the northern Murray–Darling Basin. Methods We monitored the movement of 25 acoustic-tagged golden perch for up to 3 years by using a linear array spanning ~180 km of the main river channels across the lower Gwydir system. Key results Golden perch were largely sedentary for extended periods, with movements constrained by the barrier maze that now defines the system. High flows facilitated passage over instream barriers, with the highest periods of activity occurring in spring and early summer, and to a lesser extent in autumn. Conclusion Our findings are indicative of a highly constrained and isolated population of golden perch that is now likely to be neither a source nor a sink, but is in effect a false sink perpetuated by re-stocking practices. Implications The rehabilitation of the fish community in the Gwydir and other systems in similarly poor condition throughout the Murray–Darling Basin will require major institutional and societal change.