Spawning runs of naturalised rainbow and brown trout from Lake Otamangakau, central North Island, New Zealand

ABSTRACT This study describes some of the characteristics of the spawning runs of rainbow and brown trout from Lake Otamangakau, into Te Whaiau Stream. More than 55,000 fish undergoing spawning migrations were enumerated in a fish trap between 1994 and 2013. During that period, the spawning run size increased for both species, but particularly for rainbow trout, which exhibited a concurrent decline in average weight. The timing of the runs was consistent across both species, sexes, and maturation statuses but was affected by the average weight of the fish and the run size. The level of iteroparity revealed that 40% and 55% of the runs were made up of previous spawner rainbow and brown trout, respectively. Detections of PIT‐tagged fish at the trap indicated that rainbow and brown trout could spawn up to six and eight times, respectively. Remarkably, repeat spawning fish of both species and sexes returned within a few days of the same date each year over consecutive spawning events. Rainbow and brown trout grew substantially after spawning up to the 3rd and 4th spawning, respectively. The results of this study are relevant for informing management of the popular Lake Otamangakau trout fishery, in particular achieving a balance between satisfactory CPUE and large fish size.