Lake resident and migratory smelt, Retropinna retropinna (Richardson), of the lower Waikato River system, New Zealand

Retropinna retropinna occurred in two forms in Lake Waahi, North Island, New Zealand. One form was characterized by high mean gill raker counts ( c . 28) and low mean vertebral counts ( c . 52)—similar to those of non‐migratory, lake resident, native smelt in some other lakes of the lower Waikato River system. The other form had low mean gill raker counts ( c . 21) and high vertebral counts ( c . 60)—similar to migratory smelt at five sites along the lower Waikato River down to its estuary. The lake resident form was shorter but deeper bodied than the migratory form, as indicated by significant differences in mean length and in weight‐length regression analyses. The two forms overlapped in time of sexual maturity. As in other native lacustrine smelt populations of the lower Waikato system, the resident form had more but smaller eggs than the migratory form. The egg number to body length regression and the mean egg diameter of migratory Lake Waahi smelt were similar to those of lower Waikato River smelt. Evidence for reproductive separation of the two forms is presented and mechanisms controlling their meristic and other differences are considered.