Lake Horowhenua is a coastal eutrophic lake on the west coast of the North Island. A recent survey of the lake found lower than expected fish diversity but comparatively abundant native fish populations, comprising mostly shortfin and longfin eels (Anguilla australis and A. dieffenbachii). A weir on the outlet of the lake was ifentified as a potential barrier to fish migrations, reducing fish diversity and abundance in the lake. However, large numbers of common smelt (Retropinna retropinna) were collected during this survey, indicating that the population was eighter successfully reproducing in the lake or diadromous, i.e., migrating from the sea. Previous studies have shown that lacustrine common smelt can be distinguished from diadromous populations by differences in counts of vertebrae and gill rakers, and otolith microchemistry. Horizons Regional Council requested that an analysis of smelt otoliths and relevant morphological characteristics be performed to ascertain if the Lake Horowhenua population was diadromous.