The short jaw kokopu (Galaxias postvectis) is a threatened New Zealand freshwater fish (family Galaxiidae) that migrates between freshwater habitat and the marine environment during its life cycle. The influence of this diadromous life-cycle, as well as the potential effect of introduced predatory fish on the population dynamics and habitat occupancy of short jaw kokopu were investigated in the Nelson Marlborough Department of Conservation conservancy. The distribution and abundance of short jaw kokopu were measured using spotlighting, substantially increasing their known range in this area. Juveniles (<100 mm FL) were recorded from the same stream reaches as adults using this method. Short jaw kokopu were positively associated with native forest, however they occupied a wide range of streams and habitat variables were not accurate predictors of their presence and absence. Diadromy had a large influence on fish communities at low- to mid- elevation sites. Short jaw kokopu were associated with the presence of other diadromous species, in particular other migratory galaxiids. Juvenile galaxiids were able to migrate through modified stream reaches, but the density of juvenile galaxiids significantly decreased with increasing distance from the sea. The number of juveniles recruiting to the adult population was probably influenced by the biomass of piscivorous fish species (e.g. introduced trout) and the cumulative effects of habitat modifications downstream. Short jaw kokopu and trout co-occurred at the reach scale, however short jaw kokopu were more likely to be absent when trout abundance was high. Negative interactions between short jaw kokopu and trout may be offset by diadromy, whereby individuals eaten or displaced are constantly replaced by juveniles returning from the marine environment. Conservation effort should be aimed at mitigating the deleterious effects of habitat degradation and migration barriers on short jaw kokopu and other migratory fish popUlations. Further research is needed to properly understand the effect that introduced trout have had on native migratory fish species.