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1. Freshwater fish commonly exhibit differences in reproductive seasonality within assemblages. However, the extent of variation in reproductive timing among years is less well understood, particularly in relation to environmental variation. Phenological shifts in response to environmental variation, including climate change, can differ among species and thereby change temporal resource partitioning (e.g. in the use of nursery habitat, dietary items, etc.) and potentially the strength of biotic interactions such as competition.

2. To assess interannual variation in reproductive phenology and temporal resource partitioning in an arid-land river, we collected a data set on young-of-year fish in four years within an overall span of 16 years, from 1995 to 2010. Fish species spawned in a consistent order across years; however, the onset of spawning was between 4 and 28 days earlier in 2008-2010 than in 1995. Spawning onset advanced more for later-spawning species, resulting in reduced temporal partitioning across the assemblage.

3. Based on these observations, we developed four alternate models of assemblage-level responses to alteration of flow regime based on the magnitude of phenological shifts and corresponding temporal partitioning among species. Phenological data are consistent with only one of these models.

4. Convergence and overlap in spawning time among species may decrease resource partitioning at a critical life stage and alter assemblage composition in recruitment-driven systems like arid-land rivers. Understanding factors driving convergence and overlap in spawning time among species is key to predicting effects of ongoing climate change and alteration of flow regimes on fish assemblages.