Implications of diadromy for the structuring and modelling of riverine fish communities in New Zealand

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Abstract
Development of models to describe the distribution and abundance of salmonid fishes depends on understanding the suitability of local, small‐scale, critical habitat parameters such as water depth, flow velocity, and substrate characteristics. The success of such models in predicting trout abundance depends on the populations not being limited by additional constraints on abundance by wider, large‐scale parameters, such as recruitment and migratory access. Extension of these models for use in evaluating the impacts of flow modifications to New Zealand rivers on indigenous fish populations would significantly advance biologically sustainable management of rivers. Many of New Zealand's native fish species are diadromous, requiring access to and from the sea to complete their life cycles. For this reason, abundance of native fishes is likely to be constrained by a combination of large‐scale variables such as: elevation, slope, distance from the sea, the presence or absence of explicit barriers to upstream migration, and individual species’ “urge” to migrate upstream. Successful predictive models for describing abundance of diadromous native fish will need to incorporate factors that account for these variables, Although models for estimating areas of available habitat can be based on habitat suitability criteria, when models are used for predicting abundance, they must take into account factors relating to accessibility.