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Emigration and mortality of juvenile brown trout in a New Zealand headwater tributary
Authors:Robin Holmes  John W Hayes  Weimin Jiang  Aaron Quarterman  Lawson N Davey
Institution:1. Cawthron Institute, , Nelson, 7042 New Zealand;2. Fish & Game New Zealand – Nelson/Marlborough Region, , Nelson, New Zealand
Abstract:We investigated juvenile brown trout migration and mortality in a headwater tributary of the Motueka River, New Zealand, by tracking 1000 young‐of‐the‐year passive integrated transponder (PIT) tagged fish over autumn to summer to (i) partition total loss into emigration and mortality and (ii) determine the influence of season and flow on emigration. Fish were tracked using mobile and fixed PIT tag readers. Of the 1000 fish tagged, 173 remained within the Rainy River; emigration contributed 60% and mortality 29% to loss. Only 11% of fish tagged in autumn were predicted to remain in the upper reaches of the stream by early summer, and this agreed with density data collected in a parallel study. We identified a two‐phase downstream migration pattern with early movement of large young‐of‐the‐year fish in autumn (mainly during floods). This was followed by another substantial period of movement in spring (during floods and lower flows) by fish that were initially smaller at the time of PIT tagging. The management implications for damming and fish screening in headwater tributaries are discussed.
Keywords:brown trout     Salmo trutta     passive integrated transponder tag  migration  movement  flow variation  low flow  floods  mortality  self‐thinning  young‐of‐the‐year
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