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.

Maintaining or restoring productive freshwater fisheries is a key challenge for resource managers. However, the inherent uncertainty and complexity of managing fisheries, often based on scant environmental data, make it difficult for managers and the public to reach consensus on appropriate actions. To help deal with this issue, we created a literature-based decision support system to diagnose limiting factors for stream brown trout fisheries. Once limiting factors are determined, appropriate management actions can be tailored to address them. Our Bayesian belief network (BBN)-based framework serves 2 functions: (a) It directs users to assemble a parsimonious environmental data set to inform stream fishery management, and (b) it integrates and interrogates these data to generate standardized and testable hypotheses about which environment factors are likely to limit trout productivity. The BBN has been trained on brown trout because among freshwater fish, this species has the richest literature base and is highly valued worldwide. However, the framework could be adapted for other stream fish. We applied our BBN to the Horokiri Stream, a data-rich catchment in Wellington, New Zealand. The BBN probability outputs were comparable with the conclusions of 5 experienced fishery biologists following their detailed investigation into the factors that led to the loss of the Horokiri brown trout fishery between 1951 and 1990.

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Metaphor, defined as the linguistic substitution of one phenomenon for another, is ubiquitous in environmental science and policy. In science, when used well, metaphors help to make complex and abstract ideas familiar and relatable, while also helping people orient ethically to the natural world. In freshwater science, metaphors structure many aspects of scientific and lay understanding. Yet, while metaphors are often used in environmental science and advocacy, there is a need to join up our currently diffuse understandings about how metaphors can help achieve social transformation for sustainability. Here, we focus on how metaphors are enshrined into institutions, giving them permanence and force as tools for social transformation. We explore three examples of metaphors in environmental science and activism that have ‘gone public’ to shape freshwater politics and governance in Aotearoa New Zealand (henceforth Aotearoa NZ). We focus on the origins, strategic purposes and limitations of the metaphors, the ways they have been institutionalised, and the roles that scientists in particular have played in shaping metaphorical meanings. Metaphors perform diverse political tasks, from mobilising popular support for species removal, to reorienting human obligations to rivers, through to expanding the scope of vision for river management. Scientists play key roles in shaping both regulatory institutions as well as informal norms that affect metaphor implementation. Finally, what makes a ‘good’ metaphor needs to be evaluated in context of who is mobilising the metaphor and what their broader sustainability values and objectives are. Read the free Plain Language Summary for this article on the Journal blog.