Size‐ and stage‐dependence in cause‐specific mortality of migratory brown trout

Evidence‐based management of natural populations under strong human influence frequently requires not only estimates of survival but also knowledge about how much mortality is due to anthropogenic vs. natural causes. This is the case particularly when individuals vary in their vulnerability to diffe...

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Bibliographic Details
Published in:	The Journal of animal ecology 2020-09, Vol.89 (9), p.2122-2133
Main Authors:	Nater, Chloé R., Vindenes, Yngvild, Aass, Per, Cole, Diana, Langangen, Øystein, Moe, S. Jannicke, Rustadbakken, Atle, Turek, Daniel, Vøllestad, Leif Asbjørn, Ergon, Torbjørn, Koons, David
Format:	Article
Language:	eng
Subjects:	Animals Anthropogenic factors Bayesian statistics Body Size Cause of Death dam Dam construction Data recovery Environmental effects harvesting hazard rate Human influences Hydroelectric dams Hydroelectric power Life history mark–recapture Matematikk og Naturvitenskap: 400 Mathematics and natural science: 400 Mortality Natural populations NIMBLE Norway Parameterization Salmo trutta Salmonids Spawning Trout VDP Zoologiske og botaniske fag: 480 Zoology and botany: 480
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Summary:	Evidence‐based management of natural populations under strong human influence frequently requires not only estimates of survival but also knowledge about how much mortality is due to anthropogenic vs. natural causes. This is the case particularly when individuals vary in their vulnerability to different causes of mortality due to traits, life history stages, or locations. Here, we estimated harvest and background (other cause) mortality of landlocked migratory salmonids over half a century. In doing so, we quantified among‐individual variation in vulnerability to cause‐specific mortality resulting from differences in body size and spawning location relative to a hydropower dam. We constructed a multistate mark–recapture model to estimate harvest and background mortality hazard rates as functions of a discrete state (spawning location) and an individual time‐varying covariate (body size). We further accounted for among‐year variation in mortality and migratory behaviour and fit the model to a unique 50‐year time series of mark–recapture–recovery data on brown trout (Salmo trutta) in Norway. Harvest mortality was highest for intermediate‐sized trout, and outweighed background mortality for most of the observed size range. Background mortality decreased with body size for trout spawning above the dam and increased for those spawning below. All vital rates varied substantially over time, but a trend was evident only in estimates of fishers' reporting rate, which decreased from over 50% to less than 10% throughout the study period. We highlight the importance of body size for cause‐specific mortality and demonstrate how this can be estimated using a novel hazard rate parameterization for mark–recapture models. Our approach allows estimating effects of individual traits and environment on cause‐specific mortality without confounding, and provides an intuitive way to estimate temporal patterns within and correlation among different mortality sources. With an analysis of 50 years of mark–recapture data, this study shows that fishing mortality has outweighed mortality from all other causes for large, migratory trout over the course of half a century (1966–2017), and that individual traits (e.g. body size) can affect different mortality risks in diverging ways.
ISSN:	0021-8790 1365-2656 1365-2656