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Predators reverse the direction of density dependence for juvenile salmon mortality

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Abstract

The effect of predators on prey populations depends on how predator-caused mortality changes with prey population density. Predators can enforce density-dependent prey mortality and contribute to population stability, but only if they have a positive numerical or behavioral response to increased prey density. Otherwise, predator saturation can result in inversely density-dependent mortality, destabilizing prey populations and increasing extinction risk. Juvenile salmon and trout provide some of the clearest empirical examples of density-dependent mortality in animal populations. However, although juvenile salmon are very vulnerable to predators, the demographic effects of predators on juvenile salmon are unknown. We tested the interactive effects of predators and population density on the mortality of juvenile Atlantic salmon (Salmo salar) using controlled releases of salmon in natural streams. We introduced newly hatched juvenile salmon at three population density treatments in six study streams, half of which contained slimy sculpin (Cottus cognatus), a common generalist predator (18 release sites in total, repeated over two summers). Sculpin reversed the direction of density dependence for juvenile salmon mortality. Salmon mortality was density dependent in streams with no sculpin, but inversely density dependent in streams where sculpin were abundant. Such predator-mediated inverse density dependence is especially problematic for prey populations suppressed by other factors, thereby presenting a fundamental challenge to persistence of rare populations and restoration of extirpated populations.

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Acknowledgments

The authors thank the many people who assisted with field work, particularly Jim Sotiropolous, Gonzalo Mendez, Jon Raffensperger, and Karl Dietrich. Numerous landowners graciously allowed access to study sites. John Armstrong, Matt Ayres, Doug Bolger, and anonyomous reviewers provided helpful comments on an earlier draft. This work was supported by the USDA–USFS Northeastern Research Station, NIEHS-SBRP grant ES07373, an Atlantic Salmon Federation Olin fellowship, and the Dartmouth College Cramer fund. This research was conducted with approval from the Institutional Animal Care and Use Committee at Dartmouth College and conforms to the legal requirements of the United States.

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Correspondence to Darren M. Ward.

Additional information

Communicated by Craig Osenberg.

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Ward, D.M., Nislow, K.H. & Folt, C.L. Predators reverse the direction of density dependence for juvenile salmon mortality. Oecologia 156, 515–522 (2008). https://doi.org/10.1007/s00442-008-1011-4

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Keywords

  • Generalist predator
  • Inverse density dependence
  • Predator swamping
  • Salmon restoration
  • Stream fish