Evol Ecol Res 7: 643-650 (2005)     Full PDF if your library subscribes.

A field test of the extent of bias in selection estimates after accounting for emigration

Benjamin H. Letcher,* Gregg E. Horton, Todd L. Dubreuil and Matthew J. O’Donnell

S.O. Conte Anadromous Fish Research Center, USGS/Leetown Science Center, One Migratory Way, Turners Falls, MA 01376, USA

Author to whom all correspondence should be addressed.
e-mail: bletcher@forwild.umass.edu


Question: To what extent does trait-dependent emigration bias selection estimates in a natural system?

Organisms: Two freshwater cohorts of Atlantic salmon (Salmo salar) juveniles.

Field site: A 1 km stretch of a small stream (West Brook) in western Massachusetts, USA from which emigration could be detected continuously.

Methods: Estimated viability selection differentials for body size either including or ignoring emigration (include = emigrants survived interval, ignore = emigrants did not survive interval) for 12 intervals.

Results: Seasonally variable size-related emigration from our study site generated variable levels of bias in selection estimates for body size. The magnitude of this bias was closely related with the extent of size-dependent emigration during each interval. Including or ignoring the effects of emigration changed the significance of selection estimates in 5 of the 12 intervals, and changed the estimated direction of selection in 4 of the 12 intervals. These results indicate the extent to which inferences about selection in a natural system can be biased by failing to account for trait-dependent emigration.

Keywords: Atlantic salmon, emigration, selection, selection differentials, size dependence.

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