The Plastic Portfolio Effect: Managing the Life History Composition of Populations

Many life history traits of fish are plastic. For example, juvenile salmon from the same redd that experience different habitats early in life may grow at different rates and subsequently migrate to sea at different age and size. A population with a diverse portfolio of life histories may be more stable, because the population as a whole is not as vulnerable to spatially or temporally localized events that only affect a subset of life history types. This portfolio effect applies whether life history diversity is generated by plasticity or local adaptation of unique populations within a larger stock. Given the potential to increase population stability, managing the pattern of plastic life history expression has potential application in conservation efforts. For example, enhanced life history diversity is a stated goal of many habitat restoration efforts. However, the potential for management to shift the life history composition of a population raises a number of questions, both basic and applied, that are only recently being addressed: How do changes in the life history composition of populations that are generated by plasticity of juveniles affect the habitat requirements of subsequent life stages? When will the trade-off of having a higher-diversity portfolio of life history types, thus fewer individuals expressing the “best” life history, interfere with recovery of small populations? In order to manage plastic life history diversity, how do we need to modify existing methods for population monitoring that are often focused on a subset of common or easily-sampled life history types? Is a specific life history type a legitimate target for conservation efforts if the expression of that life history is plastic? Presentations in this symposium will address these topics.