Prognostic evaluation of enhancement programs using population models and life history analysis

A primary goal of all management plans is to increase population size or growth without harming wild stocks. Stock enhancement is one of many tools available for restoration of depleted populations and should be evaluated before implementation with other conservation plans such as habitat restoration and harvest controls. For many late-maturing species, an increase in juvenile cohort size does not necessarily increase population growth significantly, even if hatchery juveniles survive and grow well and do not adversely affect wild stocks. Management efforts that focus on reestablishing healthy adult stocks and protecting subadults may restore declining populations more effectively. Managers can anticipate the effects of particular management plans or enhancement efforts by means of population models. As an example, we review an analysis of the Kemp's ridley sea turtle (Lepidochelys kempi) headstarting enhancement effort. Because sea turtles have delayed maturity and were declining primarily because of incidental drowning of large juveniles and adults in shrimp trawls, captive-rearing and release efforts were (and still are) unlikely to have an appreciable effect on population growth rates, even if headstarted turtles have the same survival and growth rates as wild turtles. Population models and life-history analysis can be very useful for establishing performance criteria and evaluation methods as well as for choosing management options that are likely to be effective. We conclude by suggesting that prognostic modeling should be one component of the improved science of enhancement, which also includes establishing of specific goals, hypothesis testing, monitoring, and evaluation.