Revised Protocol for the Adaptive Harvest Management of Mid-Continent Mallards

摘要

The population models upon which harvest regulations for midcontinent mallards are based have been in place since 1995. The basic structure of the models, alternative hypotheses of population dynamics, and support for each hypothesis (i.e., model 'weights') are subject to continuous review by parties both internal and external to the AHM process. For the last two years, the AHM Working Group (AHMWG) has been focusing on two especially important concerns about the existing models for mid-continent mallards, and is making certain revisions this year. Apparent bias in reproductive or survival rate - the current population models for mid-continent mallards share a common structure referred to as the balance equation. The balance equation is essentially an accounting tool, which predicts population size in a given year based on population size (N), reproduction (R), and survival (S) from the previous year. In theory, N, R, and S from a given year should perfectly predict N the next year. In fact, they do not (Fig. 1). Predicted population sizes are higher on average than those observed in the population surveys. The source and cause of the bias in estimates of survival and reproductive rates are unknown, but data-collection programs are being carefully scrutinized. Updating model weights - the purpose of annually updating model weights is to eventually identify the model providing the most accurate predictions over time, based on a comparison of the observed population size with those predicted under each alternative model. Model weights are highly influential in determining optimal harvest strategies because they determine the degree to which a single set of biological hypotheses (i.e., a particular model) dominates the optimization. The AHM Working Group has identified a shortcoming in the current procedure for updating model weights due to the omission of certain random errors common to all predictive models. The inclusion of these prediction errors in the updating procedure will minimize the chances of major shifts in model weights in any single year, and help ensure that model weights accurately support the model(s) with the best predictive ability. The purpose of this report is to describe the AHM Working Group's efforts to address these modeling issues. Final decisions regarding modification of the model set for mid-continent mallards will be made after the USFWS has discussed resulting management implications with the Flyway Councils, States, and the general public.