Abstracts

Presented by Scott Gilliland - Email: sgg64@mac.com

A Prescribed Take Level framework (PTL) was recently developed to assess sport harvest for North American sea duck populations. Under PTL, the predicted harvest rate that achieves maximum sustained yield (MSY) is entirely dependent on the estimate of the maximum intrinsic rate of population increase (rmax) and does not require estimation of carrying capacity (K). Published and unpublished data in combination with expert opinion was used to develop distributions for the demographic parameters required to estimate rmax. We argue that the demographic rates used in these models did not reflect optimal conditions and thus the output does not represent true rmax . We demonstrate the effect of bias in rmax on the resulting equilibrium population size relative to MSY and caution that the concept of MSY may not actually be sustainable for sea ducks. At MSY the resulting population level is approximately half of K and conditions allow birds to maintain high survival and productivity. However, this also implies that there is an excess of resources which inter-specific competitors could exploit, functionally reducing K. One key feature of PTL is the actual population size that meets the objective is unknown. Thus, management is directed towards manipulating the population to some unknown size. Because the objective population size is unknown, it is impossible to determine if the goal has actually been met. We believe the prescribed take level framework may be a useful starting point for sea duck harvest management but requires further development. We also recommend revising the target population size to be well above MSY. We recommend investments in monitoring to assess population, survival and productivity changes associated with harvest. We also suggest that work be directed at identifying and estimating additive sources of mortality and reductions in fecundity as these may be targets for management actions.