A decision analytic approach to hierarchical decisions for double-crested cormorant conflict resolution

Abstract

Double-crested cormorants ( Nannopterum auritum ) are piscivorous, migratory waterbirds that often conflict with fisheries interests in North America, stemming from depredation effects on fish populations. A new permitting process in the United States allows natural resource agencies to request permits for lethal take of cormorants in special instances like depredation of managed fish stocks. This rule does not include guidance for when cormorant take might be actionable. We developed a hierarchical structured decision making (SDM) framework for evaluating whether, when, and how much cormorant take might be necessary. A broad-scale, or “umbrella,” SDM framework established the overarching problem, objectives that likely are relevant for permitting decisions (i.e., minimizing loss of ability to harvest fish and sustaining cormorant presence on the landscape), and methods for evaluating the extent of management that might be necessary. A local-scale SDM process can then be applied for individual systems to determine the need for and magnitude of a permit request. We provide an example of such a local-scale SDM process for Brevoort Lake, Michigan, United States, where walleye ( Sander vitreus ) are sustained through stocking. We found that decisions for cormorant permitting on Brevoort Lake were influenced by uncertainties about the level of cormorant feeding on walleye, per capita level of daily cormorant consumption, number of days the cormorants spent feeding on the lake, and number of walleye stocked in the lake. This case study highlights the importance of uncertainty and risk in decisions for cormorant management, both of which can be accounted for more fully within the SDM framework.