Modeling Population Dynamics and Fish Consumption of a Managed Double‐Crested Cormorant Colony in Minnesota

Abstract

AbstractThe recovery of the double‐crested cormorant Phalacrocorax auritus throughout North America has led to increased human–cormorant competition over fishery resources and has forced managers to evaluate cormorant effects and consider management alternatives at local and broader scales. We present a method for modeling local double‐crested cormorant populations under varying levels of culling intensity based on a colony managed at Leech Lake, Minnesota, during 2005–2011. In this case study, the cormorant colony was evaluated under no, moderate, and intensive control rates and compared with the observed population response. Cormorant diets for fledged (adults and subadults) and nestling cormorants were described during 2004–2007 and 2010. Annual fish consumption and 95% confidence intervals were estimated from 1992 to 2011 using a bioenergetics model and Monte Carlo methods. Total feeding effort and fish consumption has been reduced by nearly 90% since cormorant control began in 2005 (consumption reduced from 20.01 kg/ha in 2004 to 1.98 kg/ha in 2011) and by 46–73% annually, depending on the number of birds arriving each spring and the applied culling intensity. Averaged across all years and periods, fledged cormorants consumed 0.75 kg·bird−1·d−1 and nestlings consumed 0.45 kg·bird−1·d−1. Respectively, average fledged and nestling diets were comprised of Yellow Perch Perca flavescens (61.0% and 77.4%), Coregonus spp. (12.3% and 9.4%), minnows Notropis spp. (9.9% and 2.2%), Trout‐perch Percopsis omiscomaycus (4.1% and 0.4%), and Walleye Sander vitreus (4.6% and 3.6%), though considerable seasonal and temporal variability was observed. The bioenergetics model used to estimate total fish consumption was sensitive to this variability, which was strongly associated with the dynamics of the population of Cisco Coregonus artedi. Some fish population and fishery statistics were described by changes in cormorant predation pressure, but these relationships were compromised by other concurrent management activities.Received April 1, 2013; accepted August 20, 2013