Culls effective for reducing mute swans in New Jersey

Abstract

Mute swans (Cygnus olor) have increased in New Jersey at ~7%/yr since the mid-1980s with the population peaking at 1,890 mute swans by 2005. Wildlife agencies have implemented management programs to reduce mute swan numbers to population objectives; however, evaluation of these programs is lacking. Given the detrimental impacts of mute swans on wetland habitats, we implemented a cull program in 2007 in New Jersey through shooting, mostly during mid-summer, and focused cull efforts on key molting sites. Our primary objective was to measure the impact of culls on mute swan population trends. We also examined population response of mute swans at key molting sites over time after culls began. We conducted 61 field operations at 21 sites and culled 2,038 mute swans over 11 years. We achieved a mean annual statewide cull rate of 0.15 and reduced the 2005 mute swan population 56% by 2017. The statewide mute swan population declined (P = 0.038) at nearly 8%/yr following commencement of culls. Four heavily used molting sites comprised 79% of the mute swans culled and population response varied across the 4 key cull sites. We fit a linear model using a derived annual cull rate and mute swan abundance data to describe the effect of culls on population growth (λ) and determined that a cull rate >0.09 was required to reduce (λ < 1) the mute swan population in New Jersey. Our work should benefit wildlife agencies who are considering implementing mute swan management programs.