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Does a Bioenergetics Model Accurately Predict Fish Consumption by American White Pelicans? A Case Study on Walleyes in the Tamarac River, Minnesota

The effect of piscivorous birds on fisheries is a growing concern for fisheries managers, especially when native birds consume large quantities of fish. The Red Lakes, Minnesota, fishery is one such example, where congregations of American white pelicans (AWPEs) Pelecanus erythrorhynchos forage on spawning Walleyes Sander vitreus. We quantified AWPE consumption of Walleyes on the Tamarac River, a major tributary of the Red Lakes, by using empirical diet data collected from lethally sampled birds and separately by using a bioenergetics model. Furthermore, we evaluated the diet and foraging patterns of AWPEs on the river. Camera trap data revealed that AWPEs were foraging nearly completely nocturnally, likely in response to Walleye spawning migrations, with Walleyes accounting for 98% of AWPE diets. The empirical estimate of daily fish consumption from lethally sampled birds was not significantly different from the bioenergetics estimate. Monte Carlo simulations were used to provide estimates of uncertainty in annual Walleye consumption. Based on the simulations, all estimates of annual Walleye consumption between 2014 and 2016 represented < 1% of adult (age ≥ 3) Walleyes in the system and < 2.5% of adult Walleye natural mortality. This amount of Walleye consumption by AWPEs, at current population levels, does not pose a management concern. The interactions of piscivorous birds and fish stocks have been studied since the early 20th century but have become a topic of increasing interest in the previous two decades, due in part to the expansion of bird populations. Two of the most studied piscivorous bird species in North America are the American white pelican (AWPE) Pelecanus erythrorhynchos and the double‐crested cormorant Phalacrocorax auritus. American white pelicans are large, highly visible birds and are increasing in range and abundance, doubling their population between 1979 and 2001 (King and Anderson 2005). Increasing populations coincide with aquaculture expansion (King and Grewe 2001; King 2005) and the ban of DDT. Several studies suggest that this increase in the AWPE population, in part, may be a response to previous declines caused by DDT prior to the ban (Anderson et al. 1969; Blus et al. 1974; Boellstorff et al. 1985; Donaldson and Braune 1999). However, it is unclear whether the AWPE *Corresponding author: jakegraham@u.boisestate.edu Present address: Boise State University, 1910 University Drive, Boise, Idaho 83725-1535, USA. Received September 18, 2018; accepted March 26, 2019 North American Journal of Fisheries Management © 2019 American Fisheries Society ISSN: 0275-5947 print / 1548-8675 online DOI: 10.1002/nafm.10287