A comparison of three methods to investigate the diet of breeding double-crested cormorants (Phalacrocorax auritus) in the Beaver Archipelago, northern Lake Michigan

Abstract

In order to understand the role of waterbirds in aquatic food webs it is important to first get an accurate depiction of their diet. Three methods of dietary assessment (pellets, regurgitate and stomach contents) are compared here for breeding double-crested cormorants (Phalacrocorax auritus) of the Beaver Archipelago, northern Lake Michigan. By numerical frequency (percent number), each method yielded different depictions of the diet. However, in terms of presence and absence (percent frequency) of possible prey types, stomach content data did agree with both pellets and regurgitate data. However, differences were noted between regurgitate and pellets. In terms of biomass measured (percent biomass) in regurgitate and stomachs, data gathered agreed. In essence, pellets underestimate the importance of alewife (Alosa pseudoharengus) and overestimate the importance of crayfish (Orconectes sp.) in the diet when compared to both regurgitate and stomach analysis. The non-lethal method of regurgitate collection and analysis appears most practical in assessing cormorant diet in this system. In combination with information on avian foraging ecology and prey populations, these data may be used to investigate the relationships among cormorants and their prey, and lead to a better understanding of Great Lake food web dynamics.

Comparison of Diets in the California Least Tern (Sternula antillarum browni) at Two Sites in Central California, U.S.A.

“…Employing just one method can cause researchers to over- or underestimate the presence of certain prey items, leading to inaccurate conclusions about actual diets (Lee and Severinghaus 2004; Xavier et al . 2005; Seefelt and Gillingham 2006; Siverio et al . 2011; Correia et al .…”

Diet analysis in piscivorous birds: What can the addition of molecular tools offer?

“…Nevertheless, in 2% of the regurgitated fish samples, no fish DNA could be amplified. This suggests that the tissue might have originated from nonfish prey as cormorants can also feed on other vertebrates such as frogs and invertebrates such as crayfish (Seefelt & Gillingham, ; Putys & Zarankaite, ; J. Oehm unpublished data). In fish samples where DNA could be amplified, up to four fish orders were detected in a single sample.…”

Spatial and temporal comparisons of double-crested cormorant diets following the establishment of alewife in Lake Champlain, USA

“…Double-crested cormorants (Phalacrocorax auritus; hereafter cormorants) are piscivorous, opportunistic predators that feed on a variety of fish species near their roosting areas and other nearby water bodies (Custer and Bunck, 1992;Hatch and Weseloh, 1999;Coleman et al, 2005;Seefelt and Gillingham, 2006). On many lakes where cormorants have established nesting colonies, declines in resident sportfish populations, such as yellow perch (Perca flavescens) (Burnett et al, 2002;Rudstam et al, 2004;Fielder, 2008Fielder, , 2010, walleye (Sander vitreus) (Rudstam et al, 2004), and smallmouth bass (Micropterus dolomieu) , have been observed.…”

“…Although cormorant impacts on sportfish populations have been documented in some systems (Rudstam et al, 2004;Fielder, 2010;Dorr et al, 2010), studies on cormorant foraging habits in other locations have found that cormorants feed on fish and invertebrates not targeted by commercial fishermen or recreational anglers (Craven and Lev, 1987;Ludwig et al, 1989;Seefelt and Gillingham, 2006;Diana et al, 2006;Johnson et al, 2010). Neuman et al (1997) compared cormorant diets from multiple sites in the Great Lakes region and found large variations spatially and temporally, suggesting that using diet data from one site to infer the impact of cormorants at another site is inappropriate.…”

“…With the introduction of alewife to Lake Champlain, cormorants were presented with a new prey item as observed in the Great Lakes (Neuman et al, 1997;Seefelt and Gillingham, 2006;Diana et al, 2006). This potential change in cormorant foraging pattern highlights the need for updated diet information in Lake Champlain.…”