Science: Cormorant feeding on yellow perch

            Yellow perch Perca flavescens are abundant in Dore Lake, Saskatchewan, but do not achieve sizes acceptable to anglers (total length approximately 10 inches).  Numbers of double-crested cormorant Phalacrocorax auritus, a piscivorous bird, have increased from 70 breeding pairs on Dore Lake in the 1960’s to 11,513 breeding pairs in 2006 and they consume 1.4-1.7 million kg of fish per year.  Barks et al. (2010) examined growth rates, timing of ontogenetic diet shifts and cormorant predation of yellow perch on Dore Lake to determine if they explained the lack of acceptable yellow perch. 
A total of 1,756 yellow perch were collected from littoral (<1.5 m) and offshore (>5 m) habitats from July 9, 2008 to July 28, 2008 with beach seines and small and large bottom-set gill nets.  Specimens were measured (total length and weight), frozen and aged by counting annuli on opercular bones.  Diets were characterized using stomach contents and stable isotope analysis.  Comparisons were made between littoral and offshore habitats.  Stomach contents of 122 sacrificed cormorants and 240 regurgitated boluses from flightless juveniles were analyzed for percent occurrence and composition of yellow perch. 

            The maximum size and age of yellow perch was 215 mm TL and 7.4 years.  Comparisons of overall growth patterns and body condition of littoral and offshore fish were not possible because there was too small of a sample size for older yellow perch (>3 years) in littoral habitats.  Yellow perch diets shifted, as expected, from zooplankton at age 1 to macroinvertebrates at ages 2-3 to fish at ages 3 and up.  Stable isotope analysis suggested that yellow perch move up approximately 1 trophic position from age 1-10.  Analysis of the regurgitated boluses revealed that yellow perch are the most common prey item in the diets of juvenile cormorants.  Yellow perch were the second most common prey item in the diets of adult cormorants behind ninespine stickleback Pungitius pungitius.  They were also the second largest component of juvenile and adult diets by percent composition.  Also, 94% of yellow perch consumed were less than 100 mm and 1% were greater than 150 mm.

            The results of this study verify that yellow perch attain old ages but not large sizes for the species, and they are an important component of the double-crested cormorant diet.  Cormorant preference for small fish makes it unlikely that they are directly responsible for the lack of acceptable yellow perch; yellow perch growth is regularly density dependent so predation pressure from cormorants should reduce intraspecific competition and increase growth rates.  If the cormorant population does have an effect, it is suppression of growth rates due to the changes in yellow perch behavior or selection on life history characteristics they cause.  Barks et al. (2010) suggest two possible reasons that high predation pressure on yellow perch by double-crested cormorants has not resulted in increased growth rates in yellow perch.  First, cormorant predation may be compensatory to background mortality and, therefore, would not influence population density.  The second may be competition with other, competitively superior, species prevents increased growth rates.

Implications for anglers:

            Cormorant populations are increasing on the Great Lakes and the diving birds are a common sight at many fishing spots in southeast Wisconsin.  The study above suggests that cormorants may have an impact on yellow perch populations in the area.  There are a number of game fish present in both Dore Lake and Lake Michigan: walleye Sander vitreus, lake whitefish Coregonus clupeaformis, northern pike Esox lucius and bass Micropterus spp.  However, unlike Lake Michigan, in Dore Lake there are no alewives, no stocked trout or salmon and no fishing pressure on the yellow perch population.  Given these additional factors, studies of cormorant feeding and its effect on prey populations in the Great Lakes need to be conducted.  Anglers should be vocal about the issue of cormorants on Wisconsin waters to increase attention to the problem and stimulate research.  Recording observations of the birds and taking photos may also be helpful.  Cormorants are probably eating some of Lake Michigan’s yellow perch, so anglers should try to dissuade the birds from calling fishing spots home.  Since there is not an open hunting season yet, you will have to get creative.

           

Information about cormorants on Lake Michigan

Selected definitions:

Boreal:  characteristic of the climatic zone south of the Arctic

Piscivorous:  fish-eating

Total length:  length from the tip of the snout to the tip of the longer lobe of the caudal fin

Ontogenetic: origin and development of an individual organism from embryo to adult

Annuli: yearly rings

Opercular: a hard flap serving as a cover for the gill slits in fishes

Littoral:  related to or situated on the shore of a lake

Bolus: small rounded mass of a substance, especially of chewed food at the moment of swallowing

Trophic position:  an organism’s location in a food chain

Barks, P.M., J.L. Doucette and C.M. Somers. 2010. Lack of angling-sized yellow perch in a Canadian boreal lake: potential influences on growth rate, diet, and predation by double-crested cormorants. Transactions of the American Fisheries Society. 139: 1029-1040.