Their increasing population is a big problem
Outdoors enthusiasts and anglers in Michigan started seeing cormorants in increasing numbers in the last few decades. Cormorants are fish-eating birds most associated with near-shore areas of the ocean.
A research paper by the Canadian Wildlife Service Biologists, Weseloh, Postupalsky et al. indicates that cormorants colonized the Great Lakes between 1913 and 1920. They presumably spread east from the Atlantic coastal states. Their population reportedly increased slowly until 1950. In the 1950s, their population in Michigan collapsed predominantly due to contaminant problems like DDT, much like the problem with bald eagles.
Nesting and Population Increase
A wildlife research paper by Scharf and Shugart followed recent population increases in the Great Lakes of double-crested cormorants and found a 454% increase in cormorants since 1977. Another wildlife research paper by Weseloh et al. of the Canadian Wildlife Service states, “This recent population increase is steeper than in most other parts of the species’ range and is attributed to a combination of factors: reduced human persecution, declining levels of organochlorine contaminants, and an abundance of forage-base fish, notably alewife (Alosa pseudoharengus), following decimation of Great Lakes stocks of predatory fish between the 1940s and 1960s. The decimation of predatory fish has been attributed to predation by sea lampreys (Petromyzon marinus) and human over-exploitation.”
The current cormorant survey indicates more than 12,000 nesting pairs of cormorants in Michigan and approximately 2,640 single immature/juvenile birds.
Cormorants often nest on small islands in the Great Lakes area. They make a nest of sticks, mostly on the ground. There have been cases where vegetation dies on these small islands where nesting activity has a high density. Cormorants have about four young per nesting. Cormorants are reported to typically live about 8 years, although they can live much longer.
Predation Impact
Anglers have been very concerned about the predation impact of the greatly increased cormorant population on the Great Lakes near-shore and nearby inland lakes, panfish and gamefish populations. Cormorant studies and observations of the cormorant stomach contents during control operations demonstrate that cormorants can eat relatively large numbers of juvenile game fish and smaller panfish at one time.
MDNR Fisheries research biologist Dave Fielder wrote, “It is apparent that cormorant predation is at least one factor affecting the perch population and fishery and may be the most influential force, among those examined, during this time series.”
The cormorant survey indicates one breeding pair of cormorants and their resulting fledglings for one breeding season will consume 670 pounds of fish biomass. The 2,640 juvenile birds will consume an additional 464,820 pounds of fish biomass, resulting in a total loss of over 8,504,820 pounds or 4,252 tons of fish.
Gary Gorniak, president of the Straits Area Sportsman’s Club, said, “In the early 2000s, we had several hundred to 1000s of cormorants on Brevort Lake daily. They devastated our fisheries. Since 2016 we have only been allowed to harass the cormorants, which is very alarming to us. We have seen the numbers increasing yearly. This year we have been chasing 300-400 cormorants off the lake daily during the spring.”
As a retired DNR Fisheries management biologist, I have observed cormorants as a problem during Great Lakes fish plants (typically trout) in near-shore areas in the spring. Some measures have been made to drive large numbers of cormorants away from stocking sites with non-lethal flash-bangs.
Cormorants’ Indirect Fish Mortality
Before my MDNR career, I was a Fisheries Biologist for the U.S. Fish and Wildlife Service in Ashland, Wisconsin. At the time, the double-crested cormorant population greatly increased in the Apostle Islands in Wisconsin waters of Lake Superior. I participated in a study of cormorant predation on target species of Wisconsin commercial pond netters with my former colleagues at the University of Michigan Great Lakes Research Section. We SCUBA dove on nets where numerous cormorants were. The researchers did not find much evidence of direct predation by the cormorants on the relatively large lake whitefish and lake trout. What they did find was a considerable level of gilling death of those species as they were harassed by the cormorant and sometimes swam into the large mesh of the pond nets, causing them to die from gilling as they would have in a much more lethal gill net.
Cormorant Control Measures
With concern for Great Lakes area fisheries, MDNR and the U.S. Fish and Wildlife Service permitted some control measures by concerned sportsmen’s groups on cormorants. From 2004 to 2016, Michigan exercised the use of a cormorant public resource depredation order (PRDO) to control the population. In 2016, the PRDO was halted by a federal court after a lawsuit was filed against the USFWS, which halted the control practice. The court suit maintained that the U.S. Fish and Wildlife Service had not completed the environmental assessment correctly.
Summary
Regardless of how one feels about cormorants, they greatly increase their abundance. As a result of their abundance, they have become a concern for their impact on the Great Lakes near-shore fisheries and lakes near the Great Lakes in some areas of Michigan.
