A Growing Cormorant Population on Lake Vermilion
Lake Vermilion Cormorants and the Fishery
The double-crested cormorant is a species native to Minnesota. A small number have been present on Lake Vermilion for many years. Cormorant nesting was documented on two small rocky islands in northeast Big Bay during the 1990s. Sometime around 2000, cormorants abandoned nesting on those islands and began to nest on Potato Island, a larger 0.60-acre rocky island located near the middle of Big Bay. Rapid population growth followed.
Double-crested cormorants are voracious eaters, consuming a pound of fish per day. Diet studies show they are opportunistic feeders, preying mostly on whatever species is most abundant. On Lake Vermilion, yellow perch and cisco would likely be major targets.
Cormorants have also been known to eat small walleye, although walleye usually make up a small part of their diet.
DNR test netting shows Vermilion’s perch population has been on a downward trend for several years at a time when the cormorant population has grown rapidly (see table below). Walleye growth and survival could easily be affected by reduced numbers of perch, an important forage.
The Leech Lake Experience — A Lesson for Lake Vermilion
For decades, the Leech Lake walleye fishery attracted hoards of fishermen which fueled the local economy. Three miles off the southern shore is Little Pelican Island, 3 acres of rock, sand and scruffy shrubs — somewhat larger, but otherwise similar to Potato Island in Lake Vermilion. In 1998, the number of double-crested cormorant nests on Little Pelican Island was 73. That number grew to 2524 nests in 2004.
As the cormorant population exploded, yellow perch and walleye populations decreased dramatically. Fishermen went elsewhere, leaving a huge impact on the local economy.
After an agonizingly long wait, a 5-year Leech Lake Action Plan launched in 2005. The plan was a collaboration of federal, state, local and tribal officials and included walleye restocking and cormorant reduction. In three years, over 9000 cormorants were culled, leaving about 500 nesting pairs, the management target.
Leech Lake hosted the Governor’s Fishing Opener in 2007, a statement that the fishery was recovering. And DNR test netting suggests a full recovery is underway.
How Does One Count Cormorants?
Nests on Potato Island in Big Bay
The DNR fisheries staff in Tower has been monitoring the cormorant situation on Lake Vermilion since 2004. Initially, nest counts were done every three years. However, after the sharp increase noted in 2010, counts will now be done annually.
|Survey Year||Cormorant Nests on Potato Island (DNR)||Total Cormorants Observed During Survey (SCLV)|
The only significant cormorant nesting site on Lake Vermilion is Potato Island in Big Bay. During the peak nesting season (generally June), DNR staff visit Potato Island and physically count nests. Each active nest is viewed as one breeding pair. While some cormorants nest in trees, all Potato Island nests are on the rocky ground.
Cormorants Observed During Annual Survey
The Vermilion Lake Association has been counting loons since 1983. The lake is divided into 23 territories, each with a team of trained observers. Beginning in 2010, the organization “piggy-backed” on this stable, repeatable process to survey cormorants while conducting the annual loon count in early July.
Just like loons, counting cormorants can be tricky, because both species spend extended periods underwater when feeding. An added challenge to counting cormorants is their tendency to take to the air when spooked by boat traffic.
To minimize double counting, observers keep track of cormorants in their territory and also those that flush and fly away. Cormorants that “fly-over” or those that land from another territory are not counted. Volunteers take care not to “spook” cormorants to avoid potential counting errors.