MNR study blames cormorants, anglers....

[Spiel]

Manitoulin.ca

December 19, 2007

 

 

MNR study blames cormorants, anglers equally for decline in inland lakes fishery Mindemoya Lake the worst, followed by Silver, Manitou and Tobacco Lakes

 

 

Of the 11 lakes sampled by the Ministry of Natural Resources in the spring and summer of 2005, five were deemed to be overstressed from both cormorant predation and angling pressure. On Lake Mindemoya, "cormorant consumption and angler harvest each exceeded 50 percent of total fish production (the cumulative production of all fish populations in a lake)," the report notes, making it the most compromised lake on the Island. But the fish populations of Silver Lake, Lake Manitou, Lake Kagawong and Tobacco Lake were also found to be threatened by a combination of angling and cormorant activity.

 

The ratio of cormorant impact and human influence varied from lake to lake. Cormorant consumption exceeded angler harvest in four of the lakes studied, while the opposite was true on five lakes, the report notes.

 

Data for the study was collected via aerial surveys as well as ground surveys. The former allowed the MNR to assess cormorant densities and estimate the birds' consumption levels, while the ground surveys yielded harvest rate data from anglers towards an estimate of fishing impact.

 

To establish fish populations, the ministry used a "model that predicts fish production from total phosphorous concentration," the study notes, as a more comprehensive measurement would require "intensive sampling off all fish species and multiple years of data."

 

Once the data was collected, "we compared our estimates of fish extraction by cormorants and anglers to estimates of fish production to measure the stress imposed by these two types of predators," the report explains. "Then we evaluated how the stress imposed by cormorants would exacerbate angling stress if angling harvest was not reduced to accommodate cormorant presence."

 

Cormorants were judged to consume an average of 32 percent of the potential production of medium-sized fish on Manitoulin lakes, which approaches the level documented at Oneida Lake in New York State, which serves as a benchmark for studies of this nature.

 

But while the much-maligned birds are clearly having an impact on fish, so are anglers. The study found that "recreational fishing is a substantial stress on several lakes," with anglers harvesting 52 percent of potential large fish production. In six of 10 cases, the study notes, "angling stress exceeds cormorant stress."

 

Put together, the two stressors are creating an insupportable situation on many of Manitoulin's lakes, the MNR concludes. Given cormorants' impact on medium-sized fish, which in turn hampers the proliferation of large fish, a continuation of the current take by fishermen would "result in a mean angling stress of 96 percent," the study states. "This increase is unlikely to be sustainable."

 

The scientists indicate that harvesting 50 percent of potential fish production is about the limit if a healthy population of fish species is hoped to be maintained. "Harvesting almost 100 percent of production is not."

 

 

 

The authors of the report qualify that a more thorough understanding of the interplay between cormorants and anglers would require the kind of long-term analysis conducted at Lake Oneida. "This would entail population estimates, trend-through-time patterns in abundance and vital rates such as survival, and more detail on the species and sizes of fish removed by both birds and anglers through time," they write.