The harsh winter of 2002 was a key factor in the collapse of baitfish in Lake Huron, according to a USGS report. Could last winter’s record ice cover on Lake Michigan have similar consequences?
In the study of freshwater ecosystems there is a classic debate regarding the control of baitfish populations. In simple terms, it can be boiled down to those who argue that baitfish numbers are controlled by the abundance of their food (plankton) vs. those who argue that baitfish numbers are controlled by larger fish that prey on baitfish.
In Lake Huron and Lake Michigan, this “bottom-up” vs. “top-down” debate is ongoing and complex. Baitfish, and alewife in particular, have declined in recent years. There is evidence that overabundance of Chinook salmon, which prey almost exclusively on alewife, are implicated but there have also been changes in plankton abundance and composition due to invasive species like quagga mussels. While this does not seem to account for a decline in alewife numbers directly, it has affected their condition and, as a result, salmon now must eat more to fill their bellies.
A recent paper published in the “Journal of Great Lakes Research” adds another wrinkle to the debate. Authors from the Ontario Ministry of Natural Resources and U.S. Geological Survey present analysis that links the harsh winter of 2002-2003 to the steep decline in Lake Huron alewife numbers that immediately followed. From 2003 to 2012, winters were relatively mild but alewife failed to mount a comeback in Lake Huron. This is where the top-down and bottom-up factors may have come into play. Hard winters had knocked back alewife populations in the past, but they had always recovered. Not so this time, but the changes in Lake Huron were not all bad and the decline of alewife led to resurgence of native walleye, lake trout, and certain native baitfish.
What does this mean for Lake Michigan? It is no secret that last winter was a cold one, and Lake Michigan experienced record-setting ice coverage of 93.29 percent in early March. Alewives are not native to the upper Great Lakes, and are not well-adapted for dealing with northern winters in a freshwater environment. Adult alewife sometimes die off in large numbers as waters begin to warm up after a hard winter, and young alewife can have difficulty surviving through the winter if they were not able to put on enough weight during their first summer.
For Lake Michigan, this means that last year’s crop of alewife may not contribute much to the forage base in coming years. Acoustic sampling conducted by U.S. Geological Survey in 2013 found that the 2013 alewife year-class was very poor in terms of both numbers and growth rate even before the harsh winter conditions. Declines in older alewife due to the harsh winter are also possible, but only time will tell how Lake Michigan baitfish—and the salmon and trout fishery that depends on them—weathered the hardest winter in recent history.