Competition for Food Hots up for Lake Michigan Fish

As if Lake Michigan fish do not have enough competition for resources. An Illinois-Indiana Sea Grant study has found that the diet of an invasive freshwater hydroid includes organisms that are an important food source for young-of-the-year and bottom-dwelling fish.

"Cordylophora caspia typically eats larval zebra and quagga mussels," said Nadine Folino-Rorem, Wheaton College biologist. "However, when those sources are not readily available, the hydroid can feed on other invertebrates, which potentially affects prey availability for fish." Dr Folino-Rorem, along with Martin Berg, a Loyola University Chicago biologist, studied the distribution and diet of C. caspia in Lake Michigan.

The hydroid lives in freshwater and brackish or slightly salty habitats. The freshwater colonial hydroid is native to the Caspian and Black Seas. C. caspia colonies consist of several polyps or individuals approximately one millimeter long that are interconnected by their gastrovascular cavities. Colonies grow on hard surfaces; in southern Lake Michigan, C. caspia can be found in harbors on rocks, piers, pilings, and on clusters of zebra and quagga mussels.

The researchers found C. caspia in all eight Chicago harbors sampled as well as at two offshore sites. In fact, the population of the freshwater hydroid is growing in Lake Michigan. Folino-Rorem speculates that this may be due in part to street salts washing into the lake and changing water quality. "C. caspia thrives in higher salinity," she explained.

The researchers also found that the freshwater hydroid can eat organisms – chironomids – that are two to three times its size. "This was often accomplished by working together," said Dr Folino-Rorem. "When one polyp gets a hold of a chironomid, the organism can continue to thrash about until another polyp latches onto it too. The two polyps engulf the chironomid, sometimes meeting in the middle."

C. caspia is limited in its range due to its need to colonise on hard surfaces – Lake Michigan's muddy bottom does not provide a hospitable habitat. However, the recent spread of quagga mussels may increase the amount of available substrate for attachment. Unlike zebra mussels, quagga mussels can colonise the soft, muddy bottoms found in deeper areas. According to Tom Nalepa, a NOAA biologist at the Great Lakes Environmental Research Laboratory, 99 per cent of what his team finds when sampling offshore in southern Lake Michigan waters is quagga mussels.

"They are more efficient than zebra mussels in using food resources," he said. "And they tolerate cooler temperatures. We found that the number of quagga mussels in deep and in shallow waters far exceeds zebra mussel numbers even at their peak."

For C. caspia, the spread of quagga mussels may prove beneficial in terms of expanding their range to offshore waters. For fish populations, this may prove to be more bad news.