Michigan’s rivers and lakes were once cold enough that fish were protected from some infection-causing parasites. As the Great Lakes ecosystem warms, a Michigan State University researcher is investigating new pathogens that may become relevant to the fish that live here.
Bartolomeo Gorgoglione, MSU assistant professor leading the Fish Pathobiology and Immunology Laboratory, or FPIL, found new evidence of Tetracapsuloides bryosalmonae occurring in at least three Great Lakes and several rivers throughout the Great Lakes region. This parasite is listed as an “Emergency Pathogen” by the Great Lakes Fish Health Committee and as a reportable pathogen in Michigan and surrounding states. This is because it is known to cause proliferative kidney disease in salmonid fish species. While causing no risk to humans, it has been known to heavily impact farmed and wild fish in Europe and in the western United States. To date, however, no proliferative kidney disease-related fish mortality has ever been documented in the Great Lakes area.
Warming water temperatures due to climate change have created an environment where the parasite is able to reproduce faster, Gorgoglione said. What that means for fish still needs to be studied. It’s possible the infected fish could release infectious malacospores into the water without developing proliferative kidney disease as a tolerant fish host. For now, Gorgoglione is helping the Michigan Department of Natural Resources, or DNR, identify which fish stock is infected and which rivers are free of the parasite, so they can prioritize where their farmed fish may be stocked.
“This is something completely new,” Gorgoglione said. “We don’t precisely know what it may mean for salmonid hosts in the Great Lakes and if they may be able to complete the parasite life cycle naturally and without suffering from disease, or if they will develop the disease and eventually die or become easier prey.”
T. bryosalmonae is a unique fish pathogen because it can’t be spread directly from fish to fish. Instead, it must first infect an aquatic invertebrate called Bryozoa, microscopic aquatic invertebrates that live in colonies and are also known as “moss animals.” Bryozoans then release spores that infect salmonid fish, which may release another type of spore to complete the parasite life cycle. It’s a seasonal parasite that mostly spreads during summer months when water temperatures are warmer.
Gorgoglione previously studied this parasite in Europe, working on trout immune response to proliferative kidney disease in Scotland and discovering its presence in Austria. He also confirmed its presence in Pacific salmon in Alaska. In recent years, the pathogen has caused fish mortality outbreaks in the western United States, including in 2016, when T. bryosalmonae was blamed for a massive fish kill in Montana’s Yellowstone River, leaving thousands of mountain whitefish dead.
This summer, Gorgoglione and his team set out to see if the parasite was infecting wild salmonids in the northern part of Lake Michigan. They boarded the DNR Steelhead research vessels at Charlevoix and sampled lake trout during DNR fish population surveys. They also walked in rivers and lakes around the region, sampling fish and turning over rocks and logs looking for bryozoans.
In some cases, they found bryozoan colonies attached to various material discarded in rivers, such as inside beer bottles. They also found that these invertebrates often share substrates in rivers and lakes with the highly invasive zebra and quagga mussels and have been found growing on top of their shells.
“From a scientific point of view, it’s really exciting,” Gorgoglione said. “There’s no evidence in scientific literature about the involvement of mussels in the T. bryosalmonae life cycle, so it opens up many new questions.”
So far, most of the fish Gorgoglione has tested are asymptomatic and don’t show signs of proliferative kidney disease. He believes it’s possible that fish bred in hatcheries can be essentially immunized against developing proliferative kidney disease from the parasite. Fish in the hatcheries are exposed to T. bryosalmonae when they’re kept in river water, but because the water temperature is low and they are well fed, they don’t develop the disease and they’re able to develop antibodies to fight the parasite.
There’s still more to be studied and more work to be done. Gorgoglione is using the data to warn the DNR of hot spots and areas that may be infected. He also wants to recreate adequate conditions in his lab to study the life cycle of T. bryosalmonae and monitor its effects, which would be a first in the United States. However, the project scope is costly and difficult since it involves adapting his lab conditions and maintenance to accommodate both salmonids and bryozoans. But a study of this magnitude would enable FPIL scientists to provide answers about the tolerance or susceptibility of Great Lakes fish stocks. Gorgoglione says he’s currently seeking grant funding to move forward on this research to help the DNR and to prevent future fish loss caused by warming temperatures.
