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Muskellunge Numbers Drop as Virus Invades St. Lawrence

ESF researches problem at Thousand Islands Biological Station

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(Clayton, N.Y.) - When fisheries researchers from the SUNY College of Environmental Science and Forestry (ESF) monitored St. Lawrence River muskellunge during the spring spawning run in 2003, they had more than 40 of the keystone predators to measure and tag.

Only 12 were captured in 2006.

This year, there were four.

Researchers at ESF’s Thousand Islands Biological Station on Governor’s Island in the St. Lawrence River believe that viral hemorrhagic septicemia (VHS), a highly contagious illness that has already killed thousands of fish in the river, is the culprit.

“We want to send out an alarm that VHS is killing muskellunge,” said Dr. John Farrell, station director. “There has definitely been a change.”

There are other changes in the St. Lawrence River ecosystem to consider:

  • The invasive round goby has quickly evolved from newcomer to an abundant, invasive species, found in great numbers.
  • Adult smallmouth bass, a popular sport fish that is a dominant predator in the ecosystem, are growing in size as they prey on the abundant gobies. But, in an interesting twist within the food web, the gobies prey on young smallmouth bass, which could affect the bass’ ability to thrive in the future.

ESF scientists are studying the problem in partnership with researchers at Cornell University, who are studying aspects directly related to fish health.

Before 2005, Farrell said, he and his colleagues typically saw one dead muskellunge once every few years. As the largest predator in the ecosystem, there are naturally fewer muskellunge in the river than other fish, much like there are fewer wolves than rabbits in the forest.

In 2005, the researchers came across 25 muskellunge carcasses. In 2006, they found about a dozen. That year, all the dead fish were large females, and tests later showed that several of them died from VHS. One of them was nearly 59 inches long.

“They were all females with eggs that had not spawned. Now we’re looking into the impact of losing large mature individuals from the population,” Farrell said. “We haven’t seen mortality events like this in the past.

As of the end of June, about a half dozen muskellunge had turned up dead in the river, including two in one week in mid-June. In addition, the biological station’s monitoring efforts show there are fewer young muskellunge in the river this year, and fishermen are reporting fewer catches than in the past.

About three years ago, trends indicated that the muskellunge population was on the rebound. ESF researchers credited changes in fishing regulations, increased habitat protection, public education, and a voluntary change to a catch-and-release ethic as the keys to the muskellunge’s success.

Although their size and value as a sport fish draws attention to it, the muskellunge is only one of 37 species known to harbor VHS in the Great Lakes. The round goby is particularly susceptible to the bullet-shaped virus. The goby has proven to be a popular meal for the smallmouth bass, which are getting plump feasting on the abundant goby. But at the same time, the goby are munching on the young bass, which could mean trouble for the long-term survival of the bass.

“We’re interested in the role VHS is playing in all this,” Farrell said. “Because it’s new to the system, there are a lot of questions.”