By KEVIN DUFFY
Capital News Service
LANSING — After a half-century of salmon fishing in the Great Lakes, anglers are on edge.
As numbers of native lake trout and whitefish rebound, numbers of the prized Chinook – or king – salmon have spiraled downward.
“It’s about finding a balance between prey fish and predators in this changing food web,” said Randy Claramunt, a fisheries research biologist with the Department of Natural Resources (DNR).
“We don’t want to have a fish community that’s dominated by non-native species,” he said. “But in some cases it can provide a benefit.”
The rise and fall of salmon in the Great Lakes is a cautionary tale that suggests an ecosystem cannot be restored with single-species thinking. It shows how intricate the interactions are among native and non-native species and how that affects a resource economy.
It’s a tale that began in April of 1966 when first Coho and then Chinook salmon were brought from the Pacific Northwest to the Great Lakes to eat another nonnative fish. That fish – the alewife – had invaded the Great Lakes through canals.
Its main predator – lake trout – was struggling to survive, and alewives swam in great numbers until their populations crashed. Dead alewives washed ashore, rotting on Great Lakes beaches. Then they would boom and crash again.
Populations got so big that when viewed from above, thousands of the herring-like alewife appeared as a large alien shadow beneath the water’s surface.
“I had heard about an abundance of alewives but was shocked to see one grouping that measured about 7 miles (long) by three-quarter-miles (wide) in the water,” said Howard Tanner, who led the state’s fish division when the salmon program began 50 years ago.
Tanner, now 92, wanted to control the alewives with Pacific salmon and revitalize Great Lakes sports fishing at the same time.
“To allocate the largest freshwater system in the world for commercial fishing was good but not for conservation,” he said. “What we really needed in the middle of this heartland was an exciting sport fish.”
He got one. Tanner brought the Chinook to the Great Lakes, and by 1970 Pacific salmon were thriving on an ample alewife diet. That, in turn, sparked a thriving charter fishing industry that lured anglers from hundreds of miles away.
Fast-forward 20 years and the scene began to change as other invaders started to filter-feed their way through the Great Lakes.
Voracious zebra and quagga mussels diverted energy and nutrients away from prey fish, like alewife and smelt, by eating the microscopic plants and animals – phytoplankton and zooplankton – they relied on for food.
With their food in decline, alewife numbers began to plummet. That left fewer alewives for the salmon to eat. But resource managers were still pressured to continue heavily stocking the king sport fish that had eaten those alewives when they were abundant.
DNR’s Claramunt said the remaining alewives can support only so many salmon before each population crashes,
“We considerably reduced [salmon] stocking in 1998, 2006 and 2013 in an attempt to find the balance between declining prey fish levels and the salmon population in Lake Michigan,” he said.
Claramunt studies this balance using the basic biological principle that predators are limited by what they have to eat.
In most ecosystems, one-tenth of the prey’s biomass should be predators to support a healthy predator-prey dynamic, he said.
If there are 10 pounds or less of alewives for every pound of salmon, that’s bad for the salmon because there aren’t enough alewives to eat.
But if there are 20 pounds of alewives for every pound of salmon, the populations are in balance. The salmon will have plenty to eat, and there will be enough alewives left to reproduce and replenish the salmon’s food supply.
Kevin Duffy writes for Great Lakes Echo.