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Tis the Season for Lake Trout and Whitefish
Jim Johnson
With Halloween and Thanksgiving comes spawning time for lake whitefish and lake trout in Lake Huron. And, until recently, Thunder Bay was where the action was. The DNR was out there every fall taking biological data from the spawning populations and mapping locations of the best spawning sites so they could be protected.
That's what we were doing in mid-October of 1992. We were lifting a net set on a small natural rock reef about 5 miles out of Alpena in Thunder Bay. The catch was good that day: mostly lake trout but also some whitefish. Remarkably, almost all the lake trout were wild; they had no fin clips or other marks that would identify them as having spent their early lives in a fish hatchery. These were the first wild, spawning lake trout recorded in Lake Huron since sea lampreys had exterminated them in the 1940s. Our excitement was dampened when we noticed most of the fish were rather small and riddled with sea lamprey wounds. There was one larger female in the catch, but when I lifted her out of the net to weigh and measure, her eggs fell out on the deck through a hole in her side left by a sea-lamprey attack. This was one reason Lake Huron's lake trout were so slow to recover: there were too many lampreys and lampreys focus on larger lake trout, killing them before they have the chance to spawn.
The Great Lakes Fishery Commission and the collective resource agencies on Lake Huron eventually solved that lamprey problem when they treated the mother of all sea lamprey reproduction rivers: the St. Marys River. The Hammond Bay Biological Station north of Rogers City would play a key role in that venture. Today lake trout of all ages are spawning at many locations in Lake Huron, and most lake trout today are wild.
Whitefish, a cousin of the trout, also spawn in fall. They spawn on rock reefs, as do lake trout, but are less finicky about exactly where to drop their eggs. Unlike lake trout, which gather and spawn in discrete locations of especially suitable rock, whitefish simply find a reef, find members of the opposite sex, and then 'bombs away', with eggs scattered widely over the reef, many missing the prime habitat for egg incubation. They produce many times more eggs per female as lake trout so can afford to be a bit less careful with where the eggs go.
Thunder Bay, until very recently, was a center for whitefish reproduction in Lake Huron. It also hosts several spawning sites for lake trout. And both species are using the artificial spawning reefs newly placed, just for them, near the cement plant channel.
So why are rock reefs so important to spawning? Actually, there are several ingredients to a good spawning site. Especially important are: 1) excellent water quality; 2) clean, rocky
substrate that includes lots of space between the rocks where the eggs can settle, be protected from predators, and get plenty of water circulation to oxygenate the eggs, and 3) the right kinds and quantities of food for the newly hatched fry. Until about 1990, Thunder Bay was blessed with all three of these.
Here is why not just any rock reef will do. The eggs are laid in fall but do not hatch until .about March. When hatched, the fry carry around yolk sacs, which bulge out of their abdomens and sustain them until they grow large enough to begin feeding on their own. Meanwhile, they remain in the protection of the rocks they hatched in. If the rocks fill with sediment or the water supply loses its oxygen the eggs and fry suffocate. So, reefs that support trout and whitefish reproduction are special. The State tries hard to protect them from dredging spoils and pollution.
Sadly, zebra and quagga mussels invaded Lake Huron in the 1990s and forever changed Thunder Bay's reefs by robbing them of the third ingredient of prime spawning habitat: the right kinds and quantity of food for the fry. Whitefish fry hatch at a very small and delicate size, less than half the length of a sewing needle. Their mouths are miniscule, meaning they can eat only the tiniest zooplankton, the lake's microscopic drifting animal life. Turns out, tiny plankton is just what the invasive mussels eat. So, when the yolk sac is used up, whitefish fry now face lean times. And if they survive long enough to graduate to larger prey, that's almost gone too.
Most of us have never heard of a shrimp-like critter called Diporeia, but we should have been paying attention. Because when the mussels invaded, Diporeia quickly disappeared. Diporeia, which numbered in the thousands per square yard in Thunder Bay, were the dietary staple of whitefish, young and old, but now they are gone. With no food supply, there is little chance whitefish can survive their first couple of years after hatching. Lake trout are much bigger when they hatch, and their mouths are so big they can eat stuff way out of proportion to their body size. So, they skip the plankton and go straight to Diporeia or, since the mussel invasion, to other small animals such as aquatic insects. Their ability to adapt and switch to different foods is one reason lake trout are slowly recovering in Lake Huron, and whitefish are in steep decline.
Nearly 30 years ago the DNR partnered with other agencies to tag whitefish at spawning sites all over Lake Huron. I will never forget those frozen November mornings, with whitefish porpoising around our boat, as we dipped big whitefish out of trap nets set near Sulfur Island. Thousands of whitefish were measured, tagged, and released. Those tags from Thunder Bay were later reported all over Lake Huron, especially from Harrisville north to the UP and east into Ontario's North Channel. Clearly, at that time, Thunder Bay was a primary source of whitefish for all of Lake Huron, especially the northern half. Those days
are no more and can only return if the invasive mussels loosen their grip on Lake Huron's food supply.
Be sure to visit the Besser Museum Fishery Heritage Exhibit next summer, where you can learn more about our shared Great Lakes fishery heritage.
Jim Johnson worked as a research biologist at the DNR’s Alpena Fishery Station. He and the DNR’s research vessel, the RV Chinook, as well as Boat Captain Clarence “Tuffy” Cross retired to the Besser Museum