The alewife population in Lake Michigan has declined by 80 percent in the past 20 years, and the fisheries biologists who have been managing the lake during those two decades don’t know whether to laugh or cry.
Back in the early 70s, when alewives were overrunning Lake Michigan and there were no native predators around to control them, the biologists began introducing coho salmon, a Pacific species, as an agent of alewife control. Later, chinook and steelhead salmon and brown and rainbow trout were added to the mix, along with hatchery-raised specimens of the native lake trout. These predators have done the job they were intended to do, and that is good news.
But shortly after the first introductions, sport fishermen began to notice that it was fun to catch these hatchery-raised salmonids (both trout and salmon belong to the family Salmonidae). And this led to sales of boats and motors, the development of ever more complex forms of tackle, the construction of marinas, the growth of the charter-boat business, and the opening of numerous shops selling live bait.
Add up all the money generated by these businesses and factor in the restaurant meals, six-packs, tubes of sun block, gallons of gasoline, and motel rooms sold to people on fishing trips, and you have an industry whose economic impact can be measured in billions of dollars. Like any multibillion-dollar business, sport fishing generates substantial campaign contributions, and as a leisure activity that draws large numbers of serious enthusiasts, it also produces organizations capable of flooding a legislator’s mailbox whenever government does something that seems to threaten their hobby.
What worries the biologists is, first, that this entire multibillion-dollar industry depends on the well-being of a dinky little fish that has trouble surviving a cold winter in Lake Michigan. And second, that the biologists are going to have to tell the politicians and the anglers that there may not be as many fish to catch next year as there were last year.
If you saw clumps of alewife carcasses washing into the quiet corners of our harbors last year, you might find it hard to believe that populations are down as much as 80 percent, but that number is based on the best empirical data available, and you would be hard-pressed to find a biologist who didn’t agree with it.
Alewives are schooling fish. As their numbers decline their schools get smaller and scarcer, but even now, if you see alewives at all, you are likely to see a whole bunch.
And you may still see a fair number dead on the beach. However, the numbers of visible ex-alewives are much smaller than they used to be, and the forces that turn them belly-up are much different too. Back in 1967, the huge mounds of rotting alewife carcasses on our beaches were the result of die-offs created by populations so large they outstripped their food sources. The much smaller die-offs we see now are the product of sudden temperature changes in the lake waters, changes created by our highly variable weather. Native fish can generally handle these changes, but alewives are an alien species that evolved in the open waters of the Atlantic, and the lake often catches them by surprise.
Alewife populations are currently so low that last year Illinois, Indiana, and Wisconsin cut the number of chinook salmon released from their hatcheries by 25 percent. An agreement among the three states calls for continuing at that level through 1995. Michigan has not entered into this agreement because that state is conducting a study of chinook that demands a constant rate of stocking.
Computer modeling of fish populations strongly supports the idea that alewives are not going to come back any time soon. Continuous heavy stocking of predators has built up their numbers to such high levels that even a complete shutdown of the hatcheries wouldn’t have any effect in the short term. And some of the introduced fish are reproducing successfully, so they could continue in the lake indefinitely.
Alewives eat zooplankton, the tiny animals that drift or float in the waters of the lake. The original alewife population explosion had such a powerful effect on plankton numbers that native planktivorous fish suffered major declines. Those declines have now been reversed. Populations of deep-water sculpins, yellow perch, and bloater chubs are all up. The chubs are the mainstay of the modest amount of commercial fishing that still goes on in Lake Michigan.
The biologists have been hoping that as alewife numbers go down, the predators will switch to native species like the sculpins, perch, and chubs. That way stocking can continue at high levels, anglers can catch all the fish they want, and everybody will get rich. But so far the hatchery-raised salmonids have not made the switch. Chinook, in particular, are still concentrating on alewives, and there is reason to suspect that this will not change. Alewives congregate at medium depths in open water, where the chinooks also like to hang out. The native lake trout like deeper water and so do chubs, so making the switch could be easier for the trout.
The Great Lakes Fishery Commission, a binational organization set up by treaty between the United States and Canada, is currently sponsoring a major investigation into the future of fishing on the Great Lakes, with special attention to lakes Michigan and Ontario, the two in which stocking has been most intense. The commission has formally adopted the goal of encouraging the creation of healthy aquatic ecosystems “that are based on foundations of naturally reproducing fish populations and self-regulating fish communities.” And it is further committed to “the conservation of biological diversity through rehabilitation of native fish populations, species, communities, and their habitats.”
Getting there from here is the hard part. In general, the introduced salmonids are reproducing much better than the hatchery-raised lake trout, the species that was once the lake’s top predator. Nobody knows exactly why. It could be because the alien salmonids are anadromous species. That means they live in the open lake but swim up rivers to spawn. An anadromous steelhead looking for a place to spawn has only to swim along the lakeshore until it finds a river mouth.
Lake trout spend their entire lives in the open lake and lay their eggs on rocky reefs. These reefs are scattered around the lake–most are in the north–and there is no pattern to their distribution. Therefore there is no way except pure chance for a lake trout to find one.
Fish larvae that hatch naturally on a reef may imprint on the location early in life. It may be that the age when imprinting can occur is past by the time hatchery fish are released. There are experiments under way that involve planting eggs or very young fish on suitable reefs, but it takes years for lake trout to mature, so we won’t know the results of those experiments for some time. We do know that in Lake Superior, where wild lake trout were never wiped out, the native wild fish have a much higher spawning rate than the hatchery-raised fish that have been released there.
The presence of such toxics as PCBs and DDT in Lake Michigan may be the factor that is inhibiting reproduction, but we don’t know that for sure at this point. Siltation and the detritis created by algae blooms may also be covering reefs that were once lake trout spawning grounds.
Twenty years ago, in the first flush of success of the stocking program, some fisheries biologists began to feel that they could not only manage water, they could walk on it. Two decades of experience have made them more circumspect. The stocking program did accomplish the goal that was set for it. Alewives will never be eradicated, but they are no longer a major factor in the ecosystem. The native planktivores have come back.
But 20 years of experience has taught us that an alewife-salmon system is not stable. We have also learned, yet again, that ecological change occurs on a different scale than human time. It may take centuries for the effects of a particular management scheme to work their way through a system. Twenty years is the blink of an eye.
It is wise in this situation to keep a long-term goal in mind and to judge today’s actions not by how well they address an immediate problem but by how they further that goal. You can’t think of just one thing at a time when you are dealing with nature.