Great Lakes Puzzles (or Lessons) for Ecologists.
The Great Lakes represent a model of human interference that could stand for many other water bodies on each continent, as we approach a crucial moment for our oceans, rivers and even marshes. The terrible pollutions that have wracked these water bodies are similar to many others, but have only taken place relatively recently. That means we can study anthropogenic effects more exactly, even though most of our effects are very modern by nature.
Rainbow smelt in the Lakes have been a prey fish since 1912, sustaining many predators, and also predating themselves. The walleye, Sander vitreus and salmonids, such as brook trout, Salvelinus fontinalis and various stocked Salmo species all depend on them and other young fish. Therefore, their decline is of prime importance to the recent ecosystems of all Great Lakes and the St Lawrence. The recent increase in the population of young smelt has stirred researchers into action. Indianas Purdue University (US) scientists, led by Zach Feiner and Tomas Höök, studied the situation for the 4 decades preceding 2014 and found that 2-month-old fish have been greater in numbers since the turn of the millennium. They publish in the Journal of Great Lakes Research.
The adult population of rainbow smelt is still declining however, which would lead us to think that adult predation of their own young explains these numbers. In Lake Michigan, for example, adults are now 2 (4.5cm) shorter than in 1970, but the fry are still surviving better than before. Now for the adult decline. They could be vitally affected by the other invading species that clog the Lakes. Quagga and zebra mussels have been reported massing and feeding on 100%of the plankton for many years. The need to find their zooplankton food will necessarily take the adults into deep water, away from the spawning area. Predation there or simply lack of food will be one explanation of the adults decline.
Problems abound with fishermen and invasive fish like these around the earth. Introductions cannot work in a native ecosystem, but the Great Lakes present an almost completely artificial situation. Anthropogenic change has attempted to solve problems as well as causing them, probably at the same time. When a plague of these smelt (up to 40,000 per hectare) develops in some locations, both human and animal food have been supplied for commerce. Wildlife obviously did the same, in exploiting a food resource, but the fish resource was at certain times bountiful in North America.
This species belongs in a migratory habitat, except for few locations where it is naturally landlocked. With natural restrictions, and, luckily,, fish ladders which restrict its swimming upstream, the animal has been exploited as natural food by coastal communities, but of course it is not a
big fish and so cant attract people who use rods rather than big nets! Global warming and increasing pollution have an influence nobody seems to be acknowledging properly, while dredging channels to improve flow (and help more invaders to enter) seems a preoccupation around Lakes Michigan and Erie.
Lessons to be learnt: maybe we would be better to consider much more maintenance of original ecosystems in lakes, wherever it is still possible. From Asian experience, in Baikal to the Caspian Sea, experimentation proves unsuccessful, while hopefully there are some African or South American experiences where fishermen have been happy to fish native fishes, and not concern themselves with bigger, better or more rapid growth!