Dr R.T.Leah
School of Biological Sciences
University of Liverpool
| The North American Great Lakes are one of the wonders of the world, yet have suffered massive, deleterious change since the start of the European colonisation of America. A diverse community of fishes has been depleted by overfishing, habitat despoilation, eutrophication, persistent contaminants, introduction of exotic species of fish and mollusc and perhaps even global warming. The community is very diverse to British eyes, the number of lakes and their geographical spread complicate the picture and it is difficult to appreciate the complexity of all of the factors involved. However, it is an extremely interesting and useful example to become familiar with, partly because it contains such a broad swathe of problems, and partly because of the massive effort that is being made by the US and Canadian governments to rectify the situation. The outcome of this is interesting from a scientific perspective as well as from the point of view of general environmental management. The management being attempted is nothing less than a major ecosystem reconstruction. | To date, major improvements
have been made, the ecosystem 'functions' similarly to
the original but has a different mix of species, is prone
to rapid and unstable change and is not as productive of
fish for the fisheries. Although these notes provide a basic introduction and you can read summaries in various text books, you are best advised to follow up the material on websites prepared by various organisations actually involved in fisheries management in the Great Lakes. The situation continues to change at a speed that makes it difficult for textbooks to keep up. Follow the links on aquabiol: http//www.pcweb.liv.ac.uk/fishweb for an easy introduction to the Web at its best. |
Eutrophication is a common consequence of intensified use of our environment. However, in lakes in the modern world, eutrophication does not normally happen in isolation: there can be many more factors involved. Perhaps the best example of this are the North American Great Lakes which have been greatly changed in many ways over the last 300 yrs.
Increased nutrient input into a waterbody will have a large impact on primary production and the fish biomass derived from it. Partly, increased nutrient supply increases the amount of primary production that can feed through to fish production, and partly it changes environmental conditions that can interfere with the ecology of particular species, eventually causing their complete loss. The general changes resulting from eutrophication of lakes can be summarised as taking the fish community from one dominated by fish typical of cool, clear well oxygenated waters such as salmonids through percids to a community dominated by fish tolerant of warm de-oxygenated water such as the cyprinids. This happens to also result in a shift from commercially valuable species to those of less value. From the narrow point of view of fisheries, in waterbodies where the initial conditions are very oligotrophic, then the first modest increase in nutrients may increase fish production and can be considered to be beneficial. This can also be the case in eutrophic waters containing species already adapted to eutrophic conditions. This phenomenon is taken to its extreme in some types of aquaculture where large amounts of fertilisers are used to support intensive fish production.
The case of The Great Lakes
Changes in the fisheries of the American Great Lakes (there are summaries in the latest edition of Moss 1988 or Tyler Miller 1988)
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The lakes are referred to in these notes as 'upper' (L. Superior) to 'lower' (L. Erie) which refers to their geographic position. With the other Lakes (Michigan, Huron and Ontario), they provide the most graphic example of the impact of eutrophication on a fishery although the picture is complicated by the effects of overfishing and other environmental changes. The lakes are very sensitive to inputs of nutrients (and other pollutants) because only ca 1% of the water which they contain flows out in a single year via the Saint Lawrence River. L. Superior shows the least impact of change, L. Erie the shallowest lake shows the greatest. Lakes Michigan, Huron and Ontario are intermediate. |
Fig 1 The North American Great Lakes
(For links to some Background info on Gt Lakes fish follow 
)
The factors having significant impact on the fisheries were overfishing of a selective nature, environmental deterioration in the tributary streams and rivers affecting spawning grounds, the invasion of new species and finally environmental change associated with eutrophication. It is possible that latterly, accumulation of persistent pollutants has also begun to interfere with the breeding of some species.
L. Ontario was the first drainage basin to be settled so that the impact of human interference became apparent there before the other lakes but the effects have gradually moved outwards to the others leaving L. Superior the least affected. The Atlantic Salmon (Salmo salar ) in L. Ontario was the first casualty of overfishing starting in the 1700s and disappearing altogether by 1900. It was probable that the deterioration of conditions caused by logging, sawmills and exploitation of water power in the spawning streams was as much responsible for its disappearance as the fishing pressure. Attempts to re-establish a fishery for the Atlantic Salmon in Ontario have failed principally because without the tree cover, the streams are too warm in summer to maintain suitable conditions. In the upper lakes and after the Salmon fishery had declined in Ontario, whitefish (Coregonus clupeiformis ) and Lake trout (Salvelinus namaycush ) formed the main basis of the fishery, were heavily overfished and had declined in L. Erie by 1940 and L. Huron by 1950. As the trout and coregonid fisheries were fished out, the cisco (Lake Herrings - Leucichthys sp.) populations were also fished out. A fishery for Salvelinus namaycush is now maintained at great expense by a restocking programme.
In the late 1800s two marine species, the Alewife (Alosa pseudoharengus) and the sea lamprey (Petromyzon marinus) entered L Ontario. Both species were native to the adjoining coasts and it is not known for certain why they were only successful in establishing themselves then but it probably had something to do with the changes in the overall environment as well as the building of the canal which bypassed the Niagra Falls. The lamprey, which is parasitic on large fish, reached L. Erie by 1921, L. Huron and L. Michigan by 1930 and L Superior in 1946. The Alewife was somewhat slower. Predation by the lamprey was a factor in the destruction of the populations of larger carnivorous fish which then seemed to allow massive populations of the Alewife to develop. These were so large that huge fish-kills have occurred leaving thousands of tons of fish on the lakeshore causing huge problems of odour and public health. Predation by the Alewife changes zooplankton composition and this in turn affects the algal composition. Overall it is impossible to separate out the impact of eutrophication from the other major influences but it is certain that there is increased de-oxygenation of deeper water and sediment surfaces, increased algal standing crops (total P has increased from 2-3 µgl-1 to 20-30 µgl-1). Currently the fishery is dominated in the lower lakes by percid and cyprinid fish of low commercial value and typical of warm, more eutrophic water. The latest invasive species is the Ruffe (from Europe) which has colonised Lake Superior and more recently has been found in Lake Huron and Michigan (see USG Map)
The watershed of the Great Lakes contains thousands of industries and > 60 million people It is the source of drinking water to 24 million people
1/3 popln of Canada, 1/8 of USA live within the catchment.
Despite their very large size, they are still sensitive to water-borne pollution since only 1% of water entering the system flows out of St Lawrence in an average year
In the 1950s -60s the prevalent view was `Dilution is the solution to Pollution' so that very little regard was paid to the disposal of all types of wastes and the standard of sewage treatment.
As a consequence, the main underlying problem is that of Cultural Eutrophication, which has resulted in - adverse environmental conditions, fish kills, reduction in commercial and sport fishing.
Originally it is likely that there was < 2µg/l of tot P in Lake Superior.
L Superior has changed the least in 200 yrs, greatest in Erie, intermediate in the other three - Michigan, Huron & Ontario
Various changes reflected in fish catches
Reasons for changes in the order that they became significant
Over the past century declined in Huron & Ontario, remained relatively constant in Superior, Michigan & Erie, but this apparent stability masks major changes in quality
Atlantic salmon - only present in L Ontario because of the barrier effect of the Niagara Falls
Fishing for salmon began in 1700s, was reduced by 1880 and ceased by 1900. Although intensive fishing reduced populations, it was changes due to forestry that eliminated species - waterpower, dams, sawmills sawdust, sedimentation after logging
A major effect has also been changes of water temperature after clearing of the original forests because of lack of shading in summer of small headwater streams stop. This has resulted in the failure of attempts to re-introduce Atlantic salmon.
In the upper Lakes and in Lake Ontario, after the salmon declined, whitefish (Coregonus clupeaformis) and Lake trout (Salvelinus namaycush) were both heavily over-fished - Lake Erie by 1940, Huron by 1950s. Lake trout fishery now maintained by annual stocking
Gradually shift to less valuable Ciscoes (Leucicthys artedi - Lake Herring) + others in deep water and were progressively fished out
Currently depend on percids and other fishes favoured by the changes taking place
Lake sturgeon (Acipenser fulvescens) - deliberate removal because of damage to nets plus over exploitation for caviar isinglass etc + changes in spawning habitat
particularly vulnerable with slow growth and late sexual maturity - even following a ban on commercial fishing - only common in parts of Lake Huron
The changes in tributary streams that affected salmon reproduction in L Ontario, became widespread elsewhere in late 19th and early 20th centuries. Commercially exploited spp - entered streams to breed - easy to capture during congregation below dams etc
Between 1860 and 1880 - two marine species entered L Ontario, the sea lamprey (Petromyzon marinus) & the alewife (Alosa pseudoharengus). Both could have entered previously up the Saint Lawrence but possibly environmental conditions are now more suitable for them after recent anthropogenic change. However, there was also a canal built in 1880s to connect the Hudson River, New York to the Great Lakes system. Later, the Erie and Welland Canals allowed them to bypass the Niagara Falls
The Lamprey reached Lake Erie by 1921, Lake Huron & Michigan by 1930s & Lake Superior by 1946.
The Alewife generally lagged behind, reaching Erie and Huron in 1931-33, Michigan in 1949 and Superior in 1953
Both species need deep water for part of their life cycles. Possibly as a result, neither species are abundant or problematic in Erie
Pattern of damage was one of parasitism of lamprey on large lake piscivores - lake trout, burbot (lota lota) and deepwater Ciscoes, then smaller species until the lamprey itself declined.
Now managed by lampricides in spawning streams, physical barriers or the release of sterile males.
Reduction of the piscivores apparently favoured the Alewife - aggressive predation on zooplankters, competition with young piscivores and may even feed on the young fish stopping populations recovering.
It is actually difficult to actually see what part eutrophication has played in the fisheries decline.
Another complication in the story is the colonisation of the lakes by various `exotic' species - Pink salmon - breeding, Coho salmon (Oncorhynus kisutch), the Chinook salmon, O tshawytscha, the steelhead trout (Oncorhyncus mykiss) & brown trout have all been introduced.
The increased populations of large piscivores has resulted in reductions in Alewife, and since 1983 marked increases in zooplankton and transparency of the water
Warm shallow, highly productive
Shallowest of the Great Lakes - smallest volume of water, drainage basin heavily industrialised and contains the largest population of any of the lakes.
c1750 - perhaps 100,000 people in the catchment - 12million 1980s
At one time in the 1960's, massive algal blooms had choked off oxygen to 65% of the lake's bottom
The size of the commercial fish harvest has remained relatively constant (8lbs/acre) over last 100 years, but species composition has changed many times. It is presently dominated by the Rainbow smelt and Yellow Perch
Formerly very valuable - lake trout, whitefish, lake herring, blue pike and walleye
Now none of these are abundant, some are totally absent
Summer temperature has increased, nutrients increased several fold
Species dominance has changed to opportunistic species
Total harvest has not increased as much as might have been expected - accelerated enrichment - reduced by oxygen deficits
Since 1972, $15 billion dollars has been spent jointly by Canada and the US on cleaning-up. Eutrophication has been controlled, with all the lakes meeting Phosphorus targets by 1986. The most urgent remaining problems are those to do with toxic wastes.
-decreases in phosphate, coliform bacteria, algal blooms increases in dissolved O2 and commercial and sport fishing
The rate of cultural eutrophication has been slowed in most areas and even reversed in some
All the lakes except Erie and Ontario met joint US and Canadian goals for reduction of phosphorus by 1986.
Only 8 out of 516 swimming beaches remained closed because of pollution by 1986
Results of discharges from point sources - building and upgrading sewage works
improved treatment of industrial wastes
banning/limiting of use of phosphate in household cleaners and detergents and water conditioners in the Great Lakes basins
Remaining problems
Toxic contaminants - particularly Lake Erie & Ontario
These are best illustrated by the objectives of the Great Lakes Fisheries Commission for one of the Lakes - Lake Huron is given as an example below. Similar plans exist for all of the lakes, you can track the original documents from Fishweb. From the objectives it is easy to appreciate the complexity of what is being attempted.
Objectives for the Management of Lake Huron
The overall objective for Lake Huron is to:
Specific objectives include:
References on Changes in The fish community:
There is a summary in :
Moss, B. 1988 Ecology of Freshwaters (2nd Edn)
Christie, W.J., Scott, K.A., Sly, P.G., & Strus, R.H. 1987 Recent Changes in the Aquatic Food Web of Eastern Lake Ontario Can. J. Fish. Aquat. Sci. 44: 37-52
Eck, G.W. & L. Wells 1987 Recent Changes in Lake Michigan's Fish Community and Their Probable Causes, With Emphasis on the Role of the Alewife (Alosa pseudoharengus). Can. J. Fish. Aquat. Sci. 44: 53-60
MacCallum, W.R. & Selgeby, J.H. 1987 Lake Superior Revisited 1984 Can. J. Fish. Aquat. Sci. 44:23-36
References on Problems with persistent contaminants:
Evans, M.S. (Ed.) 1988 Toxic Contaminants and ecosystem Health: A Great Lakes Focus. Wiley, New York.
Gilbertson, M. 1988 Epidemics in Great Lakes birds and mammals caused by chemicals. In Evans, M.S. (Ed.), Toxic Contaminants and ecosystem Health: A Great Lakes Focus. Wiley, New York, pp 133-152
Gilbertson, M. & Fox, G.A. 1977 Pollutant associated embryonic mortality of Great Lake Herring gulls. Environ. Pollut. 12: 211-16
Harris, H.J., Sager, P.E., Regier, H.A. & Francis, G.R. 1990 Ecotoxicology and ecosystem integrity : The Great Lakes examined. Environ. Sci. Technol., 24: (5) 598-603
Peakall, B.B. & Fox, G.A. 1987 Toxicological investigations of pollutant-related effects in Great Lakes gulls. Environ. Health Perspect., 71: 187-93.
Summary of The Main Native Fish Species in the Great Lakes:
| Lake trout | Salvelinus namaycush |
| Burbot | Lota lota |
| Walleye | Stizostedion vitreum |
| Sturgeon | Acipenser fulvescens |
| Lake Whitefish | Coregonus clupeformis |
| Shallow water Ciscos (Lake Herring) |
Coregonus artedii |
| Deepwater Ciscos | Coregonus spp |
| Slimy sculpin (deepwater) |
Cottus cognatus |
| (shallow water) | Cottus bairdii |