Science: Alewife impacts on native Great Lakes fishes

Alewives Alosa pseudoharengus, native to the Atlantic Ocean, invaded Lake Ontario via the Erie Canal in the 1860’s, then Lake Erie (1930’s), Lake Huron (1933), Lake Michigan (1949) and Lake Superior (1954).  Lake trout Salvelinus namaycush and burbot Lota lota predation contained alewife population growth.  Eventually sea lamprey predation reduced abundances of these predators, allowing alewives to proliferate.  From the 1960’s to the early 90’s, sea lamprey control and fish stocking programs allowed salmon stocks to grow and this reduced alewife populations.  Madenjian et al. (2008) examined commercial catch records, fish abundances from fishery-independent surveys and other literature to determine the effects of alewives on native fish populations in the Great Lakes.  Trends in fish abundance were compared across lakes. Lake Superior served as a baseline because alewife abundance is extremely low due to the low temperatures of the lake.  Lake Erie was not included in this study because the alewife population has a low rate of survival through the winter.

            The study found that the primary mechanism by which alewives negatively affect other fish populations is by preying on the eggs and larvae of other fishes.  Another mechanism is that a diet rich in alewives can result in a thiamine deficiency, which can cause early mortality syndrome in certain species.  A third, less important mechanism is that alewives outcompete other planktivorous fishes for food.  A proposed mechanism that this study found no evidence for is that alewives get so abundant that they physically displace native fishes from spawning areas.  Alewives had a range of adverse effects on the native fish populations examined in this study:

Slimy sculpin Cottus cognatus:

Probably have no effect.  Fluctuations in predation by juvenile lake trout have been the source of fluctuations in slimy sculpin abundance.

Lake whitefish Coregonus clupeaformis:

No effect.  The decline in their population was caused by sea lamprey and the recovery was a result of successful control of sea lamprey.

Cisco Coregonus artedi     

Minimal, if any, effect.  Degradation of key spawning areas in conjunction with overfishing lead to the collapse of cisco populations. 

Rainbow smelt Osmerus mordax

No effect.  Collapse of populations was connected to increased predation by the recovering lake trout populations.

Bloater Coregonus hoyi

No effect.  Fluctuations in abundance are probably natural cycles.

Yellow perch Perca flavescens

Can have negative effect when alewife density is sufficiently high because they feed on yellow perch larvae.  Declines are also caused by overfishing and predation by other species.

Deepwater sculpin Myoxocephalus thompsonii

Negatively impacted.  Declines connected to alewife predation on larvae and recovery occurred with decline of alewives.

Burbot

Negatively impacted.  Collapse of populations was caused by sea lampreys, but their recovery was slowed by alewife predation on burbot larvae.

Emerald shiner Notropis atherinoides

Strong effect.  Collapse of populations was caused by alewife predation on eggs and larvae.

Lake trout

Strong effect.  Collapse of populations was caused by sea lampreys and overfishing, but recovery has only occurred in areas with low alewife abundance.  Alewives have a strong effect because they prey on lake trout larvae and feeding on alewives by adult lake trout causes early mortality syndrome, reducing egg survival.

Atlantic salmon Salmo salar (Lake Ontario)

Strong effect.  Decline in population not caused by alewives but eventual local extinction was caused by them.   Atlantic salmon feeding on alewives caused early mortality syndrome which reduced fry survival and anorexia in adult fish.

            The authors also found that the severity of the primary mechanism by which alewives affect other species depends on alewife abundance and the degree of spatial overlap with other species.  This conclusion was supported by that fact that the alewives effect on other species was greatest in Lake Ontario, the Great Lake with the largest abundance of alewives.

Implications for anglers:

            Restoration of the native Great Lakes fish communities requires drastic reductions in alewife abundance.  However, alewives are a major component, if not the primary component, in the diets of popular sport fishes like trout and salmon.  So anglers have a choice:  either maintain popular sport fisheries through stocking programs at the expense of native fishes, re-establish native Great Lakes fish communities by eliminating alewives and have a sport fishery consisting of lake trout, lake whitefish and perch, or maintain the status quo.  The status quo persists because management strategies differ between the states and the federal organizations.  The Environmental Protection Agency and U.S. Fish and Wildlife Service aim is to restore the natural ecology of the Great Lakes.  The Wisconsin Department of Natural Resources works to maintain healthy stocks of trout and salmon.  Anglers could be a pivotal voice in the future direction of Great Lakes fishery management.

Selected definitions:

Laurentian:  anything referring to St. Lawrence

Planktivorous:  feeds on plankton

Fry:  recently hatched young fish that have absorbed their yolk sacs          

Madenjian, C.P., R. O’ Gorman, D.B. Bunnell, R.L. Argyle, E.F. Roseman, D.M. Warner, J.D. Stockwell and M.A. Stapanian. 2008. Adverse effect of alewives n Luarentian Great Lake fish communities. North American journal of fisheries management. 28:263-282.