A cross-basin overview reviews status and trends of prey fish from 1978 to 2016.
There were massive changes in the Great Lakes fish communities during the 20th century. During that time proliferation of sea lamprey, alewife, and smelt occurred. In the mid-20th century the collapse of native fish communities, such as lake trout and ciscoes occurred. In the late 20th century there was stocking of trout and salmon; the invasion and proliferation of zebra mussels, quagga mussels, spiny waterfleas, and round gobies; declines in Diporeia (small, shrimp-like crustacean), alewife, and rainbow smelt; and the oligotrophication of Lakes Huron, Michigan, and Ontario because of low phosphorus inputs and the cropping of phytoplankton by quagga mussels. An oligotrophic lake has a deficiency of plant nutrients, usually accompanied by an abundance of dissolved oxygen.
Given this scenario questions are asked on how similar or different are the changes in fish communities across the Great Lakes and what could be causing these changes? Michigan Sea Grant and Michigan State University Extension recently held an educational session at the Michigan Fish Producers Association Annual Conference. At the conference Chuck Madenjian of the U.S. Geological Survey Great Lakes Science Center discussed this topic and reviewed data prepared by his colleague Owen Gorman with other contributors from U.S. Geological Survey, Ohio Department of Natural Resources, New York Department of Environmental Conservation, Pennsylvania Fish and Boat Commission, and Ontario Ministry of Natural Resources and Forestry. Here is a summary of his presentation on Great Lakes prey fish:
Assessments of Great Lakes prey fish stocks have been conducted annually by the U.S. Geological Survey since the 1970s using bottom trawl surveys. The focus of the surveys has been on the prey species cisco, bloater, rainbow smelt, alewife, and round goby. Total prey fish (alewife, rainbow smelt, bloater, and cisco) biomass declined during 1978-2016 in Lakes Superior, Michigan, and Huron. Lake Ontario is now different based on a new correction factor and prey fish biomass was not available for Lake Erie.
There was a synchronous decline in coregonid (whitefish, cisco or lake herring, bloater, kiyi) biomass in Lakes Superior, Michigan, and Huron during 1978-2016 with peak biomass occurring during 1989-1992. Lake Huron showed a coregonid rebound during 2008-2012. Predation does not appear to be the primary driver of bloater dynamics during 1978-2016. Some fishery biologists believe predation on bloaters by salmon and trout is more important nowadays than during the 1980s and 1990s, but most of the diet data do not support this contention. There may be population-intrinsic factors (sex ratio); changes in climate patterns; changes in trawl catchability over time due to changes in bloater behavior or increased water transparency in Lakes Michigan and Huron.
In Lakes Huron and Michigan there was a synchronous decline in alewife biomass during 1978-2016. Alewife is the dominant prey fish in Lakes Huron, Michigan, and Ontario. It is rare in Lakes Superior and Erie. Predation has been the primary driver of alewife dynamics in Lake Michigan since the 1960s and it is likely the main driver of alewife dynamics in Lakes Huron and Ontario as well.
Rainbow smelt had a synchronous decline in Lakes Superior, Huron, Michigan, and Ontario during 1978-2016. Lake Superior peaked earlier than the other lakes in 1978. In these four lakes, rainbow smelt was an important prey species before the mid-1990s and is now a minor prey species. Predation appears to be the primary driver of rainbow smelt dynamics in Lake Superior but not in Lake Michigan.
Round goby biomass increased in Lakes Michigan, Huron, Erie, and Ontario during the 1990s or 2000s, then peaked, perhaps even decreased somewhat, and appears to have leveled off in all four lakes. Further increases in round goby biomass are not expected. Round gobies in Lake Superior are mainly limited to harbors. Round goby populations in Lakes Michigan, Huron, Erie, and Ontario now appear to be under some degree of predatory control as they are fed upon by smallmouth bass, lake whitefish, burbot, lake trout, brown trout, yellow perch, other fish and birds. There are relatively high annual mortality rates (> 60% each year) in open waters of Lakes Michigan, Huron, and Erie.