Learn more about declining Great Lakes prey fish populations

A cross-basin overview reviews status and trends of prey fish from 1978 to 2016.

The research vessel Sturgeon conducts prey fish trawl surveys on the Great Lakes. Photo: Great Lakes Fishery Commission

The research vessel Sturgeon conducts prey fish trawl surveys on the Great Lakes. Photo: Great Lakes Fishery Commission

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 ResourcesNew York Department of Environmental ConservationPennsylvania 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.

Coregonids

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.

Alewife

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

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

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.

Great Lakes net-pen aquaculture—real and perceived risks to the environment

Michigan Sea Grant addresses environmental issues surrounding net-pen aquaculture in the Great Lakes at recent Annual No-Spills Conference.

Great Lakes net-pen aquaculture—real and perceived risks to the environment

In the last several years there has been a great deal of discussion about net-pen aquaculture in the Michigan waters of the Great Lakes. Much of the attention about Great Lakes net-pen aquaculture is the generation of large quantities of fish waste from these fish production operations as well as the consequences if these fish escape into the environment. The main issue with fish waste is the release of phosphorus which is the growth limiting nutrient for primary production in freshwater ecosystems. Although some phosphorus is necessary to drive the freshwater food chain, concern arises when excess amounts of phosphorus are available which can result in significant algal blooms and other aquatic plant growth. In addition there is a concern about fish diseases and genetics, which may be the consequence of the interaction of fish raised in Great Lakes net pens and native fish in the surrounding environment.

Discussing environmental issues

To address these concerns Michigan Sea Grant was invited to speak at the 28th Annual No-Spills Conference in January 2018, to discuss environmental issues surrounding net-pen aquaculture in the Great Lakes. Currently there are seven net-pen aquaculture operations that exist in northern Lake Huron on the Canadian side of the lake. These operations are sustainably producing more than 5,000 tons of rainbow trout per year with some being sold in retail markets in Michigan. They provide 340 direct and indirect jobs with a $100 million contribution to the Canadian economy. These net-pen aquaculture operations take up a small footprint in the environment; one of these operations that produces 500,000 pounds of rainbow trout per year would fit into an average size Michigan marina.

Fish disease risks and genetic dilution can be minimized

For Great Lakes net-pen aquaculture to be environmentally sound it must have practices that prevent disease transmission and escapement of fish into the wild, as escapees could affect the genetic integrity of surrounding fish populations. These operations must also be non-polluting with minimal and recoverable impacts. With regards to fish diseases, the commercial aquaculture industry is highly regulated and is held to the same standards as state and federal hatchery programs. Fish disease risks are minimized and prevented through regulation, biosecurity, and best management practices.

In 2014 the state of Michigan stocked more than 20 million fish, produced from gametes collected from wild fish. This equated to 325 tons of fish stocked, 9 different species, 370 stocking trips, 732 stocking sites, with 100,000 miles of travel from several fish hatcheries. In comparison Canadian net-pen operations in Lake Huron typically stock one cohort, certified as specific pathogen free, then raise the fish to harvest and truck them one way to a fish processing facility. The net results are that Michigan hatcheries have a much higher risk of disease transmission than the current system for growing trout in Canadian net pens.

The Great Lakes already have rainbow trout which are non-native to the region. They were introduced by fishery management agencies years ago and many of these fish are now naturalized, spawning on their own in local rivers, with additional enhancement from government fish hatcheries. Rainbow trout produced in Great Lakes net-pen operations can be female triploids which are sterile and will not reproduce should they escape into the environment. So the risk of genetic dilution can be eliminated by use of these female triploid rainbow trout.

Low phosphorus, digestible fish diets help minimize phosphorus waste

During the height of the Great Lakes net-pen aquaculture discussion there were media reports that a typical net-pen operation with 200,000 fish would produce as much waste as a city of 65,000 people. In reality a city of 65,000 people would produce 21 times more fecal matter than a 200,000 fish net-pen operation. This same city would produce 5 times more phosphorus compared to the net-pen aquaculture operation. The city would also generate 24 kg/yr of E. coli with none coming from the net-pen operation.

Canadians have had net-pen aquaculture operations in their northern waters of Lake Huron since 1982. To help address the issue of excess phosphorus discharge from freshwater net pens, Fisheries and Oceans Canada completed a study on Freshwater Cage Aquaculture: Ecosystems Impacts from Dissolved and Particulate Waste Phosphorus. Fish receiving digestible phosphorus in specific amounts to meet their growth requirements excrete only small amounts of dissolved phosphorus. Dissolved phosphorus is most often the form of concern in impaired waters. The other form of phosphorus excreted from fish is particulate phosphorus which settles to the bottom sediments. The particulate phosphorus which accounts for the majority of the waste from net-pen operations is transported to the bottom sediments and is not immediately available for uptake into the ecosystem. In sediments it can be consumed by the benthic organisms and enter the aquatic food chain. Both dissolved and particulate phosphorus wastes produced by fish are the results of the diets they consume. The development of low phosphorus, highly digestible diets has been a tool to help minimize phosphorus waste by aquaculture operations.

The Fisheries and Oceans Canada study found that based on net-pen aquaculture production in northern Lake Huron in 2006 contributed about 5 percent of the annual total phosphorus loading to the North Channel. The study concluded that the likelihood of phosphorus additions to the environment from net-pen aquaculture operations resulting in eutrophication to Canadian freshwater environments under the current level of fish production can generally be characterized as “low.” The greatest concerns for phosphorus are in the nearshore areas where excess aquatic plant growth can foul the shorelines. In contrast, offshore phosphorus loading is of less concern and higher phosphorus concentrations may be considered a means to help mitigate declining populations of forage fish and the poor condition of sport and commercial fish species.

MSU prof seeks crowdfunding support for Great Lakes fish diet research

You can be a part of this important study by donating to support student researchers analyzing stomach samples from Lake Michigan and Lake Huron.

As we all know, the Great Lakes have changed a lot in the last decade or so. Alewife have declined, round goby are increasing, and lake trout and walleye continue to recover. Chinook salmon, the heart of Lake Michigan’s fishery, have fluctuated in numbers in the past few years, and have collapsed in Lake Huron. Our fisheries agencies must make informed decisions regarding stocking and levels to support both fisheries and conservation goals. These decisions are based in part on what those predators are eating. What predators eat is an excellent indicator of ecosystem health, and can help tell us how sustainable the fishery is.

With the tremendous help of recreational anglers, MSU together with state, federal, and tribal agencies have collected nearly 2,000 predator stomachs from around Lake Michigan and Huron. We need help to be able to analyze all of them, particularly those from Lake Michigan. MSU has a wealth of potential help in terms of undergraduate students eager to gain valuable research experience. However, funding is needed to pay these students for their work.

Would you help by contributing to this research effort?

With the help of MSU CrowdPower, any donations made at the website will go directly to the predator diet study. Any donation will help, and all donations are tax deductible.

Want to stay up-to-date on the project? 

We have several other ways to connect including:

Ludington Regional Fishery Workshop

Event Date: 1/13/2018

January 13, 2018

West Shore Community College
3000 North Stiles Road
Scottville, MI 49454

Details

Lodging

There is a block of room secured at the Ludington Holiday Inn Express for the night of January 12th. Double rooms are $75/night and are first come first serve. 

Group Code: MSU
Group Block Name: Fisheries Workshop
Reservations: (231) 845-7311

Sea Grant report on Asian carp includes educational resources

Great Lakes conservation groups will find a wealth of resources in this new publication.

By Dan O’Keefe

The Silver Carp is one of four Asian carp species that threaten Great Lakes waters.

The Silver Carp is one of four Asian carp species that threaten Great Lakes waters.

The Great Lakes Sea Grant Network, in support of the Asian Carp Regional Coordinating Committee, just released a report that contains a variety of resources for anyone working on education and outreach related to Asian carp. The report contains sections that provide basic information in addition to helping readers sift through the large amount of information available to find the best outreach products for their audience.

the cover of the report is shown

Understanding the threat

The new report details the four species of Asian carp that pose a threat to Great Lakes waters: Bighead Carp, Silver Carp, Black Carp, and Grass Carp. Each species is a concern, but Bighead Carp and Silver Carp get the most attention because they are filter feeders that eat plankton. This could result in direct competition with native gamefish or indirect effects if baitfish populations are harmed. Scientists are now employing a variety of techniques to learn more about these fish, and the report explains some of the headline-grabbing methods like eDNA monitoring and DIDSON sonar imaging.

Educational resources

The Sea Grant report includes a state-by-state list of fact sheets, articles, brochures, posters, online videos, and other materials related to Asian carp outreach. This is a great place to start if you are looking for materials to distribute at a boat show, club meeting, or other event. In the “Analysis of Education and Outreach” section, the report provides a quick reference chart that organizes materials by audience and message.

PowerPoint Presentation

In addition to a list of available materials, the report includes a set of slides that can be downloaded and used by educators around the Great Lakes region. Slides include basic life history information for each species, potential for economic and ecological harm, control attempts, and an overview of existing research and research gaps. Each slide contains comprehensive presenter notes, and the slide set can be modified to suit your audience.

Seafood HACCP Training Course

Event Date: 12/5/2017
End Date: 12/7/2017

A Seafood Hazard Analysis Critical Control Point (HACCP) Training Course that is being coordinated by Michigan Sea Grant, Michigan State University Extension, and the Great Lakes Indian Fish and Wildlife Commission will be held December 5-7, 2017 at Ojibwa Casino Resort in Baraga, Michigan. All fish processors are required to take this training if they are not currently certified.

Hazard Analysis Critical Control Point (HACCP) consists of identifying safety hazards, determining where they occur, monitoring these points and recording the results. HACCP involves day-to-day monitoring of critical control points by production employees. The Seafood HACCP regulation that is enforced by the U.S. Food and Drug Administration is based on the belief that commercial fish processors can understand the food safety hazards of their products and take reasonable steps to control them. Commercial fish processors are required either to obtain formal training for one or more of their own employees or to hire trained independent contractors to perform the HACCP functions.

The HACCP regulation requires processors to keep extensive records of processing and sanitation at their facilities.

Those completing the course will receive a Seafood Alliance HACCP Certificate issued through the Association of Food and Drug Officials that is recognized by agencies regulating fish processors.

For registration information please contact Ron Kinnunen at kinnune1@msu.edu

New video shows anglers how to remove stomachs for fish diet study

Researchers are trying to learn more about what trout, salmon, and walleye are eating in lakes Huron and Michigan. Anglers can help by donating stomachs from their catch.

Fisheries scientists around Lake Michigan and Lake Huron are working on a project led by Dr. Brian Roth at Michigan State University to understand what, and how much, different species of fish are eating. Invasive species such as round goby have damaged the environment, but they also provide food for some gamefish. Quagga mussels have reduced the amount of food in open water areas, but they also provide a food source for round goby.

Last year, much debate focused on alewife, an open water baitfish. This new study should provide better information regarding how many alewife are being consumed by different species including Chinook salmon, coho salmon, Atlantic salmon, lake trout, steelhead, brown trout, and walleye. Some species, such as Chinook salmon, rarely switch to other food sources. On the other hand, fish such as lake trout, brown trout, and walleye readily switch to feeding on bottom-dwelling fish like round goby. Sometimes.

This comprehensive effort will attempt to figure out when and where certain gamefish take advantage of round goby, alewife, and other food sources including invertebrates like opossum shrimp and spiny water flea. In order to get an adequate number of fish from all seasons of the year and all regions of the two lakes, scientists are hoping anglers can pitch in and contribute stomachs for the study. 

How to participate

  • Watch this short video to learn how to collect stomachs. It is very important not to bias the study by collecting only full (or only empty) stomachs.
  • If you are collecting stomachs after a fishing trip, be sure to collect ALL stomachs from each species that you are collecting.
  • It is not necessary to collect stomachs from every fishing trip you take, but stomachs from 2-3 trips per month would be very helpful.

What, when, and where to collect

  • What: Stomachs from all trout and salmon species, and walleye.
  • When: Now through the end of the 2019 fishing season.
  • Where: All waters of Lake Huron and Lake Michigan, including large bays like Saginaw Bay and Green Bay, but not including rivers or drowned rivermouth lakes.

What to focus on

Creel census clerks with Michigan DNR, biotechs funded by U.S. Fish & Wildlife Service, and US Geological Survey biologists will be working to collect stomachs at access sites and in conjunction with major fishing tournaments. Anglers can help these agencies to fill in the gaps by contributing stomachs from less-common species, early- and late-season catches, and fish caught at night or in regions that do not get as much coverage by agency personnel.

Some ideas to focus on include:

  • early-season brown trout
  • Green Bay walleye
  • all species in northern Lake Michigan from Grand Traverse Bay north to Manistique
  • mid- to late-summer salmon and trout from St. Joseph north to Saugatuck

All species from all areas of Lake Michigan and Lake Huron are appreciated, but these focus areas are particularly important because angler-submitted stomachs may make a critical difference in providing enough stomachs to meet sample size targets.

Materials for stomach collection include:

Data tags, list of freezer drop sites, video and full instructions are also available at www.michiganseagrant.org/diet.

2017 Summer Discovery Cruises

Event Date: 5/22/2017
End Date: 9/15/2017

Do you want to learn about the Great Lakes by being on the Great Lakes? If so, you will want to learn more about our 2017 Summer Discovery Cruises season!

For the 16th summer, Michigan Sea Grant Extension will provide Michiganders (and visitors to Michigan) with the opportunity to learn about the Great Lakes by being on the Great Lakes. Cruises depart from Lake Erie Metropark, with cruises on the lower Detroit River and Lake Erie, and Lake St. Clair Metropark, cruising Lake St. Clair.

The 2017 season offers more than 20 educational cruises around themes such as Fisheries, Wildlife, Wetlands, Shipwrecks, Lighthouses, Weather, Shipping and more. Cruises for educators wanting to enhance the use of Great Lakes content in their teaching are also provided, with stipends.

Some of the exciting cruises for the 2017 season include:

Lake St. Clair Fisheries – This is not a fishing cruise, but it definitely is a “fishy” cruise! Learn first-hand about the fish that are found in Lake St. Clair, many of which are available for hands-on examination during the cruise. We will be joined by a Michigan DNR Fisheries Biologist and rendezvous with their research vessel while out on the lake to observe fish tagging, measuring and other research operations.

Warfare on the Waterfront – The War of 1812, World War II, and even the American Civil War have all shaped the Detroit River and western Lake Erie. Long after the end of hostilities, remnants of this military presence can still be found. Join an Interpreter for an in-depth look at these conflicts, their sites and stories, and see how they impacted the region and the world.

Shipwreck at Sugar – Just under the waves off a crumbling Sugar Island dock lie the remains of a vessel sank in 1945. Travel with our resident historian to the wreck site to learn about the S.S. Seabreeze, the story of how it got there and the circumstances surrounding its mysterious sinking.

Birds, Boats & Booze (4 hour history cruise) – Many things brought people to the St. Clair River Delta Flats area. The abundant wetlands brought duck hunters and fishing. Wood boats and passenger steamers brought tourism and recreation, and Prohibition brought rumrunners and speakeasies to the region. Spend a little more time in “the flats” with us as we cruise farther up the South Channel and share a little of the past including stories of the big hotels, Tashmoo Park, Chris Craft boat building and more.

Great Lakes Science for Kids – Learn about the ecology of Lake St. Clair or Lake Erie, by using the tools a Great Lakes Scientist uses to determine water quality by studying the plants and animals of the lakes. Try your hand at using a plankton net, bottom dredge, water testing kit, underwater camera, and binoculars to discover the exciting nature of the lake and become a Great Lakes Scientist!

To learn about the Great Lakes by being on the Great Lakes, visit the Summer Discovery Cruises web site at www.discoverycruises.org for complete cruise descriptions, locations, dates and times, as well as directions on how to register for your 2017 Summer Discovery Cruises. Don’t miss the boat!

How much do lake trout and Chinook salmon really eat?

Chinook salmon have been the most important predator in Lake Michigan for decades. With baitfish on the decline, some anglers believe that lake trout are now eating more than salmon.

It takes a lot of food for a lake trout to grow this big, but a Chinook salmon eats much more on an annual basis.

It takes a lot of food for a lake trout to grow this big, but a Chinook salmon eats much more on an annual basis. Photo: Michigan Sea Grant

The food supply in Lake Michigan is not what it used to be. Invasive species like quagga mussels, nutrient reductions in open waters, and high numbers of predatory fish to feed all play a role in “squeezing” baitfish like alewife. The result can be an imbalance of predators and prey. In other words: too many mouths to feed for the amount of food available in open water.

Of course, this is a simplistic way of looking at things. Different species of fish are not the same in terms of the energy they consume and use. They grow at different rates, prefer different water temperatures, and utilize food more or less efficiently.

These factors become important when considering the total number (or biomass) of baitfish being consumed by predatory salmon and trout in Lake Michigan. Many anglers are particularly concerned about the impact of native lake trout versus introduced Chinook salmon. Both species are currently stocked in Lake Michigan, but both species also reproduce naturally.

Chinook salmon are prized gamefish with a very high growth rate and a short lifespan — they typically spawn and die at age 2.5 or 3.5 with a very few surviving to age 4. Lake trout are also a good gamefish, but they do not draw anglers to the lake in the same way that the spectacular fighting ability of the Chinook salmon does. Mid-sized trout are excellent table fare, but large, old lake trout tend to accumulate more contaminants than salmon. Lake trout can also live much longer than salmon (over 20 years), but they grow much more slowly.

Since there is a limited number of baitfish in the lake, there is a limited amount of energy (calories) available to trout and salmon. How does this all play out in terms of the amount and types of prey fish being eaten by lake trout and Chinook salmon?  To find out, I relayed some questions on the topic to two people who have been studying Lake Michigan fish for a combined total of over sixty years: Chuck Madenjian (USGS) and Jory Jonas (MDNR).

How would a lake trout and Chinook salmon compare in terms of the energy they consume each year?

C.M.: For the period of time from age 1 through age 12, annual food consumption by a lake trout in Lake Michigan averages 13 lbs. This estimate is based on the assumption that the annual food consumption for age-10 and age-11 lake trout is similar to that for lake trout of ages 7-9. For the period of time from age 0 through age 3.5, annual food consumption by a Chinook salmon in Lake Michigan averages 42.5 lbs. Thus, Chinook salmon in Lake Michigan are feeding at a rate more than three times higher than that of lake trout.

J.J.: I agree with Chuck’s summary above, but would also add that Chinook salmon grow faster, inhabit warmer waters than lake trout on average, and are much more active. All of these factors lead to increased demand for energy (food) relative to lake trout.

So, on an annual basis a Chinook salmon eats more than a lake trout of the same size. We know that lake trout live longer than Chinook salmon, though. How much does an average lake trout eat over its entire lifetime as opposed to an average Chinook salmon?

C.M.: The answer to this question partly depends on the definition of an entire lifetime for a lake trout. Based on the bioenergetics modeling by Don Stewart and others, an average lake trout consumes 143.3 lbs. of food between the time of stocking as a yearling into Lake Michigan and age 12. A Chinook salmon consumes 147.7 lbs. of food between the time of stocking as an age-0 fingerling into Lake Michigan and age 3.5, when a Chinook salmon is ready to spawn.   

J.J.: Chuck did a nice job of summarizing lifetime consumption of the two species above. When asking a question like this, it is important to consider why it is being asked. Total lifetime consumption of prey does not equate to information valuable in determining sustainability of the system. New year-classes of fish are always being produced and individual species have different life-spans and life-histories. Several generations of alewife and Chinook salmon will have cycled during the life-span of a lake trout. For example, during the lifespan of a lake trout age 12 which consumed 143.3 lbs. of prey there will have been four generations of Chinook salmon each consuming 147.7 lbs. of prey (590.8 total lbs.). Because of fluctuations in births and deaths and the lack of life-span synchrony among species, we typically summarize population levels of predators and prey on an annual basis in order to monitor for changes over time.

Fish are cold-blooded animals, so water temperature must affect how often they eat and how quickly they digest food. Do temperature preferences play a big role when comparing bioenergetics of lake trout and Chinook salmon?

C.M.: Temperature does play a role when comparing bioenergetics of lake trout and Chinook salmon. However, the main driver of the difference in consumption rates between lake trout and Chinook salmon is the difference in growth rates between the two species. In other words, the main reason for the much higher rate of food consumption by Chinook salmon compared with that by lake trout is that Chinook salmon grow substantially faster than lake trout. Average summer temperatures experienced by lake trout in Lake Michigan range from 46.4 to 50°F, whereas average summer temperature experienced by Chinook salmon in Lake Michigan ranges between 53.6 and 55.4°F. Metabolic costs typically increase with increasing temperature, and so Chinook salmon would be expected to have higher metabolic rates than lake trout. Nonetheless, the primary reason for the higher food consumption rate for Chinook salmon compared with that for lake trout is the higher growth rate by Chinook salmon compared with that for lake trout.

J.J.: Chinook salmon are also more active than lake trout, travelling large distances and generally moving around more. Combine higher activity levels with the factors mentioned by Chuck above, including higher temperature occupancy, and you have a higher demand for calories to support Chinook salmon.

Fish need energy to maintain basic body functions, chase down prey, and reproduce. Additional energy can be used for growth. How do lake trout and Chinook salmon compare in terms of their ability to use food energy for growth?

C.M.: Gross growth efficiency (GGE) is equal to growth (increase in weight) divided by the amount of food consumed to attain that growth. Thus, GGE is a measure of the efficiency with which a fish converts food consumption into its growth. According to the bioenergetics modeling by Don Stewart and others, the GGE for a 3.5-year-old Chinook salmon is 13.3%. That is, the 3.5-year-old Chinook salmon converted its food into its growth with a 13.3% efficiency. The GGE for a 12-year-old lake trout is estimated to be 8.0%. Thus, a Chinook salmon is considerably more efficient at converting food into growth than a lake trout in Lake Michigan.

Large, old lake trout are a common catch in central and southern Lake Michigan. These fish might weigh over 20 pounds and be 20 years old or older. Computer models that calculate how many baitfish are being eaten in Lake Michigan treat a 20-year-old lake trout the same as a six-year-old. Does a 20-year-old lake trout really eat only as much as a six-year-old?

C.M.: To answer this question, a growth trajectory for lake trout from ages 1 through 20 would be needed. Stewart et al. (1983) estimated mean weight at age for ages 1 through 10 only, so information on mean weight at age for ages 11 through 20 would be needed to answer this question. According to Stewart et al. (1983), annual consumption of food by an average lake trout in Lake Michigan remained relatively constant at a value of about 17.6 lbs. between the ages of 6 and 10. In other words, annual feeding rate of lake trout did not increase as lake trout age increased from 6 to 10. Mean weight at age 6 was 6.6 lbs., and mean weight at age 10 was 10.6 lbs. Thus, even though the weight of an average lake trout increased by 4 lbs. between ages 6 and 10, annual rate of food consumption by lake trout did not increase between ages 6 and 10 (Stewart et al. 1983). Note that annual weight gain by lake trout decreased between ages 6 and 10. If the annual weight gain (annual growth) continued to decrease between ages 10 and 20, a large increase in annual consumption over ages 10-20 would not be expected.

J.J.: It is true that larger fish on average require more energy than smaller fish, all else being equal. As lake trout age, the annual growth rate is much less, reducing energy demands, as mentioned by Chuck above. In more recent catch-at-age modelling efforts in eastern Lake Michigan, the mean weight of a lake trout at age 6 was 5.7 lbs., at age 10 was 9.7 lbs. (a change of 4 lbs. in 4 years) and was 11.2 lbs. at age 15 (an increase of 1.5 lbs in 5 years). By age 7 most lake trout are spawning so fish age 7-15 should be experiencing similar energy demands for spawning. Despite this, growth rate (body weight added per year) continues to decline as the fish ages.

Studies on Great Lakes salmon and trout bioenergetics were conducted back in the 1980s. Do they still hold true today with so many invasive species in the food web and changes to the strains of lake trout being stocked?

C.M.: Bioenergetics models for Chinook salmon and lake trout are sufficiently flexible such that they can accommodate changes in the Lake Michigan food web and changes in lake trout strains being stocked. Inputs to the bioenergetics models include growth of the fish (predator), temperature regime experienced by the fish, diet schedule for the fish, energy density of the prey, and energy density of the fish (predator). All of these inputs can be adjusted to more accurately reflect changes in the food web or changes in lake trout strains stocked. Bioenergetics model estimates of food consumption by Chinook salmon and lake trout are especially sensitive to estimates of growth by Chinook salmon and lake trout, so changes in growth over time would need to be taken into account when estimating food consumption by these fishes over decades of time. In the laboratory, the lake trout bioenergetics model performed equally well for both Marquette and Seneca Lake strains of lake trout, so lake trout bioenergetics was very similar among strains of lake trout. The Seneca Lake strain does inhabit slightly cooler water than the Great Lakes strains of lake trout, but this slight difference in temperatures between strains had only a small effect on food consumption. Laboratory performances of both the Chinook salmon bioenergetics and the lake trout bioenergetics model are reasonably good. On average, the model estimates of food consumption are within 5% of observed consumption.           

Now we know how much individual trout and salmon eat, but how many baitfish are eaten annually by all predators in Lake Michigan? How did estimated lake trout consumption compare to estimated Chinook salmon consumption on a lakewide basis in 2016?

J.J.: In 2016, lake trout consumed 13.7 kt of prey and Chinook salmon consumed 38.4 kt. Even though numbers of Chinook salmon in 2016 were at all time low levels lake-wide, they consumed nearly 3 times as much forage as lake trout. In 2016, the biomass of Chinook salmon in Lake Michigan was estimated to be 5.0 kt and lake trout 5.9 kt. Just four years’ prior, in 2013, Chinook salmon biomass was substantially higher at 15.7 kt and lake trout were 7.0 kt.

Until now we have only been discussing how many baitfish are being eaten, but we know that Chinook salmon depend almost entirely on alewife while lake trout can eat a variety of prey including round gobies. Have lake trout moved away from eating alewife in Lake Michigan?

J.J.: Lake trout tend to be opportunistic feeders and will take advantage of a variety of prey items, whereas Chinook salmon are more specialized preferring almost exclusively alewife as prey. Since about 2003, lake trout have been taking advantage of a relatively new prey source in Lake Michigan, the round goby. Because of increased public interest in understanding the role of lake trout as predators in Lake Michigan, a variety of new initiatives have begun to better understand this more complex predator. For the last few years, diet collections have been occurring outside of the standard spring assessments which conclude in mid-June, and on broader spatial scales. Preliminary comparisons indicate that there is a seasonal component to lake trout feeding whereby they consume larger numbers of round goby in the spring and increased dependence on alewife as the year progresses. Smelt and bloater have been abundant in the diets of lake trout in the past, but for the recent 5 years over 75% of lake trout diets have been comprised of alewife and round goby. We continue to explore new and more robust methods for keeping up with the changing trends in lake trout consumption. Some of these include evaluation of fatty acid or isotopic signatures which can represent a longer period of lake trout consumption (in the case of isotopes up to one-year). We are seeking funding to conduct broader data collection efforts to better understand changing patterns throughout the lake and in different seasons.

So, we don’t yet know exactly what percentage of Lake Michigan lake trout diet is alewife, but what was the realistic range of possible alewife consumption by lake trout in 2016?

J.J.: It’s still early, but most of us are comfortable with an average alewife diet proportion of around 50% for lake trout, which we currently use in consumption models. Preliminary investigations indicate that in the spring (April to mid-June) alewife comprise between 7% to 20% of the diet of lake trout, and from mid-June to August alewife can represent from 50% to 80% of the diet. We continue to pursue improvements to describe feeding patterns of this more complex predator in the Lake Michigan basin.

Thanks Jory and Chuck for providing detailed answers to these questions.

In summary, when anglers point out that lake trout need more food to reach a given weight they are correct. A lake trout needs about 125 pounds of food to reach a weight of ten pounds while a Chinook salmon needs around 75 pounds of food (based on differences in gross growth efficiency). However, the Chinook salmon consumes this amount of food over a very short period of time when compared to a lake trout.

In fact, a typical Chinook salmon consumes roughly three times as much food in a given year as a typical lake trout does. This is critically important because alewife (and other prey fish) reproduce and grow each year. The absolute amount of food consumed by a salmon or trout in its lifetime is therefore less important to maintaining a good predator-prey balance than its annual demand for prey.

Chinook salmon do burn through alewife much more quickly than lake trout, but that does not mean that lake trout consumption is completely insignificant. Science is always improving, and the upcoming study on predator diets is one example of an effort to better understand what lake trout are eating at different times and in different parts of Lake Michigan.

Despite the never-ending quest for better information, fishery managers must make decisions in real time based on the best available scientific information. We know that an individual Chinook salmon consumes more alewife than a lake trout does, but we also know that Chinook salmon are no longer the only important species to consider when looking at predator-prey balance in Lake Michigan. In the future, scientists will be taking a harder look at diet and consumption of other predators like lake trout, coho salmon, and steelhead.

More information:

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References

Bergstedt, R.A., Argyle, R.L., Krueger, C.C., and Taylor, W.W. 2012. Bathythermal habitat use by strains of Great Lakes- and Finger Lakes-origin lake trout in Lake Huron after a change in prey fish abundance and composition. Transactions of the American Fisheries Society 141(2): 263-274.

Madenjian, C. P., D. V. O’Connor, S. M. Chernyak, R. R. Rediske, and J. P. O’Keefe. 2004. Evaluation of a chinook salmon (Oncorhynchus  tshawytscha) bioenergetics model. Canadian Journal of Fisheries and Aquatic Sciences  61:627-635.

Madenjian, C. P., S. R. David, and S. A. Pothoven. 2012. Effects of activity and energy budget balancing algorithm on laboratory performance of a fish bioenergetics model. Transactions of the American Fisheries Society 141:1328-1337.

Madenjian, C. P., S. A. Pothoven, and Y.-C. Kao. 2013. Reevaluation of lake trout and lake whitefish bioenergetics models. Journal of Great Lakes Research 39:358-364.

Stewart, D.J., and Ibarra, M. 1991. Predation and production by salmonine fishes in Lake Michigan, 1978-88. Canadian Journal of Fisheries and Aquatic Sciences 48: 909-922.

Stewart, D.J., Kitchell, J.F., and Crowder, L.B. 1981. Forage fishes and their salmonid predators in Lake Michigan. Transactions of the American Fisheries Society 110: 751-763.

Stewart, D.J., Weininger, D., Rottiers, D.V., and Edsall, T.A. 1983. An energetics model for lake trout, Salvelinus namaycush:  application to the Lake Michigan population. Canadian Journal of Fisheries and Aquatic Sciences  40: 681-698.

Lake Superior Fisheries Workshop

Event Date: 5/24/2017

This May 24 workshop is a chance for anglers, and others to network, get updates from researchers, and agencies on Lake Superior Fisheries.

Lake Superior Fisheries Workshop planned for May 24 in Central Upper Peninsula

Do you like to fish on Lake Superior and want to know what others are catching on the lake? Are you curious how lake trout are being managed? Do you want to understand more about what invasive species are present and how they impact the Great Lakes? If the answer was yes to any of these questions, then the Lake Superior Fisheries Workshop is for you!

Michigan Sea Grant hosts a number of workshops across the Great Lakes that help inform the angling community and general public about fish populations and management. This year Michigan Sea Grant in cooperation with the Michigan Department of Natural Resources (MDNR) will host a Lake Superior Fisheries Workshop in Harvey, Mich., just outside of Marquette. The workshop will feature a variety of talks from managing agencies such as the MDNR, US Geological Survey, US Fish and Wildlife Service and Great Lakes Indian Fish and Wildlife Commission. The talks will help anglers and the general public understand what research is taking place on the lake and how it is informing fisheries management decisions. There will also be plenty of time for question and answers allowing anglers to give valuable input.

The workshop takes place 6 p.m.-9 p.m. May 24, 2017, at Chocolay Township Hall, 5010 US-41 Harvey, MI 49855. Parking is available at the Township Hall and overflow parking is available at the nearby Silver Creek Church just northwest of the facility.

Presentations will include:

  • Lake Superior Nearshore Sampling, Troy Zorn –  MDNR: Fisheries Research
  • Aquatic Invasive Species Sampling and Update, Jared Myers – US Fish and Wildlife Service)
  • Lake Superior Prey Fish Updates, Dan Yule –  US Geological Survey)
  • Lake Trout Status and Updates, Shawn Sitar – MDNR: Fisheries Research)
  • Sea Lamprey Control Status, Jessica Barber – US Fish and Wildlife Services)
  • Fishing Enforcement in 1836 Treaty Waters, Marvin Gerlach – MDNR-Law Enforcement Division
  • Fishing Enforcement in 1842 Treaty Waters, Steven Amsler and Matt Kinskern – Great Lakes Indian Fish and Wildlife Commission
  • Lake Superior Angler Creel Data, Cory Kovacs – MDNR: Fisheries Management)

The Lake Superior Fisheries Workshop is free and open to any and all interested participants.

Don’t miss out on this exciting opportunity to learn about what is happening with the Lake Superior Fisheries!