Results highlight how historically low alewife densities observed in recent prey fish assessments can affect the quality of the Lake Michigan salmon fishery.

Two U.S. Fish and Wildlife Service technicians examine and collect biological data from a Chinook salmon landed by a Lake Michigan angler.

By Chiara Zuccarino-Crowe

Fisheries scientists have discovered a few more pieces of the puzzle in the saga of Chinook salmon populations in the Great Lakes, further demonstrating the species’ ties to densities of alewife, one of their favorite prey. A study published by researchers from the U.S. Fish and Wildlife Service and U.S. Geological Survey affirmed that Chinook salmon’s growth within Lake Michigan and northwestern Lake Huron is tied to changes in alewife populations.

Chinook salmon are not native to the Great Lakes; fisheries managers introduced them to the Great Lakes in the 1960s to enhance recreational fishing and help fill the void left when lake trout populations collapsed. Since then, Chinook salmon have become a highly sought fish among recreational anglers in Lakes Michigan and Huron. When salmon populations drastically declined in the early 2000s, Michigan’s charter fishing industry experienced a downturn in catches and millions in lost economic activity. Declines in availability of prey, like alewife, were likely to blame as a result of disruptions in the lower food web of both lakes.

Fisheries managers have worked hard to rebuild salmon numbers for both ecological and socioeconomic reasons. Actions include ongoing tweaks to the numbers of salmon reared in state hatcheries for release into the lakes and monitoring of natural reproduction. This monitoring has shown a steady rise in salmon’s natural reproduction from the 1960s to the present day; now about two-thirds of the Chinook salmon population is of wild origin. Stocking decisions give careful consideration to whether there is enough food to sustain both stocked and wild fish. The ability of stocked fish to survive into adulthood is understandably important, given the resources that go into producing them and the fishery that depends on them. It is critical that the salmon have enough preyfish to eat.

Lakewide movement

Tracking tagged fish has shown that Chinook salmon move extensively around Lake Michigan and from Lake Huron into Lake Michigan, similar to movements the species makes in the north Pacific. The location where fish are stocked does not appear to be closely connected to where the Chinook salmon are ultimately caught by anglers. An exception is during the fall, when fish return to where they were stocked (or hatched as wild fish) to spawn.

“We thought, with this lakewide movement and feeding, that Chinook salmon would have the same growth rates regardless of stocking location, because they are using the whole lake to forage,” said Chuck Bronte, senior biologist at the Green Bay Fish and Wildlife Conservation Office of the U.S. Fish and Wildlife Service. “We also knew that alewife abundance varies from year-to-year and were curious if Chinook salmon growth would mirror those patterns in their favorite prey.”

Bronte leads a team of biologists and technicians that has been tagging and marking all Chinook salmon and lake trout stocked into lakes Michigan and Huron since 2010 (the program added tagging and marking of steelhead in 2017). The team has also been collecting data on these same hatchery fish, as well as wild fish, when they are captured by anglers. The program is mostly funded through the Great Lakes Restoration Initiative administered by the U.S. Environmental Protection Agency.

Tags provide essential data

The tagging method involved in this program uses coded-wire tags placed in each fish. These tags allow researchers to provide key information on when and where each fish was stocked, by what agency, and their age. Anglers provided the main means of recovery of the tagged fish, and their voluntary participation in studies like this are essential for the success of the research effort. Technicians from the U.S. Fish and Wildlife Service and the states engage with anglers at fishing tournaments and fish cleaning stations. The technicians then collect data on the length and weight of each fish, the date and location where they were harvested, and then they recover the tags from stocked fish to track the associated information.

This study evaluated data collected through this method and focused on Chinook salmon reared in hatcheries during 2011-2016, released into Lakes Michigan and Huron, and then recovered by anglers in different areas of the same lakes. Scientists ultimately used data collected from over 16,000 Chinook salmon for the study. The size and age of the recovered fish provided an indication of their growth. The scientists compared the growth among the fish from various stocking and recovery locations along with the densities of alewife in Lake Michigan each year.

Alewife density key

The analysis of the resulting data suggested that 1) regardless of where they were stocked or recovered, Chinook salmon growth (measured by average length at each age) was about the same, and 2) that year-to-year differences in salmon growth corresponded with year-to-year differences in the lakewide density of alewife in Lake Michigan. This appears to apply to the population as a whole, instead of fish stocked from different locations going to separate forage bases – or different sources of preyfish. So Chinook salmon stocked in northwestern Lake Huron exhibited similar growth patterns to those stocked in Lake Michigan. This is likely because many of those fish recovered in Lake Michigan moved there to feed following the collapse of alewife in Lake Huron in the mid-2000s. This study reinforces previous research into salmon movement and feeding patterns, and supports a lakewide management approach for Chinook salmon, with continued attention to the potential inclusion of Chinook salmon stocked in Lake Huron to the population in Lake Michigan.

“The tight relationship between Chinook salmon growth and alewife density is exactly what we’d expect in a system where food is a limited resource” says Matt Kornis, a biologist and data analyst at the Green Bay Fish and Wildlife Conservation Office and lead author of the study. “These results corroborate diet research that show Chinook salmon feed almost exclusively on alewife, and highlight how historically low alewife densities observed in recent U.S. Geological Survey prey fish assessments can affect the quality of the Lake Michigan salmon fishery.”

Predator/prey balancing act

Current research demonstrates that other salmon and trout popular for sportfishing, such as lake trout and steelhead, have a more diverse diet than Chinook salmon. This means they may adapt better to changes in the availability of a particular prey fish, like alewife. However, alewife remain a big part of the diet of all salmon and trout in lakes Michigan and Huron, so fisheries managers looking at Chinook salmon must also consider these other predators. In addition, as wild salmon and trout populations become more prominent, the balance between predator demand and available prey becomes more difficult to predict, therefore adding more uncertainty in determining appropriate stocking levels given available food.

Programs supported through Michigan Sea Grant, like the Great Lakes Angler Diary and Huron-Michigan Predator Diet Study in collaboration with scientists at Michigan State University, are helping to explore these dynamics so fisheries managers continue to have the best information available to make important decisions about stocking levels and fishing regulations.

Michigan Sea Grant helps to foster economic growth and protect Michigan’s coastal, Great Lakes resources through education, research and outreach. A collaborative effort of the University of Michigan and Michigan State University and its MSU Extension, Michigan Sea Grant is part of the NOAA-National Sea Grant network of 34 university-based programs.