Throughout the passage of time, prey fish populations have fluctuated up and down. Their story is a complex one, with scientists and recreational fishers harboring differing attitudes towards these fluctuations.
Declines in fish are very lake specific, explained Darryl Hondorp, a research fish biologist for the U.S. Geological Survey who works in the Great Lakes Science Center.
“Historically, the Great Lakes systems mainly had lake trout and a species called burbot as the top predators,” described Darryl. “These fish fed on a group of prey fish species that included ciscos, a smaller herring-like species. They also included a certain degree of shiners and sculpins which are a bottom dwelling species.”
Causes Of Decline
Darryl mentioned that the ciscos probably formed the lion’s share of the diet for the predator species in the Great Lakes. However, they greatly declined over the late 1800s and first half of the twentieth century due mainly to overfishing, but also because of the introduction of species such as the alewife and rainbow smelt.
These non-native prey fish species either outcompeted or preyed on the eggs and larva of the native ciscos. Additionally, there was a bit of habitat degradation as well.
“But overfishing was probably the primary culprit for the loss of those ciscos which were essentially replaced by alewife and rainbow smelt,” added Darryl.
Because predator populations like lake trout were also overfished at the same time, the exotic prey fish species of alewife and rainbow smelt exploded. A classic example of this is of Lake Michigan in the 1950s and 1960s.
The alewife were hyper abundant, but after winter many of them would die off because they would starve. They’d wash up on the beaches in droves, creating giant piles that would stink and ruin the beach and water quality for the lake goers.
Salmon Introduction
The states wanted to create more fishing opportunities, but they also wanted to control the populations of the exotic alewife and rainbow smelt too. Eventually, they decided to stock a few different species of pacific salmon which are native to the northwest pacific.
“Those fish took to the Great Lakes quite well and they fed heavily on the alewife which controlled their numbers and created a really profitable and economically valuable fishery,” shared Darryl.
This practice was well underway by the 1970s, but over time the Great Lakes have changed substantially with the passage of the Great Lakes Water Quality Agreement of 1972. This legislation put limits on the amount of nutrients that were coming into the Great Lakes. As nutrients declined, the overall productivity of the Lakes started to decline as well.
Limited Phosphorous
When thinking of nutrients for the Great Lakes, Darryl compared it to fertilizer for your lawn. If you put less fertilizer on your lawn, you’ll end up getting less lush grass. The particular nutrient that drives productivity in fresh water systems in phosphorous.
As the phosphorous controls kicked in, making it so that less and less phosphorous got into the Lakes, there were less microscopic organisms that fed the prey fish communities. These limits in phosphorous caused a decline in productivity, meaning that they Great Lakes could support fewer and fewer of the prey fish.
“Then during the 1990s and 2000s, we also saw introduction of zebra mussels,” noted Darryl. “Through their filtering activities, they also seem to syphon off some of the nutrient energy that was available to fish.”
Over time, Lake conditions supported fewer of the prey fish species, specifically the alewife. However, the alewife had a negative impact on some of the native fish populations. So as the alewife decreased in abundance, there’s been an increase in abundance in some of the native deep water cisco species, specifically in Lake Huron.
One example is a cisco species called the chub or bloater. The alewife probably fed on the bloaters’ eggs and larva, so as the alewife began to decrease in Lake Huron, there was an increase in bloater abundance.
Healthy Fluctuations
These fluctuations in fish populations are normal. Darryl explained that healthy fish populations fluctuate all the time, especially when it comes to prey fish. This is because some years are good for their physical and environmental conditions which support their reproductions, and then other years aren’t as good.
The decline in exotic fish species has been a concern for some recreational fisheries since the whole industry tends to circulate around those fish. Fishers like to target them, especially fishers who grew up around those salmon species in particular. The decline in alewife has been a concern for some of those anglers as well.
“However, because it’s not a native species and because the management community would like to see prey fish communities that are dominated more by native species,” explained Darryl, “the decline in exotic prey fish species is probably a good thing actually.”
The fluctuations in prey fish populations is a very complex situation, and while some recreational Great Lakes fishers may find the decline in certain species concerning, scientists like Darryl hold a different, more complex view.
U.S. Geological Survey
Great Lakes Science Center
www.usgs.gov/centers/great-lakes-science-center
