Fish Communities

One of the best ways to study aquatic health is to look at the fish community, specifically which fish species exist in a particular area and the size of their populations.

Changes in the fish community can provide insight into complex ecosystem dynamics, such as prey availability for predators, the interaction between native and introduced species and changes in food webs.

Fish are a useful indicator of aquatic ecosystem health because they can reflect changes in nutrients, water quality and habitat. For this report, we look at the overall health of several major species using data from dozens of studies and long-term research conducted in Georgian Bay.

Where Are Fish Studied?

Research agencies and institutions study fish communities all over Georgian Bay, with specific survey areas often determined by the species of fish. These include lake trout rehabilitation zones, important walleye spawning populations and study sites for muskellunge, pike and smallmouth bass. The map below shows long-term research and monitoring sites from which more than 50 years of data has been collected.

Where available, Catch Per Unit Effort (CPUE) is provided on the graph below for each location and species of fish. CPUE is an indirect measure of the abundance of target species, sometimes called catch rate.

What is Measured?

Biologists study whether different species of fish are naturally reproducing. This tells them if a population is sustainable. From years of data, they can determine population trends to see if a species is declining, improving or staying the same. However, with so many species spread over a large area, it is sometimes difficult to determine the overall health of a particular species, especially for a body of water as large as Lake Huron–Georgian Bay.

Declines in all major native species—lake trout, walleye, muskellunge, pike and others—indicate that the aquatic system has changed significantly in the past few decades. Despite restocking efforts and some habitat restoration work, the years of stress from invasive species, habitat loss and now loss of nutrients create an enormous challenge for sustaining fish populations. Added to these pressures are the effects of low water levels and climate change.

Prey Fish

Prey fish make up the majority of fish in the Great Lakes food web. Biologists study a variety of prey fish species to see how young larval fish are growing and whether there is enough food available to support them. New studies are conducted to measure the energy, or productivity, of the aquatic ecosystem, especially with the decline in nutrients, like phosphorus, that provide the foundation of the food web.

Historically, prey fish in Georgian Bay were quite diverse. In offshore waters, there was a mix of native prey fish such as the deepwater cisco (e.g., bloater), sculpin, lake herring, ninespine stickleback and trout-perch.

In nearshore waters, species like the spottail shiner, emerald shiner, young whitefish, sucker species and yellow perch were important in the diet of predators like walleye, pike and muskie. Native prey were in balance with native predators.

However, with introduced species, such as the non-native alewife and rainbow smelt, the system changed to favour an abundance of non-native prey. Even so, from the early 1970s to the early 2000s, the fish community remained stable. Stocked trout and salmon supported popular recreational fisheries and became the dominant predators in the lake, while the alewife became the most abundant prey fish.

In the late 1990s, diporeia, a benthic organism that forms the base of the food web, began to decline. Declines in zooplankton and diporeia, in part attributed to invasive zebra and quagga mussels, reduced the amount of food available for alewives. In 2003, the alewife population collapsed. Because chinook salmon fed almost exclusively on alewives, declines in the salmon population followed. By 2005, the chinook salmon population had also collapsed.

While no single species has filled the role of the alewife as the dominant prey fish, several native species, including the yellow perch, cisco and emerald shiner, have experienced population increases. The only substantial native prey species remaining in offshore waters is the deepwater cisco species known as the bloater. Overall, prey fish diversity and abundance have declined, creating a food web imbalance where there is less food energy in the system for predators.

It is likely that changes in the prey fish community are a result of top-down and bottom-up pressures in the food web. At the top, introduced and abundant salmon species consumed an excess of alewives and other prey. At the bottom, a major decrease in phosphorus offshore caused declines in the production of phytoplankton, zooplankton and benthic species. In turn, less food is now available for young fish, which threatens their growth and survival. Given this combination of pressures, prey fish are “squeezed” in the middle.

With the recent introduction of a predatory zooplankton called the spiny water flea, the food web is further changing, with unknown outcomes.

Changes to the composition and abundance of prey fish in Lake Huron present new challenges for fisheries managers. Ecological changes that used to occur over decades are now happening in just a few years. More research by agencies and institutions will be needed to investigate complex changes in Georgian Bay’s fish community, food web and water quality to help communities make good decisions about the Bay and area lakes.

Smallmouth Bass

Smallmouth bass are an important native predator of the nearshore warm-water fish community. The diet of young smallmouth bass begins at the bottom of the food web and expands to include almost all aquatic organisms as the fish mature. As a result, thriving smallmouth bass populations suggest productivity and good health in the lower food web.

In eastern Georgian Bay, smallmouth bass are one of the more abundant species, yet they too are affected by human activities, including harvesting, habitat disturbances and shoreline development. They are also sensitive to warming temperatures and changing water levels. Although some smallmouth bass populations appear to be healthy, others are less abundant, and some are actually declining, so biologists cannot confirm a definitive trend for Georgian Bay.

Northern Pike

The northern pike is a top predator that lives in nearshore waters and secluded bays. Like muskellunge, northern pike are reliant on coastal wetlands for successful spawning and nursery habitat.

Although northern pike are not considered threatened anywhere in Canada, they are as vulnerable to habitat loss as any other freshwater species. From a high of 1.6 million kilograms per year in 1900, the northern pike fishery declined to less than 0.05 million kilograms per year by the late 1960s, with the loss of nearshore spawning and nursery habitat resulting from shoreline development throughout the Great Lakes.

More recently, sustained low water levels, along with continued shoreline development, have reduced the important coastal wetlands upon which northern pike rely. Among the 10 populations of northern pike studied in Georgian Bay, some are declining, some are improving and some are unchanged.

Muskellunge

Known to many as “muskie,” these impressive fish are a native predator of nearshore environments and a highly sought after trophy fish. The nearshore waters of eastern Georgian Bay and the North Channel are thought to support the largest contiguous distribution of muskellunge populations in the Great Lakes.

Research about muskellunge can help us understand the productivity of nearshore fish communities. Because muskie require coastal wetlands to spawn, changes to these habitats can affect reproductive success and populations over time.

In Ontario, the fisheries goal for muskellunge populations is for them to be self-sustaining, and to maintain the world-class fishery present in Georgian Bay. Unfortunately, muskellunge have a low reproductive rate, grow rather slowly and have had their spawning and nursery habitat affected by shoreline development and some years of sustained low water levels.

Provincial biologists have been surveying muskellunge populations in eastern Georgian Bay and the North Channel since 1996. Their surveys confirm the widespread distribution and presence of mature muskellunge throughout eastern Georgian Bay. Nevertheless, there is continued concern over the potential for high-quality spawning and nursery habitat to become degraded and subsequently impact natural reproduction of the species.

Research shows that muskie populations in eastern Georgian Bay and the North Channel appear to be unchanging, indicating a naturally reproducing, sustainable population. But populations in southern Georgian Bay may be deteriorating. Further studies will be needed to determine each of these trends.

Researchers are calling for a strategy to identify and ultimately protect suitable muskellunge breeding habitat.

Arunas Liskauskas – Upper Great Lakes Management Unit (MNRF)

Walleye

Populations of spawning walleye are located throughout Lake Huron and usually live near a particular river. Of the 37 important walleye populations that are studied, 16 of them are found in the Georgian Bay Biosphere region. Researchers are most interested in the size of the population, spawning success and natural reproduction, as well as the age of individual fish. The age of fish helps determine if they have matured enough to reproduce.

Native to Lake Huron, and sometimes called pickerel, walleye are a valuable indicator of fish health, as they are a top predator of the cool-water nearshore community. Stable walleye populations suggest that their habitat is healthy and nearshore food webs are productive.

Walleye use different habitats throughout their life cycle. Most mature walleye in Georgian Bay move into rivers to spawn in the spring (although some spawn on shoals). They move back out to the Bay as water temperatures increase in the summer. Good-quality habitat will provide the food and shelter walleye require.

Because walleye tend to be directly and indirectly exposed to human activities, their populations can offer insights into the extent of those impacts. Given that walleye spawn in rivers, they are also good indicators of the health of rivers.

At one time, walleye were plentiful, and Georgian Bay became known for the size of its fish. However, since the early 1900s, most Georgian Bay walleye stocks have declined due to a combination of overexploitation, human alteration of rivers and waterways and the introduction of invasive species. Dams in rivers change the flow of water, and high water can threaten eggs by washing them away, while low water can cause them to dry them out. In the Moon River, for example, where walleye spawning runs used to number over 30,000 fish, these numbers have dropped more recently to only several hundred.

Since the early 1980s, efforts to rehabilitate walleye populations have included habitat restoration, stocking young fish and imposing regulations that restrict harvesting. These efforts have had mixed success, and very few walleye populations are currently sustainable.

Lake Trout

Lake trout are a top cold-water predator in Lake Huron and Georgian Bay. They are considered a useful indicator of the health of our offshore waters. Assessing their populations can provide insights into food web productivity, the presence and effects of invasive species and the availability and quality of fish habitat. They spawn in shallower waters in the fall and feed in them in the spring. In addition to their important ecological role, lake trout in Georgian Bay are caught commercially and recreationally.

Prior to 1940, numerous populations lived in the deep offshore waters. An invasion of sea lamprey (an eel-like parasitic fish), overharvesting and the decline of their major food source (the deepwater cisco) caused lake trout to collapse in all but two isolated locations: Iroquois Bay and Parry Sound.

In 1969, efforts began to rehabilitate lake trout. Sea lamprey controls were implemented across the Great Lakes. Lake trout were stocked using the same genetic strain as those found in Parry Sound, and a fish sanctuary was established. The Parry Sound population of lake trout is the only one to be considered fully rehabilitated outside of Lake Superior.

Unfortunately, lake trout populations have not re-established in most locations where they were found historically. The management goal for lake trout is to restore populations to the point where they are naturally reproducing. With reduced productivity in the offshore waters and other ecosystem changes, the prospects for lake trout rehabilitation in Georgian Bay are uncertain.