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Questioning Fish Introductions

Questioning Fish Introductions
The rapid expansion of Asian carps is testimony to the potential adverse effects of introduced fishes. Photo credit: U.S.G.S.

Ecologists and conservation biologists are reflexively against introduced (non-native) species and all about native biodiversity. It’s the dogma passed from teacher to student. The reason: introduced species often directly or indirectly adversely affect native species, sometimes to the point of their extinction. And biodiversity is essential to healthy, resilient ecosystems. So, if native biodiversity and healthy ecosystems are good, introduced species are bad. No introductions! End of discussion.

If I lived in a binary world―if, as a fishery manager and fish conservationist, I had to choose between introductions or no introductions―I would err on the side of caution and oppose introductions. But it’s not that simple. This is the 21st century, 325 million people are altering the waters and watersheds of the United States, and fisheries management is not just about ecology.

Manifold Mistakes

Examples of adverse effects of introduced species abound. I’ll restrict this discussion to aquatic systems. An important difference between inland aquatic ecosystems and terrestrial ecosystems is that non-native colonists―the first organisms to arrive in a new area―can only arrive in inland ecosystems by an act of man. Yes, once in a river or a lake connected to other lakes, an aquatic species can spread, but the initial colonization by a non-native is only by an act of man.

The rapid and continuing expansion of silver and bighead carp―the Asian carps―is a contemporary testimony of the potential adverse effects and costs of introduced fishes. Since the final distribution, abundance, and the actual effects of Asian carps are yet to be determined, let’s look where the outcome is known. A 1995 analysis of fish introductions found that extinction of 27 of 40 recently extinct (within the last 100 years) native U.S. fish species and subspecies were partly or entirely due to introduced fishes; two thirds of those introductions were intentional introductions, and most were sport species. Similar trends were found for fishes listed as endangered under the Endangered Species Act. Strong reasons to oppose introductions. And fish are just the tip of the introduction iceberg.

The Ecology of Introductions

The potential for adverse effects of non-native species is high because they can affect native species and their ecosystems in multiple ways. The effects range from the obvious to the invisible:

  • Alter habitats. Non-native hydrilla can photosynthesize at lower light levels than many native plants, allowing it to grow in deeper water and colonize larger areas of a lake; it’s lower light requirement and surface-matting growth form also allow it to shade out native aquatic plants. The term “ecosystem engineers” is appropriate for many invasive plants and animals.
  • Predation, the direct consumption of native fishes. Flathead catfish introduced into coastal rivers in the southeastern United States ravaged populations of native redbreast sunfish and bullheads.
  • Competition, either by interfering with access to a needed resource (for example, food or spawning sites) or by consumption of an essential resource. Introduced blue tilapia that excavate deep nests have been suggested to interfere with bass and sunfish spawning in Florida lakes. Alabama bass have displaced smallmouth bass in Lake Chatuge, Georgia, and largemouth bass in Lake Norman, North Carolina. Piscivores―fish-eating predators―feed on diverse forage fishes. Thus, any introduced piscivorous fish is a potential competitor of native piscivores.
  • Diseases and parasites. The Asian tapeworm that lives in common carp and grass carp can infect a dozen species of native minnows, including several federally endangered chubs.
  • Hybridization, genetic introgression. Closely related species commonly hybridize. Hybridization between introduced rainbow trout and native westslope cutthroat trout compromises the westslope cutthroat’s fitness and threatens the continued existence of some populations. Hybridization between introduced smallmouth bass and native Guadalupe bass threatened the existence of this unique native.

Fish Wars

Striped bass are a prized marine sportfish that spawns in East Coast rivers. Completion of two dams that formed Santee-Cooper Lakes (Lakes Marion and Moultrie) in the 1940s trapped spawning-migrant striped bass inland. The development of a striped bass population in Santee-Cooper Lakes proved to surprised biologists that striped bass can complete their life cycle in fresh water, and a fishery for giant, inland stripers was launched. The striped bass offered an opportunity for spectacular sport fishing and a fish that could potentially reduce large gizzard shad that were becoming abundant in many new reservoirs. Further, biologists reasoned that it was unlikely that the striped bass would have high reproductive success and become invasive.

The striped bass is a tremendous introduced sportfish, but has often blamed by black bass anglers for causing an adverse effect on those populations, although researchers rarely find a connection. Stripers grow very large on large, deep reservoirs like Beaver Lake, Arkansas, with abundant shad. Meanwhile, reservoirs like Lake Texoma, on the Texas-Oklahoma border, are much more productive, also with abundant shad populations, and produce big populations of stripers that usually don’t surpass 10 pounds. Over 100 guides work the waters of Texoma, where the daily limit is 10 fish, with no more than 2 of those fish over 20 inches.

Innovative fish culturists quickly developed procedures for spawning striped bass and producing fry for stocking. Soon, striped bass were stocked into large reservoirs throughout the Southeast.

Some of these efforts fizzled due to lack of angler interest, but other striped bass introductions, notably those in Lake Texoma and several Tennessee River reservoirs, flourished as angler interest grew and guided striper fishing developed.

Inland striped bass management was a great success story until a couple of down years for black bass fishing. In Lake Texoma, black bass anglers immediately blamed the voracious, fish-eating striper. Researchers found that striped bass ate a few black bass but far, far fewer than cannibalistic black bass eat.

Several years later, when upper Tennessee River bass fisheries temporarily faltered, black bass anglers again blamed demon striper, this time insisting the stripers were eating all the shad that were shared among the several piscivorous sport fish. The black bass anglers organized and demanded all striped bass stocking should cease. Striped bass anglers and the operators of a lucrative guide fishery demanded that striped bass stocking continue.

Tennessee Wildlife Resources Agency contracted with fisheries researchers at Mississippi State University and U.S. Geological Survey to assess whether striped bass were competing for food with and adversely affecting black bass populations in Norris Lake, the Tennessee River impoundment that was the epicenter of the controversy. Norris is a predator-rich system containing largemouth, smallmouth, and spotted bass, black and white crappie, walleye, and stripers, all of which feed on shad.

Analyzing 25 years of historical fishery data with bioenergetics models, the researchers concluded that shad production was, on average, adequate to feed all predators. However, the historical data also indicated that observed fluctuations in shad production would limit the food supply for the competing predators in three out of five years.


The Norris striped bass incident was 25 years ago. More recently in Lake Sharpe, South Dakota, walleye anglers were questioning whether introduced smallmouth bass were adversely affecting walleye growth via food competition. Similar to the Norris Reservoir study, South Dakota fisheries researchers combined diet studies and bioenergetics models to assess predation and competition.

The outcome was similar to the Norris study: smallmouth do not adversely affect walleye. But the reason was different. Smallmouth bass ate a few walleyes, and walleyes ate a few smallmouth bass. Both sport fishes ate young-of-the-year shad and were, therefore, potential competitors. The bioenergetics models revealed that elevated water temperature, not competition with smallmouth bass for shad, was what slowed walleye growth. The shad not only fueled smallmouth and walleye growth, but they also provided an abundant and readily available forage supply for the fish-hungry smallmouth. Lacking shad, smallmouth may have taken a heavier toll on the walleye.

Similar to Norris Reservoir, reduced production of shad could substantially change the outcome of this predatory drama.

Whether the fishery is in Tennessee or South Dakota or Florida or Minnesota, whether the forage is shad or sunfish or yellow perch or emerald shiners, the standing stock of sport fish is limited by available forage. The different fish-eating sport fishes all feed heavily on the prevalent forage fish and, therefore, potentially compete. Forage fish are productive and abundant and, in most years, provide ample forage for all sport fishes. They also provide a readily available food supply that helps minimize one sport species preying on another. Clearly, forage fishes have a lot to do with the quality of sport fisheries in many fisheries. But why push the limits of forage fish production and upset a well-balanced ecosystem by introducing another sport fish?


Anglers are diverse and prefer different sport fishes. Some anglers, maybe for employment reasons, move to new homes but still want familiar sport species in their new locale. Or maybe they read about an exciting sport fish and wish they had nearby opportunities. This is the realm of sociology and psychology and, thus, pure speculation by a fisheries biologist. But I do know that fisheries managers are strongly prone to attempt to provide the best fishing opportunities they can. And if that means adding another sportfish to the mix, it’s worth consideration. But fisheries managers are also legally charged with the wise conservation of fishery resources.

We live in an increasingly homogenized world. We shop in stores that have an ever-expanding diversity of goods under one roof. Why shouldn’t fisheries be the same? Why shouldn’t anglers be able to catch a popular sport fish everywhere?

Well, for starters, unlike everything else under the roof of the superstore, fish are living organisms dependent on a healthy ecosystem for their existence. And, equally important, fish influence the ecosystem within which a well-balanced community of other living organisms dwells. The superstore won’t go out of business if they stop selling, say, televisions; but a change in a forage fish or a keystone predator could disrupt an aquatic ecosystem. If you really want to catch a certain species of fish, go fish where it lives, where it is native, where it belongs.

Open Niches and Risk

An ecological niche is the part of the environment in which a species fits or to which it is adapted; that environment includes both abiotic and biotic factors and forces. Pursuant to fisheries managers intent to provide the best fishing opportunities possible for their constituents, sportfish have been introduced to “fill a vacant niche.” That saying dates back to 1960s and 70s when fisheries management was still in its infancy, but I still hear it. Indeed, striped bass were stocked into inland waters to prey on all those big, open-water shad. I question the premise of an open niche. If there was an open niche, another native species would be living in it, or an existing species would adapt to exploit it to benefit its population. Think back to the Norris Lake situation: at least six native sport species, all with different life histories and habitat associations (different niches) but all share a common food resource.

Equally false is that a fish that occupies a particular niche in its native habitat will occupy the same niche in its introduced habitat. Every action of every species of fish―where and when it spawns, what and where it eats ―is a function of the particular diversity of fish in the ecosystem. Introduce a fish into a different suite of species and they will do what they need to do within their anatomical and physiological limits to most efficiently obtain energy and reproduce. In other words, they will either not survive or they will change behavior as necessary to survive, often with adverse consequences to their new home. The outcome of introducing a species is always uncertain.

Why Introduce a Species?

If the introduction was intentional, the answer is simple: short-term human benefit, often measured in dollars or a few happy anglers (who may be counterbalanced by a bunch of angry anglers). If the introduction is unintentional, such as the more than 180 non-native aquatic species in the Great Lakes that arrived as a result of connecting the lakes to the Atlantic Ocean and via ballast water from transoceanic shipping, the answer is also dollars, but in this case dollars not spent to prevent the introduction. Author’s note: that most of these Great Lakes’ introductions were unintentional can easily be debated because managers were aware of the potential for colonization.

Brook trout are native to eastern Canada and the northeastern United States, extending as far west as eastern Minnesota. They are considered an invasive species in the West, where they most often compete with native cutthroat trouts.

When a manager or an angler considers the multiple ways that an introduced species may affect native species or the ecosystems that support them, the expense of controlling them when they become invasive, and the impossibility of eradicating them once established (western fisheries biologists have been trying to eliminate brook trout from western trout streams for at least two decades), the default should be “no introductions.” Any decision to reverse that must be made with greatest caution and informed by the best science.

Granted, not all fish introductions have been disasters; indeed, some have been successes. But even the successes can be questioned because they have facilitated a demand and expectation among some anglers that non-native fishes will continue to be available to enhance recreational fisheries. While fishery managers have done a good job of establishing expectations that diverse fishing opportunities―including the use of non-natives―will be available, I fear that some have not done as well at educating anglers about the potential adverse effects of increasing the diversity of sport fishes. Thus, legal and even well-reasoned introductions reinforce the validity of introductions to those few ignorant and self-serving anglers who take it upon themselves to introduce aquatic plants and animals to “improve” their fisheries.

Where you have good fishing with native species, don’t risk adverse effects by taking a chance with an introduced species.

*Dr. Hal Schramm is a fishery scientist and avid angler, and a frequent contributor to In-Fisherman publications on fishery science and conservation topics.

Response by In-Fisherman Field Editor Steve Ryan

Dr. Hal Schramm states that the dogma taught to ecologists and conservation biologists is straightforward. Native species are good and introduced species are bad. Quoting, “So, if native biodiversity and healthy ecosystems are good, introduced species are bad. No introductions! End of discussion.” Of course, Schramm does not end his discussion there, as he is far more educated and qualified than I am at addressing the general threats that non-native fish species may pose to an ecosystem. If required to choose between introduced or no introduced fish species, Schramm states, “I would err on the side of caution and oppose introductions.”

However, whether you like or not, northern snakeheads and bullseye snakeheads have been firmly established within limited areas of the United States for more than 15 years. They have not outcompeted native species in that time; they have not spread new diseases or parasites; they have not caused the collapse of fisheries; they have not made cross-land migrations to new waterways, and they have not snatched unattended dogs or children from quiet suburban neighborhoods.

Laugh if you may, but this was the fear spread by the press and authorities at the time. The general public pretty much accepted the hysteria hook, line, and sinker. In the process, a whole species of fish was added to the Lacey Act within months of the discovery of northern snakehead in Maryland. Just that quickly the rights of U.S. citizens were restricted. That meant if you were a hobbyist with a private aquarium, you could no longer acquire a dwarf snakehead species—perhaps one that grows no bigger than a couple inches and could not realistically be viable in the wild if illegally dumped into a local waterway. It might be a relatively small matter, but in these times of the Covid-19 pandemic, some may see comparisons between an alarmist press and grandstanding political figures who drive a narrative that may or may not be correct and remains unproven by any verified scientific studies.

In the case of snakeheads, they haven’t been proven to have had any significant negative impacts on native fish populations across the United States. Instead, they are generally recognized by experienced anglers as being more sporting than largemouth bass and coexisting with other sportfish in the same arena. So instead of having a knee-jerk reaction to an introduced fish that might not be the “prettiest” or have the most appealing name, I’d ask that people not buy into the hype. Do your own research, go out and pursue them, and then make up your own mind on the merits of this “newly” available sportfish. For those who decide that snakeheads are not for them and elect to kill every one that they catch, make certain that you are able to distinguish them from bowfin. Bowfin are a native species and should not be arbitrarily killed.

Furthermore, most anglers do not rate bowfin as being good table fare, whereas snakeheads are excellent eating. Accordingly, do not discard dead snakeheads (or bowfin) in waterways or on the shore, as it’s illegal in most locations. Furthermore, while I agree with much of what Schramm has offered in terms of the general risk that non-native species can present, I am on the other side of the fence in erring in favor of introductions and celebrating what introduced species have to offer anglers in North America. In-Fisherman touts itself as “The World’s Foremost Authority on Freshwater Fishing.” It’s printed below the magazine’s name on each and every issue. In-Fisherman is the playbook and inspiration for the multispecies angler. If we were to blanketly oppose nonindigenous fish introductions, the landscape of our country’s freshwater fisheries would be quite limited and dull in my book. Take a serious look at the six charts below from the USGS depicting the native ranges of largemouth bass, smallmouth bass, walleye, black crappies, bluegills, and yellow perch. If an area is not highlighted in yellow on those charts, the fish species is nonindigenous to those areas. They would not exist throughout two-thirds of the country if not otherwise previously introduced.

As such, if you take an all-or-nothing approach to introduced fish species, most of the United States, including the entire West and Northwest regions, along with much of the Southwest, parts of the Plains states, and much of the Northeast, would be devoid of largemouth bass, smallmouth bass, walleye, crappie, bluegill, perch, and many other popular fish species. Imagine California without record-class largemouth bass; Maine without smallmouth bass; the mighty Columbia River devoid of giant walleyes; Arizona’s Lake Havasu lacking its world-record sunfish; Idaho’s Lake Cascade without the country’s greatest trophy yellow perch population, and so on.

If you still think nonindigenous fish species are bad, here are a few of other favorites that have built a huge fanbase through the years and are among the country’s most exciting fisheries and huge sources of revenue.

Among the fishes introduced into the Great Lakes via ballast water from transoceanic shipping was the round goby, now one of the most numerous of fishes in many areas, and the target of many sportfish as forage.

The Great Lakes’ salmon and trout fishery is a $7 billion annual industry that allows tens of thousands of anglers to enjoy the thrill of doing battle with drag screaming Chinook salmon, coho salmon, and steelhead—fish whose native ranges are more than 2,000 miles away.

Striped bass growing to more than 50 pounds have been introduced in multiple fisheries across the Midsouth and South to create the excitement of a saltwater fishery in a freshwater setting. Hybrid stripers are a totally manmade creation that have the ability to fight like few fish their size. Once you’ve caught your first good-sized wiper, you’re hooked for life.

Throughout the Northwest, conventional angling and fly-fishing groups have sprung up in pursuit of tiger muskies, another manmade creation that serves as a fishery management tool.

If you like catching brown trout, welcome to the dark side of exotic fish species. Brown trout are not native to North America and yet have a huge angling following across the country. They are revered by fly-fishing, stream, and Great Lakes’ tributary anglers alike.

Speaking of exotics, don’t forget that Florida’s Fish and Wildlife Conservation Commission intentionally stocked exotic butterfly peacock bass from the Amazon into their southeastern canals as a means to control an over-abundance of other exotic fish, including multiple species of chiliads and tilapias. The introduction of butterfly peacocks in Florida is credited with generating $8 million dollars annually to the local economy, while providing thousands of hours of pleasure to anglers in search of unique and hard-fighting fish. Snakeheads could be promoted in much the same fashion.

The canals in South Florida host a mix of exotic species, besides the bullseye snakehead—clown knifefish, various cichlid species, and peacock bass, to name a few.

It is my hope that snakeheads will be accepted as yet another nonindigenous species that have made their way into our fisheries and can be utilized in much the same way as any other sportfish. Catch and release them to continue the sport, or selectively harvest them if you prefer, but above all else, don’t waste them.

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