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I feel comfortable discussing invasive species here because these are the animals that we are really concerned with the most.  Invasive implies some kind of negative impact or potential for negative impact based on past experiences. 
Here are some terms that we use to talk about the subject.  The top 5 are more or less biological descriptions. It is suffice to say that some animal is out of its historic range and probable biological range too.  Nuisance is sort of a political term and I see invasive as falling somewhere in the middle between biological and political.
Unfortunately, aquaculture represents one the greatest sources for future introductions of invasive species into North American waters.  One reason for the importing of foreign species is that aquaculturists believe they will receive a higher return on their investment if they market a species that has the mystique of being “exotic”, especially in the aquarium trade.
We’re all aware that there are some risks of bringing in foreign animals and that no aquatic systems are immune to exotics. In the northwest, introduced trout may be negatively impacting amphibians populations and Atlantic salmon have escaped aquaculture operations possibly threatening Pacific salmon stocks. In south Florida we have the walking catfish and several species of cichlids established with an exotic eel poised to enter the Everglades. And in the southwest and mountainous regions, cichlids, tropical fish, and introduced crayfish threaten the habitat of several rare desert fishes.
Aquaculture is thousands of years old in Asia, but here in the US it is relatively new. Our government got involved in the late 1800s by importing common carp and brown trout to raise for food. Presently, figures from 1995 showed that world aquaculture production reached a record of 21 million metric tons of fish and shellfish worth more than 36 billion in US dollars. China makes up 60% by weight of the world’s total (mostly in carp). The US only accounts for about 2% of the world’s total.  Here in the US over 100 species of aquatic organisms are farmed, 60% are freshwater fish of which 50% of that is catfish. The rest is mostly trout, salmon, and some sturgeon.  The dominant molluscs produced are the American oyster and the Pacific (Japanese) oyster.
The problem started hundreds of years ago when introductions occurred with the earliest European settlers. But, the earliest documentation of introductions began in the 1800s, many of which were intentional such as the Asian common carp and the European brown trout that were introduced by government agencies. So it began In 1900, the Lacy Act was the first legislation aimed at controlling unwanted introductions. In 1969, the first American Fisheries Society meeting on exotics was held headed by R.H. Stroud. This was about the time when the U.S. Fish and Wildlife Service started taking notice of problems caused by invasive species and potential problems that may cause.
In 1974, the Federal Noxious Weed Act was legislated.
In 1977 President Carter’s Order restricted all federal agencies from actively supporting projects that would increase the spread of exotics. In 1980, (A shameless plug for my office), the National Aquaculture Act stimulated the USFWS to establish the laboratory in Gainesville, Florida to oversee development of a research program that will support the use of exotics for beneficial purposes while protecting the environment. That Center is no longer functioning that way, instead is attempting to determine impacts and distributions of all nonindigenous aquatic species. In 1990, the arrival of the zebra mussel prompted Congress to pass the Nonindigenous Aquatic Nuisance Control and Prevention Act. This Act was later reauthorized in 1996 as The Invasive Species Act which focused heavily on ballast water and is up for reauthorization in 2001. Finally in 1999 there was another Executive Order to stimulate more research.  Okay, enough.
A quote from R.L. Welcomme states that any introduction made for aquaculture must be thought of as a potential addition to the wild fauna in the receiving country.  The keyword here is potential. Not always is that going to happen, but the potential can be there.
This chart shows the relative percentage of fish introductions internationally based on the purpose or reason for which they were introduced. Aquaculture is the largest, followed by Sportfishing, Unknown Origin, Accidental, Ornamental, and Biocontrol.
A majority of species have been introduced into only a few countries. Eighty-six percent of the species introduced have been recorded from 10 countries or less.  This graph is read for example the first column means that 29 countries have only had 1 species introduced, 6 countries have had only 2 species introduced and so on.  I am concerned with the farthest bar to the right that says 1 country has had 70 foreign species of fishes introduced in it and that happens to be the US.  Canada falls out at 17 foreign species, Mexico with 33, and Hawaii with 45 species of foreign fish.
Here are some selected species and the number of countries into which they have been introduced. The second name on the list is a North American crayfish that has been introduced in Europe and causing problems there.  Introductions are a two-way street. Largemouth bass have been intentionally introduced in Asia and are viewed like we view the common carp here. I believe also that the Caspian Sea is home to many North American species, but they are mostly from ballast water.
This figure charts the number of introductions over time beginning about the 1850s, when some records were kept, until the 1980s. It appears that introductions of fishes peaked into 1960s.  Did we run out of possibilities for new fishes or places to introduce them around the world?  It that possible?
As you can clearly see here there has been a steady decrease in salmon introductions worldwide.
More decreases this time in centrarchids such as basses and sunfishes.
However there has been kind of an up and down trend with a peak in the 1950s in Tilapia introductions.  My guess that it did not increase much, if any, in the 1990s.
A decrease in Asian carp too over the last two decades. I might suppose that data from the 90s would show a further decrease.
As fish introductions slowed, shrimp and crayfish increased into the 1980s. I would venture a guess that it has increased also into the 1990s.
I assume other must include mollusks such as oysters, clams, mussels, and miscellaneous pet trade species which has been steady right along over the past century.
There are almost as many ways for introductions to occur as there are introductions. But here is a list of the most well-known and documented. I found one example of shipping water very interesting.  Several years ago 3000 Pacific oysters were shipped from California to Woods Hole.  The shipment had 2 certificates stating it was parasite and disease free.  That was all well and good but only 10% of the shipment was checked which I believe is standard. After arriving at Woods Hole the water was examined that the oysters were shipped in. The oysters were also washed and the water saved. There was a total of 29 species of algae, diatoms, protozoans, and invertebrates detected.
Here is list that demonstrates the number of different species introduced and how they were introduced in the United States only.  This includes more than just fishes.
This includes some species outside the realm of commercial aquaculture by including all the gamefish that were intentionally stocked by government agencies. I have divided it into 2 groups, species foreign to the US and species native to the US that have been transplanted around the country.
The high number from South America includes much of the cichlids (Tilapia) and the tropical fish for the aquarium trade. We also get tropical fish for the aquarium trade from southeast Asia.
What makes a species a successful invader?  Every animal or species cannot possibly survive in every instance of an introduction.  Alien species that are successful often exhibit a broader flexibility in characteristics that enhances its chance for survival in a new environment. 
Impacts can be simple or complex, direct or secondary. 
Taking one step back from what impacts introductions have, what about the hatchery itself?  Hatchery effluents can have a big impacts on water quality. Depletion of water supplies would also impact the native fauna.  Many trout hatcheries use a tremendous amount of water for their flow through systems. Hatcheries tend to attract thousands of birds and mammals. In a recent 4 year period, over 50,000 birds alone were legally killed at aquaculture operations of which 20,000 were herons and egrets.  I can only imagine how many others came to an untimely end and were not reported. And there is chemical pollution with herbicides and antibiotics.
Because we know so little of the natural workings of native aquatic ecosystems, most potential impacts by introduced species are based on presumptions as to cause and effect.  But an increasing number of studies have been done that demonstrate negative effects on native species as a direct result of introduced fishes.    Some introductions may take decades to realize their full potential as an invasive species.  Some scientists believe every introduction should be treated as a potential “biological time bomb”.
This is for the commercial aquaculturist and we’ve all heard these before. But ways to prevent introductions or minimize impacts might be to build dikes for ponds, filter all hatchery effluent, don’t build in flood prone areas, assure triploidy which can be difficult, or sterilize every animal.
There are of course benefits from aquaculture. Most of us enjoy and a good catfish filet now and then or perhaps a shrimp basket. Fish are an excellent source of protein.  Most of also enjoy fishing opportunities that aquaculture has provided. This in turn supports global into the billions of dollars and also local economies.  Millions of people are able to enjoy their aquariums and bring a little of nature right into their homes.  And, of course, all this creates thousands and thousands of jobs.
Now on to some examples.  I want to begin by mentioning huge impact that invasive plants can have on ecosystems.  This is list of several plants closely associated with the aquarium trade.  It  is pretty well documented that hydrilla and parrot’s feather came from aquaculture, more specifically the aquarium trade.
Hydrilla verticillata was imported from Sri Lanka in the 1950s first to St. Louis and used as a popular aquarium plant. Hydrilla is rooted but concentrates 70% of its biomass at the surface forming a dense canopy during summer and early fall growing to depths of 15 meters. It can reproduce when detached stem fragments readily develop into new plants and attach to the soil.  Recreational boating is the most common method for the spread of this plant now. Hydrilla can alter ecosystems by shading out native submersed vegetation. The production of allelopathens by Hydrilla has been shown to negatively affect the distribution of at least one native plant here in Florida, Ceratophyllum demersum.  Researchers have reported changes in lake water chemistry after the introduction of Hydrilla in many lakes. An obvious and important impact on fish communities occurs here when open water feeding is not longer possible.
Although water hyacinths are not associated with aquaculture I just wanted to show you another example of how an invasive species has altered this ecosystem.  There’s no reason why the next invasive plant could not be brought in for aquaculture, intentionally or unintentionally.
Approximately 60% of established exotic fishes are known or presumed to be associated with the aquarium trade. Release and subsequent establishment of aquarium fishes is not restricted to the U.S. It is common in many parts of the world. The convict cichlid which was released into springs in Nevada has contributed to the demise of the native Moapa White River springfish (Crenichthys baileyi) in Nevada. Other examples are the non-native sailfin molly which overlaps with the native Ash Meadows pupfish, and the non-native shortfin molly which overlaps with the Moapa White River springfish on 2 National Wildlife Refuges. These introductions are currently being studied to determine any impacts on these 2 rare desert fishes.
The bighead carp was imported by a private farmer in Arkansas in the early 1970s to improve water quality in his culture ponds and feeds primarily on zooplankton. Several studies showed when bigheads were cultured along with catfish,  the production of catfish was depressed.  This bighead carp was collected in Lake Erie by commercial fisherman just last summer.  There have been several others collected in the lake in the last five years.  The most recent was a 17.5 kg and 937 mm bighead collected on the Canadian side last Fall. There is some preliminary data that indicates that there may be competition with paddlefish in the Mississippi River. 
The pink areas represent drainages here bighead have been collected in open waters. However, there is evidence of reproduction only in the lower Mississippi and Missouri rivers and possibly the Illinois River so far.
The silver carp was imported also into Arkansas in the 1970s by the state for phytoplankton control in eutrophic waters. It is very attractive for culture because it can filter very small phytoplanktors, often the kind that can produce nuisance blooms.  Silver carp can change the composition from blue-green algae to the more favorable green algae. But, by 1980 it had escaped into open waters of the Mississippi drainage.
It is established in the Black River in Louisiana and probably the Mississippi River in Illinois and Missouri.  Several months ago I saw a story on the news where, in the Mississippi River in Missouri, Asian carp were jumping out of the water like flying fish and landing in fishermans’ boats.  One commercial fisherman said 26 carp landed in his boat in an eight hour period.  The media saw it as some strange phenomenon, but I believe it is typical, especially of silver carp.  People were actually being injured by these flying fish.
The grass carp has kind of been a poster child for “to introduce or not to introduce” species.  They were imported by the U.S. Fish and Wildlife Service from Malaysia in 1963 to study their potential for aquatic weed control. 
The grass carp is established so far in only those 8 states and has been merely collected in open waters from 37 other states or introduced in some of them. They are illegal to possess in many states. Some states allow only triploids to be introduced. This species is still controversial because it is hard to insure all fish are triploids.
The black carp was accidentally imported with grass carp to a private fish farm in Arkansas in the early 1970’s. The next time it was imported was intentionally as a food fish and as a biocontrol agent to control snails which harbors the yellow grub, a catfish parasite in aquaculture ponds. In 1994 approximately 30 fish escaped a facility into the Osage River in Missouri after high water flooded hatchery ponds.  So far the fate of those escapees is unknown. The black carp is known to be a molluscivore and continued importation of the black carp is currently being hotly debated.
The top photo is the rudd, a native of Europe that was brought to this country late in the 1800s. It is cultured in several states and distributed to bait stores in as many as 16 states. The rudd is established in New York and Maine and possibly Nebraska and South Dakota. On the bottom is our native golden shiner, Notemigonus crysoleucas. The problem is that the rudd can hybridize with the golden shiner.  This hybridization is the first known non-salmonid intergeneric cross of an exotic and a native species.
This armored catfish, a native of South America, is established in Florida and Hawaii probably as a result of fish farm escapes and aquarium pet releases. It was found first in the 1980s in Hawaii and then later in Florida where it appears to be expanding its range rapidly. Impacts are unknown, we are just beginning to look at some of these species such as this in south Florida.
Blue tilapia, a now popular food item, has escaped aquaculture operations in North Carolina, Oklahoma, Pennsylvania, Colorado, and Texas where they were collected in open waters in each of the states.  I’ve seen what this species can do in Florida lakes by totally dominating other species in biomass.  The Mozambique tilapia, believe it or not, can be found in several geothermal springs and effluents in Idaho and is considered a pest around the world. It has been established here in Florida since the 1960s.  Both of these fishes were also stocked for biocontrol of aquatic weeds.  I want to add that many small, tropical aquarium species such as mollies and guppies have also been collected from geothermal springs of the West.
Atlantic salmon pen culture began in the 1960s in Washington and brings the risks of disease and competition to the native Pacific salmon.  There are accounts of escapees from these pens such as when nearly 100,000 escaped in the State of Washington back in 1996.  In a lawsuit brought again EPA, Atlantic salmon where considered “biological pollution”.  EPA was found in violation of the Clean Water Act.
The Asian Swamp eel, Monopterus albus, is native to tropical and temperate climates of eastern and southern Asia.  It is now well established in several canals in the Miami, Florida area over the past few years.  They can grow to about 3 m and prey on fish, crayfish, shrimp, frogs, tadpoles, etc. They are capable of breathing air and can travel short distances over land.  No one is sure how it got here.  The aquarium industry does not appear to raise them so maybe it was brought in as a food fish by some ethnic group. At this point the impacts are unknown.
Here is an assortment of animals introduced around the country linked to some step in the aquaculture process.
Corbicula fluminea  may have come in with the Pacific oyster many years ago      
Orconectes rusticus (rusty crayfish) may have been introduced into northern states from the bait industry for fishing     Melanoides tuberculatus (red-rim melania) popular in the aquarium trade, is an intermediate host for trematode parasites, the Oriental lungfluke and the liver fluke, which can affect humans. This snail has been found in 7 states including FL, TX, NV. Myocastor coypus   imported from South America for fur and to control aquatic vegetation. It’s doing a lot of damage to salt marshes in Maryland. Trachemys scripta, the red-eared slider has always been a popular aquarium pet for children. Established in several states. Illegal to sell hatchlings in US now. Potamopyrgus antipodarum, the New Zealand mudsnail, is carpeting the bottom of the middle snake river in Idaho and near Yellowstone National Park. It may compete with the few native snails there. Marisa cornuarietis (giant ramshorn) another common aquarium snail native to Central America found in FL, ID, and TX .
Pacific white shrimp have been reported as escaping shrimp farms along the Texas Gulf Coast since 1989. The greatest risk these escapees pose is the introduction of diseases. In 1995, the Taura virus from South America appeared at a hatchery in Texas where mortality was 80-90%. At the same time 2 other diseases appeared also. It is not known how the diseases got to Texas, but shipments of stock shrimp are moved globally. Another species the giant tiger shrimp (Penaeus monodon) native to SE Asia have escaped mariculture facilities in South Carolina where they were quickly collected later that same year 500 km offshore and as far south as Florida.
This species daphnia is native to Africa and Australia and may have come in with shipments of Nile perch to Texas where it was first found in Texas in 1990. It has since spread to 56 reservoirs in 15 other states.  The impact of this species is not yet known, but it has the potential to become the dominant zooplanktor because of its ability to avoid predators because of its numerous spines.
This highly, aquatic amphibian is popular in the pet trade. It is a native of Africa and is established in California since as far back as1968. It could be detrimental to native amphibians and fishes because of its voracious and nonspecific feeding habits. It is illegal to import or possess this species in California now.
This is the green mussel, Perna viridis, a tropical, marine native of the Indo-Pacific region.  Green mussels are cultured extensively in the Southeast Asia and the South Pacific where they are grown on suspended ropes and bamboo poles.  They are are very good source of protein like any other mussel.
Now we have green mussels in Tampa Bay and the Gulf of Mexico. In the summer of 1999, a power plant in Tampa reported finding them in their intake pipes. Since then they have spread out of Tampa Bay and into the Gulf of Mexico along southwest Florida. It is well established and has survived water temperature much below where it came from. This was probably not an aquaculture introduction but I thought it worth mentioning because of the potential for it to become one now that it’s here.  This species has also invaded Venezuela, Trinidad, and Jamaica in the Caribbean Sea.
It is really easy these days for anyone to purchase cultured animals from anywhere in the world thanks to the Internet. This happens to be the green-lipped mussel from New Zealand that is farmed there on suspended ropes.  Live mussels have in the past been shipped from New Zealand to California, but I think New Zealand has changed its policy and ships only dead animals now.
Hundreds of millions of dollars are spent each year in reacting to introductions after they have become a problem.  Many have suggested that being proactive is the way to go. Whoever wants to bring a new species into the country for aquaculture should spend a small fraction and do some research to determine whether that species has the potential to be invasive. This is not a new idea and I think it’s one that makes sense. A positive side to this business is that hopefully it will take some pressure off ocean fish stocks that are in real trouble.  What I have presented today is really the tip of the iceberg. There is a lot of information out there on impacts of introductions but I don’t think near enough has been done. It’s a totally open field for more research.
I’d like to finish with a list of very good books on the topic for more information and enjoyment.