Bony Fishes (class Osteichthyes) / Jawless Fishes (class Agnatha)
The secular freshwater systems all over land worldwide are sensitive to disturbance as there is often little communication allowed, and inhabitants of these systems evolve under relatively isolated environments. Exchange of freshwater species have proved extremely detrimental, as these rich ecosystems often provide a plethora of prey items for introduced predators, which are easy to catch and vulnerable to attack from the unfamiliar carnivores. And most of these top predators are bony fish --- as well as, ironically, many of the prey species driven to extinction. Many fishes are introduced as sports fish and others are acciental introductions.
eg 1 Nile Perch
Subclass: Actinopterygii (Ray-finned fish)
Order: Perciformes
Family: Centropomidae
Scientific name: Lates niloticus
A strikingly large freshwater species, the Nile Perch is also known by different local names as capitaine, mputa and sangara; it grows to 2 metres long, stretching wider than a grown man's spread arms, and is hulky, weighing to 200kg. Not being a true Perch it is nevertheless very close to a Perch in appearance, with deeply notched dorsal fins that has dorsal spines. In a single breeding cycle, 16 million eggs could be produced. It occurs in lakes and wetlands throughout the West African river systems such as the Senegal River, and also the Zaire River system plus several smaller rivers and lakes in Ethiopia.
Its introduction into Lake Victoria, the largest lake in Africa, which is bordered by Kenya, Uganda and Tanzania, was a tragedy for the one diverse ecosystem of the Lake, while from an economic perspective its status is quite disputable. As the Nile Perch is simply overwhelming in size and appetite over the other native fish species in the lake, which have evolved for thousands of years under isolation and in which a particular family of fish, the Cichlids (in particular haplochromine Cichlids) have successfully radiated to fill virtually every available niche within the segregated habitat, it has caused the extinction of more than 200 species of endemic native Cichlids through direct predation, which is a monumental loss of biodiversity for Lake Victoria. The special environmental conditions that enabled the high level of endemism (over 400 endemic species of Cichlids once) have created a fragile ecosystem as a whole in which the invasion of the Nile Perch has more than wreaked havoc in. In Lake Victoria individuals often grow to 250kg, and when fish of such a size forage they are mostly indiscriminate, so many larger Perches have taken to cannibalism quite happily. This in turn sustains the high population further, and most of the native fish are negatively affected, including the predatory fish species, as there are none that could match the Nile Perch in size and appetite. With the decline of many algae-feeding Cichlids in the originally pristine ecosystem of Lake Victoria, the Algae prospered and plundered the water of oxygen, and low oxygen levels was what many native fish could not cope with, albeit not a problem for the introduced Perches. The deeper waters are most oxygen-deficient and this drove many of the native fish into a narrower depth range, reducing the species diversity. As the Algae decomposed detritus is produced, which degrades the water quality to intolerable levels for many native fish.
On the other hand, Nile Perch is a prized food species in its native range and fisheries around the introduced lakes have modified considerably in order to take advantage of this gargantuan and open up the markets for its meat, whose quality is not bad, but it is unpopular as partly out of rote and partly the better quality of some local fish such as Carps, Lungfish, Catfish and others, whose numbers have been significantly reduced through competition with the Perch so much that the fish catch of those species have declined into a tiny fraction of all of what is now mainly Nile Perches. As the Nile Perch is hard to process and must be dried under fire, in contrast to many other food fish, only a very small portion of the fisheries have the capital and technology to process it and it is not widely marketed. Meanwhile Nile Perch is damaging nets of small-scale fisheries and driving principle food species to extinction, and therefore is causing socioeconomic problems to the three riparian countries around the Lake as well, jeopardizing the livelihood of some 30 million people.
Complete eradication of the Nile Perch is not practicable, but several solutions to the problems caused have been suggested, such as deliberately overharvesting the Nile Perch over its introduced range and granting local fisheries the technology and related capital for processing the Perch. Re-stocking of native species and captive breeding of other commercially important food species is also under way, with the hope of saving these freshwater systems under assault.
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eg 2 Mozambique Tilapia
Subclass: Actinopterygii (Ray-finned fish)
Order: Perciformes
Family: Cichlidae (Cichlids)
Scientific name: Oreochromis mossambicus
Tilapias are fishes of the tropical African lakes. The Mozambique Tilapia in particular is colourful and dapper, as in many related species. Breeding males are black with white blotches around the throat, with flame red dorsal and caudal fin fringes. Females, non-breeding males and juveniles are similar but more obviously barred and striped. Males have well developed jaws and a concave head profile.
Introduced worldwide for aquaculture, the populations that have taken root in various tropical and subtropical waters, plus more than 19 territories in the Pacific Islands, were in general deliberately released from fish farms. Tilapias are hardy fish with a high physical tolerance of the environment they are living in, and remarkably can put up with high salinities, even that of atoll lagoons. When overcrowding occurs the size of fish in the populations decrease to accommodate to lack of resources, the development of so-called stunted populations being a key survival strategy that less adaptable species will not be able to carry out. The reproductive capacity of the Mozambique Tilapia is also remarkable, with up to 1700 ripe eggs laid by one female at a time. The fry takes only several weeks to mature, and in a single breeding season there is always multiple broods. The time needed for sexual maturity varies according to environmental conditions but is generally fast.
Being omnivorous, the juvenile Mozambique Tilapia feeds on anything from insect larvae to algae, and is a generalist with many suitable food sources that enables it to establish itself in a wide geographical range where native fauna and flora differ. Adults however are mainly detritivores. It can be found in wetlands, lakes, estuaries and other waterways. In these habitats they compete with other native fish for food and nesting space, and is wiping out some of the more vulnerable restricted-range species. From its native South Africa rivers and lakes it has been transferred to Florida and may enter the Everglades, causing disasters of similar scales as that observed in Hawaii, when the Tilapia displaces Striped Mullet (Mugil cephalus), and as that in the Salton Sea area, where it poses threat to the Desert Pupfish (Cyprinodonmacularius). Eradication campaigns have started in some Pacific Islands, but have had little effect on the species.
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eg 3 Common Carp
Subclass: Actinopterygii (Ray-finned fish)
Order: Cypriniformes
Family: Cyprinidae (Carps and related species)
Scientific name: Cyprinus carpio
This large herbivorous fish, growing over a metre in length and up to 40kg in weight, is favoured as a food and ornamental fish and introduced into temperate freshwater habitats across the world. The fish has small eyes, thick lips for harvesting aquatic plants, and 2 barbels at each corner of the mouth, extremely large scales that are silverfish-green on the back and yellowish on the belly. They have a long lifespan of up to 15 years even in wild conditions. Carps live in a variety of brackish and freshwater habitats including estuaries, lakes, coastal regions with suitable food sources and wetlands. It seems to prefer stagnant waters enriched with nutrients from sewage releasing chutes and streams bordering agricultural land with fertilizer runoff, with sandy and silted bottoms, curiously rare in clear cold waters, and streams of a high gradient --- which tends to have a high flow speed. Native to Asia and Europe, the Carp has been found as widely as from China to central Europe, in the Black, Caspian and Aral Seas.
Carps feed by uprooting aquatic plants and these actions increase the amount of suspended substratum in the water in which Carps inhabit, which reduces water clarity, increasing water turbidity, and decreases the amount of vegetation cover for small fish at the river or lake bottom. Many predatory fish find it difficult to hunt in low-visibility waters, and prey species can be more vulnerable as the aquatic plant cover the natural environment offers are eradicated by the exotic Carps. They are also reproductively efficient and the Common Carp has a fecundity of up to 300,000 eggs per kilogram of body mass. Their high physiological tolerance, among other factors, enables the Carp to colonize new areas quickly --- they have been introduced into over 140 countries, and 18 of these have reported serious ecological damages caused by this introduction,mostly damage of waterways and negative impacts on the freshwater ecosystem. It is considered a pest because of its overwhelming abundance, and the destruction of habitat used by a variety of aquatic species. The Carp has been able to establish itself from escapes from collections and are spreading through the continent's freshwater systems.
The most promising approach in controlling the Carp populations is commercializing the species as a food source. Their flesh is firm and luscious if the fish are harvested from clean water habitats, and the market is being developed.
eg 4 Brown Trout
Subclass: Actinopterygii (Ray-finned fish)
Order: Salmoniformes
Family: Salmonidae
Scientific name: Salmo trutta
This species is a popular fish in commercial fisheries for aquaculture and are widely introduced and stocked also as sport fish. Averaging 80cm long and weighing up to 20 pounds, this Trout is a medium-sized salmonid that is typical of the family in many aspects. They have golden-brown backs and silvery bellies, decorated with darker spots encircled by whitish rings. In the sea the Trouts tend to go more silvery in colour, and the body colour is more uniform with less spots present. Brown Trouts closely resemble their relatives, the Atlantic Salmon and Rainbow Trout, and they occur in a wide range of habitats as well, from estuaries to oceans and lakes, but the Brown Trout prefers cold upland waters.
Introduced into the Great Lakes system by 1883, Brown trout has proliferated and began to replace the native Brook Trouts. They compete with other salmonids for food and even prey on smaller species of Trouts, apart from the normal diet of mollusks, crustaceans, and aquatic insects, which are also consumed and this decreases the amount of food available for the native fish species. Their life cycles are analogous to the other Trouts, and juveniles are territorial in their home ranges, the shallow freshwater ponds in which they have hatched --- they take 3 to 4 years to mature; however they have been found to have a longer life span than other related species, and they grow far faster, and females can produce 10000 eggs at a time. Some Brown Trouts choose to migrate to the oceans but a fair percentage remain in the freshwater system and grow to become adults, when they begin to show severe displacement and predatory effects on local fish populations. On the other hand they are wary fish and have mostly crepuscular feeding habits, and have adapted well to the environment of Lake Michigan, where, like in the other lakes, Brown Trouts have no other predators than fishermen. Control method is mainly harvesting of the Trout for food.
eg 5 Northern Pike
Subclass: Actinopterygii (Ray-finned fish)
Order: Salmoniformes
Family: Esocidae (Pikes)
Scientific name: Esox lucius
Few fish could match the Northern Pike in its formidability. The Pike family encompasses some highly carnivorous species, and the Northern Pike is one of the largest in the family. It has a single dorsal fin set well back to the rear, light tawny spots on a darker background, and is scaled over the gills; the body is slender and powerful while the snout is broad. Growing to more than a metre long and weighing up to 8 pounds, the Pike is a ferocious predator that is found all over the northern freshwater systems from the Pyrenees to Siberia, and in North America from Labrador to the Great Lakes. The Pike has managed to penetrate the Great Lakes system, posing dire threats to local and non-native fish alike due to its large appetite and predatory lifestyle. Its diet includes fish, frogs, crayfish, and even small mammals and birds --- almost anything it could get its serrated teeth on, and an adult Northern Pike can consume several times of its weight in a year, being specialized in feeding on large prey that is half their own size. In Alaska, they are harming the salmon fry, whitefish, sucker and trout populations; other Esox species occurring in the Great Lakes, for example the Muskellunge (E.masquinongy), may be threatened by hybridization with the Northern Pike as the offspring can be fertile. It is solitary and highly territorial. Juveniles are cannibalistic in times of food shortage, but in turn they are also preyed upon by a variety of aquatic insect larvae, fish, other birds and mammals.
The Pike requires weedy bays in which to hunt, and its body camouflage enables it to blend with the surroundings and ambush prey. The spawning grounds are usually near shallow marshes, and the summer heat forces them to seek the shelter of cooler deep waters. However in general they have very wide ranges of physical tolerance and have been termed one of the most adaptable of all the freshwater fish species. They are introduced as a commercial food fish to North American lakes as they fit the taste of the people well, and as a sport fish for they struggle forcefully in deep water when hooked. As its negative effects are not acute few control measures are implemented.
eg 6 Sea Lamprey
Class: Agnatha (Jawless Fishes)
Order: Petromyzontiformes
Family: Petromyzontidae
Scientific name: Petromyzonmarinus
The Lamprey is an exotic-looking, primitive fish whose ancestors are amongst the earliest of all fish to have appeared, around the Devonian Period, more than 400 million years ago. These fish have no jawbones and are in fact one of the most ancient of the vertebrates. Sea Lampreys measure up to 50cm, and are generally silvery-taupe in colour. Elongated and tubular, these fish look a great deal like Eels but are no relatives to them. There are seven gill-holes on either side of the head, and the mouth is equipped with a suctorial pad with myriads of small sharp spikes. Lampreys attack larger fish by adhering to them and rasping a cavity with its rough tongue, and an anti-coagulant keeps the blood flowing for hours while the Lamprey gorges on the meal. This way the victims die of loss of body fluids and infection of the wounds produced by the Lamprey.
Construction of canals that stretches from the Atlantic Ocean into the Great Lakes allowed the Lampreys to colonize these rich habitats. Spawning in freshwater rivers and ponds, Sea Lamprey is principally an oceanic species; in spring the adults trace the tributaries of the Great Lakes and seek out shallow gravel beds in which they excavate a depression on the river bed, and 60000 eggs can be laid by a single female. Adults die after reproduction, and the larval Lampreys exist as filter feeders on the soft sediments of the stream bottom before metamorphosing into free-swimming juveniles. These then descend to the Great Lakes and attach to large fish there, beginning to prey on them.
Historically, the Lamprey has always inhabited the lower Lake Ontario. However, the completion of the construction of the Welland Canal that connected the lower Lakes to the upper ones allowed the Sea Lamprey to exploit these new habitats. As a result, uncontrolled infestations have led to many of the fish species in the Upper Lakes to fall victim to this parasite. During the 20 months of adult life a Sea Lamprey can kill as many as 18 to 20 kilograms of fish. Around World War II, the annual catch of Lake Salmon fell as much as 90% due to Lamprey predation, and thousands of native Trouts killed by Lampreys has been washed ashore. Lamprey predation has been shown to be a main factor for the extinction of 3 endemic Great Lake fish species.
Currently a chemical named TFM (3-trifluoromethyl-4-nitrophenol) is used as a lampricide to control the numbers of the Lampreys. Specific to young Lampreys, TFM can only be effectively administered in shallow water, and is also toxic to the larvae of other native non-parasitic lamprey species. This chemical control has maintained the Lamprey population in the Great Lakes at a tenth of the peak level in the 1950s, but there are underlying concerns that Lampreys may develop resistance to this chemical and they may change their breeding grounds in order to avoid the lampricides. If this happens the millions of dollars spent on Lamprey control may have been wasted, and scientists are still trying to look for more effective and environmentally-friendly methods of control of this parasite (as lampricides may have adverse effects on local wildlife), including introducing electrical barriers to the rivers which Lampreys rely on as spawning grounds.