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Bad Cichlids?

A cura di , 2001. print format
Pubblicato
Ron Coleman, 2000

Versione Español English
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Get the full color PDF of this classic now out of print book!

Oreochromis aureus

Oreochromis aureus female mouthbrooding eggs, a common introduction in Mexican continental waters. Photo by Juan Miguel Artigas Azas.

(This article was originally published in Cichlid News Magazine, Apr-01 pp. 32-34, It is reproduced here with the permission of author Ron Coleman and Aquatic promotions).

Can a cichlid be bad? In one sense, no, it can't; all cichlids are interesting fishes. However, due to the activities of humans, sometimes certain cichlids end up in the wrong places, and that can be very bad.

Sometimes certain cichlids end up in the wrong places, and that can be very bad

Most cichlid hobbyists are well aware of the disastrous introduction of Nile Perch into Lake Victoria. The unique and fascinating cichlid fauna of that lake is forever diminished by the actions of a few well-intentioned but ill-informed people. Hundreds of species of cichlids are extinct. Exactly how many we will never know. So, cichlids can certainly be the victims of human mistakes. But cichlids can also be the agent of those mistakes, typically through accidental or even deliberate introductions.

South Florida, for example, is well-known for having a diverse fauna of introduced cichlids. Some were accidental introductions from fishfarming operations, others were released by fishkeepers and still others were deliberately planted, either as sport-fish or in an attempt to control other exotics. I have personally observed introduced cichlids in several locations including Hawaii, where cichlids are found in both freshwater and marine environments, and Texas. In both these places cichlids compete with native fishes, largely to the detriment of the native fauna.

Introducing organisms to new environments is an unpredictable endeavour. Setting aside accidental intro ductions or releases from aquariums, consider cases where good-intentioned individuals introduced cichlids for some specific purpose, e.g., to control some other fish population or as game fish. The only thing you can say for sure when doing such an introduction is that the outcome will not be entirely as expected. Why is this?

Ecologists have studied extensively the dynamics of how two species (for example, a predator and its prey species) interact. The theory and literature on these sorts of systems is enormous and relatively well-understood. The problem is that nowhere on this planet do two species interact with each other in isolation from all other species or changes in their environment. Theoretical models of three-species interactions become tricky and much harder to understand and apply. Multispecies interactions beyond three species are virtually intractable.

In one approach, ecologists attempt to understand whether a particular ecosystem is driven from the bottom-up or top-down. A bottom-up ecosystem is one in which changes to the lowest levels in the food chain cascade up to higher levels. For instance, consider a hypothetical Central American river system with wolf cichlids (Parachromis dovii) and a small cichlid like Neetroplus nematopus, often called "neets" in the hobby. Neets feed mostly on algae growing on logs, and it may be that the amount of available algae limits the population of neets. Reducing the amount of algae on logs might reduce the number of neets which in turn might affect the numbers of their predators, the wolf cichlids. Removing wolf cichlids might have relatively little effect on the rest of the system because the structure is coming up from the bottom. Alternatively, the important structuring may be topdown. It may be that wolf cichlids are keeping the numbers of neets in check, and that neets do not eat all the available algae. In that case, removing a few wolf cichlids may lead to a dramatic increase in the number of neets. Determining under which circumstances structuring is top-down versus bottom-up is an active area of research. Typically such work is done either theoretically or on very simple, linear food chains.

Now imagine the real world of a typical tropical stream or river. The trophic (feeding) structuring within such a body of water is not likely to be a simple chain; rather, it is more likely a complex web with many intricate relationships among the species. A given species may eat a variety of other species, and in turn, may be preyed upon by a variety of species. Into this mix, someone introduces a new cichlid species. What will be the outcome?

Things get even more complicated because of the flexibility of cichlids. Many cichlids are opportunistic feeders, and even those with specialized jaw morphologies will take advantage of abundant food no matter what the form. This is one reason why so many cichlids are relatively easy to keep in an aquarium: even specialists like pike cichlids or molluscivores will eat flakes or pellet food in the absence of their preferred food. Furthermore, fish change size as they get older and in doing so may change diet. Juvenile cichlids are often carnivorous but may later in life become preferentially herbivorous.

Feeding is not the only issue of concern with introduced cichlids. Competition for territories and breeding sites are equally important issues. In many tropical waters good sites to lay eggs and raise offspring are rare. One species can theoretically drive another to extinction even if the first doesn't eat the second or even compete for the same food; individuals of one may simply outcompete the other in acquiring and holding onto vital breeding sites. Alternatively, while the native fauna may be limited by access to breeding sites, the introduced fish may be much less restricted. Mouthbrooding cichlids in particular have a tremendous advantage over substrate spawning fishes when breeding sites are limiting. Mouthbrooders often need only a small, temporary spawning site, sometimes for just a few minutes, to successfully reproduce. Tilapias, mouthbrooders from Africa, are particularly adept at getting by with minimal breeding sites, and this is one reason why tilapia can be devastating to native fishes.

Tilapia are the fish of choice in tropical aquaculture because of their rapid growth rate, early reproductive maturity in as little as six months, and their ability to tolerate a wide variety of water conditions from fresh through salt water (they also taste good!). Three species are particularly important in aquaculture and introductions, Oreochromis mossambicus, O. aureus and O. niloticus; however, frequently hybrids between these species are used and it can be difficult to identify the fish encountered in any particular location.

The frustrating part of assessing the true impact of many cichlid introductions is that we typically have little or no accurate data on the abundance and diversity of fishes before the introduction. The best we can do is compare sites with the introduced fish versus those sites not yet impacted. Gratwicke and Marshall of the University of Zimbabwe attempted to determine the effects of introduced cichlids and largemouth bass (another favorite introduction to many places around the world) on a native fish fauna in the Manyame River, Zimbabwe. Largemouth bass (Micropterus salmoides) were introduced in 1932 while Serranochromis robustus, a large predatory cichlid from the Upper Zambezi River, was introduced in the early 1960s. Both species have prospered. The biggest impact appears to have been on the small cyprinids of the genus Barbus. In some cases, the number of Barbus in streams with the new predators are only 1 % of those in streams where the exotics have not invaded. Yet, for some unexplained reason, the numbers of a native cichlid, Tilapia sparrmanii, actually increased in areas with the exotic predators.

The complex biology of fishes and the ignorance and arrogance of humans combine to ensure that cichlid introductions are often devastating

Ken McKaye and coauthors (1995) present the alarming history and ongoing problems of Lake Nicaragua. Lake Nicaragua is the largest tropical lake outside of Africa. It is a dominant feature of the geography of Central America and contains at least 16 species of native cichlids, including fishes like the red devil (Amphilophus labiatus), the rose-breasted cichlid (Astatheros longimanus), Hypsophrys nicaraguensis, and Amphilophus rostratus. These fishes are a large component of the native fishery and a vital resource to one of the New World's poorest countries. In the early 1980s, in an attempt to increase the catchable fish, tilapia were introduced. In just a few years, native fishermen saw a dramatic shift in the fish composition of the lake. By 1990, at some places in the lake, exotic tilapia constituted as much as 54% of the fish being caught. Worse yet, the numbers of native cichlids are declining. An even greater peril lies ahead. Lake Nicaragua is connected by a major waterway, the Rio San Juan, to the Caribbean Sea. This is a large river which fishes, including such large species as the Caribbean bull shark (Carcharhinus leucas), move up and down, thus the river itself poses no barrier to migration. Some species of tilapia are highly salt tolerant and can survive and even breed in salt water. It is very possible that the tilapia will eventually move down the San Juan and out into the fertile coastal areas of the Caribbean where they will colonize the shallows along eastern coastline of Nicaragua that serve as vital nursery areas for millions of Caribbean marine fishes.

Together, the complex biology of fishes in the wild and the ignorance and arrogance of humans combine to ensure that cichlid introductions are often devastating with unintended consequences.

References

Citazione:

Coleman, Ron. (Aprile 25, 2003). "Bad Cichlids?". The Cichlid Room Companion. URL consultato in data Novembre 24, 2014, da: http://www.cichlidae.com/article.php?id=185.