Damage caused through introduction of alien fish

Authors: Cambray, J.A.
Journal: Hydrobiologia 500: 217–230
Year: 2003
Where: Makana Biodiversity Centre, South Africa

This South African study highlights the danger done by the transportation of fish around the world to foreign habits, for anglers.

Anglers, usually ignorant of biological processes, assume that adding more fish to a river, dam, or lake, is somehow helping nature. This practice has become so widespread that people often think that some of the invasive species are actually native ones.

The effects are far from benign:

  • The distinct genetic make-up of native fish becomes contaminated by alien fish.
  • Introduction of disease.
  • Introduction of parasites.
  • Reduction of:
    • invertebrates
    • insects
    • insects
    • small fish
  • An expert at a United Nations conference said that, second only to habitat destruction, alien species are the largest threat to biodiversity.
  • Alien species have contributed to the endangerment, or threatening of one third of fish in the USA.
  • Native species have been used as food for alien species.
  • Trout have been so widely spread around the world that anglers now have the opportunity of causing pain and suffering to them in 82 countries.
  • Alien species populations in many parts of the world are now out of control.
The author calls for the spread of alien fish by anglers to be halted urgently.


"At a UN conference on alien species in Norway in 1996, experts from 80 countries concluded that alien invasive species were a major threat to biodiversity conservation and probably the greatest threat after habitat destruction."

"One third of all endangered and threatened species in the U.S.A. are listed, at least in part, due to the action of alien species."


"Certain freshwater fish species used for recreational angling have been transported around the globe and placed in rivers, dams and lakes, frequently without environmental impact assessments or monitoring, for the sole purpose of providing ‘enjoyment’ for anglers. These introductions have resulted in subsequent loss of biodiversity in the receiving systems."

"Justification for conservation of small species, not attractive to the angling community, is difficult. Their use as forage fish for a large alien species must be discouraged."

"The angling public does not fully recognise the need to protect biodiversity ... many freshwater anglers, still believe that we can improve on the initial biotic ‘hand’ that was dealt to any river system by translocating or importing alien sport fishes."


"An angler can now travel round the world catching the same species of trout in over 82 countries! This is an extreme case of ‘homogenizing the biological resources’ of the planet."

"For some cases, such as rainbow trout, a certain status has been created for angling for this species. Many people have grown up fishing for alien species, which they now assume, are indigenous. An extreme case is that some anglers in South Africa want alien rainbow trout to be declared an ‘honorary indigenous species’ because it has been in the country for over 100 years."

"Globalisation of specialised angling equipment helps drive the spread of alien invasive fish species. Thus marketing is driving anglers’ perceptions and expectations."

"In a tourism advertisement for New Zealand (= circa 2000), under the title ‘Pure 100% New Zealand’, an angler is shown catching an alien rainbow trout! A New Zealand Grayling being caught would have been ‘Pure 100% New Zealand’ but this species is now designated extinct, one of the probable causes being introduced salmonids."

"In retrospect, there was no need to introduce bass and trout into South Africa where they now compete and prey on the juveniles of the indigenous angling species, some of which are now endangered."

"Alien fish introductions had a cascading effect and are now out of control in some countries. In Italy, conservation officials stocked alien trout into mountain rivers of national parks instead of working towards the conservation of aquatic biodiversity."


"Formerly genetically distinct stocks are now genetically contaminated."

"Alien trout will impact local biodiversity as soon as they start feeding, or possibly earlier, through imported diseases or parasites."

"Stocked alien fish such as trout and bass species can impact indigenous species through competition, predation, loss of genetic integrity or by spread of diseases and parasites"


"When stream invertebrates are reduced in abundance by an alien predator, their behaviour changes as they become more cryptic, leading to less periphyton grazing, which may depress production of benthic insects resulting in the benthos being less accessible to indigenous fish predators."

"Introduced trout profoundly affected the structure and composition of faunal assemblages in Californian High Sierra lakes. Large and/or mobile, conspicuous taxa, including tadpoles, large-bodied microcrustacean zooplankton and many epibenthic or limnetic macroinvertebrates were rare or absent in lakes containing trout."

"A study on indigenous galaxiids ... found that the best predictor of presence of galaxiids was absence of trout in over 198 sites examined. Galaxiids only existed in ‘fringe’ upstream habitats to which trout were excluded by natural barriers. Galaxiids have decreased in New Zealand streams due to alien trout impact on the invertebrate production, resulting in benthic invertebrate behaviour change. Galaxiids cannot now access the best foraging areas which reduces the food available to them."

"Loss of aquatic biodiversity by introducing alien species solely for the pursuit of pleasure needs to be urgently halted."
Fish Pain