Human Impact on Biodiversity

Fish Farming

Benefits of fish farming

• Most fish are still caught in the wild (i.e. in the open ocean or in freshwater rivers and lakes)
• However, overfishing has lead to dramatic declines in many fish populations
• Fish farms are ways of raising large numbers of fish in a small space to provide food (protein) for humans
• Fish are bred in large tanks or cages to minimise energy losses and maximise yield

Methods to maximise yield in fish farms

• Within fish farms, large numbers of fish are kept in freshwater or seawater enclosures and are carefully monitored and controlled in different ways (many of which are not possible with wild-caught fish)
  • Selective breeding ensures high quality, fast-growing fish
  • Interspecific predation is prevented using nets and cages
  • Intraspecific predation is limited by grouping fish according to their age and size
  • Water quality is carefully controlled by monitoring pH and temperature as well as removing fish waste or dead fish
  • Diet is controlled by feeding high protein fish pellets regularly to ensure rapid growth
  • Diseases and pests are prevented using antibiotics and pesticides or biological controls

The effect of fish farms on biodiversity

• Fish farms, whilst reducing the pressure of overfishing of wild fish, can have a negative impact on biodiversity in several ways
  • Predators may be attracted to the fish farms and may get caught in nets trying to reach the fish
  • Diseases can spread quickly in fish farms due to the enclosed space and number of individuals in close proximity. Diseases can also spread in the water to other species outside of the fish farm
  • If any caged fish were to escape, they can cause issues with the native species nearby (i.e. those that live in the area naturally)
  • Eutrophication can occur

Fish Farming Methods Table

Non-Indigenous Species

• An indigenous species is one which is native to a particular area
• Introducing a non-indigenous species into an area can have negative affects on the native species
• Introduction may be purposeful (for example, as a biological control) or it might be accidental (for example, if an organism escapes captivity)
• Negative effects occur due to competition, impacts on the food chain and disease
• This results in a decrease in the biodiversity of the habitat

Competition

• The non-indigenous species may compete with native species for food, water or space
• This could result in one or more indigenous species being outcompeted and possibly being eradicated
  • For example, the red squirrel is a native species of squirrel in the UK that has declined in numbers drastically due to the introduction of the North American grey squirrel
  • The grey squirrel not only carry the deadly parapox virus, which is fatal to red squirrels, but they also directly compete with red squirrels for food and nesting sites

Impacts on the food chain

• A new species in the food chain would disrupt the balance as it would provide a new food source or a new predator
• This would impact the populations of the other organisms in the food web
  • For example, the Cane toad is a species of poisonous toad that was introduced into Australia as a biological control to help combat an infestation of beetles, which were destroying sugar cane.
  • The toads then went on to infiltrate the local food webs, poisoning predators that ate them, which had a knock-on effect on other species in the food web

Disease

• New species may bring with them new diseases which may be deadly to indigenous species
• This can have minimal impact but can also be devastating for whole populations, either by total eradication or via long term effects to the health of the species
  • For example, Chalara Ash dieback (a fungus that grows on ash trees), which was brought over through imports of ash trees from Asia, is set to wipe out up to 80% of indigenous Ash in the UK

Eutrophication

• Runoff of fertiliser from farmland enters the water and causes increased growth of algae and water plants
• The resulting ‘algal bloom’ blocks sunlight so water plants on the bottom start to die, as does the algae when competition for nutrients becomes too intense
• As water plants and algae die in greater numbers, decomposing bacteria increase in number and use up the dissolved oxygen whilst respiring aerobically
• As a result there is less oxygen dissolved in water, so aquatic organisms such as fish and insects may be unable to survive
• This results in a decrease in biodiversity for the habitat

Eutrophication can reduce biodiversity in polluted water bodies