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Powerpoint Presentation: Fishing

 

Ecology Index

Ecology : Introduction
The competitive exclusion principle
Ecological Succession: Lake - Woodland Transition
Populations and Sampling
Modelling Population Growth
Biodiversity and conservation
Conservation Alternatives
The Carbon Cycle
The Nitrogen Cycle Flow Chart
The Nitrogen Cycle
Eutrophication
Methanogens and Biogas

Topic Chapters Index

 

A case study: The Peruvian Anchovy (Engraulis ringens)

This is a small (12-20cm), short-lived species maturing in 1 year. It lives in the surface waters in large shoals off the coast of Peru and northern Chile. Here there are cold currents up-welling from the sea bed bringing nutrients for phytoplankton that is at the base of the food chain.

  • The harvest of this fish doubled every year from 1955 to 1961.

  • Experts estimated the maximum harvestable yield (MSY) at 10 to 11 million tonnes per year.

  • Through the 1960s the harvest was about this level making it the biggest fishing harvest in the world.

  • Some of the anchovy were used for human food but a lot was ground into fishmeal for animal feed.

  • In 1972 there was an El Nino event that brought warm tropical water into the area.

  • The up-welling stopped, the phytoplankton growth decreased and the anchovy numbers fell and concentrated further south.

  • The concentrated shoals of anchovy were easy targets for fishing boats eager to recuperate their harvest.

  • The political will was not there to impose reduced quotas

  • Larger catches were made.

  • No young fish were entering the population (no recruitment).

  • No reproduction was taking place.

  • The fish stocks collapsed and did not recover.

 

ECOLOGY

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What can be done to stop the loss of biodiversity? Conservation

Fish stocks

Though fish farming is increasing, fishing represents the last major exploitation of wild populations by mankind.

 

What is causing the damage?

  • Uncontrolled harvesting - even if quotas are imposed they need to be policed.
  • Unrealistic and inflexible quotas
  • Insufficient data on fish populations
  • Improved technology in the fishing industry:

    Motorised fishing vessels fish further and faster
    Echo location / sonar detects fish more efficiently
    Purse seine nets can catch a whole shoal of fish
    Drag nets damage the sea bed for other species
    Factory ships process the fish at sea extending fishing time
    Freezing facilities and motorised transport has transformed fish from a local industry to an international activity.

 

The result

  • Fish populations are reduced below their recovery level.
  • Other species are being taken and killed at the same time. (e.g. dolphins caught in tuna fish drift nets)
  • Other species (e.g. sea birds) are being deprived of a food resource.

Total ban on some species imposed:

  • Peruvian anchovy
  • Pacific salmon
  • Newfoundland, Grand Banks cod
  • North Sea Herring

 

Lessons to be learned

Maximum Sustainable Yield (MSY) is based upon:

  1. the harvest rate and
  2. the recruitment rate of new (young) fish into the population.

The calculation for the MSY is based upon the principle that a population can be harvested at the point in their population growth rate where it is highest (the exponential phase).

Here harvesting (output) balances recruitment (input)

Fixed fishing quotas produce a constant harvesting rate (i.e. a constant number of individuals fished in a given period of time.

 

Problems with MSY

  1. Age structure: If all the age groups are harvested - young as well as old - then recruitment of young fish into the reproductive group will be reduced.
    The answer is to use a net with a big enough mesh size that lets the young fish escape.

  2. Limiting factors: As seen in the example above, if the limiting factors in the environment change so does the population growth rate. Limiting factors set the carrying capacity (K) of an environment. Increasing limiting factors will cause K to drop. Fixed quotas cannot cope with this.

  3. Data: For MSY to work accurate data in fish populations is needed (population size, age structure, recruitment rates). Usually these are not well known.

What is required?

  • Nets with bigger mesh size.
  • Regulated fishing methods.
  • More data on fish populations (e.g. by fish tagging investigations - see mark and recapture).
  • Constant monitoring to observe changes in environmental factors.
  • Policing of fishing industry - respect of quotas
  • International agreements
  • Greater exploitation of fish farming - but this is not without its own problems (space, diseases and pollution are all associated with intensive fish culture).

 

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