Sustainable Yields


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•  Evaluating the methods used to estimate the size of commercial stock of marine resources

•  Analysis of the effect of population size, age and reproductive status on sustainable fishing practices

In ecology, sustainability is the capacity for a biological system to remain diverse and productive indefinitely

  • A sustainable yield is the amount of a natural resource that can be taken from an ecosystem without reducing the base stock

Maximum Sustainable Yield

In fishing, the maximum sustainable yield (MSY) is the highest proportion of fish that can be removed from the total population without jeopardising this maximum yield in the future

  • The MSY will represent the optimum balance between reproductive growth rates and losses to harvesting and death
  • The MSY should be half the carrying capacity of the species, as this is the stage when population growth will be highest
  • Harvesting above the MSY will soon reduce the possible sizes of future yields (not enough base stock to fully resupply)

Estimating Commercial Stock

Calculating the MSY requires the collection of data over many years – this data can be obtained in a number of ways:

  • Commercial stock can be estimated via capture and release methods, echo sounders and analysis of fish catches

1.  Capture-Mark-Release-Recapture

  • Fish populations can be estimated via the Lincoln index using the capture-mark-release-recapture method
  • Fish may be captured via electro-shocking (discharging an electric pulse into the water to stun nearby fish)
  • This method may be used in rivers and lakes but is not viable in oceans (recapture numbers are too low due to fish mobility)

2.  Echo Sounders

  • Echo sounding involves the transmission of sound waves in water to identify the sizes of schools of fish via sonar
  • Echo sounding is a non-invasive way of collecting data, but is only useful if the species of fish form shoals

3.  Analysing Fish Catch Data

  • Data collected from fish catches (e.g. age profile of captured stock) can be used to estimate population sizes
  • The accuracy of data relies on cooperation of the fishing community – misrepresentation of data will lead to false estimates
  • Because many fish stock occupy international water, effective data collection may also require international cooperation
  • If fishing operators do not comply with all regulations, data will be biased causing miscalculations of stock size

Example of Fish Catch Data

fish catch data

Sustainable Fishing Practices

There are a number of factors that must be considered when developing and implementing sustainable fishing practices

  • These include considerations of the population size, age and reproductive status of the targeted fishing stock

1.  Population Size

  • The maximum sustainable yield should be half the carrying capacity of the species (this is the point of highest growth rate)
  • Harvesting above this threshold (overfishing) will lead to a decline in population size over time
  • Limiting total allowable catch sizes and registering the number of fishing vessels in use are ways to control population size
  • Certain oceanic regions may also be cordoned off for biological conservation (prevent population decline)

2.  Age

  • If a population is growing, then the relative proportion of younger fish will be higher (greater potential for future reproduction)
  • If a population is in decline, then the relative proportion of older fish will be higher (higher probable mortality rates)
  • Introducing mesh size restrictions on nets can allow younger fish to evade capture (as they are smaller in size)
  • Also, using fish farms instead of open water fishing allows for a level of control over the age at which fish are harvested

3.  Reproductive Status

  • Efforts must be taken to ensure fish are able to reproduce and repopulate stock lost to fishing practices
  • If there are insufficient numbers of fish of a suitable reproductive age (too young or too old), population numbers will fall
  • Closed seasons are often declared to allow for uninterrupted breeding periods via which fish species can repopulate
  • Specific exclusion zones may also be introduced at these times to avoid fishing in breeding areas

Sustainable Fishing Practices

sustainable fishing