Several stages occur during fisheries exploitation such as under-exploitation, overexploitation. Several fisheries are overexploited. In this unit, we would define these terms, state the different forms of overfishing, effects of overexploitation, principles of resource exploitation (Sutherland, 2001), measures needed to exploit conservatively and methods for assessing exploitation level.
Under intense exploitation, most fisheries experience the following sequence: undeveloped, developing, fully exploited, over exploited, collapsed and with appropriate management measures, rebuilding.
Underexploited fisheries refer to underdeveloped or new fisheries. This is believed to have a significant potential for expansion in production.
Moderately exploited fisheries are those that are exploited with a low level of fishing. They have some limited potential for expansion in total production.
Fully exploited fisheries: The fishery is operating at or close to an optimal yield level with no expected room for further expansion.
Overexploited fisheries: Fisheries are exploited at or above a sustainable level in the long term with no room for further expansion and at a risk of collapse or stock depletion. Over exploitation or overfishing is the removal of aquatic living resources to levels that cannot sustain viable populations.
Overexploitation can lead to resource depletion, threaten or endanger species of fish or wildlife. Over exploitation affects fisheries directly and indirectly. Direct effects are associated with target and by catch species.
Forms of Over fishing
According to the Community-Based Coastal Resource Management Centre (2003), there are different forms of overfishing which include:
Economic overfishing- Increasing fishing intensity does not translate to more catch or profit. There is a maximum level at which the environment can produce. When the limits of production are reached, the catches begin to reduce. Increasing fishing intensity leads to dwindling catches.
If the fishing intensity is increasing, fishing cost such as labour, materials, capital and fuel increases.
If the catch is greater than the cost, there is a profit or resource rent (Total catch – Total costs) Recruitment overfishing occurs when fishing is so intense that the parent stock are reduced in numbers and can no longer produce a normal stock in the next year. This is called recruitment overfishing.
This can be prevented by the provision of sanctuaries for such fish. This affects the ability of the species to replenish the population. Gears can physically disturb the habitats and sediments, catch both target and non-target species.
Growth overfishing occurs when fish are caught at an uneconomic size; for instance when a lot of small-sized fish are caught in the absence of large fish. This form of overfishing occurs at the species level.
Ecosystem overfishing occurs when several species are overfished. It is difficult to understand when an ecosystem overfishing occurs because when one species is over fished, another one will take over but eventually, the ones that take over may also become consumed.
There are three ways an ecosystem can be overfished:
- Destruction of the primary producers such as the mangroves, coral reefs, sea grass beds
- Decrease in secondary producers
- Decrease in primary producers and decrease in biomass of the fisheries
Malthusian overfishing: Limited resources cannot support increasing populations. This implies that fish is being harvested beyond the maximum sustainable yield.
Effects of Over-Exploitation
Indirect effects of over fishing or over exploitation include ghost fishing where a fishing gear left or lost at sea continues to fish or restrict movement of fish causing starvation, laceration, suffocation and infection.
Indirect effects also cause atrophic cascading effects in which top level predators are removed affecting the whole ecosystem.
Overfishing has decreased catches for many fisheries, negatively impacted ecosystem health and the sustainability of stocks. Human food supplies are reduced due to reduction in catches. Overexploited fisheries may be unable to meet the demand for fish oils.
Depleted fisheries have stocks with catches well below historical levels, irrespective of the amount of fishing effort being exerted.
Recovering fisheries are those with increasing catches after being depleted. They are yielding less than their maximum potential owing to excessive fishing pressure in the past. According to FAO (2008), 2%, 52%,19%, 8% and 1% of the fish stock were underexploited, fully exploited, overexploited, depleted and recovering respectively.
Principles of Resource Exploitation (Sutherland, 2001)
Population increase can be exploited and population should be exploited at the rate they increase (Caughley & Gunn 1995).
Density dependence is essential. If due to density dependence, reduction in population causes increase in breeding output or survival, then the resulting increase may be exploited. Density dependence is central to sustainable exploitation (Ricker 1954, Schaefer 1954).
Quantifying density dependence is difficult to measure because of sampling errors (Shenk etal,1998).
Sustainable exploitation involves reducing population size and depends on a growing population which can be achieved by reducing populations to take advantage of the density dependent increase in survival or breeding output. Exploited populations must be lower, even when exploited sustainably.
Sustainability has many conflicting definitions depending on the objectives.
It is better to monitor the population than the harvest: It is easier to measure changes in numbers of individuals exploited but determining changes in population sizes is better for adjusting the exploitation level (Lande etal.,1997). It is the population size that really matters.
Quotas are unstable. Populations fluctuate, estimates of a sustainable quota maybe faulty or the quota exceeded. When the population declines, the quota becomes an increasing proportion of those remaining which can drive the population further downward.
Monitoring can prevent overexploitation. There is a natural variation in the population size. It is difficult to reduce agreed quotas.
Increasing effort is simple, reducing it is painful. Over exploitation of populations continues even when reducing effort should produce greater long-term yield (Ludwig etal,1993).
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Measures Needed Fisheries Exploitation Conservatively
It is better to restrict effort rather than quotas.
Only exploit populations when they exceed a threshold size (Lande etal,1997).
Adopt rotational management in which areas are exploited for a period and then left (Myers etal,2000).
Exclusion zones in which fishing is not allowed (McCullough, 1996).
Protection of gravid and juvenile individuals.
Protection of water from pollution
Inclusion of fisheries impact assessments in environmental impact assessment for activities which impact fishery or other aquatic resources.
Methods for Assessing Exploitation Level
1. Surplus production method
This method determines how catch varies with effort. Data required are numbers or biomass exploited per year and effort. This method has been used mainly for fisheries.
2. Yield per recruitmethod
This method considers recruitment in determining long term sustainable strategy. It is used mainly for fisheries and forestry.
3. Robins on and Bed for model
This method entails calculation of maximum growth and yield if population is 0.6 of expected population size. The data required average at first reproduction, annual birth rate, age at last reproduction.
This method is used when nothing is known about a species. It has been used mainly for Tropical forest mammals and birds.
4. Linking yield to recruitment and mortality
Data required for this method are recruitment and mortality rates. No data are needed for trends or effort. It does not apply to populations in equilibrium only to populations at sustainable population sizes. The method is used occasionally for mammals
5. Adjusting to population changes
This method loosens and tightens regulations to adjust to population changes. It demands very little data on population. It emphasizes population sizes but may be difficult to respond to environmental changes.
6. Comparing demography across sites
This method relates exploitation intensity to density or population change. The data required are exploitation level and population density (or population change at a range of sites).
The method focuses on populations and can be carried out in the short term without detailed ecological studies. It is necessary to know whether results are affected by other interactions, e.g. environmental factors
7. Reducing to a fixed fraction of exploited population size
This method maintains population at a proportion of (e.g. 60%) of unexploited population. Data required are likely unexploited population size, current population size and the method can be used with very little data.
This method is useful for data-deficient populations. However, the method requires estimates of unexploited population and is difficult to use for variable populations.
8. Full population model
Creates full model of major components of the population and examines consequences of different harvest levels. It requires full data on strength of all density-dependent processes. It is about the best method and can be used to examine the consequences of other changes.
9. Adaptive Management
This method uses models to determine where doubts occur and the consequences of these doubts. Experiments are undertaken to reduce uncertainty. It involves continually improving knowledge and management.
Data from experiments are used. The main strength is that it continually improves knowledge.
In summary, most fisheries all over the world are fully exploited or depleted. Overfishing has many effects therefore measures are required to assess exploitation levels and use resources conservatively.
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