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Any intelligent fool can make things bigger, more complex, and more aggressive. It takes a touch of genius -- and a lot of courage -- to move in the opposite direction. E. F. Schumacher
The next set of articles will address the basic quantitative concepts underpinning the practical management of living renewable resources. The content may appear to be somewhat theoretical and complex, but a proper understanding of these concepts is essential if one is to appreciate the basis for modern quantitative fisheries management. Given the immense complexity of biological systems, the conceptual framework presented here represents a tremendous simplification of true resource dynamics or even of modern concepts of ecosystem function. This simplification, often referred to as the "single species" approach, is necessary because of the limitations imposed by the poor quality of the available data in most fisheries. Nevertheless, this "simple" theory still leads to complicated resource behaviour, with a number of counterintuitive results.
The material in these articles will also deal with bioeconomics and socio-economic considerations. For example, what is the optimum harvesting strategy? the one that leads to maximum yield, the one that provides maximum financial benefit, or perhaps the one that best supports the livelihood and economic security of coastal communities? These issues are contentious because frequently competing approaches and solutions imply benefits for some interest groups and losses for other interest groups.
Quantitative concepts used in the management of marine resources are confusing. A marine resource is like a factory. It produces product, in this case fish. However, because these fish are produced by other fish, it is conceptually difficult to distinguish the producers; i.e. the mass of fish in the sea, from the products; i.e. the amount of fish produced and caught. The former are typically referred to as the standing stock while the latter are often referred to as the sustainable yield. Given this conceptual conundrum, and other counterintuitive ideas about the dynamics of renewable resources, we resort to an analogy to convey the key concepts. Nevertheless, the reader should anticipate that certain conceptual obstacles will have to be overcome to understand some of the ideas in quantitative fisheries science.
In modern fisheries science a key assumption underlying many management decisions is that an unexploited fish resource has a natural population size which is determined by the carrying capacity of the environment. It is assumed further that although there may be changes in the environment from year to year which cause the population size to fluctuate around the carrying capacity, the average population size does not really change, when viewed over hundreds or thousands of years. The attached figure shows three states of a fish resource: a pristine unexploited state, a state of maximum sustainable utilisation, and a severely overexploited condition.
A: The pristine (or unexploited) state of the resource.
The water in the container represents the fish population. The maximum capacity of the container is equivalent to the population carrying capacity. Although the fish population may not be changing (viz. the water level is not changing) there is a lot happening in the population. Each year, fish are born and enter the population via the recruitment stream shown on the diagram. These young fish are the result of reproductive activity by mature fish, hence the stream of water representing recruits is pumped from the water in the container, back into the same container. At the same time that this is going on, fish are dying. Under natural conditions death occurs due to disease, or predation. One can see that for the water level in the large container to remain constant, the amount of recruits must exactly balance the number of fish that are dying. At this stage no fishing is taking place.
B: Maximum sustainable yield.
If the level of water in the container is reduced substantially, then the situation shown in part B of the diagram occurs. Now the number of fish dying naturally will be less, as represented by less water flowing through fewer holes. However, the number of recruits coming in at the top does not change. This is represented by the pump still working at full capacity. The amount of recruits entering the population is therefore larger than the amount of fish dying. Left to its own devices, the water level would increase back to its carrying capacity level. The only way to keep the water level at the same distance below its carrying capacity would be to remove the excess water from the recruitment stream using a bucket (i.e. harvest the excess production). This excess production is known as sustainable yield, and if it is not all removed, then the water level will increase. For this reason, the sustainable yield is also called the surplus production. For each resource there is some population size at which the surplus production/sustainable yield reaches a maximum value. This population size is called BMSY, and the corresponding maximum value of sustainable yield is known as the "maximum sustainable yield" and is abbreviated as MSY.
Three dynamic states of a fish resource: A) - the pristine unexploited state, B) - the state of maximum sustainable utilisation, and C) - a severely overexploited condition. Dynamics are depicted either as water flows into and out of containers, or in terms of surplus production and population biomass on a graph.
C: Overexploitation:
Part C shows what happens if the water level in the large container is reduced too low. Now the inlet pipe of the pump is only partially submerged in water, and as a result the recruitment stream is reduced in size. Biologically, this means that the number of reproductive adults has been reduced below a critical level, and for the first time commercial recruitment has decreased. In this situation, the difference between the recruitment stream and the natural mortality streams gets smaller, and as a result the sustainable yield is substantially reduced. Even the removal of a small amount of water will stop the water level from rising. For example, a few years ago the South African pilchard resource had not recovered despite the low catches (less than 40 000 tons) that were taken from this resource during the 1970's and the 1980's, and even though a maximum annual catch of 400 000 tons is on record for this resource. A simple explanation is offered by the water analogy, i.e. the resource biomass was simply depleted to a level at which it was generating a very small sustainable yield.
The x-y graphs on the right-hand side of the figure represent exactly the same process as described above, and allow one to determine more precisely the population size which produces maximum sustainable yield, and the value of MSY.
It should be clear from this analogy that the largest catch experienced in the history of a fishery is not necessarily an indication of the MSY of the resource, and that in most cases MSY will be substantially smaller than the maximum catch experienced. In terms of the water analogy, a very large container can be filled by a very small stream, given enough time. The historic catches could just be a process of emptying the container, and bear no relationship to the size of the incoming and outgoing streams, which is what determines MSY. Another consequence of severely depleting renewable resources is that they can take a very long time to recover back to their maximum sustainable yield potential, even if a moratorium on fishing takes place. The resource recovery path of an unproductive resource which has been severely depleted and is then completely protected involves relatively slow growth at first, followed by a period of relatively faster growth. As the resource size approaches its carrying capacity, the growth rate slows down again.
Different resources will have different sustainable yield characteristics. For example, in a highly productive resource like anchovy, one expects the value of MSY to be quite a large percentage of the carrying capacity (10%-20% would be large) compared to long lived species like whales and rock lobsters where the MSY would be a smaller percentage of the carrying capacity.