17.16: Maximum Sustainable Yield

Essential to Know

• Fishing initially reduces the size of a fish stock. However, because the reduction results in greater food availability for the fishes remaining, the rate of production normally increases.
• The additional biomass produced represents an excess over that needed to maintain the population. This excess can be harvested safely.
• As fishing increases, stock size is reduced to a critical level known as the “maximum sustainable yield,” at which the production and reproduction rates of the population are just sufficient to balance the removal rate from predation and fishing. If fishing yield is increased and continued beyond the maximum sustainable yield, the population can no longer sustain itself and collapses.
• Maximum sustainable yield is difficult to establish because fish stocks vary as a result of year-to-year climate-induced changes and changes caused by other factors, such as diseases.
• Maximum sustainable yield also depends on the age structure of the population and on the degree of age selectivity in fishing methods used. Harvesting older fishes tends to increase the sustainable yield because the remaining younger fishes are faster-growing, but it also tends to reduce the breeding population because the younger fishes are sexually immature.
• Maximum sustainable yield is usually established by using one year’s data for stock size and reproductive success to project the survival of adults and young into the next year. Unexpected events, such as disease outbreaks, can render such estimates inaccurate and inadequately protective.
• Most fisheries are managed at a yield 20% to 40% below the estimated maximum sustainable yield to allow a safety margin. This safety margin may not always be adequate, but any safety factor means that fishes that could be harvested will not be.

Understanding the Concept

Although fisheries and shellfisheries provide only a small fraction (about 1%) of the world’s total human food supply, they provide a much larger percentage of its required protein. In many areas, the oceans are the only significant source of protein.

The world’s total fish catch is currently approximately 90 million tonnes per year, about one-third of the estimated global annual fish production. Some optimistic biologists believe that the global fishery catch could be raised by a factor of as much as 10. These optimistic estimates are based on the assumption that existing fisheries can be exploited to the maximum possible extent, that many new stocks of fishes will be discovered, and that many currently underutilized exotic species, including invertebrates such as sea cucumbers and sea urchins, will be fully utilized. Unfortunately, about 85% of major marine fish stocks are currently depleted, overexploited, or fished at their biological limits. Consequently, the global total fish catch has begun to decline from its previous maximum level. Many fisheries have been closed or severely restricted in attempts to reverse this damage.

The oceans do not appear to have the potential to help solve the world’s food supply problem. However, if fisheries are to continue sustaining human populations in areas that have historically depended on them, and if seafood is to continue supplying the same proportion of human food, the world’s fisheries must be managed carefully. The goal of such management must be to catch the maximum amount of seafood that can be taken from the oceans without damaging individual species or marine ecosystems. To fully meet this objective, we would need to harvest species selectively at low trophic levels instead of the currently consumer-desired species, such as tuna. Furthermore, we would have to learn to farm the sea as we do the land, eliminating undesirable plants and animals from ocean farms.

Neither approach is likely to be fully acceptable, at least in the foreseeable future. Consequently, the principal approach of fishery management, which will probably continue for many years, is to manage the fishing of each individual species that is targeted by fishers. The specific objective is to maximize the total amount of the species caught (the yield) while ensuring that the standing stock does not decline to levels that cannot sustain the yield in the future. Thus, the goal is to manage each species to ensure the maximum sustainable yield.

To determine the maximum sustainable yield, we must have a good understanding of the species’ life cycle. Consider how fish stocks respond to fishing. The stock (or biomass) of a fish species is limited primarily by its food supply and its predators. If human or other predators harvest more of the species than is normally taken by its natural predators, more food will be available for others of the species. If excess food is available, the species will reproduce and grow to use this food until the population is again at a size where food availability limits further growth. Exceptions occur when the excess food is consumed by competitor species, but in most cases, if we harvest a species and reduce its stock somewhat, the total production of the species will increase. If we harvest an amount equal to this increased production each year, the population will remain stable, but at a lower number than before we began to harvest. If we harvest more than the increase in production, the stock will progressively decline.

As a stock declines with increased fishing, a critical level, generally 35% to 70% of the original stock, is reached at which the remaining stock becomes so small that it is barely able to grow and reproduce fast enough to replace fishes removed from the population by predation and fishing. If fishing continues to increase beyond this critical point, the stock will begin to decline precipitously (Fig. CC16-1a), and unless the catch is reduced, the stock will collapse to a very low abundance and will not recover. The critical level at which the species is just able to replace the stock lost to fishing is the maximum sustainable yield.

Figure CC16-1. Conceptual model showing (a) the changes in fish stock size with increasing fishing, and (b) fishing yield as a function of fishing effort. Note that moderate fishing can normally produce high sustained yields because the rate of reproduction initially increases as the stock size is reduced and more food becomes available to younger fishes. However, if the fishing effort and yield are increased further, the stock size is reduced to such a level that there are no longer enough individuals to sustain a high rate of reproduction, and the stock collapses.

One of the most important characteristics of a fishery is the “fishing effort,” which is the number of boat or person days of fishing expended. As a typical fishery develops, fishing effort increases as more boats and fishers target the resource species. At first, yield increases rapidly, but as the maximum sustainable yield is approached, fishing effort increases faster than yield because stock size is reduced (Fig. CC16-1a). Each boat must fish longer to catch the same amount of fish. Consequently, either the cost of each fish caught rises, or the catch and income of each fisher declines. Therefore, for economic reasons, the optimum harvest level of a species may be well below its maximum sustainable yield. Fishery management often controls fishing effort to control yield (Fig. CC16-1b).

If the maximum sustainable yield is exceeded in an uncontrolled fishery, fishing effort often rises dramatically as stocks decline. Fishers targeting the species try to protect their livelihoods by increasing their efforts in an attempt to maintain their individual historical catch levels. This dynamic occurs both with technologically advanced fishers targeting a regional resource, such as North Atlantic cod, and with subsistence fishers in island communities, whose growing populations lead to increased fishing effort for local reef fishes.

Changes in the age structure of populations subject to fishing also affect maximum sustainable yield because age structure influences the population’s reproductive capacity. Like children, young fishes increase their biomass with time faster than adults do, even if they consume similar amounts of food. If larger adult fishes of a particular stock are preferentially targeted by fishers, the excess food and decreased competition from larger fishes will enable a greater number of young fishes to grow successfully. Because young fishes gain weight or biomass faster than adult fishes do, a greater number of young fishes in the population will cause an increase in the rate of biomass production, even if the available food supply remains the same. If large fishes are preferentially removed, the stock gains more biomass per year, and the maximum sustainable yield is increased. Hence, fishery management commonly attempts to control the size of harvested fishes by establishing minimum mesh sizes for nets, requiring the release of captured small individuals, or other methods.

Although it is generally advantageous to reduce the average age of a fish population to increase the maximum sustainable yield, the stock can be adversely affected if too many large fish are removed. Only the larger fish of most species are of reproductive age. Therefore, reducing the number of large fishes also reduces the breeding population.

Clearly, establishing maximum sustainable yield is a difficult task that requires knowledge of stock size, age structure, and reproductive process for each species. It is further complicated by the natural year-to-year variability of fish populations and the requirement that maximum sustainable yield be established before or early in a given year. The natural variability of fish populations from year to year is very large, at least for some species, due to climate variation, disease, and other factors. For example, a relatively small change in water temperature may alter the timing of a phytoplankton bloom. In turn, this change may cause an entire year class of larvae to die if they depend on the timely availability of this food. In addition, because diseases are present in fish populations, the equivalent of epidemics can occur and decimate the stock. Therefore, fish stocks can vary dramatically from year to year and on even shorter timescales, and the maximum sustainable yield will vary accordingly.

If the size and condition of the stock were continuously monitored and known, the maximum sustainable yield could be continuously adjusted to accommodate changes. However, data cannot be gathered and analyzed quickly enough to do this. Consequently, maximum sustainable yields generally are estimated from the previous year’s data. Estimates of the stock of adults and number of juveniles that enter the population are made each year. In addition, estimates are made of the survival rates of both adults and juveniles through the coming winter. These data are used to project what the stock will be in the following year, and the projection is then used to estimate maximum sustainable yield and set fishing limits for the following year. If an unexpected event occurs that adversely affects the population after the maximum sustainable yield has been estimated and fishing limits established, the permitted yield may be high enough to damage the stocks. If the unexpected decrease in the stock is recognized early enough in the fishing season, emergency measures can be taken to reduce fishing efforts. However, the stock size is often not well known until after the fishing season ends.

To account for uncertainties and variability in maximum sustainable yield, most fisheries set the permissible catch 20% to 40% below the estimated maximum sustainable yield. This practice leads to conflicts because some people feel that this safety margin is not enough to ensure protection of the stock, and others feel that part of the resource is being wasted because the maximum sustainable yield is not fully used.