Ecological Issues & Applications Sustainable Harvest of Natural Populations Requires Being a “Smart Predator”
Although the advent of agriculture some 10000 years ago reduced human dependence on natural populations of plants and animals as a food source, more than 80 percent of the world’s commercial catches of fish and shellfish is from the harvest of naturally occurring populations in the oceans (71 percent) and inland freshwaters (10 percent). When humans exploit natural fish populations as a food resource, they are effectively functioning as predators. So what effect is predation by humans having on natural fish populations? Unfortunately, in most cases it is a story of overexploitation and population decline. The cod fishery of the North Atlantic provides a case in point.
For 500 hundred years the waters of the Atlantic Coast from Newfoundland to Massachusetts supported one of the greatest fisheries in the world. The English explorer John Cabot in 1497 discovered and marveled at the abundance of cod off the Newfoundland Coast. Upon returning to Britain, he told of seas “swarming with fish that could be taken not only with nets but with baskets weighted down with stone.” Some cod were five to six feet long and weighed up to 200 pounds. Cabot’s news created a frenzy of exploitative fishery. Portuguese, Spanish, English, and French fishermen sailed to Newfoundland, and by 1542 the French sailed no fewer than 60 ships, each making two trips a year. In the 1600s, England took control of Newfoundland and its waters and established numerous coastal posts where English merchants salted and dried cod before shipping it to England. So abundant were the fish that the English thought nothing could seriously affect this seemingly inexhaustible resource.
Catches remained rather stable until after World War II, when the demand for fish increased dramatically and led to intensified fishing efforts. Large factory trawlers that could harvest and process the catch at sea replaced smaller fishing vessels. Equipped with sonar and satellite navigation, fishing fleets could locate spawning schools. They could engulf schools with huge purse nets and sweep the ocean floor clean of fish and all associated marine life. In the 1950s, annual average catch off the coast of Newfoundland was 300,000 metric tons (MT) of cod, but by the 1960s the catch had almost tripled (Figure 14.27). In 15 years from the mid-1950s through the 1960s, 200 factory ships off Newfoundland took as many northern cod as were caught over the prior 250-year span since Cabot’s arrival.
The cod fishery could not endure such intense exploitation. By 1978 the catch had declined to less than a quarter of the harvest just a decade before. To protect their commercial interests in the fishery, the Canadian and U.S. governments excluded all foreign fisheries in a zone extending 200 miles. But instead of capitalizing on this opportunity to allow the fish populations to recover, the Canadian government provided the industry with subsidies to build huge factory trawlers. After a brief surge in catches during the 1980s, in 1992 the North Atlantic Canadian cod fishery collapsed (see Figure 14.31).
The story of the North Atlantic cod fishery is an example of the rate of predation exceeding the ability of the prey population to recover; and unlike natural predator–prey systems, there is no negative feedback on the predator population. (Despite the economic consequences of the collapse of the fishery, humans do not exhibit a numerical response to declining fish populations). Unfortunately, the story of the North Atlantic cod fishery is not unique (Figure 14.28). Often following the collapse of one fishery, the industry shifts to another species, and the pattern of overexploitation repeats itself. Over the past decades, however, there has been a growing effort toward the active scientific management of fisheries resources to ensure their continuance. The goal of fisheries science is to provide for the long-term sustainable harvesting of fish populations based on the concept of sustainable yield. The amount of resources (fish) harvested per unit of time is called the yield . Sustainable yield is the yield that allows for populations to recover to their pre-harvest levels. The population of fish will be reduced by a given harvest, but under sustainable management, the yield should not exceed the ability of natural population growth (reproduction) to replace the individuals harvested, allowing the level of harvest (yield) to be sustained through time.
A central concept of sustainable harvest in fisheries management is the logistic model of population growth (Chapter 11, see Section 11.1). Under conditions of the logistic model, growth rate (overall numbers of new organisms produced per year) is low when the population is small (Figure 14.28). It is also low when a population nears its carrying capacity (K) because of density-dependent processes such as competition for limited resources. Intermediate-sized populations have the greatest growth capacity and ability to produce the most harvestable fish per year. The key insight of this model is that fisheries can optimize harvest of a particular species by keeping the population at an intermediate level and harvesting the species at a rate equal to its annual growth rate (Figure 14.29). This strategy is called the maximum sustainable yield .
In effect, the concept of sustainable yield is an attempt at being a “smart predator.” The objective is to maintain the prey population at a density where the production of new individuals just offsets the mortality represented by harvest. The higher the rate of population increase, the higher will be the rate of harvest that produces the maximum sustainable yield. Species characterized by a high rate of population growth often lose much of their production to a high density-independent mortality, influenced by variation in the physical environment such as temperature (see Section 11.13). The management objective for these species is to reduce “waste” by taking all individuals that otherwise would be lost to natural mortality. Such species are difficult to manage, however, because populations can be depleted if annual patterns of reproduction are interrupted as a result of environmental conditions. An example is the Pacific sardine (Sardinops sagax). Exploitation of the Pacific sardine population in the 1940s and 1950s shifted the age structure of the population to younger age classes. Before exploitation, reproduction was distributed among the first five age classes (years). In the exploited population, this pattern of reproduction shifted, and close to 80 percent of reproduction was associated with the first two age classes. Two consecutive years of environmentally induced reproductive failure (a result of natural climate variations associated with El Niño–Southern Oscillation [ENSO]; see Chapter 2) caused a population collapse the species never recovered from.
Sustainable yield requires a detailed understanding of the population dynamics of the fish species. Recall that the intrinsic rate of population growth, r, is a function of the age-specific birthrate and mortality rate (Chapter 9). Unfortunately, the usual approach to maximum sustained yield more often than not fails to consider adequately the sex ratio, size and age class structure, size and age-dependent rates of mortality and reproduction, and environmental uncertainties—all data that is difficult to obtain. Adding to the problem is the common-property nature of the resource; because it belongs to no one, it belongs to everyone to use as each of us sees fit.
Perhaps the greatest problem with sustainable harvest models is that they fail to incorporate the most important component of population exploitation: economics. Once commercial exploitation begins, the pressure is on to increase it to maintain the underlying economic investment. Attempts to reduce the rate of exploitation meet strong opposition. People argue that reduction will mean unemployment and industrial bankruptcy—that, in fact, the harvest effort should increase. This argument is shortsighted. An overused resource will fail, and the livelihoods it supports will collapse, because in the long run the resource will be depleted. The presence of abandoned fish processing plants and rusting fishing fleets support this view. With conservative, sustainable exploitation, the resource can be maintained.