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The Effects of Overfishing on Marine Biodiversity

by Mercedes Lee and Carl Safina

Originally published in Current: The Journal of Marine Education, 13: 5-9, 1995.

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"The last fallen mahogany would lie perceptibly on the landscape, and the last black rhino would be obvious in its loneliness, but a marine species may disappear beneath the waves unobserved and the sea would seem to roll on the same as always." (Ray, p. 45)

Overfishing occurs when fish are being caught faster than they can reproduce and replace themselves. Overfishing can affect biological diversity. Biodiversity is the diversity of living things, and can be thought of as occurring on three levels: genetic, species, and ecosystem. Genetic diversity is the genetic variability that occurs among members of the same species. Species diversity is the variety of species found in a community or ecosystem. And ecological diversity is the variety of types of biological communities. An additional means of categorizing biological diversity is functional diversity; the variety of biological processes characteristic of a particular ecosystem. These units of diversity are interrelated. As Thorne-Miller and Catena explain,

In the face of environmental change, the loss of genetic diversity weakens a population's ability to adapt; the loss of species diversity weakens a community’s ability to adapt; the loss of functional diversity weakens an ecosystem's ability to adapt; and the loss of ecological diversity weakens the whole biosphere’s ability to adapt. Because biological and physical processes are interactive, losses of biological diversity may also precipitate further environmental change. This progressively destructive routine results in impoverished biological systems, which are susceptible to collapse when faced with further environmental changes. (Thorne-Miller and Catena, 1991, p. 10)

Genetic Diversity

Genes are the materials that codify the characteristics and functions contained within an organism. Genetic diversity enables a species to persist in the face of environmental changes that occur naturally. If a species population is large or dispersed over different geographic areas, it is virtually assured of having abundant genetic variation. Abundant genetic variation within a species "increases its potential for successfully evolving in response to long-term environmental changes" (Ehrlich, 1988, p. 24).

Selection pressures, whether natural (such as predation and competition for food) or human-related (such as fishing), can shape the heritable adaptations of a species and thus alter its characteristics over time. Fishing mortality can be a form of environmental change that selects for and alters certain physical and developmental characteristics of a population of marine animals. In other words, fishing activities can cause evolution.

The frequency of occurrence of certain genes in a fish population can be altered by overfishing in two ways: if fishing activity applies a selective pressure; and if the fishing activity applies a random pressure so great that the population is driven low enough to reduce genetic variability.

Heavy fishing pressure can change the genetic characteristics of a population by selecting for or against certain genetically heritable traits like size at first sexual maturity (Policansky, 1993). This can happen, for example, when the larger fish in a population are selectively overexploited. Removing the larger fish over time results in favoring the survival of smaller fish that mature at an earlier-than-average age or smaller-than-average size. If heavy fishing removes most fish early in their reproductive life, individuals that mature younger or smaller than average are at an evolutionary advantage: the fish that survive and do more of the reproducing (e.g., the smaller-at-maturity ones) are able to pass on their genes to future generations. The genetic variability of the population is changed from its former state to now containing a larger proportion of individuals that are genetically encoded to begin reproducing at a smaller size and/or younger age. Fishing can in this way inadvertently exert a pressure to selectively breed toward miniaturization or early maturation.

Fishing pressure selecting for smaller-sized fish can be found in the case of Pacific pink salmon. Over time, with about 80 percent of the spawning fish being caught, catch data registered a decrease in the average weight per fish. After evaluating and accounting for other factors, such as environmental causes, researchers concluded that fishing pressure was the cause in miniaturization of the pink salmon (Law, 1991).

Fishing activities could also apply a pressure so great that the population goes low enough to lose genetic variability simply because there are not enough individuals in the gene pool to carry the full range of variability that once comprised the population. Genetic changes within a population undergoing intense fishing pressure can be measured within as little as ten years. An example is the orange roughy, a fish which does not mature until it is 20 years old, and can live as long as 50 years. A large spawning aggregation was found off New Zealand in the early 1980s. After ten years of heavy commercial fishing of adults, the total biomass of orange roughy declined 60-70 percent. Genetic studies revealed that genetic diversity within the orange roughy population decreased significantly during this time period (Smith, et al., 1991).

One example of overfishing having induced early maturation in a population can be found in the Northeast Arctic cod. In this case, the trawling practiced was indiscriminate, intensively exploiting all age-classes of the cod. Fishing of Arcto-Norwegian cod on their feeding grounds since the onset of trawling in the 1930s gradually caused the breeding stock to become younger overall (Sutherland, 1990). A "large change in mortality imposed by fishing generate[d] a big selection pressure for early maturation irrespective of any change in size-at-age" (Law, 1991, p. 36). Between the 1930s and 1950s, the fish were known to mature between the ages of nine and 11, and "immature individuals had roughly a 40 percent chance of surviving from age 3 to 8 years" (Law, 1991). Overall mortality was increased by this fishing pressure to such a level that it significantly reduced the chances of the breeding-age cod reaching their spawning grounds to two percent. As a result, remaining faster-growing cod entered the breeding stock, and as such, "there has been a gradual shift toward earlier maturation" (Sutherland, 1990, p. 814). The cod now mature when they are about seven or eight years old.

Species Diversity

Species richness, that is the numbers of species per area, and the pattern of their distribution under normal stresses, is used to assess species diversity (Thorne-Miller and Catena 1991).

Fishing-related activities can in some cases actually add species to a given ecosystem. There are numerous examples of fish introduced to natural waterways for food or recreation, or accidental introductions of fouling organisms, symbionts, and diseases associated with transfer of creatures for aquacultural activities. Shellfish are the basis of many introductions to marine environments for commercial cultivation. Many times, these, and other introductions, have negative consequences for native organisms.

For example, the Japanese oyster was introduced to help boost British Columbia's declining commercial shellfish fishery, which had been based on the native Olympia oyster. Competition from the Japanese oyster and other introduced species exacerbated the decline of the Olympia oyster. While the native oyster is still extant, it can no longer be considered ecologically functional, and the shellfish industry there is now based on the introduced species (Lipton et al., 1991). Overexploitation and introductions have also been a source of severe problems for the sole native oyster species of France, the European oyster. In 1979, a disease from cultivated oyster stocks of the European oyster in Washington was inadvertently introduced to France's native oyster population (Lipton, et al., 1991). The persistence of this disease has undermined subsequent oyster introductions of various species as well. It is not known whether any original stocks of the European oyster continue to exist in the waters off France.

Overfishing can deplete biological diversity by causing extinctions. While no marine species is known to have gone extinct due solely to fishing, the Atlantic gray whale was hunted to extinction, and other marine mammals were placed close to extinction by overexploitation. For example, between 1920 and 1986, the population of humpback whale was reduced to five percent of its former level (Butman, et al., 1993). Several fish species are being reduced to very low levels by fishing, especially species that have concurrent habitat problems, such as many sturgeons, several North American salmon stocks, and the totoaba of the Gulf of California, suggesting biological extinction may become a possibility.

Overfishing can affect biological diversity by reducing species richness. When an animal's population is depressed to such low levels that the species no longer fulfills its role as prey, predator, or competitor in the ecosystem, it has essentially become ecologically or functionally extinct. This can have the effect of relaxing competition or predation, allowing other species to become more dominant in the ecosystem. This affects the naturally evolved numerical and functional relationships--which may be called the ecological integrity--among species in a community.

Overfishing of wrasses and triggerfishes off the coasts of Haiti, the U.S. Virgin Islands and Hainan Island, China, provide an example of how overexploitation can disrupt predator-prey relationships. Wrasses and triggerfishes feed on sea urchins. Overexploitation of these wrasse and triggerfish populations resulted in sea urchins reproducing unchecked. As sea urchins are herbivores dependent on algae as a major food source, the increased population of urchins over-grazed the areas' seagrass beds to the point of obliteration (Norse, 1993). Somewhat conversely, removal of most herbivorous reef fish from some Caribbean coral reefs appears to have had consequences during a natural die-off of algae-eating urchins there. With the urchins reduced to very low levels and few herbivorous fish to compensate for their absence, algae overgrew corals, causing large-scale mortality, with consequences for the coral-dependent community (Robertson, 1991).

The effects of overfishing on humans, as top predators, is a good indicator, on a qualitative level, of when the richness of species is diminished and a biological community becomes changed. For example, as much as ten pounds of unwanted creatures are killed for every pound of shrimp caught in the southern U.S. This bycatch, according to the President's Council on Environmental Quality, has contributed over the last 20 years to an 85 percent decline in the Gulf of Mexico population of bottom fishes like snappers and groupers--which themselves support commercial fisheries. Some people who once fished for adult snappers in the Gulf have been forced to fish for other species or driven out of the fishing business altogether. Georges Bank once supported one of the richest cod and haddock fisheries in the world. However, decades of overfishing drove these groundfish to such low populations that spiny dogfish and skates now dominate the ecosystem. This ecological shift may well be permanent, as the recovery of cod and other groundfish populations may not be possible if they are unable to successfully compete with the spiny dogfish, skates, and other opportunistic species to regain their ecological niche. Fishermen and fisheries managers are now discussing the possibility of redirecting the overcapitalized fishing fleet to target the now-dominant dogfish and skates; but uncertainty remains as to whether sufficient markets can be found for these species, which were once considered "trash" fish. Ironically, humans suffer the major effects of overfishing long before the animals themselves completely vanish.

Ecosystem Diversity

Analogous to species diversity, the number of ecosystems and pattern of their distribution can be used as a measure of ecosystem diversity. While we know of no examples where fishing activities eliminated an ecosystem, there are several examples where fishing activities have resulted in major reduction in the regional distribution of ecosystem types over large areas.

Mariculture--the farming of economically valuable sea life--is a fishing activity that has significantly altered coastal and estuarine habitats in many parts of the world. Along the coasts of Ecuador and Thailand, for example, fish farms have replaced mangrove habitats over fairly extensive areas. In order to build aquaculture facilities to raise shrimp and fish, mangroves are dug out and replaced with ponds, eliminating essential nursery habitat for many fishes. While mangrove ecosystems have not disappeared on a global scale, on a regional level there have been significant reductions in the total area of mangroves; a form of ecosystem depletion.

There are other examples of ecosystem diversity being affected by fishing activities that destroy habitat upon which complex communities of marine organisms rely. Overfishing of herbivorous fishes on coral reef complexes in the Caribbean, as in the example above, has directly resulted in coral reef die-offs. The use of dynamite and cyanide in Southeast Asia to catch reef fish for local consumption and the aquarium trade has also killed off significant expanses of coral reefs. Trawling, where heavy nets are dragged along the ocean floor, can also damage seagrass or rocky habitats, physically dislocating or crushing fish and shellfish, undermining structural needs, and disrupting food availability for creatures such as shellfish and groundfish.

Conclusion

The dance of life operates in diverse, strange, and mysterious ways. Understanding the biological processes influencing the functional relationships of marine organisms, species, and whole communities has practical implications for understanding the consequence of human actions. As a species reliant on the biological productivity of oceans, in order to optimize the benefits humans can gain from this vessel of life, we need to know how to minimize the negative consequences of our actions for other living things, and for future generations.

Even with better understanding of the human factors influencing marine biological diversity, fisheries management is inevitably a matter of politics, not science. Management will not succeed in preserving future options unless it is both scientifically informed and ethically responsible.

References

Butman, C.A., J.T. Carlton, and S.R. Palumbi. 1995. "Whaling Effects on Deep-Sea Biodiversity," Conservation Biology. 9:462-464.

Ehrich, P. 1988. "The Loss of Diversity: Causes and Consequences," pp. 21-27, in E.O. Wilson, ed., Biodiversity. Academic Press, New York.

Knowlton, N. 1992. "Thresholds and Multiple Stable States in Coral Reef Community Dynamics," Amer. Zool. 32:674-682.

Lipton, D., E. Lavan, and I. Strand. 1991. "Economics of Mollusk Introductions and Transfers: The Chesapeake Dilemma," paper presented at the 1990 Annual meeting of the North American Shellfish Association. (updated 1991).

Law, R. 1991. "Fishing in evolutionary waters," New Scientist. 2 March:35-37.

Robertson, D.R. 1991. "Increases in surgeonfish populations after mass mortality of the sea urchin Diadema antillarum in Panama indicate food limitation." Marine Biology. 111:437-444.

Smith, P.M., R.I.C.C. Francis, and M. McVeigh. 1991. "Loss of genetic diversity due to fishing pressure." Fisheries Research. 10:309-316.

Pain, S. 1990. "Deep-sea fishing dries up the gene pool." New Scientist. 1 December, p. 31.

Plicansky, D. 1993. "Fishing as a cause of evolution in fishes." In T.K. Stokes, J.M. McGlade, and R. Law (eds.). The Exploitation of Evolving Resources. pp. 2-18. Lecture Notes in Biomathematics 99. Springer-Verlag, Heidelberg.

Sutherland, W.J. 1990. "Evolution and fisheries." Nature. 344:814-815.

Thorne-Miller, B. and J. Catena, 1991. The Living Ocean, Understanding and Protecting Marine Biodiversity. Island Press, New York.