OUP user menu

Predation and Competition: The Impact of Fisheries on Marine-Mammal Populations Over the next one Hundred Years

Douglas P. DeMaster, Charles W. Fowler, Simona L. Perry, Michael F. Richlen
DOI: http://dx.doi.org/10.1644/1545-1542(2001)082<0641:PACTIO>2.0.CO;2 641-651 First published online: 17 August 2001


We discuss the potential for commercial fisheries to adversely impact ≥1 population of marine mammal by the end of the 21st century. To a large degree, patterns over the last 50 years regarding human population growth, success and failure in marine-fisheries management, and life history and status information on marine mammals are the basis for 6 predictions. First, annual worldwide landings of fish and shellfish by the end of the 21st century will be less than 80 million tons. Second, virtually all of the predictions regarding species composition and energy flow within a marine community, based on models developed to date with incomplete information on species abundance, food habits, genetic effects of fishing, and variability of predator food habits, will prove wrong on a decadal or longer time scale. Third, the most common type of competitive interaction between marine mammals and commercial fisheries will be that in which commercial fisheries adversely affect a marine-mammal population by depleting localized food resources without necessarily overfishing the target species of fish (or shellfish). Because of this, the number of extant populations and species richness of marine mammals will be reduced by the end of the 21st century, and coastal populations and species will be affected more negatively than will noncoastal species. Fifth, predator control programs designed to reduce local populations of marine mammals will be common without changes in existing forms of fishery management. Finally, protein from marine mammals will become a more important component of the human diet than it currently is.

Key words
  • commercial fishery
  • competition
  • marine mammals
  • predation

… animals living in the water, especially the sea waters, are protected from the destruction of their species by Man. Their multiplication is so rapid and their means of evading pursuit or traps are so great that there is no likelihood of his being able to destroy the entire species of any of these animals.Jean Baptiste Lamarck

Lamarck (1809 cited in Roberts and Hawkins 1999) may have been asked by some well-intentioned symposium organizer, as we were, to make predictions about resource management 100 years into the future. Lamarck was clearly wrong concerning this particular prediction over the next 100 years, and we now remark at his naïveté. There is no reason to think that scientists reviewing our predictions 100 years hence will see any less humor in our assessments. Therefore, we have used this exercise of predicting future events primarily as a venue for our recommendations for the next few generations of scientists and managers regarding the management of marine mammal–fishery interactions.

Numerous lines of thought suggest that commercial fisheries have the potential to affect adversely ≥1 population of marine mammal by the end of the 21st century. Herein, we have dealt with biological (or indirect) interactions, as opposed to direct interactions (e.g., incidental mortality—Beverton 1985). We also have chosen not to address the interesting issue of whether marine-mammal populations will adversely affect ≥1 population of commercially important marine resources (i.e., finfish and shellfish). To a large degree, we have used patterns observed over the last 50 years regarding human population growth, success and failures in marine-fisheries management, and life history and status information on marine mammals to develop predictions and recommendations regarding the future status of mammals occupying marine environments.

Environmental Context

Human population growth

Since the industrial revolution, the human population has increased from 1 billion to more than 6 billion people (United States Census Bureau, International Data Base, Washington, D.C.). After 1950, the human population continued to increase, although the rate of increase has slowed (Fig. 1). Based on this projection, the size of the human population will stabilize in about 2070. By 2020, the human population will likely exceed 8 billion, and it could be as large as 12 billion by 2050. Sadly, the human species does not seem to have the capacity to regulate its own population size, other than by the regulatory mechanisms described by Malthus (1798 cited in Dhamee 1996 at http://landow.stg.brown.edu/victorian/economics/malthus.html) in the 18th century (i.e., war, famine, pestilence, and plague). Such regulatory mechanisms are not conducive to humans avoiding the tragedy of the commons (Sandvik 1998), which does not bode well for marine-mammal populations.

Fig. 1.

Human population growth rates between 1950 and 2050 (United States Census Bureau, International Data Base)

Currently, 50% of the world's human population lives ≤60 km from the coast, and it is likely that this figure will increase to 75% by 2020 (Intergovernmental Panel on Climate Change, in litt.). Based on these trends, coastal waters likely will become increasingly polluted over the next 100 years. As reported by Roberts and Hawkins (1999), 95% of marine-fish catches come from the continental shelf regions. Therefore, a related prediction for the next century is that the amount and quality of fish as a human food source will diminish. Implications of these trends for marine mammals will be discussed in the last section of this paper.

Marine-fisheries management and marine mammals

In the past 50 years, marine-fishery production has increased from just <20 million tons/year to 80–90 million tons/year. However, the rate at which marine-fishery landings have been increasing has dropped dramatically in recent years from almost 10%/year to zero (Grainger and Garcia 1996). In the Atlantic Ocean, landings actually have decreased in absolute terms since about 1983, with no evidence that the decline in landings is slowing. In other areas (e.g., Mediterranean Sea and Indian Ocean), landings continue to increase, although the rate of increase is declining. The National Research Council (Anonymous 1999) estimated that about 74% of individual stocks of fish and shellfish have either been overfished (30%) or are being fished at or near the maximum long-term potential catch (44%).

Globally, 28% of fishery products in 1996 were used as protein supplements for animal feed (Anonymous 1999), whereas marine fisheries contributed roughly 14 kg of food/person. If one includes current by-catch estimates with landings of target species, it is likely that humans remove about 100 million tons of biomass/year from the world's oceans. Some have argued that this figure is likely equal to the total production of ocean ecosystems and the “maximum long-term potential catch of marine animals” (Anonymous 1999:3). Others suggest that it is well beyond what may be realistically sustainable (Fowler 1999a; Fowler and Perez 1999; Fowler et al. 1999).

Key lessons have been learned about fisheries management as a result of the collapse of numerous fisheries worldwide in the 1960s. Butterworth (1999:5) noted that “oversubscribed industries create undue pressure to delay necessary catch reductions by advancing socio-economic arguments, with results that can take decades to rectify. Fisheries management is gambling, intelligently.” The international move to “limited entry fisheries” is a sign that fishery managers are learning how to manage social “momentum” and limit fishing effort, at least in some fisheries. On the other hand, Dayton et al. (1995) maintained that fishery managers have a long way to go before their practices can be considered sustainable. That is, managers must take a “more balanced scientific approach that considers both Type I (i.e., falsely rejecting the null hypothesis) and Type II (falsely accepting the null hypothesis) errors and the relative risks of various management alternatives, especially with regard to whether other non-target components of the ecosystems deserve protection” (Dayton et al. 1995:224).

The proposition that the current level of human-related removals of marine fish and shellfish is sustainable at the 100-year level remains debatable. Many experts in the field of marine fisheries have argued that the current level of human-related removals is not sustainable and that fishery managers need to reduce overall fishing mortality (Anonymous 1999). Fowler (1999a, 1999b) has developed an approach in which fishery “management action could be guided by frequency distributions of empirical examples of sustainability” (Fowler 1999b:25). That is, managers could use the observed harvest rate by marine predators on marine prey to estimate maximum harvest rates that are sustainable at ecosystem and biosphere levels, on a very long time scale (i.e., an evolutionary time scale). Such an approach, if implemented, would lead to a dramatic reduction in worldwide fishery landings on an annual basis.

Reducing catches is not the only topic before managers, and implementing change of any kind needs guidance. It is important to identify appropriate management measures, the kinds of changes that are advisable, and then the extent of change that is needed. Alternative fishery-management approaches could be integrated into current fishery practices and would be more sustainable than many of the current practices or approaches (Table 1).

View this table:
Table 1.

By the end of this century, the planet likely will be inhabited by an additional 6–10 billion people. If all existing food sources for humans contributed evenly to sustaining as many as 12–16 billion humans, then fishery production would have to double or triple. If 100 million tons of annual production is an optimistic estimate of all that we can harvest sustainably with existing guilds of marine species, this will not happen. It is more likely, given the presumed social and economic pressure associated with expanding human populations, that marine ecosystems will continue to be overexploited, which in the long run will provide less protein for the human population, rather than more. At the same time, many populations of marine mammals protected from direct exploitation will increase in abundance as they recover from having been overharvested in the past century. Therefore, by the end of the 21st century, annual worldwide landings of fish and shellfish likely will be <100 million tons. Severe political and social pressure from some segments of the fishing community will cause the purposeful removal (or at least reduction in density) of other top predators (e.g., marine mammals and seabirds) from the marine ecosystem as a way to solve the problem of an imbalance between worldwide demand for fishery products and production of marine ecosystems available for human consumption. This pressure will be justified by the argument that such removals will leave a greater fraction of annual net production for human consumption. This prediction is based on observations that calls for culling top-level predators to enhance commercial fisheries have already been heard, and historically, this has been a common management strategy in terrestrial settings.

Status of Marine-Mammal Populations

There are 124 extant species of pinnipeds, cetaceans, and sirenians (Rice 1998). At a minimum, adequate information about the ecological status of their populations should include data on population structure within each species and population size. To fully understand the role of marine mammals in the marine environment, information on distribution, abundance, trends in abundance, and current population level relative to carrying capacity (or, lacking information on carrying capacity, relative to population levels before recent human disturbances) is required. This information is not available for most species. Further, to evaluate the potential for competition between commercial fisheries and marine mammals, one also needs to know age- (or size-) specific information on energy needs and food habits. The available information on energy needs and food habits of marine mammals is incomplete. So how do we proceed with this evaluation?

Several authors have ventured boldly where few others have dared to go (Anderson and Ursin 1977; Green 1977; Innis et al. 1981). For example, Trites et al. (1997) addressed the issue of competition between fisheries and marine mammals for prey and primary production in the Pacific Ocean. Thereafter, an international scientific organization concerned with the North Pacific ecosystem created a working group that produced a report on abundance and seasonal distribution of marine mammals and seabirds by subregion of the Pacific. Tamura and Ohsumi (1999) estimated total food consumption by cetaceans in the oceans. Because of complex ecosystem interactions that are not well studied or well understood, the uncertainty associated with estimates of marine-mammal abundance and food habits, and the logistical difficulty and expense of collecting this information, predictions from these models remain untested with empirical data (J. W. Young, in litt.). Therefore, it is unknown if it is possible to adequately model marine ecosystems and thus produce reliable predictions about how reducing the biomass of 1 species will affect the biomass of another species. Furthermore, for these very reasons, virtually all of projections based on models developed to date with incomplete information likely will prove wrong on a decadal or longer time scale (other than the most general predictions, such as fish species are the largest consumer of fish species in any marine ecosystem). To a large extent, this is why Fowler (1999a, 1999b) has recommended using nature's “Monte Carlo” experiment as a better viewing port into the inner workings of complex systems such as marine ecosystems.

It is beyond the scope of this paper to summarize the status of all marine-mammal populations. Recent summaries by Read and Wade (1999) on the status of marine mammals in waters off the United States, Perry et al. (1999) on the status of 6 species of endangered large whales, Clapham et al. (1999) on the status of large whales, and Gerber et al. (2000) on recent trends in managing endangered populations of large whales are recommended. However, 1 generalization from these works and others (e.g., Best 1993) is that we know substantially more about marine-mammal species that inhabit coastal waters (e.g., gray whale, Eschrichtius robustus; most pinniped species; marine otters; sirenians; polar bear, Ursus maritimus), than we do about pelagic species (fin whale, Balaenoptera physalus; sei whale, Balaenoptera borealis; minke whale, Balaenoptera acutorostrata; all beaked whales, Ziphius, Berardius, Tasmacetus, Indopacetus, Hyperoodon, and Mesoplodon; sperm whale, Physeter macrocephalus; ribbon seal, Histriophoca fasciata). For example, out of 153 marine-mammal stocks in United States waters, we have trend data for 26% and abundance data for 66% (Read and Wade 1999). However, by taxon, we have trend data for 10% and abundance data for 60% of odontocete stocks, whereas we have trend data for 74% and abundance data for 96% of pinniped stocks (Read and Wade 1999). Presumably this reflects the dependence of pinnipeds on pupping and hauling out on land (or ice), which makes them logistically easier to study than odontocetes, many of which are pelagic throughout their lives.

In addition, more populations of marine mammals are either stable or increasing than decreasing (at least for those populations where data exist). For example, for populations where trend data are available (Read and Wade 1999), 27 populations are increasing, 5 are decreasing, and 7 are stable. Also, Best (1983) reported that for 12 populations of large whales where trend data are available, 10 are increasing in numbers. This does not mean that management of marine mammals has been successful across all taxa (Clapham et al. 1999); however, it does suggest that many populations of marine mammals have recovered to some extent from overexploitation in the 19th and 20th centuries and will continue to increase well into the next century.

Of greatest concern in trying to predict the degree to which commercial fisheries will adversely impact marine-mammal populations in the 21st century is the lack of information on pelagic species of marine predators in general, including nonmammalian species (e.g., squid) but particularly pelagic species of cetaceans. At present no credible population estimates or trends in abundance exist for any population of sperm whale. The same is true for the complex of beaked whale (Ziphius, Berardius, Tasmacetus, Indopacetus, Hyperoodon, and Mesoplodon) species. Consequently, it is not possible to reliably predict current biomass of prey needed to sustain these populations. Further, it is not possible to test if the food habits of recovering populations of large whales have changed over time because most of the food-habits information available for large whales was collected as part of commercial whaling operations before the moratorium on commercial whaling established in 1985–1986.

By the end of the 21st century, it is likely that at least crude time series of abundance estimates will exist for virtually all populations of marine mammals. This bold statement is predicated on recent advances in remote survey technology (visible- and invisible-wavelength technology and acoustic technology) and the growing interest of the general public in developed countries in supporting research on, and the protection of, marine mammals. It is not obvious that the same can be said for many of the other marine predators that are not of commercial value. Further, it is not at all clear if human society in the near future will value marine science sufficiently to fund necessary research to collect the information needed to make reliable predictions about human impacts on marine ecosystems (E. L. Miles, in litt.).

Competition Between Commercial Fisheries and Marine-Mammal Populations

Beverton's (1985:4) broad overview of the different types of interactive systems between commercial fisheries and marine mammals formed the basis of our analysis. Beverton (1985:4) noted “the conflict of interests arising from the interactions (real or supposed) between marine mammals and fisheries has, indeed, become something of a cause celebré in recent years.” That certainly has not changed in the last 15 years and likely will not change in the next 100 years. Also, as noted by Beverton (1985:4) “the world into which these mammal populations are recovering is not the same as it was when they were at their height decades ago.”

Although Beverton (1985) described 6 types of interactions, for simplicity, we only summarize 3 types that are most important to our predictions and recommendations. Existing data are inadequate to show beyond reasonable doubt that proposed interactions are the primary mechanism driving the system in each case; therefore, the marine mammal–fishery interactions presented below are intended to represent an underlying process rather than a classification of a particular interaction. It is also likely that other anthropogenic forces (e.g., global climate change) and nonanthropogenic forces (e.g., Pacific decadal oscillation) are important in determining the long-term state of marine ecosystems. The degree to which multiple factors influence the resulting species composition of marine communities remains an important question.

Steller sea lion and the groundfish fishery of the North Pacific

In this type of interaction (Fig. 2), the commercial fishery is not overfished as defined in current approaches to management. However, the fishery causes a significant reduction in biomass of the target species relative to an unfished condition and causes localized depletions of key prey species that result in a reduction in the foraging efficiency of the marine-mammal predator. This type of interaction likely will be the most important of the 3 in the future, and it primarily will affect smaller marine mammals, such as marine otters and pinnipeds. We make these projections because, in this type of interaction, many such fisheries are being well-managed as judged by the current definition of overfishing, and we expect many if not most fisheries in developed countries will adopt these practices in the future. Further, distributions of most commercial fisheries are coastal, as are distributions of most populations of marine otters and pinnipeds. The reason we believe that small species of marine mammals will be most involved in this type of interaction is because they are the ones that must balance energy needs, including those of their young, on a relatively short time scale (i.e., days), and in a relatively small area, compared with the situation for other marine-mammal species, such as whales. If the scale of the localized depletion caused by a fishery is on the same scale as the foraging area of the marine mammal, then commercial fishing likely will affect adversely the marine-mammal population with which it competes.

Fig. 2.

Conceptual model of the interactions in the Steller sea lion–groundfish fishery

Harp seal and cod fishery of the North Atlantic

In this type of interaction (Fig. 3), the fishery has overharvested the target species as a whole, or a widely distributed subpopulation, and caused it to become severely depleted. Because of its former commercial value, a management need exists for the stock to recover as quickly as possible. However, when the target species of fish also is an important prey species for a marine-mammal population, the fishing community often puts pressure on managers to minimize predation pressure by reducing the predator population (even though the actual cause of the decline may not be related to predation pressure by the marine mammal). This interaction generally is associated with marine-mammal populations that have broad food habits, such that the collapse of the commercial target species does not cause a similar collapse in the marine-mammal population. Interactions of this type also figure heavily in the minds of many when the marine-mammal population is large (e.g., western North Atlantic population of harp seal), even if the marine-mammal species preys on the target species to only a minor extent. Nonetheless, even a low level of predation by a large marine-mammal population could adversely affect the recovery of the fish species of interest. A similar form of interaction has been proposed to describe the interaction between populations of California sea lions (Zallophus californianus) and harbor seals (Pagophilus groenlandicus) that prey on endangered runs of salmonids and between sea otters (Enhydra lutris) and certain species of shellfish along the west coast of North America. This interaction will be common if fishery managers allow widespread overfishing. Further, it will occur when anthropogenic (e.g., pollution) and natural factors result in severe population declines in commercially important species of fish and shellfish.

Fig. 3.

Conceptual model of the interactions in the harp seal–cod fishery

Hawaiian monk seal and commercial lobster fisheries in the North Pacific

Unlike the previously described type of interaction, the marine-mammal population (or ≥1 age or sex class in the population) in this scenario (Fig. 4) is dependent on the health of the target fish species. Therefore, the collapse of the target species results in the collapse of the marine-mammal population or retards its recovery, making extirpation more likely. In this situation, the commercially harvested species typically has a much greater reproductive potential than does the marine-mammal population. Consequently, recovery potential for the commercially important fish species far exceeds that of the marine-mammal population. In such situations, extirpation of the marine mammal is much more likely than that of the species of commercial interest.

Fig. 4.

Conceptual model of the interactions in the Hawaiian monk seal–lobster fishery


We have made 3 general predictions regarding competition between marine mammals and commercial fisheries over the next 100 years. First, in the 21st century, annual worldwide landings of fish and shellfish will remain ≤80–90 million tons. Because of the imbalance between worldwide demand for fishery products and production of marine ecosystems available for human consumption, severe pressure will exist to remove, or at least reduce the density of, top predators (e.g., marine mammals and seabirds) from the marine ecosystem to leave a greater fraction of the annual net production for human consumption. Second, virtually all current predictions regarding species composition and energy flow within a marine community will prove wrong on a decadal or longer time scale because these predictions were based on models that were developed with incomplete information on abundance of marine mammals, energy needs, food habits, genetic effects of fishing (Law et al. 1993), and life history and abundance of prey species. Third, the most common type of competitive interaction between marine mammals and commercial fisheries will be that in which commercial fisheries adversely impact a marine-mammal population by reducing the overall biomass of a target species and then prosecuting the fishery in such a way that food resources of marine mammals are locally depleted without necessarily overfishing the target species of fish or shellfish. This type of interaction primarily will affect smaller marine mammals that are coastal in distribution, such as sea otters and pinnipeds.

It is unclear how human society will respond to the changes likely to occur in the future. If commercial fisheries are managed using current management practices, it is likely that changes will occur in the species composition of marine communities that are more rapid and more severe than previously observed. This is based on recent work (Fowler 1999a, 1999b; Fowler and Perez 1999; Fowler et al. 1999) that suggests that if humans remove biomass from marine ecosystems at rates that are unprecedented in nature, ecosystems that experienced degradation (Pauley et al. 1998) will eventually collapse or change in an unpredictable manner with undesirable consequences. The literature on management science suggests that harvest practices of humans have led to instability in prey populations relative to the stability in prey populations not used by humans, which have survived the unavoidable filter imposed by natural selection (Fowler 1999b; Fowler and Perez 1999). Improvements in sustainability could be achieved if the principle of maintaining system processes (Fig. 5) were applied within the normal range of natural variation (Fowler and Perez 1999; Fowler et al. 1999). In all cases, human harvest rates far exceed nonhuman harvest rates. As recommended by Fowler (1999a, 1999b) and Fowler et al. (1999), information about the variation in consumption rates among other predatory species could be used to guide fisheries to achieve harvest levels that are within the usual range of natural variation. Such a practice likely would minimize the disruptive influence of fisheries on community structure in the marine environment.

Fig. 5.

Frequency distributions for the amount of prey consumed at various levels of predation in the Bering Sea (adapted from Fowler and Perez 1999). a) Consumption of pollock (log10 tons per year) by 6 species of marine mammals compared with commercial takes by fisheries. b) Consumption of finfish by 20 species of marine mammals, and by commercial fisheries. c) Total removal of biomass in this ecosystem by 20 species of marine mammals in contrast to humans (fisheries). d) Consumption of biomass for the entire marine environment comparing that for 55 species of marine mammals with the total take by fisheries. e) Expansion of the comparison of (d) to the level of the biosphere to compare biomass consumption by the 55 species of marine mammals with the ingestion of biomass by the entire human population

Although changes in the composition of the marine community due to short- and long-term changes in environmental regime have been a persistent feature throughout the evolution of this planet, the speed with which these communities change when exposed simultaneously to natural and anthropogenic factors likely will cause localized extirpations and species-level extinctions at an unprecedented rate in the future. In part, this will be due to the mechanism described by Estes (1979), where long-lived species, such as marine mammals, have not evolved life-history mechanisms to adapt to sudden and major changes in their environment. Because changes may coincide with periods of time where harvest levels of marine products will fall well below the current level of 80–90 million tons, local societies may turn to predator control to minimize nutritional and economic stresses suffered by their members. Unless major changes are made, the current recovery of marine-mammal populations will be replaced by regular periods in which conventional forms of management will exercise predator control. The impact of managing marine mammals at levels well below their carrying capacities likely will lead to high levels of local extirpation and species-level extinctions. This likely will occur to species that occupy coastal habitats.

Perhaps it is best to conclude with a succinct list of predictions regarding the future status of marine mammals. We apologize to those offended by predictions that are largely unacceptable by today's standards, but we submit that today's standards will not matter 100 years from now. We predict that considerably fewer extant populations and species of marine mammals will exist by the end of the 21st century than now, and that coastal populations and species will be impacted proportionally harder than noncoastal species; that predator-control programs designed to reduce or eliminate local populations of marine mammals will be common without changes in existing forms of fishery management; that protein from marine mammals will be a more important component of the human diet than it currently is; and that harvest levels of marine resources will be less than the current 80–90 million tons/year. If fishery managers adopt management strategies that are sustainable at spatial scales that encompass entire ecosystems and time scales that approach the century mark, our predictions will be less likely to occur. If ecosystem-friendly management regimes are not adopted, the degree to which ecosystem health will diminish relative to the current standard will be severe. That is, the rate and number of species-level extinctions for all marine species, not just marine mammals, will continue to increase, the marine environment increasingly will become degraded, and our ability to use marine resources to feed ourselves will be compromised severely.


We thank P. K. Anderson for arranging the symposium on how marine mammals will fare over the next 100 years. The symposium was successful because of his hard work and diligence, as well as the fact that all of the speakers took their assignments seriously and presented interesting and compelling talks. We also thank the American Society of Mammalogy for sponsoring the symposium as part of their annual meetings. Finally, we thank P. K. Anderson, T. Klinger, L. Mazzuca, and M. Smolen for their review of early drafts; their comments improved the text considerably.

Literature Cited

View Abstract