Human activity has evoked considerable harm to aquatic ecosystems and is the cause of global changes in marine life interactions (Jackson et al., 2001). The concept of fishing suggests that all fish species populations contain a surplus of biomass available for harvest which may be extracted without permanent damage to the ecosystem, however this concept holds untrue to current global fishing practices. Overfishing is a method of anthropogenic overexploitation of aquatic environments, which is the reduction of marine biomass below such a mass that the remaining populations are unable to be replenished and ecosystems begin to lose productivity (Beamish et al., 2006). This overexploitation directly and indirectly contributes to the degradation of Earth’s oceanic ecosystems, including, but not limited to, kelp forests, coral reefs, seagrass beds, temperate estuaries, inland freshwater systems, and offshore benthic communities on continental shelves (Jackson et al., 2001). The global overfishing of large predatory species has not only caused extinctions, extirpations, and massive decline in these species populations, but has had cascading trophic effects through the marine trophic levels (Scheffer et al., 2005). Considerations of the longevity of fish populations apply directly to overfishing, as older fish as well as younger fish must remain present in the ecosystem to maintain population integrity (Beamish et al., 2006). The processes by which overfishing occurs and the widespread nature of its repercussions are significant to the understanding of aquatic ecosystems.
An Overview of the Effects of Overfishing
There are numerous anthropogenic mechanisms of disruption for aquatic ecosystems including, but not limited to, overfishing, pollution, mechanical habitat destruction, the introduction of invasive species, and climate change. However, overfishing is the first action (Figure 1) and the most devastating action by humans in the sequence of historical events to be a source of aquatic environmental disturbance (Jackson et al., 2001). With the exponential acceleration of human population growth, there has been an intensification of the anthropogenic disturbances to marine environments. There is unregulated exploitation of biological resources at increasing geographical scales for the purpose of human commercial activities (Jackson et al., 2001). This overexploitation has resulted in numerous cases in the extinction or extirpation of entire trophic levels from ecosystems, which significantly
Figure 1: The historical sequence of human disturbances to coastal ecosystems, beginning with fishing as step 1, and with all disturbances leading to the result of altered ecosystems (Jackson et al., 2001).
degrades the resiliency of the ecosystems to natural and human disturbances. The effects of this exploitation are synergistic, thus the overall impact is remarkably greater than the sum of the impacts of each individual disturbance (Jackson et al., 2001). Fisheries can cause the depletion of both their target and non-target species through indirect mechanisms in the ecosystem (Coll et al., 2008), thus the synergistic effect can be attributed to the cascading trophic effects which occur following an ecosystem disturbance (Scheffer et al., 2005).
Trophic cascades can be the indirect effect of either overfishing a large predatory species for a top-down impact on the pelagic food web, or overfishing of smaller species for a bottom-up impact on the food web. In either scenario, the trophic cascade concept holds that the removal of one species from the ecosystem will have effects which extend throughout trophic levels separate from that of the lost species (Scheffer et al., 2005). The top-down effect can be illustrated through the historical collapse of the Canadian cod populations, Gadus morhua. Off the coast of Nova Scotia in the late 1980s and early 1990s, the cod population as well as other large predatory fish populations rapidly declined as a result of the overexploitation of these species by Canadian fishing industry (Scheffer et al., 2005). This
process virtually eliminated the top predators in the ecosystem’s trophic web, and was the start of the trophic cascade. The loss of top predators led to a drastic increase in the populations of their prey species; these were the benthic invertebrates, such as northern shrimp and snow crab, and small pelagic fish. As a continuation of the top-down effect, the large-bodied zooplankton species, the prey of shrimp and crab, subsequently declined. With the reducing feeding of zooplankton, the phytoplankton species became more abundant, which led to the final observed effect of lower nitrate levels as nitrate was depleted more quickly by the phytoplankton populations (Scheffer et al., 2005). Experimental reproductions of this top-down process have been held in lakes with the same result, indicating undoubtedly the effects of top-down trophic cascades following anthropogenic overfishing in aquatic ecosystems (Scheffer et al., 2005). Similarly, trophic cascades were relevant in the case of overfishing in the Black Sea (Daskalov, 2002). The aquatic environment conditions in this area have significantly declined, due to an increase in the populations of phytoplankton and jellyfish, and a decrease in large fish populations. Following the trend illustrated with Canadian cod, the pelagic predators in the Black Sea, such as bonito, mackerel, bluefish, and dolphins, were severely overexploited by human fisheries, which was the initiation of the top-down trophic cascade (Daskalov, 2002). The reduction of the apex predator populations decreased their control over the planktivorous fish, which subsequently increased in population size. The increased consumption of these herbivorous fish reduces the zooplankton biomass, thus increasing the zooplankton grazing population, the phytoplankton (Daskalov, 2002). These scenarios of uncontrolled fisheries, as seen in the Canadian fishing industry and the Black Sea, can lead to detrimental effects on aquatic ecosystems.
Contrastingly, the bottom-up variety of trophic cascades involves the process of removal of biomass and energy at lower trophic levels, which decreases the secondary production of mass and energy at the higher trophic levels (Coll et al., 2008). The catches by fisheries are a net export of biomass and energy which can no longer be of use in the aquatic trophic web. There is an increasing number of unsustainable fisheries, with the total catch per capita existing at an estimated twice the moderate sustainable level of fishing, being the sustainable level to which secondary production may be reduced without catastrophic trophic effects (Coll et al., 2008).
Types of Overfishing
Longevity overfishing is a specific category of overexploitation which pertains to the removal of large numbers of older age groups of aquatic species (Beamish et al., 2006). This
process is particularly important for the commercial fisheries on Canada’s Pacific coast to take into consideration, where the majority of species in the area have lifespans of thirty years or longer. The current management of long-lived species, such as the sablefish, operates under the assumption that young fish have the same ecosystem productivity per unit of biomass as the older fish, and thus the species is managed on a basis of biomass and not longevity (Beamish et al., 2006). However, this assumption is incorrect for the sablefish and numerous other long-lived aquatic species. It has been proven that when younger fish have lower productivity per biomass than older fish, the overexploitation of the older fish can irreversibly damage the productivity of the ocean ecosystem; in this case the ecosystem will never recover from an extended period of low productivity (Beamish et al., 2006). Thus, it is necessary for the Canadian coastal fishing industries to adjust their management strategy to account for longevity as well as biomass, in order to prevent the permanent disruption of these marine ecosystems. The concept of longevity is one of three distinct types of overfishing (Beamish et al., 2006). The second type, referred to as recruitment overfishing, reduces the older members of the population to such a level that the juvenile fish are incapable of reproduction at a rate to replenish population. The third type, referred to as growth overfishing, occurs when fish are caught at such a young age that the population as a whole is reaching only a small fraction of its potential growth age or size (Beamish et al., 2006). Of these three concepts, longevity overfishing has only recently been recognized, since numerous species of fish were not known to attain great age until the mid-1980s. Overfishing in all of its forms as a mechanism of anthropogenic disturbance leaves devastating effects in numerous different types of aquatic ecosystems (Beamish et al., 2006).
Impacts on Individual Aquatic Ecosystems
A marine ecosystem particularly impacted by the effects of anthropogenic overexploitation is kelp forests, which provide habitat for numerous fish and invertebrates. Kelp forests in the northern hemisphere have widely shown a decrease in the number of trophic levels (Figure 2) and a deforestation of the kelp as a result of massive herbivore population increases; this is due to loss of apex predators in the ecosystem from overfishing (Jackson et al., 2001). The sea otter is one of the predators specifically in the Northern Pacific kelp forest ecosystem. The action of sea otters preying on sea urchins controls the sea urchin
population and prevents the overgrazing of kelp (Jackson et al., 2001). However, with decline of sea otter populations, this mechanism of control is lost. The Sea otter populations are decreasing with the increased predation by killer whales, which cannot feed on their former main source of food; the killer whale’s main diet consists of organisms which are being overexploited by humans, seals and sea lions (Jackson et al., 2001). Similarly in the Gulf of Maine kelp forests, the Atlantic cod, among other large predatory fish, prey on sea urchins. In the 1920s, the development of mechanized fishing equipment led to a rapid decline in the Atlantic cod population, as well as the extinction of other predatory fish species. These larger fish have been ecologically replaced by smaller species, which do not control the sea urchin population as efficiently, resulting in the decline of the kelp forests (Jackson et al., 2001).
The Trophic Impact of Overfishing on Kelp Forest Ecosystems
Figure 2: Simplified trophic webs indicating the differences in top-down trophic interactions between kelp forest ecosystems without overfishing (A) and with overfishing (B) (Jackson et al., 2001).
Coral reefs, the most diverse oceanic ecosystems, provide complex environments for tens of thousand of marine species. For at least half a million years, there is evidence of highly stable coral reef ecosystems, providing proof that the current disruptions are a result of human interference (Jackson et al., 2001). The corals in theWestern Atlantic reefs suffered a dramatic decline in the 1980s due to the rapid expansion of a macroalgae population. This expansion was a result of a sea urchin, the last major herbivore, being locally extirpated
following the previous extirpation of numerous other large herbivorous fish species (Jackson et al., 2001). Additionally, decades of overfishing sponge predators has led to increased competition between marine sponges and coral colonies in coral reef ecosystems (Loh et al., 2015). In Caribbean coral reefs, sponge-consuming fish require additional laws for fishing restrictions, given that the loss of these species from the coral reef ecosystems increases the competitive interactions between sponges and corals. The overgrowth of corals by sponges leads to the inhibition of gas exchange and water flow through the ecosystem, impacting the thousands of species that reside in coral reef systems (Loh et al., 2015).
Similar to coral reefs, seagrass beds provide habitat for a diverse assemblage of marine species and indicate evidence of millenia of stability prior to human disruption (Jackson et al., 2001). Due to the overfishing of the large herbivorous species centuries ago, the seagrass ecosystems are now more vulnerable to disturbances such as disease and increases in sedimentation and turbidity. In America and Australia, the essential herbivorous species now absent from these seabeds is green turtles, a species overexploited by European colonists (Jackson et al., 2001).
Due to anthropogenic exploitation, as well as pollution, temperate estuaries are the most deteriorated marine ecosystems (Jackson et al., 2001). The disturbances include increased sedimentation and turbidity, increased hypoxia episodes, the extirpation of sea grasses and other dominant suspension feeders, decreased oyster reef habitat, outbreaks of jellyfish, and a shift in ecosystem primary production from benthic to planktonic production. Specifically the mechanical overharvesting of oyster reefs beginning in the 1870s may be the cause of the recent massive ecosystem degradation (Jackson et al., 2001). The decreasing populations of oysters in the estuaries led to oyster fishery collapse in Chesapeake Bay, which through cascading trophic effects subsequently led to the symptoms of eutrophication and outbreaks of oyster parasites in the twentieth century. In addition to oysters, there are numerous other suspension feeding bivalves which graze on plankton in estuaries and limit blooms of phytoplankton, thus limiting eutrophication. The loss of these species due to overfishing is catastrophic for the temperate estuary ecosystems (Jackson et al., 2001).
Inland Freshwater Systems
Inland freshwater systems suffer from anthropogenic exploitation equally to, if not more than, marine ecosystems, and yet they lack attention from the media and conservation groups (Allan et al., 2005). An assortment of direct and indirect impacts occur from overexploitation, and result in freshwater ecosystems supporting species which, on average, are more threatened than those residing in marine ecosystems. The indirect impacts include altered flow of water, habitat fragmentation, habitat degradation, pollution, and invasive species (Allan et al., 2005). The direct impacts, mainly the extirpation of species, cause a disrupted state of these ecosystems which subsequently result in increased vulnerability to the indirect impacts, such as disease outbreaks and invasive species. Most inland fisheries are currently relying on species which are already being captured at their biological limit, or over the limit to overexploitation (Allan et al., 2005).
Offshore Benthic Communities
Offshore benthic communities on continental shelves cover a higher percent of the ocean floor ecosystems than all of the previously discussed aquatic ecosystems combined. The overfishing of commercially important species in these ecosystems for centuries in Europe and North America, and more recently globally, has resulted in the ecological extinction of numerous formerly abundant large marine species (Jackson et al., 2001). The extensively fished commercial species include, but are not limited to, cod, halibut, haddock, turbot, flounder, plaice, rays, scallops, cockles, and oysters. The conversion of human practices from hook-and-line to industrialized fishing vessels has devastated these marine ecosystems (Jackson et al., 2001).
Conclusions and Future Directions
Recognizing the complex network of interactions between humans and marine environments is crucial to protecting species and the resources they provide. Both the root anthropogenic causes and their global effects on marine environments must be investigated to fully understand the mechanisms and extent of these changes. In order to protect all of the individual types of aquatic ecosystems, such as kelp forests, coral reefs, seagrass beds, temperate estuaries, inland freshwater systems, and offshore benthic communities on continental shelves (Jackson et al., 2001), all of the differences between these ecosystems must be taken into consideration when legislation on overexploitation is created. Due to the
impacts of trophic cascades, there must be legislation globally in order to protect not only the species already endangered from overfishing, but the all aquatic species in the ecosystem which may be affected by the loss of an apex predator. The process of global responses to current events on a regional case-by-case basis is insufficient to provide the widespread changes which are needed to stabilize these ecosystems. Alternatively, these issues must be addressed globally through a series of experiments testing on the scale of whole ecosystems, not only the individual species or trophic level (Jackson et al., 2001); the success of integrated management of multiple aspects of the ecosystems must be tested in order to have a more complete view of the impacts of overfishing and the solutions required to restabilize the environments following trophic cascades, and to prevent permanent detrimental effects (Scheffer et al., 2005).
Dana S. Halay
A Method of Anthropogenic Overexploitation of Aquatic Ecosystems