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  • Published on: 15th October 2019
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Coral Reefs are located in shallow subtidal ecosystems in the tropical oceans and provide many different resources to marine organisms and humans as well. They have a geologic component which is the deposition of calcium carbonate by the corals, mollusks, and algae that are present there (Kleypas et al. 2001). However, it is reported that they have been declining drastically all around the world (Hughes et. Al. 2011). Even though there are efforts being put towards restoring coral reefs the decline is still present. This is important to take into consideration because they provide various services such as shoreline protection, food, beauty and the list goes on (Birkeland 1997). In fact, it is estimated that the coral reefs contribute over US $375 Billion per year of ecosystem services (Pandolfi et. al. 2005). Also, approximately 850 million people live within 100km of a coral reef and 275 million live within 30km of a reef meaning that many depend on the reefs (Oguro et al. 2015). Because of the important resources that the reefs offer, human overexploitation of them causes a serious problem. It is found that the reefs are a fragile ecosystem where, if key functional elements are removed, then the consequences for the reefs can affect the entire ecosystem in a negative way. Most of these issues have to do with the overharvesting of resources and organisms which will be discussed. First, the overharvesting of trees in nearby areas can have an indirect effect on the reefs due to the possibility of sediment runoff into the reefs. Next, the fishing that is done in the reefs is destructive and can cause physical damages of reefs. Lastly, unregulated fishing leads to the depletion or extinction of key species which causes macro-algae blooms which outcompete the corals, decreased reef resilience and outbreak of consumers of corals such as the crown of thorns starfish.

Deforestation leads to sediment runoff:

Due to anthropogenic causes such as settlements, industry, and infrastructure, deforestation of local forests, that are near coral reefs, have occurred. For example, an increase in tourism leads to the construction of hotels and coastal development. To build all of this, the trees and mangroves are overharvested or completely wiped out. Between 2000 and 2005, approximately 2.4% of the humid tropical forests were lost due to these factors which are found in coral reef countries such as Brazil, Indonesia, and Cambodia (Hansen et. al. 2008). Because there is no mangrove forests or seagrass beds which act as a buffer, the sediments, nutrients, and pollutants enter into the waters where the corals are present (Waycott et. al. 2009). High concentration of sediments can weaken and kill corals and other benthic organisms. They decrease the effectivity of zooxanthellae to photosynthesize which will stunt coral growth (Alongi et al 2005). Above all, the most increasing problem is that because of these excessive levels of nutrients, there are blooms of phytoplankton. This blocks light from reaching the corals and can cause an abundant growth of algae and seaweed on top of the seabed. Extreme eutrophication leads to hypoxia which leads to a dead zone which consequently causes an ecosystem collapse. The number of these cases documented has increased from 10 in 1960 to 169 in 2007 (Selman et. al. 2008).

Destructive Fishing Techniques:

It is globally known that a tremendous amount of fishing is taking place everywhere to feed humans. The coral reefs are also one of the main sources of food for people. In fact, over 91% of the people who live within 100km of a reef are found in developing countries (Donner & Potere 2007). Because they are in developing countries they are most likely poor and rely on fish as their main food source (Fenner 2012). Pandofi and other researchers reported that fishing is the main contributor to the major problems of coral reefs long before other anthropogenic causes (2003). The techniques in overharvesting the fish cause physical damage to the corals itself impacting the whole community. One of these fishing methods is the use of explosives to kill or stun the fish. Fox and Caldwell reported that with small blasts the coral was able to establish themselves. However, in severely bombarded areas even though enough time was given and the presence of coral larvae is there they were not able to show any significant recovery in that area. Arguing that even human intervention for those areas are not helpful and control should be established to prevent the use of explosives in the first place (Fox and Caldwell 2006). There is also the use of cyanide to stun and capture the fish live for aquarium markets or for selling as live reef fish. The cyanide can bleach corals and polyps and then the corals themselves are destroyed by fishers as they reach for the stunned fish (Mous et al 2000). Mous, however, found that cyanide fishing does not contribute to coral degradation as severely when compared to other fishing methods. Blast fishing accounts for approximately 3.75m2 per 100m2 of reef yearly (Pet-Soede 1999). Mous said that compared to his observations with cyanide fishing, this is 75 times worse than their best estimate, and 5-6 times worse than their worst estimate, of the destruction of corals through fishing. He notes that overexploitation is a bigger concern than cyanide fishing. Nevertheless, damaging fishing techniques are still present and being used which are causing destruction to the reefs. The reason why these are used is that they allow for easy harvesting of fish which leads to another problem: overexploitation.

Consequences of the overexploitation of fish markets:

Newton reports that there is widespread unsustainability in all coral reef fisheries. 55% of the 49 island countries are exploiting their coral reefs and the total landings of the fisheries are 64% higher than can be sustained right now (Newton et. al. 2007). This means that the rate of exploitation in coral reefs is on a increase and there are consequences associated with that. Mumby demonstrated that even with the removal of a single species of fish, such as: parrotfish, there are detrimental effects across the whole ecosystem (Mumby et. al. 2006). Parrotfishes are a dominant grazer in the coral reef community. With the overharvesting of this fish, it no longer is able to keep the algae population in check. Due to the overgrowth of algae, “algae shading” occurs, which is algae forming a layer above the sea. Box and Mumby performed tests using different types of algae and observing its effects. The L. variegata species caused a loss in coral tissue and mortality rates increased from 0-50% in half a year. Simply the presence of the algae reduced the growth of juvenile corals decreasing it to 60% compared to the that of control corals. Shading by the algae D. pulchella species caused 99% growth inhibition (Box and Mumby 2007). This causes serious coral vulnerability because the juvenile corals are not able to efficiently grow and can lead to whole colony death. Box and Mumby state that this happens due to interference competition and is what most likely contributes to the dominance of macroalgae on many Caribbean reefs.

Even though fishers primarily seek large, predatory fish; as the population of the large fish declines, fishers move down to also smaller fish in the area, overexploiting the reef even more. This process is called fishing down the food chain (Pauly et. al 1998). Pauly states that fishing down the food chains ends with a period associated with declining catches. In other words, there are simply no more fish left to catch that is profitable. This shows that this practice is unsustainable, as the populations are never able to reestablish. These types of reefs that are overfished and consist of low numbers of small fish are prone to not only the algal overgrowth mentioned before, but are also less resilient to stressors as well. This means they are now vulnerable to disease. Raymundo explored this issue and surveyed various reef spots and discovered that there was a negative correlation between disease prevalence and fish diversity in the coral reefs. He claimed that heavy fishing does lead to cascading effects. Species once held in control by predation and competition are released and no longer regulated by other species (Graham et al 2003). Without the presence of fish diversity, many higher order interactions which are responsible for stability are gone (Raymundo et. al. 2009). As mentioned, algae overgrowth which occurs from the lack of top herbivory fish, is a contributor to the weakness of corals. Along with this weakness, there is evidence found by Raymundo to show that some species of fish, the butterfly fish, which is usually ignored by fishers, are hosts and a vector for disease. This species of fish is abundant in overexploited areas because of the lack of predator control. They specialize in hard corals and carry some form of a disease that will kill the corals.  Raymundo found that in areas where fishing is regulated or not present completely such as in marine protected areas the reefs are tropically diverse and the amount of disease is scarce compared to overharvested coral reefs (Raymundo et. al. 2009). This is not the only problem that can arise from low diversity in coral reefs ecosystems from overexploitation. Another problem has to do with predators that feed on the corals themselves.

Crown-of-thorns starfish outbreaks:

Due to the fishing exploitation that was going on, Dulvy, Freckleton, and Polunin set out to document the biodiversity consequences of predator removal in coral reefs. They documented predatory reef fish densities, coral-eating starfish densities and coral reef structure along 13 islands in Fiji. It was already known that the crown-of-thorns starfish feeds upon live corals and is known as one of the main causes of disturbances by a specific species (Birkeland and Lucas 1990). Dulvy found that lightly fished reefs, that had higher predator densities, were dominated by organisms that would build hard coral reefs. Areas that were overfished and had low predator densities, were infested with the starfish and dominated mainly by fast-growing, non-reef building species such as turf algae. In these heavily fished areas, the predator densities declined 61% and starfish densities increased by three orders of magnitudes (Dulvy et. al. 2004). It is suggested that the reason for the outbreak of starfish growth is due to the plankton blooms that provide food for the starfish larvae to grow. The consumption of coral by the starfish results in mass coral mortality for island observed (Dulvy et. al. 2004).

Marine Protected Areas and Possible Solutions:

Reefs at Risk Revisited state that unsustainable fishing is the most influential threat to coral reefs. Where more than 55% of all the world’s reefs are threatened by this, 30% are considered highly threatened (Burke et. al. 2011). There was a common suggestion to combat the reef decline from all the papers that were reviewed. Conservational practices with the use of marine protected areas, or MPA for short. These are tools used in coral reef management and conservation. There is a form of an MPA called a: no-take marine reserve (NTMR). These areas are completely protected from all activities that would take something from the protected habitat. A review of different studies showed an increase of fish biomass in these types of MPA by 413%, and a 200% increase in density (Sobel and Dahlgren 2004). However, For MPAs to significantly save coral reefs, they need to be established in areas where fishing pressure is heavy, yet this occurs primarily in developing countries as mentioned earlier. Establishing an NTMR prohibits fishers to catch food to feed their families. In order for a no-take area to be effective there must be local compliance in these areas. Therefore, there must be some incentive provided for community fishers to be able to benefit from this (Fenner 2012). Blackwood, Hastings, and Mumby have created mathematical models to predict the time of recovery with the implementation of lowered or restricted fishing habits. It was found that complete coral recovery will approximately take 20 years with no fishing effort. However, if it is increased to 0.25 then it will take more than 60 years. The model illustrates that drastic management would be effective in helping restore coral reefs (Blackwood 2012).

It was also mentioned that deforestation was also a contributor to coral degradation due to the sediment runoff. This means that the protected coral reef needs to have management on the land as well (Allison et. al. 1998). Similar to MPA, there is a necessity to implement drastic management principle to protect the land which protects the reefs as well. By preserving the wetlands, mangrove and seagrass at the coast will filter and trap the sediments and nutrients that can trickle down into the reefs. If the area cannot be preserved or heavy mining will occur the in the area than the erosion and runoff can be controlled by used diversion channels and sediment traps (Burke et. al. 2011).

Reefs at Risk Revisited provides some practical way that can help the coral reefs as well. Fishery business can have season closures to protect the sites where fish breed and allow the population to rebound. Restrictions can be put on the type of fishing practice is used or the amount of fish people are able to catch. Sustainably tourism should be practiced as well by educating the tourists of the importance of local reefs and business should avoid selling corals or seafood that is not sustainably harvested (Burke et. al. 2011).

Conclusion and future research:

This review has provided a small glimpse of current problems that are impacting the coral reef ecosystems specifically through the perspective of overharvesting resources or organisms.  It was found that deforestation of trees and mangroves, which normally acts as a form of protection for the coral reef habitats, leads to sedimentation and excess nutrient runoff into the reefs which increases mortality rates (Waycott et. al. 2009). Then, due to the need for fishing, fishers have been using unsustainable fishing practices such as explosives and poisons that are severely physically damaging the reefs. Due to the strength of some explosions corals are not able to even grow back there anymore (Fox and Caldwell 2006). Next, overharvesting the predatory and herbivory fish causes detrimental cascading effects on the ecosystem. One example is the macroalgae blooms which competes with juvenile corals (Box and Mumby 2007). Another was decreased reef resilience which made corals more susceptible to becoming diseased and dying off. Lastly, with the lack of predatory fish and increased algae blooms there are more reported outbreaks of crown-of-thorns starfish which eat on the coral itself (Dulvy et. al. 2004).

It should be mentioned as well that even though marine protected areas are effective and protect the reefs from overfishing they are limited. They depend on local community enforcement and participation from everyone (Christie 2016) They cannot help what goes on land by human interactions there. Also, they cannot protect from abiotic factors such as sea temperature rise, ocean acidification, hurricanes, or oil spills (Fenner 2012). All of these factors are influenced by humans as well and are harming the reefs.

There are many challenges associated with the conservation efforts of coral reefs. However, it must be done because millions of people depend on the reefs for food, shoreline protection and other services it may offer (Fenner 2012). To further guide some research, the MPAs need to be improved. This is because they heavily rely on local efforts to enforce the areas. Poor developing countries usually do not have success in MPA because of lack of communication. The Coral Triangle Initiative on Coral reefs, Fisheries, and Food Security propose to increase the efforts to implement vertical integration from the regional level to the local level (Christie et al. 2016). They mention that it is difficult but possible over time. What this allows is the ability for a single entity in control of the conservation efforts rather than trying to regulate it through multiple parties. Lastly, another area of focus could be on the studies of the direct cause of the crown of thorns starfish. Dulvy mentions that it may be both regional and local factors that cause outbreaks. He says that it may be worth studying the starfish dynamics and thresholds in Fiji and other Indo-Pacific regions where outbreaks may occur (Dulvy et al. 2004). This can help focus allocate time, energy, and funds on conservational efforts that cause a direct positive impact in the certain location to help preserve reefs from pest outbreaks.

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