What are the effects of fishing on biodiversity and ecosystem functioning in the marine environment?

What is Biodiversity?

Biodiversity is defined as “the variability among living organisms from all sources and ecosystems and the ecological complexes of which they are a part this includes diversity within species, between species and between ecosystems.” (United Nations, 1992). Human factors such as fishing can influence biodiversity in terms of community composition, species abundance, species richness, species biomass as well as the distribution and balance of flora and fauna within an ecosystem(Forester and Machlist, 1996).

What is Ecosystem Functioning and Services?

The function of ecosystems can be quantified by the services and processes that the ecosystem provides. These processes and services provided by an ecosystem can be summarised by Provisioning services, regulating services, cultural services, and supporting services. Ecosystem products such as food, water, shelter, clothing, or genetic resources are encompassed under provisioning services. Regulating services are service that aid in the regulation of the ecosystem such as climate, water, and natural disasters regulation. Supporting services are required for the rest of the services and functions to run; these include biomass production, oxygen production, and carbon sequestration. As well as nutrient cycling and providing habitats for organisms. Finally, cultural services are functions that the ecosystem provides which benefit humans. These include recreation, spiritual improvement as well as learning and aesthetic services.

The Impacts of Fishing on Biodiversity and Ecosystem Functioning

Anthropogenic impacts are vital drivers of change within the marine environment. These influences can create pressure and even loss of the ecosystem services and biodiversity in the marine environment (Coll et al., 2016; Fu et al., 2015; Halpern et al., 2008, 2012; Sale, 2008; Worm et al., 2006). Fishing has been shown to be one of the most significant influencers of marine ecosystems, and it has been suggested that it is also the greatest pressure on biodiversity (Coll et al., 2016). Fishing pressures in marine environments can significantly alter the community composition, the abundance of flora and fauna of the area, in addition to the overall species richness (Russ, 1991). Fishing has also been shown to decrease the biomass of individual organisms, specifically apex predators as they are most commonly targeted due to their high biomass. A decrease in biomass and abundance of top predators, like piscivorous fish can cause trophic cascades and has been shown to be a dominant feature in exploited ecosystems (Russ, 1991), for example, due to loss of predators of cownose rays through overfishing, there was a consequent increase in cownose ray abundance. This increase caused a dramatic decrease in the Cownose ray’s prey; North Carolina's bay scallop stock which eventually collapsed the commercial scallop fishery in that area (Myers et al., 2007). This increase in ray population may have been caused by an Allee effect where there is positive density dependence on personal fitness, wherein the growth rates of individuals is increased by the increased density of the population. Overfishing, that is fishing above a sustainable level, can have a pronounced effect on the biodiversity of an ecosystem along with its essential functions and services. Overfishing causes a direct reduction of targeted species and can result in trophic cascades which ultimately lead to a decrease in biodiversity. For example, removal of key species with ecosystems can alter the structure of the habitats (Coleman and Williams, 2002). As most of the time, the largest of the fish are targeted during fishing, the size structure and size diversity of the community might be altered (Rago et al., 1998).

Unfortunately, the overall impact of fishing on biodiversity and ecosystem services is hard to quantify, due to many variables such as differences in the ecosystem, location, habitat, as well as other factors. Another important boundary to quantifying the impact of fishing on the marine ecosystem is the illegal, unregulated, and unreported fishing (IUU) which occurs all around the world. The IUU means that the true impact and intensity of the fishing is not easily quantifiable.

As fishing methods which physically alter the environment are thought to be the main threats to ecosystem services (Mak et al., 2005), different methods of fishing and the various impacts they have on different organisms, ecosystems and their services will be examined.

The Effect of Different Fishing Methods on Biodiversity and Ecosystem Functioning.

Apex Predator Fishing

Apex predators are predominantly hypercarnivores, which means that more than 70% of their diet consists of prey items that are at the top of the ecosystem trophic pyramid. Apex predators are often targeted during fishing on coral reefs, as their high biomass often makes their catch a lucrative business. It has been quantified that anthropogenic effects in the marine environment have led to a decline in 90% of the apex predator abundance (Myers and Worm, 2003). It has also been suggested that the most significant influencer on the apex predators is commercial fishing (Jones et al., 2009). The removal of apex predators creates instability in the ecosystem food web, as they can affect the behaviour and population dynamics of the organisms in the lower trophic levels and therefore the biomass of these organisms (Dulvy et al., 2004). In response to fishing pressure in Coral Reefs, the biomass of critical groups of organisms has been shown to change with both light and intensive exploitation. Under both light and intense fishing pressures, the biomass of piscivorous fish (fish which eat other fish) decreases the most, with the biomass of herbivorous and invertebrate feeding fish also decreasing. Invertebrates biomass has been shown to increase, probably due to not being directly targeted during light and intense fishing (Jennings and Polunin, 1996). In unfished coral reef environments 54% of the biomass was comprised of apex predators such as Sharks and Jacks, whereas, in areas which had increased fishing pressure, apex predators (mainly piscivorous fish) accounted for only 3% of total biomass (Friedlander and DeMartini, 2002). It was also found that increased fishing pressure, resulted in a 260% reduction in overall biomass of fish, and fished areas were dominated by herbivores(Friedlander and DeMartini, 2002). The reduction in these large apex predators can cause a loss in overall ecosystem biomass which is a reduction in an ecosystem supporting service as biomass production is a crucial ecosystem function although it can cause an increase in lower trophic biomass production. A decrease in biomass of top predators, like piscivorous fish can cause trophic cascades, where the removal of one high trophic species impacts the abundance of the prey species. Another impact of fishing on coral reef apex predators is bycatch, whereby a non-targeted species are caught. It is thought that around 50% of the Elasmobranch bycatch goes unreported, this makes it hard to quantify the exact pressure fishing places on apex predators with the estimated landings of apex predators at around 1.5 million tonnes (Stevens et al., 2000). Bycatch of apex predators means that even if a sustainable fishing limit is set for these apex predators, the bycatch is not quantifiable and therefore the sustainable quota is not likely to be met.

Cyanide Fishing

       Cyanide fishing is a method of fishing in which sodium cyanide is squirted into the fishes’ habitat to stun the fish. This fishing method is practised in coral reefs to capture fish for the aquarium trade and occurs most frequently in Asia. It is not a widely discussed fishing method, and although this process of fishing is illegal in most areas, it is thought that it is still widely used. Approximately 1 million kilograms of cyanide has been used in fishing on the Philippine reefs (Mak et al., 2005).

        It has been found that cyanide fishing not only stuns the fish targeted but can also impact the surrounding area, in particular, corals. Exposure to cyanide has been shown to cause the bleaching ( the dissociation of symbiotic zooxanthellae ) in corals and therefore affects the photosynthesis of the zooxanthellae (Jones and Hoegh-Guldberg, 1999). Cyanide treated reefs have been shown to have a 40% reduction in the density of zooxanthellae compared to control reefs (Jones et al., 1999). Medium doses of cyanide cause coral bleaching while light treatments cause discolouration. It has also been found that high doses of cyanide, comparable to those used in cyanide fishing, caused complete mortality of the coral. It was also shown that respiration rates were dramatically reduced by as much as 90% after cyanide exposure (Jones and Steven, 1997). A reduction in respiration has implications for the supporting ecosystem services of a coral reef, as loss of the zooxanthellae and reduced respiration mean that less carbon is being sequestered. Coral reefs are an important carbon sink, and around the world account for 111 million tonnes of carbon per year, any reduction in this amount would be a reduction in vital the ecosystem function of carbon sequestration. The removal of fish for the aquarium trade through cyanide fishing may also reduce the biodiversity of the coral reefs, as certain desirable species may be over-taken from the reef. This will alter the community structure of the reefs.

Blast Fishing

       Blast fishing is a fishing method whereby explosives such as dynamite are used to stun the fish, using shockwaves so that they can be easily retrieved. Although this practice is illegal in many places, it is thought that blast fishing still occurs in around 40 countries. It is a particular problem in Tanzania where explosives are cheap, easy to obtain, and it provides a lucrative income (Slade and Kalangahe, 2015). However, the blast fishing is altering the structure of the coral reefs which are a key source of revenue for Tanzania in the form of tourism. Tourism is a crucial cultural ecosystem service, so by altering the reef this essential ecosystem service may be impacted. A FAO and NGO led team was implemented in 2015 to bring awareness to the issues of blast fishing in Tanzania. However, some studies have shown that there has not been a reduction in the occurrence of blast fishing even with this scheme in place (Katikiro and Mahenge, 2016).

        Dynamite or blast fishing not only stuns the fish but also physically changes the habitat, primarily when used in the vicinity of coral reefs. The dynamite blast causes parts of the calcium carbonate skeletons of coral to shatter. Which, often causes mortality of the hard coral. The main source of restoration is larval supply from other corals in the vicinity as long as they have not been impacted. Recovery depends on many factors including the intensity of the blast fishing (Roberts, 1997).  Blast fishing is particularly detrimental to corals as many coral species are not able to recover very well from disturbance which physically alters the habitat compared to disturbance which does not change the physical environment(Connell, 1997).  The disturbance caused by blast fishing can cause a significant alteration in the community structure in coral reefs, communities which were once dominated by calcium carbonate hard coral structures shift to a composition dominated by algae and soft-bodied corals such as Xenia spp. (Roberts, 1995). This shift may be because soft corals have higher fecundity and more modes of larval dispersion than hard corals. Once soft corals have colonised an area, it is unlikely to shift back to its original composition as soft corals hinder the colonisation of hard-bodied corals(Maida et al., 1995). This change in structure leads to a reduction in biodiversity as the soft corals do not provide the same level of protection and habitat as hard corals(Inoue et al., 2013). Also, it can cause a reduction in the biomass of coral reef fishes in the area as reef fish numbers and size is influenced by the coral cover and reef complexity among other things (Roberts, 1995).

Trawling

Trawling is the process of pulling a net through the water column often with a beam or other mechanism to keep the net open, being dragged across the sea floor. Trawling can impact the marine environment in multiple ways and have impacts on the biodiversity of the benthos and the ecosystem functions it provides. The effects of trawling with equipment like otter trawls and beam trawls have been a cause for concern since 1376 when the changes to the seabed caused by trawling were brought before the British Parliament (De Groot, 1984). Some of the major issues with trawling; include the capture of non-target species, the ploughing of the substrate, alteration of the benthos and sediment resuspension. Trawling has also been shown to drive long-term changes to the composition and structure of the benthos (Jones, 1992). Direct impacts of trawling have been shown to influence the abundance, biomass and species richness as well as other indicators of biodiversity. Sedentary fauna such as anemones, sponges, and bryozoans were found at higher abundances in areas of un-trawled benthos compared to trawled areas due to the lack of physical disturbance in un-trawled areas, which could damage these organisms. Biodiversity and number of niches were found to be greatest in un-trawled regions (McConnaughey et al., 2000). It has also been found that species richness was higher in un-trawled areas were as species evenness (similarity) was higher in trawled sites. These finding may be due to un-trawled zones providing a more complex habitat compared to trawled areas (Collie et al., 1997).  Trawling has been shown to have an indirect effect on the length at age of certain species such as Pleuronectes platessa or the European Plaice. Plaice were found to be smaller in areas of gravel than in areas of sand; it is suggested that this is because Plaice in gravel habitats are more likely to feed on echinoderms and molluscs which are negatively impacted by trawling. Therefore, leading to a decrease in prey and hence a decrease in growth of these Plaice. Whereas, sand-living Plaice feed predominantly on polychaetes which are positively impacted by medium intensity trawling, therefore their growth is enhanced. This suggests that trawling may influence an ecosystems biomass production either negatively or positively depending on the substrate.  This may have a severe consequence for crucial ecosystem services such as food production (a key ecosystem product) and may lead to a decline or increase in commercial fish production (Shephard et al., 2010). Discards from trawling constitute a significant driver of change in the marine environment (Dayton et al., 1995). Non- targeted species, which are caught as bycatch are sometimes used as alternative food sources but are very often thrown back into the sea. It has been calculated that for some areas, just under 80% of discard would not survive (Hill and Wassenberg, 2000) and therefore the discard of this dead material can impact the ecosystem in several ways (Dayton et al., 1995). One impact of trawling discard may be an increase in the abundance of Elasmobranchs and other scavengers such as seabirds which learn to associate trawlers with food as they scavenge on the “free food”(Furness et al., 1992). This behaviour may impact the marine food web in the area as increased predator abundance could decrease the abundance and biomass of organisms of the lower trophic levels, this may influence the fisheries landings of that area and therefore the key ecosystem function of food production. Almost all the continental margins are now being trawled, however, the range of trawling is being expanded as new technology is developed. Increasingly trawling is occurring in the deep sea. Trawling has been shown to reduce overall biodiversity by half as well as impacting many ecosystem services such as the organic carbon turnover which showed a 37% decrease in areas that had been trawled as well as a decrease of over a half of organic matter. The abundance of fauna has been shown to decrease up to 82% when the pressure of deep sea trawling is present (Pusceddu et al., 2014). Some deep-water fish stocks are being exploited to collapse, so much, so that deep sea fishing (in its current form) is considered by some as unmaintainable (Davies et al., 2007).

Dredging

Dredges have similar effects to trawls as they also create physical disturbance to the seabed by either digging the sediment or by using jets of water to lift the substrate. The largest dredges can leave 1 by 0.4 m grooves in the sediment, and some boats can dredge upwards of 30 dredges at the same time, therefore creating a large impact on the benthos. Dredges are more effective at slower speeds than trawls, therefore, spending more time in contact with the seabed.(Kaiser et al., 1996)

Conclusion

Fishing is a major driver for change within different marine habitats, with fishing having a significant impact on the biodiversity, in terms of reducing the species richness and increasing the species evenness.  Fishing has also been shown to change the community structure and even the genetic resources of an environment by the removal or change in essential species.  Fishing also has an impact on the abundance and biomass of individual species which are both supporting ecosystem functions. This variation in biomass of certain species can have a knock-on effect further down the food web. Thus, fishing can be responsible for altering the entire community structure. Fishing can also influence other ecosystem functions such as carbon sequestration and nutrient cycling. Fishing can also physically modify an environment such as coral reefs, which not only impacts the fauna and flora inhabiting that area but also on humans who use some marine habitats to fulfil cultural and social function such as recreation. Some fishing methods can alter the environment so much that these cultural services are no longer viable.  

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