Course:EOSC270/2021/Ecological Impacts of Finfish Aquaculture

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What is Finfish Aquaculture?

Finfish aquaculture is the cultivation or farming of captive marine finfish in the ocean using open-net pens, ponds, closed systems, and more recently, recirculating systems[1] . The umbrella term ‘finfish’ refers to marine fish with fins, as opposed to shellfish, and encompasses salmon, carp, tilapia, cod, trout, etc.

Aquaculture is a global method used in hopes of decreasing stress on wild fish populations as a result of overfishing and the increasing demand as global population increases[2]. Capture fisheries, a term which refers to the harvesting of both marine and freshwater naturally occurring resources, has been a relatively stable industry since the late 1980s at around 90 million metric tonnes of production. On the other hand, aquaculture yields have increased by 33 million metric tonnes between 1985 and 2004 alone[2]. While Asia is currently responsible for 89% of global aquaculture production[3], Gentry et al. have mapped the potential for further growth of marine aquaculture for nations including Africa, Europe, Middle East, North America/Caribbean, Oceania, and South America[4] (See Mapping the global potential for marine aquaculture. Genry et al. ).

Nowadays, most aquaculture facilities rely on open net pens, however technological developments in this ever-growing industry[4] may be leading towards the increasing use of recirculating systems. Recirculating systems reuse a portion of the water necessary for the cultivation of finfish through both mechanical and biological filtration. The amount of water that is recirculated varies depending on the institution and can be broken down into four categories[5]. Flow-through systems consume 30m3 of new water per kilogram of fish produced per year, whereas low level recirculating aquaculture systems (RAS) consume 3m3, intensive RAS consume 1m3 and super intensive RAS use as little as 0.3m3[5]. 

Red Tilapia (red perch) aquaculture ponds in Thailand.
Open net pens are the dominating method of containment for finfish aquaculture. Other forms of containment include ponds, closed systems, and more recently, recirculating systems.
Recirculating systems used in finfish aquaculture aim to decrease annual water use. Water is filtered then reintroduced to the tanks through means of mechanical and biological filtration.

While aquaculture may seem like a suitable solution to declining wild marine fish populations combined with a growing population, the ecological impacts of finfish aquaculture must also be considered.  The three main problems this form of aquaculture presents are: damage to farmed and surrounding species[6], high nitrogen levels due to large amounts of waste[7], and deposition of excess food and fecal matter[8]. A subsequent result of these threats is the potential for eutrophication, which can be defined as the “excessive growth of plant material in the presence of high nutrient concentrations”[9]. The ecological impacts of high nutrient content can be devastating to the surrounding environment and we will explore these consequences on various scales.

How does finfish aquaculture impact marine ecosystems?

Aquaculture farms cause excess feed and untreated waste to build up on the sea floor, creating "dead zones" or hypoxic conditions in the sediment. This excess feed and waste also get transported to surrounding areas via currents.

Finfish aquaculture impacts marine ecosystems at various spatial and temporal scales.  Looking at “caged” aquaculture, the artificial (human) activity and the natural ecosystem exist in an open system.  So not only are surrounding organisms impacted, but there are repercussions for the farmed fish themselves[6].  By examining the impacts at different scales, it will become clear how problems such as hypernutrification, hypoxia, biochemical demands and benthic enrichment, all come together to alter the marine environment.

First, it’s important to recognize the heavy overlap between scales.  Marine ecosystems are dynamic; momentary events can have lasting impacts, and localized changes can cause cascading effects on increasingly larger areas.  Keep this in mind when unpacking the relationship between finfish aquaculture and marine ecosystems.

Eutrophication occurs when an excess of nutrients are injected into the system by an aquaculture farm. This can cause algae populations to quickly get out of control, manifesting as potentially toxic algal blooms affect large bodies of water.

Internal Impacts - Immediate Environment

Spatial scale: ~100 m, Time Scale: minutes

The key impact here is oxygen depletion, hypoxia, as the rate of oxygen consumption of the farmed fish is not in balance with the oxygen replenishment from new water entering the system via ocean currents, etc. [8].  This is a threat to the aquaculture system and other species that inhabit the area.

Local Impacts - Surrounding Area

Spatial scale: ~1 km, Time scale: minutes - one tidal cycle

With open aquaculture cages, the surrounding water uptakes 60-80% of the nitrogen excreted in fish waste[7].  The main impact here is benthic deposition, as all the fecal matter and unconsumed food end up on the sea floor.  Silvert[8] explains that high deposition rates cause detruits to get trapped in the sediment and far surpass the feeding demand of benthos.  In decomposing so much excess organic matter, the benthos consume far more oxygen.  This oxygen depletion, or hypoxia, causes a shift towards anaerobic process dominance. Enriched fish food also stimulates benthic microbial metabolism, which has sulphuric by-products[10] that can become toxic to surrounding fish, both wild and farmed[6]. These factors, along with other inorganic chemicals introduced, alter the sediment chemistry and create an environment where bacterial mats are likely to form and the system is not able to naturally regulate itself.

As the demand for finfish aquaculture grows, so does the demand for fish food. Typically wild populations of smaller pelagic fish are caught to be turned into pellets to feed the aquaculture farms. This practice may be compromising the populations of these wild fish.

Regional Impacts - Larger Body of Water

Spatial scale: ~10+ km, Time scale: one tidal cycle - entire season

As stated by Silvert [8], eutrophication is the main aquaculture impact at this scale. The overabundance of nutrients injected into the system by the aquaculture farm can contaminate the larger body of water as it naturally circulates.  This can cause widespread algal blooms which block sunlight, deplete oxygen and can even be toxic.  Under these conditions the marine environment becomes uninhabitable for many species.

Global Impacts

Spatial scale: ~100+ km, Time scale: years

One key component here is the reliance of finfish aquaculture on fishmeal.  Wild anchovy, sardines, herrings and other small pelagic fish are harvested to make feed for larger farmed finfish[11].  In 2006 nearly 40% of farmed fish were fed with commercial feed[12].  With the world’s steadily growing population and demand for seafood, these wild populations are being exploited beyond appropriate levels.  As fishing is a global market, this indirect impact of finfish aquaculture is leading to overfishing of smaller pelagic fish across the world.  Further impacts on these smaller fish’s ecosystems will worsen with prolonged exploitation.

Summary of Impacts

Overall, the issue boils down to the inability of the natural environment to keep up with, and properly absorb, all the waste produced by finfish aquaculture.  This leads to imbalances of organic and inorganic chemicals, dominating organisms, and nutrient availability.  While this may inherently allow some species to thrive, the overall marine ecosystem becomes unstable and less conducive to a productive environment.

What is the extent of the problem?

The Past, Present, and Future of Finfish Aquaculture

Ecosystem Changes: Southeast Asia Case Study

Finfish aquaculture activity, when improperly conducted and managed, can severely damage the ocean environment, and particularly coastal ecosystems. Such damage can include waste disposal, habitat degradation or destruction, and pathogen invasions[13]. Honing in on habitat modification, we see the impacts of this practice when considering finfish aquaculture in southeast Asia. In countries such as the Philippines, Malaysia, and Indonesia, mangrove habitats in coastal wetland ecosystems have been converted into aquaculture ponds to farm milkfish[13]. Specifically, the mangrove habitats are being removed to provide space for aquaculture ponds, which causes many ecological deficits[14]. These mangrove trees are a significant species within their environment, as they provide a variety of ecosystem services[14]. Such ecosystem services include: sediment trapping, coastal protection, water treatment, controlling floods, etc.[13]. Therefore, the removal of mangroves results in various issues regarding these services, such as flood management, among others. Evidently, finfish aquaculture has impacts not only on intertidal flora and fauna, but on humans as well.

Intensive aquaculture ponds in Indonesia rest where mangrove forest habitat once existed.

Past vs. Present

When considering how finfish aquaculture has evolved over the past decades, one may think of advances in sustainable methods of farming, or improvements to the actual materials used in farm construction. A major change in aquaculture separating the past from the present is the food provided to the finfish. In the past, traditional aquaculture practices made use of wastes, byproducts, and natural food as the primary nutrient sources[15]. Presently, modern aquaculture farms use agro-industrially manufactured feed, which is essentially processed food pellets for finfish[15]. While superficially, this feed may not seem to be problematic, and only aids farmers in terms of time and money being saved, it actually has harmful effects on the nearby environment, such as fish species loss, reduced water quality, and other deficits that are associated with eutrophication[16]. Along with expansion of the aquaculture farms, the increasing use of this pelleted feed has resulted in an intensification in these aquaculture areas that may result in such eutrophication[15]. Specifically, excess fish feed that remains uneaten in the water introduces excess nutrients into the environment[15]. While modern technology and advancements may make certain practices easier, they also may have many environmental risks associated with them.  

Prognosis for Future  

Finfish aquaculture has a variety of environmental impacts which reshape communities globally. Returning to the mangroves case study, continued habitat modification will greatly harm mangroves and the ecosystem as a whole, impacting birds, fish, and humans alike[17]. If aquaculture growth maintains its current trajectory and results in continued mangrove forest exploitation, there may be many adverse effects on the land itself, as well as its inhabitants. Along with many fauna losing their homes, a similar fate could be met by people who live in coastal communities in southeast Asia. Without mangroves, these communities will be more susceptible to environmental catastrophes such as rising sea levels and tsunamis[17]. Although finfish aquaculture has negative implications for the future, there are certain practices to be put in place to ensure that the future is not completely bleak.

Given the impact, what are the solutions?

Finfish aquaculture needs to significantly alter its trajectory to handle growing food demands, as well as increased environmental strain. Aquaculture, according to the Agriculture Organization of the United Nations, is currently a source of food, employment and revenue in many countries and communities[18].

On a local level, British Columbia faces many upcoming problems from aquaculture. A report in 2018 by the Minister of Agriculture’s Advisory Council on Finfish Aquaculture (MAACFA) proposed some recommendations on the sustaining of salmon and the need for improved farming practices. Due its prominence in British Columbia’s economy, Aquaculture in BC will still be practiced, meaning that the primary issue is the risk to wild salmon populations presented by salmon aquaculture. Their recommendations include strengthening “the precautionary approach to regulating salmon farming in B.C. to reduce the potential risk to wild salmon[19], as well as better spatial planning, more First Nations and community involvement, adopting newer technology and methodologies, and more research into pathogens and sea lice. Overall, British Columbia can turn towards practicing more sustainable aquaculture, and to better plan for the long-lasting effects it has on the local environment and food supply.  

Three possible solutions for local fisheries include culture system improvements, feed strategies, and species selection[20].  

Recirculating aquaculture systems (RAS), are a type of aquaculture system that treats and reuses wastewater. Parallels can be drawn to fish tanks, which already filter and cycle the water to promote the fish’s health. RAS also reduce land and water usage, due to the water being recycled back in. Aquaponic systems are derived from the combination of RAS and hydroponics, essentially allowing the waste to be used to grow plant life. In addition, It is recommended to take steps such as selecting sustainable sites, and utilizing water circulation to maximize water quality[21]. New feed strategies revolve around finding a viably commercial replacement of fish meal and fish oil, such as animal products and plant proteins. Another method involves the recycling of processing waste into viable food sources, as well as the use of rendered animal products such as meat and bone meal. Due to climate change and other environmental factors, selective breeding has become more popular in order to increase growth rates or disease resistance, and therefore increase fish output. A semi-related topic, though controversial, is the use of genetic modification. This would allow, for example, finfish to develop in half of the usual time. A 2020 report further confirmed that integrating aquaculture production systems with livestock and agriculture, along with alternative sources of protein, water quality treatment and microbial management of farming systems were all key to the sustainability of aquaculture[22].

On a global scale, the Food and Agriculture Organization of the United Nations listed key topics such as ecological and economic sustainability, biodiversity conservation, the impact of climate change, new technologies, and possible policy opportunities as different aspects that should be addressed .[18] This includes changes such as implementing policies that encourage the implementation of better information technologies, while also matching the transformation of the industry as it move towards cleaner and more sustainable practices. Government policies allows the industry as a whole to evolve and adapt, instead of continuously catching up. Increasing public awareness of how crucial fisheries are to the global food supply, while also educating about the physical impacts of fisheries and the steps taken to improve is another way to get more public awareness, and therefore ensuring that fisheries get more ownership of their international role. Fisheries around the world must also respect these new policies that allow aquaculture to become less intrusive towards the environment. Fisheries should also develop better spatial management mechanisms to better combat shifts in species distributions.

These sustainable practices need to be held up consistently in order for the entire global industry to grow, change and adapt as a whole. Countries and communities all over the world need to take action to secure their future food supply, as well as the health of the global ecosystem.


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