The relative contributions and ecological impacts of aquaculture and capture fisheries

Historically, the oceans were considered limitless and thought to harbor enough fish to feed an ever increasing human population. However, the demands of population growth, particularly in poorer countries, now far outstrip the sustainable yield of the seas. At the same time as fishing has become more industrialized and wild fish stocks increasingly depleted, aquaculture production – fish and shellfish farming – has grown rapidly to address the shortfalls in capture fisheries. With this rapid growth aquaculture has come under intense scrutiny and criticism as environmentalists fear that it could cause significant environmental problems and further impact wild species that are already over-exploited.

Indeed, both capture fisheries and aquaculture must have environmental costs, all human activities of significant scale do, but it is necessary to fairly evaluate and compare the ecological and economic impact of both. In fact, a thorough analysis shows that the ecological threat of aquaculture is much lower than continuing to supply the majority of fish protein from wild capture owing to aquaculture’s greater control over production, harvest, processing and transport, which results in less wastage and reduced energy demands.

Fish is a vital source of food for people. Fish is man’s most important single source of high quality animal protein and provides approximately 16% of the animal protein consumed by the world’s population (FAO, 1997). It is a particularly important protein source in regions where high-quality protein from livestock is relatively scarce: fish supplies less than 10% of animal protein consumed in North America and Europe, but 17% in Africa, 26% in Asia and 22% in China (FAO, 2001). The Food and Agriculture Organization of the United Nations (FAO) estimates that about 1 billion people worldwide rely on fish as their primary source of animal protein (FAO, 2001).

Fish also has substantial social and economic importance. Over 36 million people are employed directly through fishing and aquaculture (FAO, 2001), and as many as 200 million people derive direct and indirect income from fish (Garcia and Newton, in press). The FAO estimates the value of fish traded internationally to be US$ 51 billion per annum (FAO, 2001). Consumption of food fish is increasing, having risen from 40 million tonnes in 1970 to 86 million tonnes in 1998 (FAO, 2001), and is expected to reach 110 million tonnes by 2010 (FAO, 1999). Increases in per capita consumption account for only a small portion of the increase in total demand. It is the growing human population in many countries in Asia, Africa, and South America that is primarily responsible for this steadily increasing demand for food fish. These statistics illustrate that a consistent source of fish is essential for the nutritional and financial health of a large segment of the world’s population.

Today, fish is the only important food source that is still primarily gathered from the wild rather than farmed, with marine capture historically accounting for more than 80% of the world’s fish supply. Total landings from marine fisheries increased approximately five-fold in the 40-year period from 1950 to 1990 (Mace, 1997). More recently, however, capture fisheries have not been able to keep pace with growing demand, and many marine fisheries have already been over-fished. From 1990- 1997, fish consumption increased by 31% while the supply from marine capture fisheries increased only 9% (FAO, 1999). This has intensified the pressure on the harvesters, which has translated into increased pressures on, and over-fishing of, many commercial fisheries. Nearly half of the known ocean fisheries are completely exploited (FAO, 1999) and 70% are in need of urgent management (MacLennan, 1997).

As fisheries become depleted and fish get harder to catch, many fishermen and governments have responded with increased investment in equipment and technology to fish longer, harder, and farther away from their home ports. These efforts have resulted in what is essentially an ‘arms race’ within the marine fishing industry, both in the addition of greater numbers of people and ships but also in better technologies (MacLennan, 1997). Radio and satellite navigation allow fishermen to better locate fishing grounds, while new fish-aggregating devices intensify the harvests. These changes put immense pressure on fish stocks and leave fewer regions out of reach so that fish can reproduce unmolested. This decreases the reproductive capacities of fisheries, thus exacerbating the effects of over-harvesting.

Indeed, capture fisheries have advanced to the point where newly discovered fish populations can be put under severe stress more quickly than regulators can collect needed biological data and impose catch limitations. Based on the current assessment of over-exploitation of many fish stocks, and overcapacity and over-capitalization of many fishing fleets, Mace (1997) concluded that many capture fisheries would probably not be commercially viable without significant government subsidies. However, the private and public investment in increased infrastructure creates a financial inertia that makes it more difficult to reduce the pressure on fisheries (Speer, 1995).

Consumer tastes and demand in the developed countries have largely contributed to the problem. Increasing demand for top predators, such as swordfish or tuna, has put severe pressure on existing stocks. The average size of fish caught for some species has dropped until there is now a significant need to impose minimum size limits, or capture moratoria, to allow these and other species to reach reproductive age and size before being removed from the population. The hunt for certain species also affects non-target species through their inadvertent capture, known as ‘by-catch’. Longline fishing for swordfish and other billfishes may significantly diminish the populations of many shark species, which are known to have slow reproductive rates and thereby slow recovery rates.

Trawling technologies also capture a large amount of by-catch, known as ‘trash fish’. Alverson et al. (1994) estimated that ocean fishing results in about 28.7 million tonnes of by-catch annually, most of which is simply discarded. These figures are very likely low estimates of total wastage, as by-catch figures are often under-reported, and statistics do not include fish lost to spoilage, undetected mortality under the surface, and ghost fishing through lost equipment that continues to catch fish (Alverson et al., 1994). For certain shrimp species, the by-catch is often composed of a high percentage of juveniles of commercially important species, compounding the impact on both present and future fisheries production. Nance and Scott-Denton (1997), when analysing a 5-year survey of trawling operations in the Gulf of Mexico, found that only 16% of the total catch was commercially valuable shrimp, while 68% was unintended by-catch, mostly juvenile finfish.

In some areas of the Gulf of Mexico, it is estimated that for every 1 kg of shrimp harvested, 10 kg of other species are caught and discarded. High profile examples of by-catch conflicts, such as the capture of sea turtles by shrimp trawls and of dolphins by purse-seines targeting tuna, have drawn severe criticism by environmental groups and consumers. But it is consumer demand that has fuelled this conflict, as tuna and shrimp are the number one and number two seafood demand categories in the developed countries.

Humankind also places severe indirect strains on ocean fisheries. The World Resources Institute reported that about 51% of the world’s coasts are at high or moderate risk of degradation (Speer, 1995; FAO, 1999). Since approximately 90% of the marine capture fisheries depend on coastal habitats for young fish to develop, direct and indirect losses of nursery habitat and negative effects on water quality have a devastating impact on commercial fisheries.

To meet the ever-increasing demand for fish, aquaculture has expanded very rapidly and is now the fastest growing food-producing industry in the world. The proportion of the total fish supply produced by aquaculture increases yearly. By the year 2030 it is estimated that over half of the fish (FAO, 2000) consumed by the world’s people will be produced by aquaculture (Figure 1). Total aquaculture production increased from 10 million tonnes of fish in 1984 to 38 million tonnes in 1998 (FAO, 2001), and a growth rate of 11% per year has aquaculture on a pace to surpass beef production by 2010. Not only is the total amount of fish being produced important, but also how and where it is produced is important. While 80% of cattle are raised in industrialized nations, fish farming has been growing almost six times faster in developing countries than in developed countries. The FAO states that “As an inexpensive source of a highly nutritious animal protein, aquaculture has become an important factor for improving food security, raising nutritional standards, and alleviating poverty, particularly in the world’s poorest countries.” Indeed, in those areas where the need is greatest, the contribution of fish and shrimp farming is expected to increase. For instance, the FAO estimates that small-scale aquaculture production in Africa will significantly increase by 2010; in fact, fish and shrimp production in Africa has already grown by about 400% between 1984 (37,000 tonnes) and 1998 (189,000 tonnes).

Rapid growth of aquaculture has led, in some cases, to environmental problems and conflicts over limited resources. One problem widely publicized by non-government organizations and environmental groups has been losses of mangrove forests (Naylor, 2000). Mangroves are extremely productive coastal ecosystems and their decline has indeed been extensive–as much as 55-60% of the original forests have already been lost. However, most of that loss is due to clearing for rice production, grazing, urban development, fuel, construction materials, wood pulp, and tourism; conversion to shrimp farms accounts for less than 10% of the decrease (Boyd and Clay, 1998). In fact, the vast majority of new shrimp pond construction does not affect mangroves because these areas have proven to be unsuitable for shrimp production due to acid soils and high construction costs. Mangrove buffer zones now are protected in many new shrimp farm developments, and replanting has become common.

‘Biological pollution’ is a term that has been used to describe the potential of introduced species on natural populations. Its recent usage has primarily been in the context of Atlantic salmon (Naylor et al., 2000). Atlantic salmon (Salmo salar) is the main salmon species reared artificially. Total aquaculture harvest of this fish in 1999 was about 800,000 mt or about 2.7% of total world aquaculture production (FAO, 2000). Over 94% of the world’s Atlantic salmon adults are in aquaculture production facilities and 6% in the wild (Gross, 1998). Recently the vast literature on the potential impact of Atlantic salmon from aquaculture sites on wild salmonid populations were comprehensively reviewed and analysed by Gross (1998). The author reported that along with potential negative genetic and ecological effects, Atlantic salmon aquaculture does offer some benefits for wild populations, but these benefits are often overlooked. In developed countries there has been a significant shift in consumer preference from wild Atlantic salmon, and other wild salmonid species, to farmed Atlantic salmon. Increased availability has decreased prices, resulting in decreasing harvest pressure on wild stock. Gross’s conclusions were that aquaculture is not the root cause of the current poor state of wild salmonid fisheries and conservation. The author reported that there are two primary causes, mismanaged capture fisheries and habitat destruction, which have resulted in wide-scale extirpations, depletions, and loss of biodiversity in both Atlantic and Pacific salmonids, and that this occurred long before commercial salmon aquaculture appeared in the 1970s.

Figure 1. Foodfish supplied by capture fisheries and aquaculture.

Recent criticism has also centred on the use of fish meal in aquaculture diets. Naylor et al. (2000) reported that aquaculture is “a contributing factor to the collapse of fisheries stocks world-wide.” The authors further state that with aquaculture expansion, “ever increasing amounts of small pelagic fish would be caught for use in aquaculture feeds to expand the total supply of commercially valuable fish.” However, in truth, fish meal production has changed very little over the past 20 years. Adele Crispold (personal communication) from the FAO explains that market forces have simply reallocated the use of a fixed amount of fish meal, but have not actually changed the total amount of pelagic fish harvested or fish meal produced. The percentage of total fish meal production used for aquaculture feeds has indeed increased from 10% in 1988 to 35% in 1998. However, the large majority of fish meal is still used in livestock feeds and for fertilizers, while the actual amount of fish harvested to produce fish meal has remained relatively constant at about 30 million tonnes per year (FAO, 1999). A statistical analysis of FAO data over the past 15 years indicates that there is no statistical relationship (P>0.80) between aquaculture production, harvest rates for pelagic fishes and fish meal production (Figure 2). A shift in fish meal use toward aquaculture may actually represent an environmentally friendly use of this resource, as fish are more efficient feed converters than the primary users, terrestrial livestock.

Naylor et al. (2000) also proposed that certain types of fish, particularly salmon and shrimp, are actually net consumers of fish, requiring as much as 3 kg of fish in their feed to produce 1 kg of farmed fish. Overall, these species represent a relatively small proportion of total aquaculture production (Figure 3). Furthermore, to evaluate these values fairly, they must be compared to these products if sourced from wild harvests. Forster (1999) points out that, based on classic values of energy flows, 10 kg of forage fish are required to produce 1 kg of a carnivore (such as salmon) in the wild. If by-catch values are taken into account, at least another 5 kg of fish can be added to the equation. Based on these considerations, even if farmed salmon or shrimp do utilize 3 kg of fish to produce 1 kg of weight gain, this would actually represent a significant ecological advantage compared to 10-15 kg of forage fish needed in the growth and capture of 1 kg of wild salmon or shrimp. Also, when considered in toto, aquaculture is a huge net producer, generating 3.5- 4.0 kg of food fish for each kg of pelagic fish (live weight) used in fish meal production (Figure 4). Importantly, the efficiency of aquaculture production will improve further. As an industry, aquaculture is still in its relative infancy, thus knowledge of the nutritional requirements of most fish species is rather limited compared to poultry and other livestock. Naylor et al. (2000) noted that livestock feeds on average “contain only 2-3% fish meal.” However, 20 years ago, fish meal was also the preferred source of protein for poultry feeds, just as is the case for some aquaculture species today. Reduced reliance on fish meal for poultry feeds came as a result of nutrition research, particularly the quantification of requirements for individual amino acids and energy needs as well as the rigorous evaluation of alternative ingredients.

Figure 2. The relationship between aquaculture production, pelagic fish landings, and fish meal production from 1984-2000 based on FAO data.

Figure 3. The proportion of total aquaculture production accounted for by different taxonomic groups.

Figure 4. Relationships between fish meal consumed, calculated live weights of pelagic fish used, and weight of aquaculture products produced.

The search for alternative ingredients is already a research priority for aquaculture for exactly the same reason: the desire to minimize feed costs. In channel catfish diets, the proportion of fish meal in the feed has decreased from 8-10% in 1990 to less than 3% currently, based on an improved knowledge of their nutritional requirements (Robinson, 1996). Several other species can also be successfully fed with similarly low contents of fish meal (Allan et al., 1999. Other factors caused by the relative immaturity of the industry will also greatly benefit from continuing research efforts. In salmon production the introduction of vaccines has reduced the amount of antibiotics used per kilogram of salmon cultured by over 97% (Klesius et al., 2001).

Increased use of animal by-product meals to decrease aquaculture’s use of fish meal has also been proposed (Tacon et al., 1998). Due to concerns over BSE, such rendered products are available at relatively low costs. While their use in other ruminants is of great concern, the evolutionary distance between ruminants and cold blooded fish and crustaceans could possibly provide a safe outlet and use for these products (Tacon and Forester, 2000). However, significant research would be required to ensure consumer safety.

In an earlier paper, Naylor et al. (1999) concluded that, due to a reliance on fish meal, aquaculture of these species is being subsidized by the marine ecosystem. However, all human food production is eventually ‘subsidized’ by aquatic or terrestrial ecosystems. The production of some aquaculture species is indeed partially fuelled by primary and secondary productivity within the marine system, but fish caught in the oceans have been entirely subsidized by the marine ecosystem. Even the ‘cultural species’ identified by Naylor et al. (2000) as net producers, such as carps, tilapia, and catfish, do not actually convert food to flesh with higher efficiency than other species such as salmon or shrimp. They are, in fact, only ‘subsidized’ by different ecosystems–the freshwater ecosystem in the form of natural food items or terrestrial ecosystems through the production of feed ingredients, such as corn or soybean, each of which has its own ecological costs. Prudent and proper use of fish meal under certain situations may actually be advantageous for the environment. Due to its extremely high nutritional quality, i.e. the proper balance of amino acids and fatty acids, and extremely high digestibility, the use of some fish meal in the diet can reduce waste production in the culture system compared with completely plant-based diets.

The demand for fish meal could potentially be met by improved use of by-catch from wild capture fisheries (Howgate, 1997). The amount of by-catch killed and discarded annually is estimated to be between 18-40 million tonnes (FAO, 1996)– approximately the total amount of fish currently harvested for fish meal production (30 million tonnes). There is also a significant amount of fish currently wasted due to the intentional discarding of part of the catch. This occurs when fishermen wish to save limited quota at times when prices are low or when they practice ‘high grading’–discarding smaller fish of low value to create capacity for species that achieve a higher price on the market (FAO, 1999). For some capture fisheries, as much as 40% of the total catch is discarded. In aquaculture there is much more control over production, harvest, processing and distribution (Howgate, 1997), and these practices seldom occur.

Capture fisheries and aquaculture should not be considered in isolation. In certain areas some supposedly ‘wild harvest’ fisheries are actually highly dependent on an aquaculture phase to produce young fish needed to maintain current capture rates. In Alaska, for instance, aquaculture is basically outlawed. However, without the aquaculture production of seedstock, Alaska’s wildharvest salmon and oyster industries could not supply a fraction of the total production currently generated. According to Coates (1997), the divisions between aquaculture and capture fisheries will rapidly fade and, in many regions, have already gone. In fact, the best hope of providing fish to meet future demands will likely be co-ordinated partnerships of aquaculture, managed wild fisheries, and wise protection and management of coastal zones and ecosystems.

Studies that do not weigh the relative costs and impact of the different sources of fish are overly simplistic and not constructive. Skewed conclusions can cause negative public opinion that could impede environmentally responsible aquaculture and its ability to supply the projected 35 million tonnes of aquatic foods needed to meet the difference between demand and capture (FAO, 2001).

Unfounded negative media coverage could further stifle aquaculture development in rural and lowincome areas where its potential impact is greatest. In a recent report, FAO (2001) stated that “irrespective of whether inaccurate information is generated deliberately to promote a specific cause, or inadvertently through ignorance, it can have a major impact on public opinion and policy making that may not be in the best interest of either the sustainable use of fisheries resources or the conservation of aquatic ecosystems.”

There are not too few fish–there are too many people. If agriculture had not developed to increase the production of terrestrial livestock, we would never have been able to support the current human population. A similar juncture has been reached or passed in fish supplies. Although per capita consumption has not increased substantially, population growth has increased to the point that capture fisheries alone can fill only two thirds of the current demand for fish, thus almost all future demand will have to be met by aquaculture.

According to the FAO (2001), “there do not seem to be any insurmountable obstacles to the continued growth of aquaculture”. Both aquaculture and capture fisheries cause environmental impacts, which can be substantially reduced through further research and improved management. However, if aquaculture is unfairly assigned a negative label through unbalanced ecological assessments, its potential contributions to present and future food securities could be severely compromized. This could be especially devastating in regions where high-quality protein is needed most. If aquaculture development is unfairly impeded, the increasing deficit between wild harvest rates and total demand for fish will actually increase the pressure to capture more fish from the wild and further devastate stocks of many marine fish species. These consequences on both human and fish populations would seem to go against the stated intentions and missions of many of the groups currently attacking aquaculture.


by James H. Tidwell 1 and Geoff L. Allan 2
1 Aquaculture Department, Kentucky State University, Frankfort, Kentucky, USA
2 New South Wales Fisheries, Port Stephens Fisheries Centre in Nelson Bay, Australia

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