One of the ocean’s greatest services is its fisheries that provide a critical part of the human diet worldwide. Seafood now makes up 17 percent of the animal protein the world consumes and that demand is expected to double in the next two decades. At least half of the global fish supply comes from wild-caught marine fish—87 percent of which are estimated to be fully or over exploited. Aquaculture (fish farming) makes up most of the remaining balance.
Many fisheries have close links to particular ecosystems. Approximately 95 percent of all commercially important fish species depend on coastal habitats such as mangrove forests, coral reefs, seagrass meadows and saltmarshes, mud flats, and rocky shores at some point during their life. Numerous studies have looked at the value of fisheries in different habitats, as these “fish factories” generate critical resources, economic gain and a source of employment worldwide.
The abundance of fish on coral reefs is surprising, as many of the world’s coral reefs are found in nutrient-poor waters. Their rich bounty is an apparent anomaly. In fact, this productivity is also fragile. Reefs are susceptable to many pressures, including pollution, sedimentation, climate change and overfishing. If too many fish are taken from a reef, productivity of the entire system is compromised. By the time most larger fish have been removed from a reef the complex ecosystem is more vulnerable to impacts from from diseases, tropical storms, algal overgrowth and other pressures.
By contrast, when fishing is limited, or well-managed, coral reefs have the capacity to feed coastal populations, generation after generation. This is important for small island countries where locals often have few protein options since agricultural soil is often poor and land is scarce.
Mapping Ocean Wealth: Coral Reef Fisheries
The global map of fish catch considers four elements:
- the reef productivity
- local fishing intensity
- international fishing for key target species
- management influences
The expected background productivity of reefs made a simple allowance for lower productivity on Caribbean reefs, as well as the reduction in productivity, which is observed in areas where human impacts degrade reefs. We then predicted local fishing pressure, assuming a direct correlation with the size of local human populations, further influenced by possible access to larger markets.
We also recognized that not all reef fishing is local. There is also a worldwide harvest of very high value species: sharks, large groupers and snapper, in particular, are now of sufficient value in markets, notably in East Asia, that they support a truly global fishery. Rarely sustainable, and often illegal, such fishing for key target species affects almost every coral reef in the world, with remoteness offering little protection. The final minor modification to our model was to take into account reef management. For this we factored in the role of no-take fishing reserves. In the model, we assumed that these would have increased fish populations inside (which could not be fished), but that such reserves would also enhance fishing opportunities in a halo around their perimeter.
Mangroves provide a unique space for fish and shellfish. Their complex structure is a natural shelter—a safe space to breed, or to live and grow away from predators in early life. Mangroves also have abundant light, nutrients and oxygen which enable prolific growth not only of the trees, but also of algae and plankton. This powers prolific food chains.
Some animals, such as highly sought-after mud crabs, may spend all their lives in the mangroves. Many others, from shrimps to snappers, just use mangroves in their early lives before migrating offshore.
Many mangrove fishers are artisanal—fishing small volumes to feed their families and earn small amounts of cash at markets. They use traps, lines and cast nets to gather a mixed haul of fish, shrimp and crabs. Such fisheries can generate 100 to 1000 kilograms of fish per hectare every year. Other fishers target key and high value species, notably crabs, cockles, oysters and shrimp. These generate lower volumes but high values, particularly if they are close to important markets—some fishing grounds, from Fiji to Hong Kong generate, over US$2,000 per hectare year on year.
Many of these fishers are fully aware of their reliance on mangroves, however they do not always have the political influence to halt the destruction or conversion of mangroves to other uses, such as aquaculture, which can generate high incomes in the short term, but at the cost of multiple incomes, food security and jobs in the artisanal fishing sector.
Larger commercial fisheries operating offshore often overlook the mangroves on which they depend. Key among these is the offshore shrimp industry. Vast quantities of shrimp are trawled from the seabed in areas from Northern Australia and Malaysia to French Guyana, and all depend heavily on the supply of adult shrimp that began their lives as larvae among the tangled roots of coastal mangroves.
Mangroves: Mapping Ocean Wealth
Our initial global model of mangrove fisheries was built up from a detailed review of hundreds of studies from around the world, and informed by an expert panel. In terms of natural productivity, the most important areas are those with high input of freshwater and nutrients notably focused around estuaries, deltas and lagoons, particularly in the wet tropics. Fishing effort of course is uneven, but centered in areas where high populations live close to mangroves, or where smaller fishing populations may nonetheless have access to urban markets.
Our output map shows that high value mangroves are found on every continent and in most mangrove countries worldwide. Mangroves are of highest value precisely where humans and markets are concentrated. Although not surprising, decision-makers often overlook these factors: the apparent gains to be had from converting mangroves to urban, agriculture or aquaculture are rarely, if ever, weighed against the huge costs of reduced food security and livelihoods.
The improved understanding that comes from this work offers new opportunities to protect and better manage mangroves worldwide. The sustainable provision of food for vulnerable coastal populations is a benefit that cannot be replicated easily. Economic values are only part of the story of course, but existing studies show values ranging from hundreds to many thousands of U.S. dollars for just one hectare of mangrove every year. Such values are sufficient, in many areas, to counter demands for clearance or conversion of mangroves to aquaculture or coastal development. In many places, the derived values are sufficiently large to justify considerable mangrove restoration efforts in the many areas where they have been lost.
Oysters are consumed by humans as food. What is not always documented is that oyster beds also provide fisheries enhancement of other consumable marine life. The complex arrangements of oyster beds provide nursery and nesting sites for fish. Juvenile fish avoid predation by hiding among the beds increasing fish biomass. Oyster beds also act as attachment points for macroalgae and a variety of invertebrates.
Burrowing shellfish can also cycle nutrients within the sediments, enabling conditions that are important for many other species that reside within and above sediments.
Mapping Ocean Wealth: Oyster Goals
We compared the numbers of fish and crustaceans in or directly adjacent to oyster reefs to numbers away from reefs. This approach enables us to quantify of the “enhancement” role that the reefs play. The same species still survive without the oysters, but where there are healthy reefs they thrive. Often, larvae or sub-adults make up the throngs of fish around oyster reefs: many species find refuge here in their early life. Accordingly, even fish caught far from oysters may still depend on the complex sheltering structure of the oysters. We sought to quantify this life-long enhancement benefit from oyster reefs. Uniquely, we also sought to build a model that would capture the variability in this enhancement, and enable us to build the same uncertainty in our estimates.
We found that nineteen species were enhanced by oyster reefs in the Gulf of Mexico, while twelve species were enhanced on the Eastern seaboard of the US. One hectare of oyster reef in an estuary in the Gulf of Mexico will generate millions of extra larvae in the ecosystem. Only a fraction will reach adulthood, but a fraction of a very large number is still a large number. From the perspective of fishers, it looks like this—one hectare of healthy oyster reef in a place like Matagorda Bay will generate an extra 32,000 crabs of fishable age, just from a 100 meter –by-100 meter patch.
Scaling up to the bays and estuaries on the Gulf and Mexico and the Eastern seaboard of the US, we have a good picture of the total oyster reef extent in 31 of the major bays, and from these alone we find that oyster reefs are generating in the order of 185,000 (plus or minus 45,000) metric tons of fish to the ecosystem, year on year. These are additional to populations that would occur in the absence of such reefs.
While some of the most important small-scale fisheries and recreational fisheries take place close to shore, many commercial fisheries take place further from shore, including benthic fishing (fishing for species closely linked to seabed habitats, such as cod) and pelagic fishing (fishing for species that are free ranging in the open waters, such as tuna).
The Mapping Ocean Wealth team conducted two regional studies that can be found on our demonstration site.
Salt Marshes and Seagrass
Gently undulating green meadows of grass-like plants carpet vast areas of calm shallow seabeds in both tropical and temperate regions. Just like grasslands on terra firma, these seagrass beds capture the sun’s energy, generating food and oxygen, and providing a critical home for fish and other creatures. Lacking the drama of vertiginous reefs, they are often overlooked, and many have been lost both through pollution and by being torn up by trawlers scraping the seabed for flatfish and shrimp, or scouring by anchors and the propellers of pleasure boats.
We still have much to learn about seagrasses, indeed the best global map of seagrasses is simply a patchwork of local studies, full of gaps where no one has really looked. Scientists still disagree on whether there is as little as 170,000 square kilometers or as much as 600,000 kilometers of seagrass in the world. To understand global values, we need first to focus in and understand exactly what seagrasses may be delivering locally.
Set against this paucity of data, we located 11 studies from the coastal waters of temperate Australia. All of these had looked at the differences in abundance of fish over seagrass beds compared to unvegetated adjacent waters. Focusing on just 12 commercially important species, the results are astonishing. A single hectare of seagrass is generating some 30,000 additional fish to the community, equivalent to one kilogram of fish for every square meter. Using simple market values such a seagrass bed, just one hectare would generate a commercial fishery enhancement worth some US$24,000.
Large areas of the world’s temperate shores are fringed by marshes. Too cold for mangroves, these are dominated by shrubs, herbs or grasses, growing over deep mud and cut through by myriad winding channels. Although exposed to the air for much of the day, they are occasionally covered by tides. Countless fish and shellfish benefit from their shelter and their rich productivity.
The MOW team is beginning to study the relevance of saltmarshes to fish production, but thus far the best knowledge comes from the work of others. One example comes from a team working in Galveston Bay looking at the abundance of three crustacean species. The brown shrimp and the white shrimp are among the most important commercial shrimp species in the USA. They are harvested from vast trawling grounds around the Gulf of Mexico and offshore of the US East Coast, with a total harvest of over 100,000 metric tons each year. A third species is the blue crab, whose commercial harvest around the shores of the Atlantic coasts of the Americas is of tremendous importance, but which also supports a large recreational fishery. Adults of all three species tend to spawn in waters near the coast, and young larvae float in the plankton before settling on the seabed right up in the coast and in estuaries.
Studies have shown that these three crustaceans all benefit immensely from the presence of saltmarsh. Each hectare of marsh in Galveston Bay is estimated to generate an extra 235 kilograms of shrimp and 170 kilograms of blue crab compared to the shallow waters nearby.
Top image: © Ian Shive. Image credits: © Jeff Yonover, © Jeff Yonover, © Mark Spalding, © Peter Frank Edwards, © Nick Hall, © Mark Spalding, © Ami Vitale