# Oyster Reefs as Natural Sea Defences

IELTS Academic Reading - 13 questions - 20 minutes - Matching information, Diagram labelling, Short-answer questions

Source: Written by Testmigo.

## The passage

You should spend about 20 minutes on Questions 14-26, which are based on the passage below.

**A** For over a century, the usual answer to a crumbling coast has been a wall. A 2015 study led by Rachel Gittman of the University of North Carolina found that sea walls, bulkheads and breakwaters already harden 14 per cent of the tidal shoreline of the United States. Such barriers guard the land directly behind them, but at a cost. The National Oceanic and Atmospheric Administration (NOAA) warns that they may cause erosion on their seaward side and shrink the habitat available to fish, and it expects a third of American shorelines to be hardened by 2100. Engineers have therefore begun to promote 'living shorelines', which stabilise the coast with natural materials such as plants, sand, rock or oyster shells.

**B** An oyster reef is constructed by the animals themselves. Oyster larvae swim freely for two weeks or more before fixing themselves to a hard surface, such as an existing oyster shell, after which the young oysters are known as spat. Each generation settles on the shells of the one before, so a reef slowly rises from the seabed as a rough, three-dimensional mound full of crevices. As waves roll across this uneven surface, they lose much of their energy before they reach the shore, which is why NOAA calls such reefs 'living breakwaters'. Unlike a concrete wall, a healthy reef can also grow. In 2014 a team led by Antonio Rodriguez of the University of North Carolina reported that intertidal reefs could build upwards by as much as 11 centimetres a year, quickly enough to keep pace with rising seas.

**C** Yet such reefs have largely disappeared. A 2011 survey of 144 bays and 44 ecoregions, led by Michael Beck of The Nature Conservancy, concluded that roughly 85 per cent of the world's oyster reefs had been lost, chiefly through over-harvesting and disease. Chesapeake Bay, on the US east coast, shows how fast the damage was done. In the 1850s more than 1.5 million bushels of oysters were taken from the bay every year; three decades later, the annual catch had reached 20 million. Two parasites, Dermo and MSX, have also killed many oysters, and the population is now thought to be under 1 per cent of its former size. Australia suffered a similar collapse between the mid-1800s and the early 1900s, when oysters were dredged both for food and for lime production. Of 118 places where the Australian flat oyster once formed reefs, just one remains.

**D** The loss affects water quality as well as coastal defence. Oysters feed by pumping water through their gills and trapping plankton, and a single adult is often said to filter 50 gallons a day. However, Matthew Gray of the University of Maryland Center for Environmental Science has pointed out that this is close to the maximum recorded in laboratories, where temperature and food are ideal; in the wild, an oyster more probably filters between 3 and 12.5 gallons. Even so, the combined effect can be huge. According to the Chesapeake Bay Program, the bay's oysters in the late nineteenth century could filter a volume of water equal to the whole bay in three or four days, whereas today's population takes nearly a year.

**E** Large-scale restoration is now under way. The 2014 Chesapeake Watershed Agreement aimed to restore oyster reefs in at least ten of the bay's tributaries by 2025, and nine have now been restored. Much of the young stock is raised in hatcheries: the University of Maryland's Horn Point Laboratory produced 1.78 billion baby oysters in 2016. In New York, the Billion Oyster Project, founded in 2014, plans to return one billion oysters to the harbour by 2030. Restaurants donate their empty shells, which are left in the open air for months to remove bacteria. This cured shell is combined with engineered concrete to form reef structures, which are seeded with larvae that settle and turn into spat.

**F** In Australia, the Reef Builder programme, a partnership between The Nature Conservancy and the national government, restored more than 40 hectares of shellfish reef through 13 projects in six states between 2021 and 2023. In the United Kingdom, where native oyster numbers have fallen by 95 per cent since the mid-nineteenth century, the Native Oyster Network supports restoration across the UK and Ireland. One such effort, the Solent Oyster Restoration Project, started in 2015 by hanging cages of mature oysters beneath pontoons so that they could release larvae into the water. The team has since found 97 different species living in the cages, among them juvenile spiny seahorses, sea bass and critically endangered European eels.

**G** Oyster reefs cannot replace sea walls everywhere, however. NOAA notes that living shorelines suit estuaries, bays, rivers and other sheltered coasts, and are not usually found on beaches facing the open ocean. Height matters, too. At Little Toms Cove in Virginia, the US Geological Survey monitored oyster castles, a type of constructed reef. When the castles stood above the water or were only slightly submerged, they reduced wave energy by 39.7 per cent; when they were fully covered, wave energy behind them was actually 38.6 per cent higher. Still, NOAA reports that living shorelines tend to cost less than hard defences to install and maintain, and they provide habitat and cleaner water, which a wall cannot. On sheltered coasts, the most effective defence may be one that builds itself.

## Questions

### Questions 14-18: Matching information

The passage has seven paragraphs, A-G. Which paragraph contains the following information? NB You may use any letter more than once.

14. an example of artificial reefs making wave conditions worse

15. a reason, other than eating, why oysters were collected

16. a prediction about how much coastline will be protected by hard structures

17. the claim that a commonly quoted figure is unrealistic under natural conditions

18. a reference to animals found living in a restoration structure

### Questions 19-22: Diagram labelling

Label the diagram below. Choose NO MORE THAN TWO WORDS from the passage for each answer.

19. What the waves lose as they cross the reef

20. Young oysters fixed to the top of the reef

21. Layer left in the open air for months before use

22. Blocks forming the base of the reef

### Questions 23-26: Short-answer questions

Answer the questions below. Choose NO MORE THAN TWO WORDS AND/OR A NUMBER from the passage for each answer.

23. In natural conditions, what is the largest amount of water an oyster is likely to filter in a day?

24. How many baby oysters did a Maryland university hatchery produce in one year?

25. Which animal at serious risk of extinction has been recorded in the Solent cages?

26. What type of man-made structure did government scientists study on the Virginia coast?

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