Threats to Shellfish Reefs
Bridget Hott
There is no catchy title or anecdote when it comes to talking about the current state of shellfish reefs. This marine ecosystem is the most threatened of all shallow-water habitats – even more than coral reefs and mangroves. As much as 85% of the world’s oyster reefs have disappeared in the last 200 years. So, what exactly is responsible for all this loss?
Close-up of an oyster bed. Photo: Unsplash.
Overharvesting
If you have been following along with the Marine Ecosystem Diaries, it should be well evident that one of the main threats to marine ecosystems is the unquenchable greed of the human race. One example of this is shown through the over-harvesting of the Chesapeake Bay oysters, a body of water located on the east coast of the United States of America. For centuries before the settlers arrived in the Americas, oysters were a main component of native diets. A vast network of oyster reefs weaved throughout the bay, supporting life of all sorts, both on land and in the water. In fact, the bay was named by the Algonquin Indians, with the word “Chesapeake” meaning “great shellfish bay.” Between the years 1860 and 1920, all of this changed. Dredging for oysters resulted in a 75% population loss, with the rest nearly wiped out towards the end of the century. At the peak of the harvesting industry, roughly 120 million pounds of oyster meat was collected each year - that’s not including the weight of their shells. At a similar time in Europe, oyster harvesting was seeing alarming rates with 700 million oysters consumed in one year in London alone and shell piles in France that amassed over 1 trillion shells, a true testimony to the productivity of shellfish reefs. Both of these reef systems are still trying to recover from the massive hits they took over 100 years ago, and while progress may be happening, it is extremely slow.
Habitat Degradation
Harvesting and habitat degradation tend to go hand in hand these days, and there is no reason to believe this would be any different with shellfish reefs. The current methods of shellfish harvesting are actually very old, unchanged, and outdated techniques that require scraping oysters off the reef for collection. Dredging and tong harvesting are among the most popular methods for harvesting oysters, which are extremely detrimental to the habitats. Harvesting with these methods often leaves reefs with impacts that can take decades to recover from, and continual dredging can entirely flatten three-dimensional reef structures. As a result of these methods, previously topographically-diverse shellfish reefs have become thin reef layers on the seafloor, which diminishes the growth rates of the very organisms that are being harvested.
Tampa Bay Estuary Program. Photo: Unsplash.
Reduced Water Quality
A remarkable aspect of shellfish reefs is their natural ability to filter water and create productive ecosystems. Unfortunately, and a tad ironically, one of their biggest threats is the increasing rate of poor water quality. NOAA states, “runoff and erosion from industry, farming, and development contribute to lower salinity, low oxygen levels, and silt overload.” The increased influx of toxins and nutrients from these activities weakens the health of oysters and other shellfish in the area, which lowers their capability to clean the water around them, resulting in an extremely difficult-to-break cycle of worsening water quality. A prime example of how pollution effects shellfish reefs is shown in the New York Harbor. In the 1600s, the New York Harbor was the largest provider of oysters worldwide. By the 1900s, oyster reefs had all but vanished from the waters, largely in part to the huge uptick in pollution entering the waterways from constant coastal development and the industrialization of America. At the present, over 100 years later, water quality is improving and oysters are returning, although they are still deemed unsafe for consumption. Hopefully, a lesson has been learned and the oysters will be able to clean up the harbor and allow more reefs to grow.
Collected oyster shells. Photo: Mark Stebnicki.
Other Factors
It has been well established that overharvesting, habitat degradation, and poor water quality are at the forefront of threats towards shellfish reefs, but there are still plenty of other threats faced every day. Disease has also been a major player in the decline of shellfish reefs, and as a result of having lost 85% of the worlds shellfish reefs to other threats, there is only a very small genetic pool left for regrowing oyster reefs and fending off illnesses. For example, Crassotreae gigas is the main oyster species that is farmed worldwide and is responsible for over 98% of oyster production. This type of restrictive diversity is extremely worrying in the face of such pressing threats. Diseases, such as the Pacific Oyster Mortality Syndrome, have been a major threat to the oyster industry as it quickly spreads through reefs without the resilience of a diverse gene pool. Within the Chesapeake Bay alone, the average mortality rate from disease was 29% from 1985 – 2004. Fortunately, this number dropped to 17% in the next five years and is showing continual signs of improvement, but the road to recovery is still long for shellfish reefs.
Climate change is also at the forefront of declining shellfish reef habitats, as oceans warm and waters become more acidic. The entire concept of a shellfish is threatened by increasing pH levels, since the very thing that grants their survival becomes nearly impossible to build and weaker than needed. Shells are made of calcium carbonate (CaCO3), the same thing coral reefs are made of. Ocean acidification threatens the life of all shellfish and the existence of shellfish reefs as a whole, as well as the 300+ species that live within them.
Australia’s Untold Environmental Disaster
For a deeper look into what all this means for the ocean and the world, take a look down under at the shellfish reefs of Australia. Australia has had a particularly hard time when it comes to protecting their shellfish reefs. Of course, most of the damage was done before anyone even knew the value of these habitats. Like the rest of the world, Australia lost its shellfish reefs during the British colonisation period over 200 years ago. Unlike the rest of the world, Australia was hit particularly hard, losing 99% of their shellfish reefs. Scientists are worried about the trickling effects that losing shellfish reefs will have on the world, since the destruction of their reefs lead to a “release in stored carbon, worsening climate change, and increasing coastal risk.”
An oyster shell on a beach. Photo: Trygve Finkelsen, Pexels.
Shellfish reefs tend to be out of sight and therefore out of mind, but their value to our world is irreplaceable, and while it will take dramatic intervention to restore these habitats, it is all doable with a bit of elbow grease from around the world. Without stronger conservation efforts, we can expect to see a decline in fishery productivity, water quality, and climate predictability. Our homes and our lives will be in greater danger and many marine ecosystems will be pushed closer to collapse and extinction. Through stricter regulations on harvesting and harsher penalties for pollution, we can give shellfish reefs a better chance to regrow into the vast structures that once ruled the coastlines and supported lives all over the world.