High temperature makes hairy crabs thin? I wish this was just a rumor

The west wind blows, the crab legs itch, it’s the season for eating hairy crabs again!

However, this year, many people may not have hairy crabs on their Mid-Autumn Festival tables. A few days ago, "Yangcheng Lake hairy crabs may miss Mid-Autumn Festival" became a hot search. This is not only because the Mid-Autumn Festival is relatively early this year, and the crabs are not ready yet, but also because the weather is too hot and the crabs are not growing meat .

Can we still eat delicious hairy crabs this year? Will hot weather really make hairy crabs thin?

Why don't hairy crabs grow meat?

It’s true that crabs will become thin due to heat!

The most suitable water temperature for crabs is 18℃～25℃. When the temperature is above 30℃, their appetite will gradually decrease , their physical fitness will also decline, and they will be prone to illness. Under the scorching sun this summer, the water surface temperature in some areas may reach 40℃. The crabs are too hot to eat and naturally cannot gain weight. Moreover, crabs are omnivores, and their feed often contains small fish. During high temperatures, crabs have a poor appetite. If they do not eat the small fish in time, they are prone to spoilage, causing the water quality to deteriorate, which in turn affects the growth and development of crabs.

Crabs need to molt five times before they can become adults, and their size doubles after each molt. The fourth molt of crabs usually occurs in July, and they can weigh more than 2 taels after molting. However, this year, the molting time of crabs tortured by high temperatures will be delayed by 20 to 30 days, and the crabs that have not completed the fourth molt weigh less than 1.5 taels.

Crab is trying to shed its shell

Many crabs died before they could successfully molt . Some crabs were forced to run to the shore by the high temperature and eventually died of dehydration; some crabs died of disease - the high temperature caused the water plants to rot, the oxygen content in the water to decrease, the harmful substances of ammonia and nitrogen to increase, and the bacteria to multiply. Eventually, the crabs with reduced immunity would be infected with diseases such as shell ulcers.

Although the autumn wind has started and the temperature is not as high as before, the large-scale listing of hairy crabs this year still needs to wait until October, and the hairy crabs that can be served on the table during the Mid-Autumn Festival will not be as fat and strong as in previous years. Everyone should also be mentally prepared that the size of hairy crabs this year is likely to be smaller than in previous years , and due to the high mortality rate, the total production of hairy crabs will drop by about 20%. As a result, the prices of hairy crabs with fat crab roe, full crab roe, and full meat will inevitably rise.

Other seafood is also hot

The high temperature not only tortures us and makes hairy crabs become thinner, but also affects the farmed fish, shrimp and shellfish in China's coastal areas.

The suitable temperature for sea bass is 20℃～28℃, but the water temperature in Guangdong's sea bass breeding area can reach 35℃, which greatly affects the feeding and growth of sea bass, and even causes them to suffer from heat stress and be more susceptible to diseases. High temperature plus low pressure causes the dissolved oxygen content in the water to drop quickly, especially in the middle and lower water bodies where sea bass live. Farmers need to install oxygenators in fish ponds to reduce their deaths.

Another problem caused by high temperatures is the high salinity of seawater . When it is hot and rainy, the water evaporates quickly and the salinity of the seawater rises. Once it exceeds the tolerance range of fish and shrimp, it will cause losses. For example, the salinity of seawater adapted to white shrimp is 10‰ to 20‰, but in the white shrimp production area in Taizhou this year, the salinity of the water reached 32‰, the immunity of white shrimp was reduced, and Vibrio spread faster. Fortunately, the pathogens of white shrimp will not be transmitted to fish. These shrimp were eventually salvaged and dumped into nearby fish farming areas, becoming a delicious meal for yellow croaker (shrimp:?).

Don’t waste it! This is the end of the white shrimp...｜skretting.com

High temperatures can also cause large-scale deaths of marine shellfish in the summer. For example, at a water temperature of around 30°C, the massive spread of the oyster herpesvirus (OsHV-1) can cause large numbers of oysters to die. The abalone we often eat, the wrinkled abalone, has a suitable seawater temperature range of 10°C to 22°C. When the water temperature exceeds 28°C, they cannot live normally. In Fujian, their main production area, due to the continuous high temperature and unstable dissolved oxygen content in summer, the mortality rate of wrinkled abalone is as high as 25% to 60% , causing huge losses every year.

Mussels grilled on a Vancouver beach | Alyssa Gehman

This isn't just happening in our country.

Last year, under the extremely high temperatures in the summer, tens of thousands of mussels, clams, starfish and barnacles were killed by the heat on the beaches of Vancouver, Canada. Some even cracked open to reveal the meat inside, giving off the smell of roasted seafood, as if it were a huge "open-air barbecue". Scientists estimate that 1 billion shellfish died in this high temperature . Shellfish have a strong ability to filter water and are very important in the ecosystem. Their death will trigger a chain reaction, exposing more organisms to a catastrophe.

Try to avoid the heat

In order to ensure that the plump hairy crabs can be served on the table smoothly, crab farmers will also adjust their breeding methods according to their living habits.

A comfortable aquatic environment can reduce the impact of high temperatures. As the temperature rises, crab farmers will gradually increase the water level in the crab pond and add and change water in a timely manner. In extremely high temperatures, they may even add ice cubes to the water . In order to avoid hypoxia, aerators are also installed in the crab pond to increase the amount of dissolved oxygen in the water.

The aquatic plants in the crab pond are also very useful. They can help purify the water and provide a place for crabs to cool off. However, crab farmers need to strictly control the variety, quantity and density of the aquatic plants to ensure their growth and prevent them from rotting and accumulating, while also allowing crabs to have sufficient living space.

As the saying goes, "the size of a crab depends on the water plants." The water plants in the crab pond are crucial to the health of the crabs.

In addition to the environment, food is also very important. The crab feed must pay attention to the combination of meat and vegetables. During the high temperature stage, crab farmers will slightly reduce the proportion of meat, add more green fodder, and feed in small amounts and multiple times to prevent animal feed from rotting and water deterioration. When the temperature drops a little and it is time to fatten and yellow, they will add meat to the crabs to make them gain weight . In addition to these staple foods, crab farmers will also add minerals and vitamins to the water, and some crab farmers will also add garlic and sugar to the feed (the seasonings have been added) in the hope of helping them molt smoothly.

However, these methods are only temporary solutions and can only reduce the impact of high temperatures on crab production to a certain extent. As temperatures continue to rise, not only hairy crabs and other aquatic products are at risk, but many delicious foods, from kiwis and citrus to coffee beans, even the production of peppers will be greatly reduced under high temperatures - protecting the environment and slowing down climate change are the biggest challenges we face.

References

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[7] Zhou Xiaowei, Lu Changmei. A brief discussion on crab farming methods and water quality management[J]. China Agricultural Information, 2014(01):149.

[8] m.cnhubei.com/content/2022-08/08/content_14964663.html

[9] Hu Geng, Liu Debin, Wang Jiawei. Current status of abalone farming in my country and management measures for abalone farming during high temperature period[J]. China Fisheries, 2015(05):76-77.

[10] LIN Si-heng. Molecular mechanism of summer mortality and high temperature stress response of Haliotis discus hannai[D]. Graduate School of the Chinese Academy of Sciences (Institute of Oceanology), 2016.

[11]https://www.washingtonpost.com/world/2021/07/08/canada-sea-creatures-boiling-to-death/

[12] www.davidpublisher.com/index.php/Home/Article/index?id=25946.html

Author: Cat Tun

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