Mother carps in Halda not spawning amid severe heat

 A sharp rise in temperature and salinity in the water of River Halda is obstructing mother carp fish from laying eggs during the peak breeding season (March-July), say marine biologists.

An intense heatwave is currently sweeping over the country with temperatures hitting record highs in recent decades, disrupting public life as well as natural habitats.

"The extreme heat is causing physiological changes in mother fish, as well as disrupting the ecosystem of the Halda — the sole natural spawning ground for carp-like fishes (Rui, Katla and Mrigel carp) in South Asia," Md Manzoorul Kibria, a Halda researcher and professor at the Zoology Department at Chittagong University, told The Business Standard (TBS). 

 According to researchers, flash floods occur in Halda if it rains with thunder on full moon nights between the Bangla months of Chaitra and Ashar. If the physicochemical factors and the temperature (25-28 degrees Celsius) are favourable, carp fish lay eggs. 

The mother fish usually lays eggs in the Halda, flowing through Chattogram and Khagrachhari, between mid-April and June. Mother fish lay eggs in phases that locals call "Jo".

"One 'Jo' has already gone, another is going on. But the temperature of the water is much higher than normal, hindering the physicochemical factors needed for spawning. Hence, mother fish are not releasing eggs," said Manzoorul Kibria.

The threat to the well-being of mother fish in Halda has been increasing in recent years as rainfall decreases and the temperature rises due to climate change. 

The salinity of Halda River water is also increasing as the sea level rises due to global warming, pushing more salt water into the river system.

Expressing concern about the ecosystem of Halda, Manzoorul Kibria said, "According to the Met Office, the temperature is four degrees higher than normal in Chattogram, which has a direct effect on mother fish, full of eggs. If this condition continues, physiological changes will occur in the mother fish. Besides, water quality will change and the river ecosystem will be destroyed."

The Halda expert also claimed that the natural characteristics of Halda are being destroyed due to the construction of water retaining walls and blocks along the two sides of the river.

Salinity in the Halda has also emerged as a major problem for fish over the years.

 According to Chattogram Water Supply and Sewerage Authority (Wasa), as the water flow of Kaptai Lake decreases, salty sea water is entering the Karnaphuli and Halda rivers with the tide. 

Maqsood Alam, chief engineer of Chattogram Wasa, told TBS, "According to the Environmental Protection Act, water is drinkable if it contains 150 to 600 mg of salt per litre. But up to 1700 mg of salt was found in the river water in Wasa's Mohora project." 

Chattogram Wasa is implementing several water supply projects, including Mohora, to ease the acute water crisis in the port city. 

The issue of rising salinity has also been confirmed by the fishermen of Halda.

Ashu Jaldash, a fisherman in the North Madarsa area who has been collecting eggs in the Halda for about 40 years, told TBS, "The river water is hot and salty now. The fish will not release eggs in this situation. We are waiting for rain, that is when the fish will release their eggs." 

Dr Mohammad Shafiqul Islam, head of the Biology Department of Chattogram Cantonment Public College, told TBS, "Carp fish are comfortable in temperatures ranging between 22 and 30 degrees Celsius. A rise in temperatures will change the reproductive behaviour of mother carp such as delayed breeding, decreased fertility, delayed ovulation, and decreased egg production due to hormonal imbalances."

"High temperatures can also affect the food chain of the river ecosystem. Many aquatic plants and animals that are sensitive to high temperatures can die. As a result, the food source for carp can decrease, reducing their growth rate and overall river ecosystem health," he said.

In addition, the increased metabolic rate of fish at high temperatures increases the oxygen demand, thereby reducing the oxygen level in the water and disrupting the respiration process, which can reduce reproductive success, Shafiqul Islam added.

Mohammad Ali Azadi, another Halda researcher and former professor of the Zoology Department of Chittagong University, said, "When flash floods occur amid sufficient rainfall, the water temperature drops to 27-29 degrees Celsius for 14-16 hours — a time frame very suitable for spawning. Mother fish are fully ready to lay eggs but waiting for that suitable environment."

With adequate rainfall, there is a possibility of fish laying eggs on 2-8 May, 16-22 May, 1-6 June, and 16-19 June, Muhammad Ali Azadi added.

In 2020, the government declared the River Halda as Bangabandhu Fisheries Heritage, meaning fish or aquatic animals cannot be caught in this river.

However, during the breeding season every year, fertilised fish eggs may be procured at specific times under the supervision of the Department of Fisheries