DHAKA, Bangladesh — This time of year, Mohammad Shamsuddin normally earns about $120 a month working with the crew of a fishing boat off the coast of Bangladesh.

But on Monday, the central government imposed a 65-day national ban on coastal fishing — the most restrictive ever in Bangladesh, a poor and densely populated country where fish play a central role in the economy and diet.

Mr. Shamsuddin, 30, promptly reduced by about a third the amount of food that he buys for himself, his wife and their three children.

“But I won’t be able to run my family for the next two months with this little amount of savings,” he said by telephone from Bhola District, about a 155-mile drive south from the capital, Dhaka. “And when the savings run dry, my life will be a nightmare.”