Matsysavathara - Lord Descends as Divine Fish

Srimad Bhagavatam - Matsyavathara (Lord Descends as Divine Fish)  

Why did Lord Narayana descend as divine fish? Who is King Satyavratha? What was the divine fish’s request to Satyavratha?

Once, during the middle of kalpa, Brahma accidentally took a small nap during his creation work. Seizing this opportunity, demon Hayagreeva stole Vedas from Brahma and hid them deep inside the ocean.   

To retrieve back Vedas that are essential for the creation, Srihari took the form of a fish. He killed Hayagreeva and restored Vedas to Brahma.

Again, at the end of Kalpa and just before the deluge, Srihari took the form of a fish to protect King Satyavratha, the future Manu.

Once King Satyavratha of Pandya dynasty was performing his morning ablutions at Krathamala river. While holding the water in his palms a small fish appeared and said "Satyavratha, protect me from predators”. 

The King took pity on the fish and transferred it to his kamandal (carry pot). To his surprise, the fish grew in size and sought a bigger space. 

The king transferred the fish to a larger tub, to well, pond, and finally to the ocean. The king wondered who this fish could be and thought it must be none other than Srihari and offered his prayers.

Then Srihari, in fish form said, “Seven days from now, the world will cease to exist on account of deluge" (pralaya). Get into a large boat along with Saptarishis, Vedas, seeds of every variety, one pair of animals. I shall take the boat to Meru Parvatha and offer protection.   

As instructed, the king and others were waiting on a boat when a divine fish appeared with Vasuki as rope. The divine fish went around the boat and gave protection until the water subsided. The divine fish clarified doubts raised by Satyavratha. 

Satyavratha, in his next birth, was born to Vivasvan (Sun God) as Shraddadeva. In the present kalpa, Shraddhadeva is also known as Vivaswatha Manu.  

Vivasvan is son of Sage Kasyapa and Aditi (daughter of Daksha Prajapathi), grandson of Saptarishi Marici and great grandson of Brahma. 

Source: Bhagavatha Mahapurana