"Keal sardines" refers to Indian Oil Sardines (Sardinella longiceps), which are a staple, highly nutritious, and affordable fish commonly consumed in the Indian state of Kerala. In Malayalam, they are known as Mathi or Chaala. 
Here are the key details about Kerala sardines:
Nutritional Value: They are rich in Omega-3 fatty acids, calcium, protein, and Vitamin D, making them excellent for heart and bone health. Because they are small and often eaten whole, they are a significant source of calcium.
Role in Cuisine: They are a beloved staple, often referred to as "the poor man's fish" due to their low cost and high availability. They are commonly prepared as a spicy curry (using Kudampulli or Malabar tamarind) or fried (Mathi fry).
Cultural Importance: They are considered a cornerstone of Kerala cuisine.
Issues: Due to high demand, local supplies sometimes dwindle, leading to imports from places like Oman.