Eight core industries have witnessed a 2.6 per cent decline in their growth rate in December 2012 due to drop in production of coal, natural gas and fertilizers. It was 4.9 per cent in the same month of previous year.According to official data released in New Delhi on Thursday, the cumulative expansion of the eight industries — crude oil, natural gas, cement, coal, electricity, steel, petroleum refinery products and fertilisers — was down at 3.3 per cent in April-December 2012 against 4.8 per cent in the same period previous year.The eight industries have a weightage of 37.9 per cent in the overall Index of Industrial Production.AIR correspondent reports the growth of eight core sector industries had declined to 1.8 per cent in November last year.