The reason for decline in fish-eating birds is biomagnification of toxic substances like DDT and other pesticides from the agricultural fields.
The pesticides are applied in the fields for protection of crops. These get washed off to water bodies with irrigation water and rain and are indirectly consumed by the fishes in the water, thereby entering the food chain. These toxic chemicals get accumulated and their concentration increases with each trophic level. Thus, on reaching fish-eating birds, their concentration will be increased many folds.
The high concentration of DDT interferes with calcium metabolism in birds and causes thinning of eggshells and their premature breaking, ultimately causing decline in bird population.