The Baltic Exchange's dry bulk sea freight index, which tracks rates for ships carrying dry bulk commodities, hit a one-week low on Friday and marked a second straight weekly fall as rates across all segments weakened.The index, which factors in rates for capesize, panamax and supramax shipping vessels, slipped 36 points at 987 points, its lowest level since Jan. 9. It was down 5.8% for the week.The capesize index fell 89 points to a one-week low of 1,393 points and lost 3.8% this week.