Dec 20 (Reuters) - The Baltic Exchange"s dry bulk sea freight index slipped for an eighth straight session to its lowest level since mid-April, hurt by a decline in rates across its smaller panamax and supramax vessel segments. * The overall index (.BADI), which factors in rates for capesize, panamax and supramax vessels, was down 8 points, or 0.3%, to 2,371, its lowest since April 15. * The capesize index (.BACI) added 80 points, or 2.9%, to 2,807, snapping an eight-session losing streak. * Average daily earnings for capesizes, which transport 150,000-tonne cargoes such as iron ore and coal, increased by $670 to $23,283. * Singapore iron ore futures rose for a third straight session, buoyed by renewed optimism over demand for the steel-making ingredient in the short term as signs emerged that top steel producer China is back to ramping up output this month. * The panamax index (.BPNI) lost 88 points, or 3.6%, to its lowest in a month at 2,356. * Average daily earnings for panamaxes, which carry 60,000-70,000 tonne coal or grain cargoes, decreased by $790 to $21,204. * The supramax index (.BSIS) fell 33 points to 2,436, its lowest level in over two weeks.