The Marine Species Population Index provides an assessment of the average changeover time in the populations of 217 species of marine mammals, birds, reptiles, and fish. The index represents the average value of six regional ocean indices. More pronounced declines are seen in the southern oceans, attributed to the fact that major losses and degradation of marine ecosystems in the industrialized world took place prior to 1970.
Marine species are generally more difficult to monitor than terrestrial ones. Assessments are therefore based primarily on fishery catches, and the monitoring of land breeding species (e.g. turtles, birds and seals). However, these species are over-represented in the index, which should have a far greater proportion of invertebrate species.