The World Wildlife Fund (WWF) and Zoological Society of London (ZSL) just released a new report detailing a 49% decline "in the size of marine populations between 1970 and 2012."
This follows their 2014 report which states in part that over the past 40 years:
Populations of mammals, birds, reptiles, amphibians and fish have declined by an average of 52%. Freshwater species populations have suffered a 76% decline, an average loss almost double that of land and marine species."