Populations of marine life, such as marine mammals, birds and fish, have declined by 49 per cent in just four decades, The World Wildlife Federation found. Some fish species were even found to have declined by almost 75 per cent. The Living Blue Planet Report tracked 5,829 populations of 1,234 species. The authors attribute the population drops to habitat loss, global warming, pollution and overfishing. "Our ocean -- that seemingly infinitely bountiful, ever awe-inspiring blue that defines our planet from space -- is in crisis," Marco Lambertini, director general of WWF International, explains.