Put down that tuna fish sandwich.

New research published in Marine Ecology Progress Series Journal finds that predatory fish such as grouper, tuna, swordfish and sharks are in deep trouble, having declined by two-thirds in the 20th century.

With predatory fish often the top choice for seafood lovers, overfishing these species can have detrimental consequences for ocean ecosystems. Because when the big fish are away, the little fish will play. The study, “A Century of Fish Biomass Decline in the Ocean,” finds that with carnivorous fish like grouper and tuna on the downswing, small prey species such as anchovies and sardines have seen a population uptick.

With 55 percent of the decline occurring in the past 40 years, the sharpest dip seems to have been between 1970 and 1990, according to the study. Since then, the numbers have leveled off somewhat.

“This does not mean, however, that conditions have started to improve globally; we found no indications of increase in biomass of predatory fish,” the study states. “There may be regional improvements; however, this is not evident yet at a global scale.”

A collaborative effort led by Villy Christensen, a professor at the University of British Columbia’s Fisheries Centre, this study supports what marine scientists have known for some time: Predatory fish are in trouble.

“Predators are important for maintaining healthy ecosystems,” Christensen told Scientific American. “Also, where we have had collapses of the larger fish, it has taken many decades for them to rebuild.”

According to the International Union for Conservation of Nature Red List of Threatened Species, 11 percent of tuna, 12 percent of grouper, and 24 percent of sharks and ray species are threatened with extinction.