The seafood on your dinner plate might have a very messed-up backstory.

A new survey led by the University of Florida has exposed the widespread practice of killing dolphins, sea lions, seals, and otters to use as bait in global fisheries. The grim reading can be found in the scientific journal Frontiers in Marine Science.

Researchers discovered that fisheries in at least 33 countries use more than 40 species of aquatic mammals as bait. This fishing practice is most common in Latin America and Asia, although it's seen on a smaller scale among fishermen working in waters around the Mediterranean and Australia.

Nevertheless, the seafood caught using this practice could potentially be exported to North America or Europe. 

In most instances, aquatic mammals are used as bait because they are accidentally swept up in the net's bycatch. Many species such as dolphins are illegal to catch or culturally taboo to eat, so fishermen will cover their tracks and chop up the animal to use as bait. However, the practice isn't always a result of bycatch. Some fishermen were found to actively hunt aquatic mammals to use as bait because they are considered an effective meat to attract sharks.

Since most of this activity is clandestine and kept hush, the real figures are likely to be even higher than this study reports. These figures are simply the instances that have been officially documented by scientists and conservationists.

They discovered that common bottlenose dolphins (T. truncatus) were used as bait in at least 17 of the countries. Other widely killed species include the spinner dolphin (Stenella longirostris) and the Amazon river dolphin (Inia geoffrensis).

Killing for use as bait is considered a direct threat to a number of species, such as the Ganges river dolphin (P. gangetica) and the Chilean dolphin (Cephalorhynchus eutropia). In fact, nearly 27 percent of the species in the study are listed as threatened, critically endangered, or near threatened.

It’s a big and often hidden problem, but there’s still some hope. The researchers hope their work, along with other studies, could help policy-makers target certain areas and bring this grisly practice to a stop.

"For scientists already working on species and locations identified as 'hot spots' in this review, organized efforts should begin right away to estimate these numbers," lead author Dr Vanessa J. Mintzer, from the University of Florida, said in a statement. "It took years to determine that the hunt for botos was unsustainable and now conservation actions need to be expedited. We need to identify other affected populations now to facilitate timely conservation actions."