A tangled fishing line around a stranded dolphin doesn't necessarily mean that fishing gear was the cause of death, but fishing gear is responsible for many marine mammal deaths each year. In 2005, the World Wildlife Fund estimated that about 308,000 whales, dolphins and porpoises are killed by fishing gear each year.
One major culprit in cetacean deaths identified by the WWF was the gillnet, a special type of fishing net designed to trap fish by their gills. But larger marine mammals have difficulty detecting these nets with their echolocation, and they frequently become entangled in the netting or ropes and drown.
The National Marine Fisheries Service recommends that recreational fishers reel in their lines if they see a dolphin, and to change locations if one appears. Boat captains should put their vessels in neutral. People should never feed dolphins, the NMFS warns, because it teaches them to approach humans and beg for food, putting them in dangerously close proximity to fishing gear and boat propellers.
The NMFS also encourages fishermen to use circle hooks that curve sharply backwards, and which pose less of a threat to dolphins and sea turtles. Corrodible hooks, made out of any metal other than stainless steel, are also advisable since they break down after a time if they're lost at sea.
Not all injuries to dolphins by humans are accidental. The National Oceanic and Atmospheric Administration is currently investigating recent incidents in the Gulf of Mexico where people have deliberately harmed dolphins. Dead dolphins have washed up with gunshot and stab wounds, and their bodies were sometimes found mutilated.
"This is gruesome really," Moby Solangi, a biologist with the Institute of Marine Mammal Studies, told CNN in November. Solangi had performed necropsies on all the slaughtered dolphins. "It's not only killing them, it's also mutilating them. And the parts that are taken are disposed of -- not next to the animal, they are taken. And they are not worth anything."
Between 2002 and 2012, there were 12 documented cases of dolphins found with gunshot wounds, but it's unclear if the animals were killed by a firearm or shot after death.
2012 was an especially brutal year. Last January, a dolphin with a gunshot wound washed up on Deer Island, which is near Biloxi, Miss. A dolphin with a screwdriver impaled in its head turned up near the Florida-Alabama border in June. In September, another dolphin with a gunshot wound washed up on Elmers Island, La. In October, a mutilated dolphin washed ashore in Alabama with one of its flukes severed. November was particularly nasty, with three dead dolphins showing cuts and bullet wounds found near Ocean Springs, Miss.
Under the Marine Mammal Protection Act, a person convicted of harming, harassing or killing a wild dolphin faces up to $100,000 in fines and a year in jail per violation.