Not surprisingly, the discovery rate varies by species, depending on such factors as habitat preference, physical size, and the sociality of the animals. Your odds of finding a dead sperm whale are slightly better than average (about one body for every 30 deaths), of finding an offshore striped or spinner dolphin are far worse (less than one in 200). The paper doesn’t assign a number to bottlenose dolphins, no doubt because of their complicated demographics in the northern Gulf. But presumably nearshore dolphins would have one of the best rates of discovery (the highest rate given in the paper is about one in 16), and offshore dolphins perhaps among the worst, with dolphins on the shelf lying somewhere in the middle.
Regardless of which numbers you pump in, the paper suggests that thousands of Gulf dolphins are dying.
This frightening math makes determining the provenance of the 130 stranded animals all the more urgent. As I’ve said before, the dolphin communities that have made their homes in the Gulf’s bays, sounds, and estuaries are small and semi-isolated, and the death of even a few babies can have outsized effects on the group. The shelf and offshore populations are larger but not vast, and the death of hundreds, let alone thousands, of animals would far exceed the government’s estimate of what they can reasonably sustain.
To be sure, the Conservation Letters paper is a first take, and investigators will have to account for greater monitoring efforts made since the Deepwater Horizon blowout, among other things. But it demonstrates the need to start thinking beyond the shore, especially if investigators determine that the BP spill may be responsible.
As the authors note, “Many media reports have suggested that the spill caused only modest environmental impacts, in part because of a low number of observed wildlife mortalities, especially marine mammals.” In the Exxon Valdez case, the government came up with a multiplier to account for the numbers of undiscovered dead animals that the oil giant was liable for. If BP is to blame for even a fraction of these strandings — not only of the bottlenose dolphins this year, but of the whales and dolphins found during the spill itself — the government will have to face the same grim mathematics now.