Rice's whale (Balaenoptera ricei), also known as the Gulf of Mexico whale, is a species of baleen whale endemic to the northern Gulf of Mexico. Initially identified as a subpopulation of the Bryde's whale, genetic and skeletal studies found it to be a distinct species by 2021. In outward appearance, it is virtually identical to the Bryde's whale. Its body is streamlined and sleek, with a uniformly dark charcoal gray dorsal and pale to pinkish underside. A diagnostic feature often used by field scientists to distinguish Rice's whales from whales other than the Bryde's whale is the three prominent ridges that line the top of its head. The species can be distinguished from the Bryde's whale by the shape of the nasal bones, which have wider gaps due to a unique wrapping by the frontal bones, its unique vocal repertoire, and genetic differences.
It is a medium-sized baleen whale that grows up to in length and weighs up to . The Rice's whale inhabits a restricted stretch along the continental slope in the northeastern part of the Gulf of Mexico between depths of off the coast of western Florida, although some whales have been sighted in the northwestern portions and the species may have inhabited a wider distribution throughout the Gulf in historical times. It does not migrate but remains within this area year-round. Little is known about the feeding behavior of Rice's whales, but data from a tagged individual revealed a diel vertical diving pattern, in which the whale spends most of the day feeding at or near the seafloor at depths of up to and night at the surface. The whale's diet remains unknown, but lanternfish and hatchetfish are suspected prey.
The Rice's whale is on the brink of extinction and, alongside the vaquita, is one of the most endangered cetaceans in the world. It is listed as Critically Endangered in the IUCN Red List and protected under the United States Endangered Species Act. The best population estimate is 33, with as little as 16 mature individuals, and the population is continuing to decline. The reasons why the species' population declined to its current state remains poorly understood, but scientists believe that the industrialization of the Gulf of Mexico and the increase of anthropogenic activities within its habitat are primary contributors; unlike most baleen whales it is unlikely that whaling had an impact. Today, the Rice's whale's main threats are related to industrial and commercial activities within its habitat, including oil pollution, ship collisions, and underwater noise from seismic surveys and vessel traffic. It has also shown to be especially vulnerable to local catastrophic events such as the 2010 Deepwater Horizon oil spill, which single-handedly killed nearly twenty percent of the species' population.
Taxonomy
Discovery and naming
The Rice's whale was likely first documented by the Yankee whaling ship Keziah in 1790, which reported the chasing of a "finback" in the Bay of Campeche. At least 49 additional encounters of "finbacks" were reported throughout the Gulf of Mexico during the 18th and 19th centuries. Scientists generally agree that the "finbacks" were most likely Rice's whales, as the species is the only baleen whale that resides in the Gulf year-round. While fin whales (of which the name "finback" applies to today), blue whales, sei whales, and common minke whales have been sighted in the Gulf of Mexico, their presence is extralimital and they only appear in the area as occasional wanderers. The Rice's whale was first scientifically recognized by late cetologist Dale Rice in 1965, but as a local population of the closely related Bryde's whale. The extent of its modern geographic isolation was uncovered during the 1990s. During the 20th and early 21st centuries, this group of supposed Bryde's whales was vernacularly referred to as the "Gulf of Mexico whale".