One of the most endangered species of marine mammals lives within two hundred miles of our laboratory in San Diego, California. No, it is not the blue whale, the largest marine mammal, which now numbers in the thousands and appears to be recovering at a healthy rate ( Barlow 1995). Instead, it is the vaquita (Phocoena sinus), the smallest marine cetacean, which also has a very small range in the uppermost portion of the Gulf of California (Brownell 1986). Imagine an entire porpoise species living in an area smaller than the Great Salt Lake. This slender porpoise is distinguished by color patterns that look like it is wearing both lipstick and mascara. Its other distinguishing feature is a tall dorsal fin that is unusually large for an animal its size. The size of the dorsal fin may be an adaptation to this special habitat, some of the hottest marine waters in the world. Most porpoises live in cold water, as does the closest relative to the vaquita, found in the much colder waters of Peru (Rosel et al. 1995). Some feel that the extra large dorsal fins allow vaquita to dump extra body heat.
Vaquita are in danger of extinction because they get caught in fishing nets and drown (Rojas Bracho and Taylor, 1999). Fishing provides the sole source of income for many people living in this part of rural Mexico. When it was found that vaquita had little genetic diversity (Rosel and Rojas-Bracho, 1999) some felt that the vaquita was likely already doomed and that changing fishing practices would be wasted efforts. Simulation models were used to show that the low diversity was much more likely a result of this species’ natural rarity, than from a loss incurred during the recent decline (Taylor and Rojas Bracho, 1999). The immediate concern with low diversity is that lethal genes will be exposed: the very small population size means that close relatives must mate with one another, an effect called inbreeding depression. It is not surprising that vaquita might be naturally rare, as they are so seldom seen that they were not described as a species until 1958 (Norris). Populations with low abundance lose genetic diversity through a process called genetic drift. If the vaquita have already survived many years at low abudance, then they have in some way adapted to the low genetic diversity, perhaps by purging the bad genes through natural selection over long periods of time. Even though vaquita remain imperiled because gill netting remains high in their small range, we feel assured that they are not doomed from a genetic perspective.
Mexico formed its first recovery team to plan to save vaquita, and the SWFSC has three members on that team (Taylor, Barlow and Brownell). We continue to actively collaborate with our Mexican colleagues, using science to help them choose appropriate recovery strategies.
Brownell, R. L., Jr. 1986. Distribution of the vaquita, Phocoena sinus, in Mexican waters. Marine Mammal Science 2:299_305.
Rojas-Bracho, L. and B. L. Taylor. 1999. Risk factors affecting vaquita (Phocoena sinus). Marine Mammal Science 15:974-989.
Rosel, P. E., M. G. Haygood and W. F. Perrin. 1995b. Phylogenetic relationships among the true porpoises (Cetacea: Phocoenidae). Molecular Phylogenetics and Evolution 4:463_474.
Rosel, P. E. and L. Rojas-Bracho. 1999. Mitochondrial DNA variation in the critically endangered vaquita, Phocoena sinus (Cetacea: Phocoenidae). Marine Mammal Science. 15: 990-1003
Taylor, B. L. and L. Rojas-Bracho. 1999. Examining the risk of inbreeding depression in a naturally rare cetacean, the vaquita (Phocoena sinus). Marine Mammal Science 15:1004-1028.