Short-beaked common dolphins are the most abundant cetacean species off the coast of California. They are year round residents although their distribution changes with the season. In fact, during winter it is not uncommon to see schools of short-beaked common dolphin from our lab. The dolphins are most likely following their prey, which moves inshore during winter months.
The short-beaked common dolphin has the highest mortality rate of all cetacean species impacted by the drift gillnet fishery that operates off the coast of California. Only one management unit is recognized for the animals inhabiting these waters and it includes animals inhabiting the waters off the coasts of Oregon and Washington as well. The management unit is called the CA/OR/WA stock (Carretta et al., 2004). However, available data on dorsal fin coloration patterns, contaminant concentrations and reproductive seasonality suggest there may be more than one stock. To further evaluate whether the CA/OR/WA stock represents one population of animals, we analyzed a small, exploratory data set of mitochondrial DNA control region sequences. Our data set included samples representing four putative populations within the eastern North Pacific: the central eastern tropical Pacific, the northern eastern tropical Pacific, the Southern California Bight, and the central and northern coasts of California. Our results suggest that there is genetic distinctness between the putative California populations (Chivers et al., 2003, Chivers et al. 2005). Thus, supporting the existing biological evidence that more than one population exists within the CA/OR/WA stock. Because our results are for a relatively small data set representing a large population (> 300,000 animals) with high haplotypic diversity (98%), we will be expanding the study using our exploratory data set for the experimental design. We have samples available for this genetic study to examine the question of population structure on a finer scale than with any other data set. Additional knowledge about short-beaked common dolphin population structure will help us better evaluate the impact of the driftnet fishery on these animals and to better understand their ecology and life history.
Carretta, J., K. Forney, M. Muto, J. Barlow, J. Baker, B. Hanson, and M. Lowry. 2004. US Marine Mammal Stock Assessments: 2004. NOAA-TM-NMFS-SWFSC-375. 323pp.
Chivers, S. J., R. G. LeDuc, and K. M. Robertson. 2003. A feasibility study to evaluate using molecular genetic data to study population structure of eastern North Pacific Delphinus delphis. NOAA, NMFS, Southwest Fisheries Science Center Administrative Report LJ-03-12. 14pp.
Chivers, S.J., N.M. Hedrick and C.A. LeDuc. 2005. Genetic analyses reveal multiple populations of Delphinusdelphis in the eastern north pacific. Poster presented at the Biennial meeting on the Biology of Marine Mammals. December, 2005. San Diego, CA .