Spinner dolphins are the second most frequently killed species in the yellowfin tuna purse-seine fishery, and the number of animals killed during the past four decades has significantly reduced their abundance in the eastern tropical Pacific (ETP). There are three stocks recognized in the ETP and two of them are recognized as subspecies: the eastern spinner (Stenella longirostris orientalis) and the Central American spinner (S. l. centroamericanus). The other stock is referred to as ‘whitebelly’ spinners, which are thought to be hybrids of the eastern spinner and Gray’s spinner (S. l. longirostris). Gray’s spinner dolphin is found in the central and southern Pacific. The eastern spinner has been declared ‘depleted’ under the Marine Mammal Protection Act of 1972.
Like the spotted dolphin, thousands of biological samples have been collected and used to study stock structure and to quantify life history characteristics of the population. The collection of biological samples was due to the observer program implemented by the NMFS in the early 1970s. All studies were designed to better understand the dynamics of the spinner dolphin population, their ecology and the impact of the fishery upon them. Current research has included analyses of molecular genetic data to better understand stock structure in the ETP as well as life history studies analyzing age data for female eastern and whitebelly spinner dolphins and fetal mortality rates to better understand the reproductive potential of female spinner dolphins.
Chivers, S. J. and D. P. DeMaster. 1994. Evaluation of biological indices for three eastern Pacific dolphin species. Journal of Wildlife Management 58:470-478.
Dizon, A. E., Perrin, W. F. and P. A. Akin. 1994. Stocks of dolphins (Stenella spp. And Delphinus delphis) in the eastern tropical Pacific: A phylogeographic classification. NOAA Technical Report NMFS 119. 20pp.
Perrin, W. F. and J. R. Henderson. 1984. Growth and reproductive rates in two populations of spinner dolphins, Stenella longirostris, with different histories of exploitation. Rep. int. Whal. Commn (Special Issue 6):417-430.