The pantropical spotted dolphin (Stenella attenuata attenuata) has been the most frequently killed species of dolphin in the yellowfin tuna purse-seine nets. There are two stocks of dolphin recognized within the eastern tropical Pacific: the northeastern and the western/southern as well as a second subspecies, the coastal spotted dolphin (S. a. graffmani), which is managed as a separate stock. The abundance of all stocks has been significantly reduced by the purse-seine fishery, which has killed millions of dolphins during the past four decades. The impact of the fishery on dolphin populations was documented in the late 1960s (Perrin, 1969) and was one of the issues that led to passage of the Marine Mammal Protection Act in 1972.
Early in the 1970s, NMFS implemented a program to place observers, or biological technicians, aboard fishing vessels to document fishery activities and dolphin kill. Thousands of biological samples were collected and used to study stock structure and to quantify life history characteristics of the population. All studies were designed to better understand the dynamics of the spotted dolphin population, their ecology and the impact of the fishery upon them.
Concern that the dolphin population was not recovering even though mortality had been low for many years, led to passage of the International Dolphin Conservation Act (IDCPA) of 1997 to conduct three years of population abundance surveys and a series of studies to examine physiological stress in the dolphins. More detail about these studies (and references to publications) can be found on the IDCPA program web site. In addition to being involved in IDCPA research, our group is currently involved in research to better understand the cow-calf bond and how the fishery may impact it, the reproductive potential of female dolphins by estimating fetal mortality rates, and the stock structure of coastal pantropical spotted dolphin.
Currently, designation of spotted dolphin, Stenella attenuata, populations in the Eastern Tropical Pacific (ETP) is based upon morphological and distributional data, with three geographic stocks recognized. Nevertheless, critical gaps exist in our knowledge about the taxonomic status, population subdivision, and location of stock boundaries for this species. We are using genetic markers, both mitochondrial and nuclear, to determine the intraspecific population structure and phylogeography of coastal spotted dolphins in the ETP and to test whether the recognized stock divisions are supported by genetic findings. So far, results argue for the existence of at least six distinct coastal populations, which should be treated as separate units for management purposes, and suggest the possible existence of male-biased dispersal among the coastal strata.
Additional data are being collected in order to refine the location of boundaries between stocks, confirm the role of sex-biased dispersal in gene flow processes, and shed some light into the social structure of these animals.
spotted offshore dolphin
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Perrin, W. F. 1969. Using porpoise to catch tuna. World Fishing 18:42-45.
Perrin, W. F., J. M. Coe, and J. R. Zweifer. 1976. Growth and reproduction of the spotted porpoise, Stenella attenuata, in the offshore eastern tropical Pacific. Fishery Bulletin 74:229-269.
Perrin, W. F., P. A. Sloan, and J. R. Henderson. 1979. Taxonomic status of the ‘south-western stocks’ of spinner dolphin, Stenella longirostris, and spotted dolphins, S. attenuata. Rep. int. Whal. Commn 29:175-184.
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