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Pregnancy patterns of pantropical spotted dolphins (Stenella attenuata) in the eastern tropical Pacific determined from hormonal analysis of blubber biopsies and correlations with the purse-seine tuna fishery

Abstract

The northeastern offshore population of the pantropical spotted dolphin (Stenella attenuata) in the eastern tropical Pacific remains listed as depleted under the Marine Mammal Protection Act, and recent estimates of abundance show that recovery has been slow. One hypothesis for the slow recovery is that continued chase and encirclement by the tuna fishery negatively affects reproduction. Insufficient life-history sampling in this region over the last two decades makes traditional estimates of reproductive rates impossible. Here, we examine the current reproductive patterns of these dolphins by measuring blubber progesterone (BP) concentrations in biopsy samples to assess pregnancy state. BP was quantified in 212 biopsies from female offshore spotted dolphins sampled between 1998 and 2003 in the northeastern tropical Pacific, and we found that 11.5 % of the biopsied females (mature and immature) were pregnant. The relationship between pregnancy and fishery exposure was analyzed, and we found that pregnant females were exposed to significantly less fishery activity than non-pregnant ones (p = 0.022), suggesting that the fishery may have an inhibitive effect on pregnancy. Spatial analysis indicated that pregnancy was more aggregated than random (p < 0.05) at a scale up to 180-nmi, with the highest proportion pregnant in the mouth of the Gulf of California, an area with relatively low reported fishery activity.

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Notes

  1. 1.

    Other plausible explanations are summarized in Gerrodette and Forcada (2005) and include indirect reduction of dolphin survival, underreporting of dolphin by catch, and fundamental shifts in habitat conditions.

  2. 2.

    The ratio of usable samples (those with sufficient blubber) out of all females sampled, 212/320 was particularly low for this sample set because many of the samples have been subsampled for other studies (population genetics and immuno-histochemical staining to characterize protein composition).

  3. 3.

    Because there was no a priori stratification of the biopsy data, we did not compare the proportion pregnant between these regions with this sample set.

  4. 4.

    Given sufficient sampling, it is likely that one would find intermediate values indicating a transition between pregnancy states or perhaps as a signal of ovulation. However, because these events are short in duration relative to the amount of time a female spends in pregnancy, lactation, or as an immature animal over her lifetime, they would be rare events and therefore difficult to detect via biopsy sampling.

  5. 5.

    The duration of each dolphin set is on the order of one to 2 h with problematic sets lasting longer (Curry 1999).

  6. 6.

    Currently, we do not know how or whether biopsy sampling is selective relative to age. We do know that calves under a year old and mothers with calves under a year old were not targeted for biopsy sampling. With this restriction, one would expect that the biopsy data would be, like the fishery-kill, biased high compared to the fraction of pregnant females within the actual population because female calves cannot be pregnant and mothers with calves are disproportionally less likely to be pregnant compared to other adult females (lactation generally suppresses ovulation). However, it is unknown whether this bias is as large as that resulting from the fishery-kill; consequently, it is uncertain whether the differences in selectively of the two sampling methods (i.e., fishery by catch and biopsy) are solely responsible for the large differences in proportion pregnant.

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Acknowledgments

We like to thank our amazing marine mammal observers and field personnel, especially Juan Carlos Salinas and Ernesto Vazquez, for obtaining many of the biopsies used in this study. Additional thanks go to the crew and officers of the NOAA ships David Star Jordan and the McArthur II for their adept service over these years. Also this project would not have been possible save for the tireless efforts of the tuna boat observers, who over many years brought home quality data despite arduous working conditions. Finally, we thank Wayne Perryman and Bill Perrin for their insightful recommendations of both the research and this manuscript. This research was funded by NOAA Fisheries under the International Dolphin Conservation Program Act.

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Correspondence to Nicholas M. Kellar.

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Communicated by U. Siebert.

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Kellar, N.M., Trego, M.L., Chivers, S.J. et al. Pregnancy patterns of pantropical spotted dolphins (Stenella attenuata) in the eastern tropical Pacific determined from hormonal analysis of blubber biopsies and correlations with the purse-seine tuna fishery. Mar Biol 160, 3113–3124 (2013). https://doi.org/10.1007/s00227-013-2299-0

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Keywords

  • Pregnant Female
  • Pregnancy State
  • Pregnant Animal
  • Tuna Fishery
  • Spotted Dolphin