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Pelagic Pupping: Reproduction in the World's Largest Fish

Summary

Whale sharks (Rhincodon typus), the world’s largest fish, occupy tropical and sub­tropical waters worldwide yet are only regularly observed at a few locations throughout their range. Of the thirteen known aggregation sites, all are dominated by small, immature sharks1. To date, scientists are still unable to reliably locate any reproductively viable individuals and have no knowledge of breeding habits or pupping locations. Consequently, current research is dominated by our understanding of juvenile whale sharks.

As technology improves along with our understanding of the global ocean, this bias is bound to change. A recent whale shark study in the Gulf of Mexico reported the extensive movement of one sub-adult female to the mid-South Atlantic, which contrasted sharply with the restricted movements of 34 juveniles tagged in the same study2. Subsequently, the authors proposed the Mid-Ocean Ridge Pupping Hypothesis (MORPH), suggesting mature females give birth in the open ocean near oceanic ridges and may explain our extremely low encounter rate with these reclusive mothers.  However, many questions arise from this hypothesis. Why would these individuals make energetically costly migrations to such an oligotrophic environment to give birth? Where does mating occur, and why hasn’t this been observed? Why are juveniles dominant in every known aggregation site to date? These questions are becoming increasingly important as global elasmobranch populations experience serious declines3,4, and an estimated 17% of shark species, including whale sharks, are listed as critically endangered, endangered, or vulnerable to extinction5.

Surprisingly, we are still unable to address these questions and have no knowledge of the reproductive individuals that sustain global whale shark populations. However, evidence from survey planes and fishing vessels indicates individuals of this elusive and unstudied demographic of adult whale sharks are present seasonally in the shelf break waters less than 100 miles south of Cape Cod (S. Thorrold & G. Skomal, pers. comm.). Thus, to fill this knowledge gap, I propose to employ a comprehensive movement ecology toolset, including satellite tagging, stable isotope, and genetic marker techniques, to address movements, habitat associations, and behavior of these reproductive individuals that effectively occupy our backyard. 

Over the last 5 years, our research group has assembled the world’s largest whale shark telemetry program in the Saudi Arabian Red Sea and demonstrated some incredible movements that make a significant contribution to our understanding of whale shark ecology as a whole (Fig. 1)6. We are confident that our methods for this work are effective as we have demonstrated great success in deploying >150 tags for our megafauna tagging programs, and our results exhibit standard reporting rates for this technology (78% and 73% tag report rate for whale sharks and basking sharks, respectively)7. My lab has also made significant contributions to marine biogeochemistry by devising new compound-specific isotope techniques that enable finer interpretation of isotopic patterns for fish movements8. Finally, our collaboration with the Reef Ecology Lab (KAUST) enables access to a myriad of genetic analyses that can facilitate especially useful insight into population relatedness, connectivity, and genetic exchange among regions and ocean basins9. Specifically, my experience with these techniques in the lab Camrin Braun 2013 COI Proposal and the field makes me confident that I can successfully lead and execute a study to elucidate the ecology of adult whale sharks in the north Atlantic.

To initiate my thesis research, I propose to conduct a multi-day field trip to the Canyons region (70 miles southeast of Woods Hole) to deploy satellite tags and collect tissue samples for genetic and isotopic analyses. Although my advisor has provided me with an initial set of tags for this work, I do not have access to funding for hiring a boat to deploy these tags. Thus, I am also submitting a funding proposal to the National Geographic Young Explorers program and plan to apply to WHOI’s Ocean Life Institute and Access to the Sea programs during the next cycle. Finally, we recently initiated the Tagging of Oceanic Teleost and Elasmobranch Megafauna (TOTEM) project as an effort to raise additional funds to increase our tagging efforts. To date, we have secured over $60,000 for our collective tagging projects.


1Rowat D, Brooks KS. J Fish Biol 80 (2012). 2Hueter RE et al. PloS One 8 (2013). 3Dulvy NK, et al. Aquatic Conservation 18 (2008). 4Ferretti F, et al. Ecology Letters 13 (2010). 5IUCN (2012). 6Berumen ML, Braun CD et al. PloS One (submitted). 7Musyl MK, et al. Mar Ecol Prog Ser 433 (2011). 8McMahon et al. PNAS 109 (2012). 9Saenz-Agudelo, et al. Molecular Ecology 18 (2009).

Last updated: December 4, 2013