Atlantic Bluefin Tuna Spawning in the Slope Sea: larval growth and retention

Christina Hernandez, Biology
Joel Llopiz, Biology
Glen Gawarkiewicz, Physical Oceanography

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Abstract

The Atlantic bluefin tuna (ABT) is a highly valuable species, both commercially and ecologically, and debates over its conservation and exploitation have become highly contentious.  This large-bodied, highly migratory species has gained broad attention from the media and conservation groups as a species in serious decline; the species is listed as endangered and the western stock is managed under a rebuilding plan.  The recent discovery of a spawning ground in the Slope Sea has led to a needed revision to the accepted life cycle of western ABT, which could have management and conservation implications.  The conditions experienced by larval tuna spawned in the Slope Sea is a critical gap in understanding the relative contribution of these larvae to the productivity of the western ABT stock.  We propose to test two hypotheses about the Slope Sea spawning grounds: (1) larval growth rates (and, thus, the abiotic and biotic conditions influencing growth) do not differ between the Slope Sea and the Gulf of Mexico, and (2) ABT spawn in the Slope Sea because larvae are likely to be retained in favorable habitat for their larval duration.
 
We will have the opportunity to participate in upcoming NOAA cruises in the Slope Sea, with ichthyoplankton sampling led by scientists at the Northeast Fisheries Science Center (NEFSC).  Samples of larval ABT will be brought to the lab at WHOI to estimate their growth rates; the same analyses will be performed on available ABT larvae from the Gulf of Mexico, and rates will be compared.  Additionally, oceanographic drifters with temperature sensors will be deployed at stations in the Slope Sea with relatively high abundance of ABT larvae.  Drifter tracks over 28 days will show likely trajectories of ABT larval patches, as well as the temperature conditions experienced by the larvae.  Based on the typical reproductive strategy of ABT, the drifters are expected to be retained in the warm waters of the Slope Sea, and to be associated with warm core rings.  By synthesizing growth and transport data, we hope to provide evidence about the relative contribution of the Slope Sea spawning ground to the productivity of the western ABT stock.  This study will build on existing relationships between WHOI and NEFSC scientists, and the proposed work will add a valuable biophysical perspective to the fisheries science cruises that will be working to revise the stock assessment for ABT.  Ultimately, we hope to leverage this work towards a larger comparative study of larval ABT growth and survival in the Gulf of Mexico and the Slope Sea.