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Seeing where tagged billfish go and learning about their diving behavior gives us fascinating insight into these animals’ behavior. However, IGMR tag data are also beginning to give us an understanding of not only where billfish go but also why. IGFA Representative and Stanford student Martini Arostegui had the opportunity to work with IGMR data to investigate patterns of blue marlin movement in the Pacific with the assistance of Dr. Aaron Carlisle (IGFA’s funded postdoctoral position at Stanford). Hawaii has seen more IGMR events and tag deployments than any other location. This is in large part due to the fact that genesis of the IGMR was the venerable Hawaiian International Billfish Tournament (HIBT) and the collective guidance of IGFA Trustee Emeritus Peter Fithian, IGFA Representative Bob Kurz and Dr. Barbara Block. 2009 saw the first deployments of satellite tags on marlin at the HIBT for what was then called the Great Marlin Race and tagging has taken place there every year since. Multi-year deployments at the HIBT provided a rich dataset that allowed Arostegui and Carlisle to examine how prevailing oceanographic conditions affect horizontal and vertical movements in blue marlin. Working together they analyzed tag data from 2009-2012. One thing the scientists realized immediately was that there were several years where blue marlin readily made trans-equatorial movements where they travelled south from Hawaii and ventured into the southern hemisphere. However, there was also a year when no tagged marlin crossed the equator. Although results at this point are preliminary and pending publication, Arostegui and Carlisle believe that the El Niño Southern Oscillation (ENSO) which causes the El Niño/ La Niña cycle may be a contributing factor that dictates if and where blue marlin cross the equator into the southern hemisphere. Under neutral ENSO conditions, trade winds blowing from the southeast advect surface waters westward in the central equatorial Pacific. To fill the void, deeper water rises to the surface in a process called upwelling. If you look at an ocean in cross section, it’s not a homogenous mass of water. Water near the surface receives Thermocline/Oxycline Deeper water is colder and, because it is not well mixed, contains less oxygen. At the intersection of these two water masses is a steep gradient of decreasing temperature and oxygen saturation called the thermocline and oxycline, respectively. And the depth at which the thermocline/oxycline occurs can be strongly influenced by upwelling. Cooler SST from the effects of upwelling and vertical habitat compression inhibit the ability of marlin to migrate. 64 BISBEE’S CONSERVATION JOURNAL Q1/Q2 • 2016


BCJ_Q1/2_2016
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