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considerable sunlight and is well mixed from wind/ wave action. As a result these surface waters are warmer and well oxygenated. In contrast, 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. Neutral ENSO oceanographic conditions are therefore characterized by moderate upwelling in the eastern and central equatorial Pacific. In this area of upwelling, sea surface temperatures are lower than in adjacent tropical waters, with the thermocline/oxycline closer to the water’s surface. During the neutral ENSO conditions that were experienced in 2012, the 25°C isotherm extended (i.e., the sea surface temperatures blue marlin prefer) as far as 141°W in the equatorial Pacific. During this period, two of the nine blue marlin that were tagged off of Hawaii crossed the equator. One of the marlin crossed the equator at 147°W, while tag issues precluded the researchers from determining where the other marlin crossed the equator before the tag popped up south of the equator. During an El Niño event, trade winds in the eastern tropical Pacific relax and, as a result, upwelling subsides considerably, which allows warm waters from the western Pacific to slide eastward. Decreased upwelling not only creates warmer sea surface temperatures but also deepens the depth at which the thermocline/ oxycline occurs. In short, the eastern and central tropical Pacific warms up a good deal during El Niño and there is warm and well-oxygenated water deeper in the water column than in neutral ENSO and, especially, La Niña conditions. Oceanographic conditions at the equator in 2009 indicated the occurrence of a moderate El Niño and, during November of that year, the 25°C isotherm extended as far as 123°W. Of the five blue marlin that were tagged at that year’s HIBT, three crossed the equator at approximately 148° W, 141° W and 132° W. La Niña episodes are the polar opposite of El Niño. During La Niña, trade winds strengthen significantly producing extreme upwelling in the eastern and central equatorial Pacific with very cool sea surface temperatures and a very shallow thermocline/oxycline. In addition, strong trade winds transport this cooler water a considerable distance to the west along the equator. Strong La Niña conditions occurred in 2010 and not one of the 10 marlin that were tagged at that year’s HIBT crossed the equator. So what does this have to do with how marlin move around? Because they are tropical/ subtropical creatures, blue marlin have a fairly narrow thermal tolerance in regard to water temperature. Satellite tag data from blue marlin During the neutral ENSO conditions that were experienced in 2012, the 25°C isotherm extended (i.e., the sea surface temperatures blue marlin prefer) as far as 141°W in the equatorial Pacific. During this period, two of the nine blue marlin that were tagged off of Hawaii crossed the equator. El Niño - 2009 Event Oceanographic conditions at the equator in 2009 indicated the occurrence of a moderate El Niño and, during November of that year, the 25°C isotherm extended as far as 123°W. Of the five blue marlin that were tagged at that year’s HIBT, three crossed the equator at approximately 148° W, 141° W and 132° W. BISBEE’S CONSERVATION JOURNAL Q1/Q2 • 2016 65


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