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in the Pacific has revealed that they spend the majority of their time in water warmer than 24°C (78°F). Blue marlin in the Pacific also display split diving behavior, where they spend the majority (≈60%) of their time in the upper five meters of the water column and 30-35% in 25-100 meters. Blue marlin are capable of occasional dives to depths in excess of 300 meters, but these dives are rare events. The combination of low oxygen concentration and cold water temperature have been found to prevent many species from making extended deep dives below the thermocline/ oxycline. As with other “cold-blooded” fishes, cold water temperatures slow a marlin’s heart rate and low oxygen reduces its metabolic rate. Taken collectively, cold, oxygen-deficient water is not prime habitat for blue marlin and, as such, they largely avoid it. Areas that have a shallow thermocline/ oxycline compress the amount of viable habitat (warm, well-oxygenated water) to the uppermost portion of the water column. Previous work done by other billfish scientists has demonstrated this phenomenon of habitat compression in the eastern tropical Atlantic and Pacific and how it constrains billfish diving behavior. However, Arostegui and Carlisle believe that habitat compression along the equator may affect if and where blue marlin cross it. Of all the ENSO phases, El Niño appears to be the most conducive for blue marlin to undertake trans-equatorial migrations. Warm, well-oxygenated water flowing from the western Pacific warms equatorial waters and also deepens the thermocline/oxycline. These conditions are favorable for the typical diving behaviors that blue marlin exhibit. Also, because warm water extends so far to the east during El Niño conditions, it also appears to allow blue marlin to cross the equator further to the east than in neutral ENSO conditions. Conversely, Arostegui and Carlisle believe that La Niña conditions may be the least favorable ENSO phase for blue marlin to cross the equator. The position of the 25°C isotherm during the 2009 El Niño and the 2010 La Niña demonstrates how radically equatorial sea surface temperatures are affected. During the 2009 El Niño the 25°C isotherm’s western equatorial boundary was 123° W, while during the following year’s La Niña it extended out to 165° W—a difference of approximately 5,000 kilometers (3,106 miles). In addition to very cool sea surface temperatures, extreme shoaling of the thermocline/oxycline and subsequent habitat compression may have served as a physical barrier to marlin making trans-equatorial migrations. La Niña - 2010 Event 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. Neutral ENSO conditions such as those experienced in 2012 have oceanographic conditions that are intermediate relative to El Niño and La Niña, but still appear to be somewhat favorable for blue marlin to cross the equator into the southern hemisphere. However, because of cooler water and shoaling of the thermocline/oxycline in the equatorial Pacific associated with prevailing upwelling conditions, marlin that crossed the equator did so farther west along the equator than did those that crossed during the 2009 El Niño event. Thus, Arostegui and Carlisle believe that ENSO conditions not only influence the likeliness of trans-equatorial migrations, but also the longitudinal location of where blue marlin cross the equator. While habitat compression associated with La Niña conditions may be a factor that affects trans-equatorial movements of blue marlin in the Pacific, previous research has shown that blue marlin and other billfish species exist in other areas that exhibit habitat compression. In fact, habitat compression in some areas may be favorable for billfish because it also concentrates their preferred prey. Arostegui and Carlisle believe sea surface temperatures may determine whether or not thermocline/oxycline shoaling becomes an unfavorable environment for blue marlin. The intertropical convergence zone (ITCZ) is an area just north of the equator (10°N) that between June and December acts as the “meteorological equator” between hemispheres. The ITCZ is an area of low pressure that experiences upwelling but also has elevated sea surface temperatures relative to those at the equator. During the 2009 El Niño, the thermocline/oxycline at the ITCZ were both shallower and had steeper gradients than adjacent equatorial waters yet the three marlin crossed this area of habitat compression utilizing the upper, mixed portion of the water column that had favorable water temperatures. Similarly, during the 2010 La Niña, one tagged marlin made an eastward migration 10°N of the equator presumably because sea surface temperatures and the shoaling thermocline/oxycline were unfavorable for an equatorial crossing. Yet, even with significant habitat compression in the ITCZ, this particular fish continued to stay in the upper mixed layer that had warm sea surface temperatures of up to 28°C for two and a half months. 66 BISBEE’S CONSERVATION JOURNAL Q1/Q2 • 2016


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