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Great Marlin Race expands knowledge of billfish behavior

By BY DONNELL TATE/Harbor Report - | Jan 8, 2021

The original Great Marlin Race was initiated in 2009 by Dr. Barbara Block with Stanford University and International Game Fishing Association (IGFA) Representative Robert Kurz, with the guidance of Peter Fithian, IGFA trustee and chairman of the Hawaii International Billfish Tournament, to commemorate the 50th year of the HIBT. Tournament teams, individuals or clubs sponsor satellite tags. Each tag is programmed to pop off the fish 204 days after it is deployed.

Once the tag pops off, information is transmitted to a satellite and then to scientists at Stanford University. The data is then processed so that anglers can see graphical representations of the track of each fish on the IGFA Great Marlin Race.

In a tournament, the tag that surfaces farthest from where it was initially deployed wins the race for that tournament. The overall winner of the annual IGMA is the fish that travels the furthest in all the participating IGMA events for the year, from Oct. 1 to Sept. 31, based upon when tags pop off.

Follow the IGFA Great Marlin Race with the latest satellite tag pop-ups on the IGMR website. Real-time satellite tag pop-ups are added to the site as they happen. Anglers can learn about each marlin and their route of travel over the 240 days while at liberty. Visit www.igfa.org and click on the IGMR Science logo.

Satellite tagging reveals interesting behavior of Pacific blue marlin. IGFA Conservation Director Jason Schratwieser stated, “Satellite tags are greatly enhancing our body of knowledge on billfish behavior due to their ability to collect massive amounts of data, and because they do not need to be recovered to obtain the information they collect.”

They have learned that blue marlin tagged at all the IGMR races exhibit a consistent pattern in their depth distribution. The marlin spend the majority of their time (around 60 percent) in the top 16-33 feet of the water column. When they are not near the surface, they primarily occur between 165-330 feet below the surface.

At the Hawaii International Billfish Tournament in Kona (the longest-running IGMR event), tagged marlin have consistently exhibited this bimodal depth distribution across all of the years of races.

The depth range of blue marlin is likely constrained by the thermocline, the boundary between relatively warm water surface water and deeper, colder water below. This is likely due to the fact that blue marlin are quite temperature-sensitive, and they are unable to spend long periods of time below the thermocline.

They rarely are in waters less than 79 degrees Fahrenheit. However, blue marlin do make periodic deep dives to depths in excess of 1,155 feet, but these forays into colder, deeper waters are generally brief, and the marlin quickly return to the surface layer after these deep dives.

This bimodal depth distribution likely represents a day/night pattern, where fish generally dive deeper and spend more time at depth during the day and remain near the surface at night. One theory behind this behavior is that the marlin are highly visual predators, and they dive deeper during the day so they can better see their prey, silhouetted at the surface, while remaining invisible to them below.

Temperature and depth data provide valuable information about what part of the water column the fish use and the oceanographic structure of the water column. Light levels are used to estimate the location of the fish using techniques similar to 17th century explorers.

These are some of the preliminary discoveries that scientists have made during the last several years.

They are also beginning to understand broader patterns in migratory behavior that appear to be linked to major oceanographic processes such as El Nino and La Nina events.