Even after years of intense interest and some focused study, billfish biology and ecology remain mostly mysterious given the logistical difficulty of studying them. Although each region of the globe has its challenges, Atlantic billfish—specifically blue marlin, white marlin, sailfish and the roundscale spearfish (a white marlin look-alike that was recently put on the map by the GHRI and its NOAA collaborators)—pose many hurdles to scientists trying to quantify their numbers and study their behavior to develop more effective conservation methods.
The key challenge comes from their migratory nature. Unlike some fish species, which form large schools and migrate in known seasonal patterns, billfish tend to be largely (though not completely) solitary, have expansive territories and their travel patterns are poorly understood. This is compounded by the sparse distribution of billfish. With the possible exception of sailfish, they are so few in number that it is difficult to observe enough fish to determine if the observed migratory behavior of a few animals is typical of the whole species.
This migratory nature also complicates the implementation of management plans, since a given fish may easily travel through multiple national jurisdictions. This means conservation efforts must be agreed upon by a significant number of parties. In the Atlantic alone, there are 48 countries involved in taking billfish, either recreationally or commercially.
Furthermore, not all countries report landings, and research suggests that the great majority of billfish landings come as bycatch in commercial longline fishing operations, perhaps as much as 70 percent. It all adds up to mean accurate stock assessments and effective conservation measures are still an area of constant debate.
Technology, however, is helping researchers make progress on many of these fronts. One of the most effective tools in exploring billfish movements and shedding light on billfish behavior has become the PSAT (pop-up archival satellite tag) tracking devices that record fish movements, diving patterns, water temperature, and other parameters over the course of several months, and then upload this information via satellite for researchers to decipher. The tracks recorded by these devices help researchers piece together important facts about other behaviors, such as feeding and spawning.
In the continued effort to gain a detailed understanding of billfish movements, Dr. Guy Harvey, along with GHRI and NOAA scientists Dr. Eric Prince and John Hoolihan, are working together to tag and track this species in the central Caribbean and eastern tropical Pacific. It is usually difficult to keep the satellite tags on these fast-moving fish for more than two to three months, but two fish (estimated at 120 and 140lbs. each) recently tagged by Guy off his home base in the Cayman Islands have provided six-month tracks, an uncommonly long length of tracking time for this species.
To say the least, the results have been revealing. First, despite being tagged at the same place, the two fish wandered off in opposite directions. Second, although neither fish ran off to far regions of the Atlantic based on where they were tagged and where the tags popped up, analysis of the detailed tracks shows they covered some impressive distances. In six months, one fish travelled approximately 3,077 nautical miles and the other nearly twice that much (about 5,939 nautical miles). Of note here is that if you measured just the straight-line distances between the tagging location and the site where the tag popped up, these were only 313 and 131 nautical miles, respectively. The lesson here is that traditional tagging methods for billfish based on simple tag and recapture efforts can provide a highly inaccurate view of how far a fish travels. The challenge to researchers, of course, is that satellite tags and associated satellite time are pricey (about $3,800 per tag), so tagging large numbers of fish to properly decipher their migration patterns is expensive research.
Still, satellite tracking efforts such as these are the key to helping fill in the huge gaps in our knowledge of billfish. And, although the conservation challenges are significant, progress is being made. Some really excellent studies led by Dr. Prince at NOAA have utilized satellite tags to track blue marlin and revealed a remarkable alteration in the depths these fish use depending on ocean oxygen conditions. Basically, Dr. Prince and his colleagues found that blue marlin tracked in the eastern tropical Atlantic (off West Africa) spent much more time in shallower waters compared to fish tracked in the western North Atlantic (off Florida and the Caribbean), which spent more time in deeper waters.
One of the group’s key findings was that this “compression” of depth use by eastern Atlantic fish was related to the much lower oxygen levels that occurred in deeper waters in that area compared to the waters off the western North Atlantic. In other words, the low oxygen below certain depths forced the blue marlin into shallower waters in the eastern Atlantic, where they could become concentrated and, therefore, more exposed to commercial fisheries.
Most anglers recognize the importance of billfish, both as apex predators and as top targets of recreational anglers. As predators, these fish play an important role in the ocean’s intricate food web. As highly prized targets of anglers, they constitute an immensely valuable economic commodity. In fact, numbers from the International Game Fish Association peg the financial impact of U.S. recreational billfish anglers at well over two billion dollars every year. By any measure, they are fish worth fighting and worth fighting for.