Sims et al. (2009) used a fast-acquisition global positioning system (Fastloc GPS) tag with remote data retrieval to track long-term movements of three Ocean Sunfish (Mola mola) captured and tagged off southern Portugal. This allowed tracking in near real time with position accuracy of <70 m. These fish are known to dive to depths of at least 472 m and can often remain at deep depths for long periods, only returning occasionally, and then often only briefly, to the surface. The system implemented permitted reception and transmission of location information whenever the fish came close to the surface.
According to Sims et al., their study represents the first demonstration of long-term (>90 days) GPS tracking of a large pelagic fish and shows the great potential for this technique, whereby GPS-quality location data are retrieved remotely via conventional Argos satellites. By freeing researchers from the restriction of working on estuarine or nearshore species in order to physically recover tags to download GPS acquisition data, this technique offers tremendous potential for tracking large pelagic fish species that surface relatively frequently. Although the high spatial accuracy of this technique for fish in open ocean habitats has practical applications for both fisheries and conservation, it will not be suitable for fish species that remain in deep water after tagging.
In the early part of the 20th Century, Ocean Sunfish were sometimes described as active swimmers, but in other cases described as sluggish, inefficient swimmers, passively carried by ocean currents. Modern tracking studies of these fish using attached acoustic transmitters, acceleration dataloggers, and satellite-linked archival transmitters have demonstrated that they are active swimmers both horizontally and vertically. Sims et al. (2009) found that the Ocean Sunfish they tracked covered around 10 to 20 km per day, a rate that is comparable to the movements of pelagic sharks. GPS track integration with current direction/strength maps showed individuals often headed into and across prevailing currents associated with mesoscale eddies. These data confirm that Ocean Sunfish are not passive drifters, but rather, are in fact active swimmers with movement rates within the range observed for other pelagic fishes. Search-like movements occurred over at least three distinct spatial scales. At fine scales, Ocean Sunfish spent longer in highly localised areas with faster, straighter excursions between them. These "stopovers" during long-distance movement appear consistent with finding and exploiting food patches.
Sims et al. (2009) suggest that there would be great value in pursuing long-term GPS tracking of other large pelagic fish species--such as tunas, billfish, and sharks--that surface relatively frequently and have high conservation priority in many ocean regions where a greater understanding of when and why they use certain habitats could enhance management.
