Archival Pop-up Tag Studies of Pelagic Fishes and Sea Turtles in the Pacific Ocean: Treasure Trove of Data
Mike Musyl 1, Yonat Swimmer 1 and Rich Brill 2
1University of Hawaii, Joint Institute of Marine and Atmospheric Research, Pelagic Fisheries Research Programme, Kewalo Research Facility, 1125-B Ala Moana Blvd., Honolulu, HI 96822
2Virginia Institute of Marine Science, PO Box 1346, Gloucester Point, VA 23062
Integrating the latest technology into their research strategy, scientists from the National Marine Fishery Service Honolulu Lab, and the University of Hawaii, Pelagic Fisheries Research Program, are tackling many questions about the ecology and management of pelagic fish and turtle species in the Pacific Ocean using archival pop-up tags. These tags contain a veritable treasure trove of immediately useful ecological information that can be used to help solve a number of important management questions such as delimiting stock boundaries, identifying possible spawning areas, and investigating the post-hooking survivability.
An oceanic white-tip shark is fitted with a harness
drilled through the dorsal fin to which an
Archival Pop-up tag is attached.
Photos courtesy of Phil White and
Mike Musyl
A blue shark is fitted with an Archival Pop-up tag.
Telemetry from the tag helps to determine
the mortality and morbidity of released sharks
from fishing gear.
Figure 1. When one of the tagged sharks died
and sank, the Archival Pop-up tag jettisoned and
uploaded its data.
Archival pop-up tags are fishery independent and thus do not require fishermen to physically return tags because data are downloaded via the Argos System of polar orbiting satellites. After a pre-programmed pop-off date or if the animal dies and sinks, the tag automatically jettisons and floats to the surface, whereby it transmits archived data to these satellites. The tag is equipped with several “fail-safe” measures to ensure successful downloading of collected data (e.g. pressure, temperature, and a daily geolocation estimate). Changes in light intensity can be used to calculate times of dusk and dawn from which longitude can be estimated from local noon, while latitude can be estimated by day length.
The type of tag selected for our studies depends on the research questions at hand. For example, we are trying to determine if blue sharks and turtles survive after release from longline fishing gear. If a shark was implanted with a conventional archival tag and it died and sank, we would get no data back. Instead, we chose Archival Pop-up tags to answer our questions about morbidity and mortality in post-released blue shark (and other pelagic species) in the Pacific Ocean because these tags can still provide data in the event of a mortality. Moreover, the data prior to transmissions of data will enable detection of a mortality and allow for differentiation between a shed tag and a dead animal. One of our tagged sharks died and sank, and its tag automatically jettisoned and uploaded its data. According to the temperature and depth chart (Figure 1), the shark showed some apparently normal vertical behaviors for the first five days, then expired. We are confident that this tag's data clearly represented a mortality event—the system worked just like we thought it would for showing the mortality of tagged individuals (i.e., the tag's pressure-sensitive depth-release mechanisms worked properly at about 1200m).
Obviously we don't view the mortality as a failure because we expected results like this for some of the deployments. We faced a Hobbesian choice in this study. On one hand we didn't want to tag moribund sharks that were obviously going to die, because that wouldn't provide useful information. But we also didn't want to tag only extremely vigorous animals, because that would skew our conclusions as to rates of post-release mortality. Dr. Chris Moyes, a colleague at Queen's University, will conduct analysis of blood samples to look for biochemical correlates of post-release survivability; with some luck, his data should confirm that the tagged shark was in bad shape when released. In view of the results to date, we envisioned archival tags deployed on turtles should act in a similar fashion, allowing researchers to determine unequivocally the mortality of released animals.
Preliminary findings have started surfacing (literally) in the initial stages of research projects intended to determine, among other things, the survivability of bigeye tuna (Thunnus obesus), yellowfin tuna (Thunnus albacares), blue marlin (Makaira nigricans), bigeye thresher shark (Alopias supercillious), blue shark (Prionace glauca), longfin mako (Isurus paucus), oceanic white-tip shark (Carcharhinus longimanus), swordfish (Xiphias gladius), and sea turtles (Green Chelonia mydas), Leatherback Dermochelys coriacea, Loggerhead Caretta caretta, olive ridley Lepidochelys olivcea) caught and released from commercial longline gear. We are using model PTT-100 Archival Pop-up tags in our projects to resolve a number of environmental and behavioral details about the animals, including:
1. daily depth distribution and horizontal and vertical movement patterns;
2. the effects of oceanographic conditions on the sharks' vulnerability to longline gear;
3. the survival rates of sharks captured and released from longline gear; and,
4. stock identification, dispersal and possible fishery interactions.
Last November we traveled to Costa Rica where the incidental capture of sea turtles (primarily olive ridleys) in fishing gear is very high. Our goal was to investigate whether free-swimming green turtles exhibited different behaviors than longline caught and released animals. We affixed archival tags to control animals (n=3) (i.e., caught by divers free swimming) and ones caught directly from longline gear (n=4) (Figure 2). Although horizontal movement patterns for the two groups appeared to be similar and indicated neritic behavior, the vertical behavior differed. While turtles that had been hooked rarely made dives greater than 100m depth, control animals frequently made dives beyond 200m depth. Further studies such as these will provide solid baseline data in which to infer the mortality and morbidity of turtles released from fishing gear.
As for the mortality of tags, there are perils out there besides loss due to sinking, implosion, shedding and shark nuptial bites. We got an e-mail from a Japanese longliner to let us know they had pulled in one of our tagged sharks. The funny thing was, no data had uploaded from the tag even though it should have already. We contacted them to ask about it, and it turned out the crew had put the tag in a tackle box or something, and stored it in the wheelhouse. We figure the tag couldn't establish a link with the satellite because its signal couldn't penetrate the metal of the box or the wheelhouse roof. We asked the ship's crew to place the instrument outside and the tag started to transmit archived data on temperature and depth experienced by the shark for nearly one month.
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