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Researchers believe tagged marlin was eaten by a shark

By Staff | Mar 8, 2018

Majestic as they are, marlin are still part of the oceanic food chain and are subject to predation. A satellite tag deployed on a striped marlin in the Seychelles Islands, as part of the IGFA Great Marlin Race, reported its data back to the Stanford University lab, coordinated by Stanford University Professor of Marine Science Dr. Barbara Block, in partnership with the IGFA.

In this particular case, it appears that the marlin was killed by a shark five days after it was tagged and released. Unfortunate as this is, the tag data gave insight on what type of animal might have preyed on the marlin.

Dr. Jonathan Dale from the Stanford/Hopkins Marine Station gave his interpretation of what most likely happened.

Occasionally, satellite tags that are deployed on marlin are ingested by a predator feeding on a marlin.

Analysis of the transmitted tag data provides insights into diving behavior, water temperature and light level. All three can provide useful clues.

Frequently, after a predation event, the depth data reveals a change in diving behavior. Billfish typically show very consistent diving behavior during the course of a deployment, and a sudden shift in diving behavior is a good indicator that the tag is in a different animal. Another piece of evidence regarding a predation event comes from the light data.

Tag data that shows prolonged periods of darkness can indicate that the tag is in the stomach of another animal. Eventually, the fish will egest the tag, it will pop up, and the light data will once again follow the natural light cycle.

While depth and light data are sufficient to identify a predation event, the temperature data can differentiate between an ectothermic and an endothermic animal. When a tag is attached externally to a fish, the temperature data shows similar patterns as the diving behavior in that temperature will get colder as the animal dives into colder water.

When an ectothermic animal ingests a tag, there is little change in this relationship between depth and temperature data, although the temperature data will change a little slower due to being inside the fish and a process known as thermal inertia.

Endothermic animals, on the other hand, have the ability to generate and maintain heat within their bodies.

When an endothermic animal ingests a tag, the temperature data becomes more independent of the depth, with less variation in the temperature data due to the internal heat of the animal.

Although it is not possible to determine the exact species of animal that has ingested a tag, the most likely endothermic fish large enough to ingest a tag are lamnid sharks such as mako and white sharks.