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Eyes in the Ocean: Measuring Post-Release Behaviour of Pelagic Predators by Alina Hussey

What is post release behaviour?

Post release behaviour is just as it sounds… the behaviour of the animal after it has been captured, handled, and released. Certain factors may result in an increase in behaviour modification, for example, if an injury is sustained from fishing gear or if tagging procedures are performed incorrectly. In sharks, post-release behaviour is difficult to monitor, as they swim off into their enormous underwater domains, making observation impossible to sustain for the necessary periods of time needed to recognize behavioural differences.

Why is post release behaviour important?

When handling large pelagic fish, it is imperative that the stress of the animal be a major priority, not only for the reduction of bias in the analyses of the data retrieved from tags, but also in consideration of adequate animal health and wellbeing. Whether the shark was captured by longline for sampling because of scientific fishing, or in an instance of bycatch, measures must be taken to examine post-release behaviour to determine if any irregularities occur. If these irregularities can be identified, then steps can be taken to reduce stress caused by capture and handling, and to further refine fisheries release systems, and study techniques.

{A tiger shark being released after a quick workup - Photo by Chelle Blais]

What is a satellite tag?

One method of tracking the movement of marine animals is the pop-up satellite archival tag (PSAT), which detaches from the animal at a predetermined time, floats to the surface, and transmits collected data by use of the Argos satellite system. As a result, PSAT tags do not have to be physically retrieved. PSATs record the depth, pressure, ambient water temperature, and light-level data in a format which is time-stamped. It is important to note that these tags are very small to prevent interference with the behaviour of the animal.

[ An unused PSAT tag here at the Shark Lab ]

Satellite tags and post release behaviour – how does it relate?

Since PSATs can be used to track the vertical movements of sharks, data retrieved from these tags can indicate irregularities in their movement through the water column. Satellite tags have been used to see if sharks fail to survive after release, and this can be seen if the tag goes down beyond a certain depth or remains the same for an extended period of time.

Another key reason for tracking post release behaviour is to ensure that the data being used for research from electronic tags is reflective of the real behaviour of the animal when it is in the wild. Determining if irregularities in behaviour exist, in what capacity, and for how long, will demonstrate how much data needs to be removed so that only normal behaviour is present.

In addition, satellite tags can provide information that can have a big impact on the enhancement of marine management within fisheries. As mentioned, post release mortality, or the number of animals that die following release, can be estimated from satellite tag data. When this data is compared to behavioural indicators of animal health (for example, eye reflex, wounds, body reflex), and the two are found to be correlated, a strong metric is created which can be of great use to get a real understanding of the actual numbers of shark mortalities for a given fishery. In short, satellite tags can provide resource managers with easy-to-understand data sets with straightforward information that have the potential to make a big difference. If mortality levels are too high, greater pressure can be applied to modify fishing practices, or to improve handling practices when releasing.

Satellite tags essentially serve as “eyes in the ocean”, helping to develop a better understanding of these post release behaviours to improve marine management, and further lessen the impact of fishing practices, all whilst maintaining the integrity of the science.

[How we satellite tag sharks - A similar method is used for PSAT tagging - Video by Chelle Blais]

References and further reading:

Bowlby, H. D., Benoît, H. P., Joyce, W., Sulikowski, J., Coelho, R., Domingo, A., Cortés, E., Hazin, F., Macias, D., Biais, G., Santos, C., & Anderson, B. (2021). Beyond post-release mortality: Inferences on recovery periods and natural mortality from electronic tagging data for discarded lamnid sharks. Frontiers in Marine Science, 8.

Hoolihan, J. P., Luo, J., Abascal, F. J., Campana, S. E., De Metrio, G., Dewar, H., Domeier, M. L., Howey, L. A., Lutcavage, M. E., Musyl, M. K., Neilson, J. D., Orbesen, E. S., Prince, E. D., & Rooker, J. R. (2011). Evaluating post-release behaviour modification in large pelagic fish deployed with pop-up satellite archival tags. ICES Journal of Marine Science, 68(5), 880–889.

Hutchinson, M., & Bigelow, K. (2019). Quantifying post release mortality rates of sharks incidentally captured in Pacific tuna longline fisheries and identifying handling practices to improve survivorship. In: WCPFC Scientific Committee 15th Regular Session. WCPFC-SC15-2019/EB-WP-04, Pohnpei, Federated States of Micronesia

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Talwar, B., Brooks, E. J., Mandelman, J. W., & Grubbs, R. D. (2017). Stress, post-release mortality, and recovery of commonly discarded deep-sea sharks caught on longlines. Marine Ecology Progress Series, 582, 147–161.



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