Juvenile natural mortality is a hard parameter to estimate, but it can be done with pop-up archival tags

New research published today (9th May 2018) uses electronic tags to highlight the risk of shark mortality through unintentional capture in fishing gear. Associate Editor, Andre Punt, comments on the new paper by Benson et al., Juvenile survival, competing risks, and spatial variation in mortality risk of a marine apex predator.

One of the most influential parameters in any population dynamics model is the rate of natural mortality, as it determines productivity and hence the sustainable level of population removal. Natural mortality is perhaps the most difficult demographic parameter to estimate for marine populations, with the rate for juveniles being even more difficult to estimate than that of adults and sub-adults. Benson and colleagues report on a novel use of an existing technology, pop-up archival satellite tags (PATs), to estimate juvenile natural mortality, and apply it to white sharks in the north eastern Pacific Ocean.

There are few methods to estimate natural mortality in the field. These have involved conventional tagging, the use of ‘smart tags’, and approaches such as acoustic telemetry. However, these methods have been applied primarily to estimate natural mortality for adults and sub-adults for most species.

Juvenile white shark tagging-Kevin Weng
John O’Sullivan of the Monterey Bay Aquarium (left) and Dr. Christopher Lowe of the California State University-Long Beach Shark Lab (right) prepare to release a tagged juvenile white shark in Southern California. Data from 37 sharks tagged since 2002 generated new insights into causes of mortality and mortality rates for juvenile white sharks in the Northeastern Pacific waters of California and Mexico. Photo © Kevin Weng/Virginia Institute of Marine Sciences.

PATs, in contrast, have been used primarily to examine movement patterns of fish and marine mammals, and to determine the survival of animals that have been incidentally caught in fisheries. Benson and colleagues developed a method that involves interpreting the final hours on the tag of depth, temperature and light data to assess whether animals that were not captured in fisheries have died. Estimates of survival rate were estimated using the Kaplan-Meier estimator, which allows for some animals to be right-censured because they were still alive when the tag popped up.

Benson and colleagues tagged 44 white sharks using PATs off the coast of California, USA, and Baja, Mexico, of which 37 provided usable data.  Two of the sharks were assessed to have died of natural causes, including one that was likely consumed by a predator. Nine of the young-of-the-year sharks interacted with fisheries, of which six died, one was released alive, and two were reported missing.

Overall, the annual survival rate of juvenile sharks off California was estimated to be 0.632 (SE 0.146), with interactions in fisheries assessed as the primary source of mortality. Three of four sharks tagged off Mexico were caught in fisheries. The mortality risk for sharks tagged off California declined with increasing length, which may be due to increased familiarity with fishing gear.

The sample size was fairly small (37 animals in total), which led to a fairly imprecise estimate of mortality and an inability to estimate survival for sharks in Mexican waters. Furthermore, the data were collected over a fairly long period of time, during which predator populations may have changed in abundance and several fisheries have experienced regulation changes. However, the estimates of juvenile survival obtained by Benson and colleagues are the first for white sharks estimated using satellite tracking data. As such, these estimates will provide analysts a basis from which to develop models for this population of white sharks and help to partition mortality between natural and fisheries-related causes.

PATs have been placed on many marine species, including several of conservation concern, and for which estimates of mortality may be unavailable. The approach of Benson and colleagues could be used to obtain such estimates with sufficient precision to help conservation and fishery managers challenged with developing management measures to achieve management goals.

The full article, Juvenile survival, competing risks, and spatial variation in mortality risk of a marine apex predator is free to read for a limited time in Journal of Applied Ecology.

Find out more about the research here.

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