Abstract
Identifying factors influencing survivorship is key to understanding population persistence. Although satellite telemetry is a powerful tool for studying remote animal ecology and behaviour it is rarely used for demographic studies because distinguishing the death of the animal (individual mortality) from failure of the tag (mechanical tag failure) has proven difficult. Southern elephant seals present an opportunity to separate tag failure from animal mortality thanks to the availability of large tracking datasets, broad knowledge of demographic rates, and because for these large animals, satellite tags are known not to influence mortality rates. A key rationale for investigating satellite telemetry to estimate mortality as compared to using traditional Capture-Mark-Recapture methods is the potential for obtaining spatially and temporally specific information, particularly while the animals are at sea and largely unobservable. We used satellite tag data from 182 seals from Isles Kerguelen, deployed between 2004 and 2018. Of these, 76 (42%) tags transmitted for the full post-moult foraging trip (max. 265 days for females and max. 305 days for sub-adult males) with the remaining 107 tags (58%) ceasing transmission at sea. We found that contrary to expectations, behavioural choices seem not to influence tag failure rates by mechanical means, rather the signals we detected seemed to align with previously described variation in mortality between groups. There was evidence, albeit limited, for an increase in tag failure for adult females in years with negative Southern Annular Mode (lower Southern Ocean productivity). We speculate that this increase in failure may suggest higher mortality in these years. Also, males using the Kerguelen Plateau had higher tag failure rates than those in the sea-ice zone, perhaps indicative of higher mortality. We suspect that these differences in tag failure rates between groups reflect variation in predator exposure and foraging success. This suggests satellite telemetry could be used to infer mortality events for southern elephant seals while they are at sea.
Highlights
Knowing when and why animals die provides key insights into the processes that shape populations
We found a tag failure rate of 40% for the first 200 days of deployment which increased to 70% at day 265 and 90% at day 305
The daily tag failure rate of one unit failing every 7–8 days per 100 tags deployed before 200 days may at least in part be due to intrinsic software or hardware failure
Summary
Knowing when and why animals die provides key insights into the processes that shape populations. Finer scale temporal estimates of mortality are more challenging due to the need to regularly monitor a population’s status which is especially difficult for animals that perform long migrations into remote regions. Tracking studies have the potential to quantify individual animal mortality even in remote regions (Hays et al, 2016). Tracking studies were used to estimate mortality in three raptor species by quantifying how animal migration routes and behaviour affected individual survival and calculating the effect of these factors on population growth rates (Klaassen et al, 2016). The main obstacle in the use of tracking data to estimate mortality rates is that even in the absence of catastrophic battery failure (Smith et al, 2018), an unknown proportion of tags will stop transmitting due to failure of the tag itself, which can be as high as 25%, at least in terrestrial studies (Hofman et al, 2019)
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