Implications of tag positioning and performance on the analysis of cetacean movement

Abstract

Background: Satellite-linked animal-borne tags enable the study of free-ranging marine mammals. These tags can only transmit data while their antenna is above the surface for a sufficient amount of time. Thus, the position of the tag on the animal's body will likely influence the quality and the quantity of location estimates. We explored the effects of tag placement and tag performance on the analysis of cetacean movement, by deploying two identical Argos tags 33 cm apart on the dorsal fin of a male killer whale in Norway in January 2017. Results: The highest placed (top tag) generated 540 location estimates, while the lowest placed tag (bottom tag) generated 245 locations. In addition, the top tag generated locations of higher quality, with less than 50% of the location estimates in Argos class B (the class with the highest estimated uncertainty), compared to the bottom tag (90% Argos class B locations). The distance between two reconstructed paths ranged from 81 m to 31 km. The path based on the top tag was 1.5 times longer, yielding a higher average speed and more extreme turning angles. The estimated uncertainty around the top track was smaller than that of the bottom track. Switches between searching and travelling behaviour, based on data from the top and the bottom tags, occurred at different positions and times. A significant relationship between core utilization areas and a simulated environmental variable was detectable at a finer spatial scale using data collected by the top tag compared to the bottom tag. A literature search yielded no evidence that tag performance or tag placement is commonly discussed in killer whale telemetry articles. Conclusions: The differences in quality and quantity of location estimates from our two tags had a substantial effect on derived movement metrics, behavioural inferences and significance of a simulated environmental variable. These differences in tag performance are likely linked to the height difference in tag placement of 33 cm. We suggest that tag positioning on free-ranging marine mammals and tag performance should be considered as a covariate in telemetry studies, especially at a fine scale.

Asset Tracking Whales—First Deployment of a Custom-Made GPS/GSM Suction Cup Tag on Migrating Humpback Whales

“…This allowed for longer exposure times of the tag and better acquisition of GPS. The position of telemetry tags is known to influence location results [47]. The first whale was moving with constant swim speed and constant dive times in a steady southeasterly direction.…”

Killer whales are attracted to herring fishing vessels

“…Killer whales Orcinus orca are highly mobile, globally distributed predators. In Norway, killer whales interact with industrial purse seine herring fisheries by feeding around the nets (Similä 2005, Rikardsen 2019. A recent estimate suggests that 15 056 killer whales (CV = 0.293, 95% CI = 8423−26 914) inhabit the northeastern Atlantic (Leonard & Øien 2020), with more than 1100 known individuals in Norwegian waters (https://www.norwegianorca-id.no).…”

“…Killer whale movements in Norway have been associated with their primary prey species: Norwegian springspawning (NSS) herring (Clupea harengus), which is the largest herring stock in the northeastern Atlantic (Drage sund et al 1980, Similä et al 1996, Kunin gas et al 2014, Jourdain et al 2019. A large portion of the NSS herring stock often overwinters close to the Norwegian coast (Nøttestad & Axelsen 1999, Huse et al 2010, Rikardsen 2019. These winter aggregations have attracted large numbers of killer whales since the 1980s and humpback whales Mega ptera novaeangliae since 2011 (Similä et al 1996, Jourdain & Vongraven 2017.…”

“…The tags measured 54 × 46 × 20 mm and were surface mounted with 2 subdermal 68 mm titanium anchors. Tags were specifically deployed close to the middle of the dorsal fin, as this position yields better position data compared to a lower-placed tag (Mul et al 2019). All killer whales were adult males, with the exception of 1 adult female.…”

A Satellite-linked Tag for the Long-term Monitoring of Diving Behavior in Large Whales

“…Extrinsic factors like bad weather or biofouling can also affect data recovery by limiting satellite transmissions. The biases associated with these irregular dive summaries are related to the degree of irregularity and the scale of the behavior being studied (81,82), but a more adequate characterization will require dedicated research. For these reasons, RDW tags should be considered to provide a relative index of dive behavior, rather than a continuous and complete record.…”