Marine turtles are charismatic species that have garnered support due to their listing on the International Union for Conservation of Nature Red List of Threatened Species (Campbell and Smith, 2006; IUCN, 2021). Recently, some populations have demonstrated stabilization and even growth, while others are still facing declines (Wallace et al., 2011; Mazaris et al., 2017; Barrios-Garrido et al., 2020). To promote the recovery of declining populations as well as the continued success of growing populations, integrated conservation and management strategies that protect both individuals and key habitats are needed (IUCN/SSC Marine Turtle Specialist Group, 1995; National Marine Fisheries Service and U.S. Fish and Wildlife Service, 2008). In an effort to determine the strategies that will be most beneficial for aiding in this recovery, experts have identified key questions, research priorities, and knowledge gaps that need to be addressed. Priority topics range from demographics to nesting strategies to health parameters. Of particular importance is the topic of spatial distribution (Hamann et al., 2010; Hays et al., 2016; Wildermann et al., 2018). Indeed, understanding how individuals utilize space throughout time is imperative for identifying key areas to protect, as well as for assessing whether current and proposed protective measures are well placed to conserve species (Cooke, 2008; Gredzens et al., 2014; Lea et al., 2016; Santos et al., 2021).

Several research approaches have been used to study the spatial distribution and habitat use of marine turtles, such as mark-recapture studies (Blumenthal et al., 2009a; Llamas et al., 2017), visual transect surveys (Gorham et al., 2016), submersible camera recordings (Auster et al., 2020), drone surveys (Sykora-Bodie et al., 2017), stable isotope analyses (Bean and Logan, 2019; Pearson et al., 2019), epibiont identification (Silver-Gorges et al., 2021), biotelemetry studies (Berube et al., 2012; Santos et al., 2021), and a combination of the aforementioned techniques (Scales et al., 2011; Haywood et al., 2020; Siegwalt et al., 2020). Biotelemetry in particular, or the tracking of individuals, has emerged as a common method for obtaining habitat use information of species (Cooke et al., 2004; Hussey et al., 2015; Hays and Hawkes, 2018). Most marine turtle tracking studies have utilized satellite telemetry, with hundreds of studies tracking thousands of individuals (Hussey et al., 2015; Hays and Hawkes, 2018). Less common and perhaps underutilized in marine turtle biotelemetry is acoustic telemetry (Hussey et al., 2015). This method is widely used in fisheries and has the ability to provide fine-scale spatial information on a scale of tens to hundreds of meters at a lower cost than other biotelemetry technologies, depending on receiver set up and detection ranges (Heupel et al., 2006; Hussey et al., 2015; Zeh et al., 2015; Matley et al., 2021b).

Acoustic transmitters are attached to or implanted within an individual and emit coded ultrasonic signals which are logged by receivers. Individuals can be actively followed and tracked with a single receiver or tracked passively via an array of receivers moored throughout the study site. Because individuals must pass within the detection range of a receiver to be detected, acoustic telemetry provides presence-only data and studies are spatially constrained by the receiver array, limiting their applicability for highly migratory species. Passive array designs as well as study site characteristics will dictate what questions can be answered and how the data is analyzed and interpreted (Heupel et al., 2006; Heupel and Webber, 2012; Whoriskey and Hindell, 2016). However, since signals transverse through water easily, acoustic telemetry allows for near-continuous monitoring of marine turtles, who spend 91–97% of their time submerged (Renaud et al., 1995; Schmid et al., 2002), As a result, acoustic telemetry provides higher numbers of detections, improved resolution, and lower positional errors than other tracking techniques, such as satellite telemetry, which only transmits location data when turtles surface (Hazel, 2009; McClellan and Read, 2009; Hart et al., 2012; Whoriskey and Hindell, 2016). Therefore, when within the range of receivers, acoustic telemetry presents an opportunity for effectively collecting spatial information for marine turtles (Matley et al., 2021a; Pillans et al., 2021).

While acoustic transmitters themselves are low-cost compared to other biotelemetry devices, installing and maintaining an array of receivers and facilitating data retrieval can be quite costly (Whoriskey, 2015; Zeh et al., 2015). However, acoustic receivers are not species specific (Lea et al., 2016; Matley et al., 2019), which allows multiple taxa to be tracked simultaneously within the same receiver array, enabling cost-sharing among projects to reduce the financial burden to any one study (Selby et al., 2019; Bangley et al., 2020; Reubens et al., 2021). Tens of thousands of passive receivers exist throughout the world and as more are deployed, opportunities for collaborations and data sharing emerge (Hussey et al., 2015; Whoriskey, 2015; Whoriskey and Hindell, 2016). Organizations such as the Ocean Tracking Network (OTN) and FACT Network create regional and global networks that facilitate data sharing among members and encourage standardized data collection and analysis protocols. While challenges still exist in creating compatibility between transmitters and receivers of different manufacturers, these networks are working toward increasing the spatial scale of studies as well as the overall capacity of acoustic telemetry to address questions related to the ecology of marine species, such as marine turtles (Whoriskey, 2015; Bangley et al., 2020; Young et al., 2020; Matley et al., 2021b; Reubens et al., 2021).

To provide marine turtle researchers and managers with a comprehensive understanding of how acoustic telemetry can contribute to marine turtle conservation, we conducted a systematic literature review to investigate how acoustic telemetry has been used to study the ecology of marine turtles. We document the extent of marine turtle acoustic telemetry studies, identifying the regions, species, life stages, and approaches that have received the most attention. We detail the contribution of acoustic telemetry studies to the knowledge of marine turtle ecology and its application toward understanding complex concepts important to aiding their conservation, while also highlighting challenges and identifying knowledge gaps associated with acoustic telemetry. The approaches and limitations discussed here are applicable to many marine taxa and present a broad overview of the relevancy of acoustic telemetry to marine conservation.

A systematic literature search was conducted on the 28th of January 2021 following the Preferred Reporting Items for Systematic Reviews and Meta-Analyses (PRISMA) standard (Moher et al., 2009) to identify studies that have utilized acoustic telemetry to obtain information on marine turtles. The search was performed within EBSCO Host and Google Scholar using the below search string:

(“chelonia mydas” OR “caretta caretta” OR “eretmochelys imbricata” OR “lepidochelys kempii” OR “lepidochelys olivacea” OR “natator depress^∗” OR “dermochelys coriacea”) AND (“acoustic telemetry” OR “acoustic transmitter^∗” OR “sonic telemetry” OR “sonic transmitter^∗”).

The above string was also searched in Web of Science (“All Databases”), prefaced with “TS = ” per the database’s search requirements. An additional, broader search was conducted in Web of Science using the search string:

TS = [(“marine turtle^∗” OR “sea turtle^∗”) AND (“acoustic telemetry” OR “acoustic transmitter^∗” OR “sonic telemetry” OR “sonic transmitter^∗”)].

For inclusion, papers must have been published in peer-reviewed academic journals and have utilized acoustic telemetry (also referred to as sonic/ultrasonic telemetry) to gain insight and knowledge on marine turtles during any life stage. Studies in laboratory settings and/or those focused solely on the efficacy of acoustic tag use were excluded, as well as reviews. The original search strings returned 316 unique results. After applying the selection criteria, 39 published journal articles fell within the scope of our review (Supplementary Figure 1). Backward and forward searches were conducted to identify additional papers for inclusion, which required scanning the reference sections of all 39 original papers, as well as scanning the titles of all articles that cited the selected papers. The same selection criteria were applied when determining whether additional articles should be added. In total, 48 studies were selected for inclusion in this review.

To understand the spatial, temporal, and demographic trends of existing marine turtle acoustic telemetry studies, we extracted the following information from each article: date of study and publication, study location, type of tracking method used, number of individuals successfully tracked, species, life stage, sex, duration of tracking, transmission interval, and transmitter attachment method, fate, and retention. It was also noted whether studies conducted range tests to estimate the detection probabilities of the receivers used in the study. Occasionally, the same tracking data was used in multiple published studies for different analyses. Individual turtle parameters that were the same between studies were omitted from our analysis, such as species, sex, life stage, and transmitter details. We used the life stage specified by the study; if the authors did not identify the life stage, individuals were classified as “unknown” due to varying growth rates and age at sexual maturity across species and populations (Avens and Snover, 2013). The study period was defined as the overall time between the first and last tracking events included in a study’s analysis. For active telemetry studies, tracking duration was considered to be the cumulative time an individual was actively followed, while for passive studies it was defined as the time between which the individual was released or the study began and when the study ended or the transmitter was evidenced as being dislodged. Some studies limited their analyses to a specific period of time (e.g., a period when all passive receivers were operable). For these studies, we included only the tracking duration used in analyses. Transmitter retention was only determined for those known to be lost.

Additionally, the research question(s), study goal(s), and major findings of all 48 studies were reviewed and grouped to identify the overarching broad research topics that acoustic telemetry studies have addressed, as well as to explore common and novel approaches utilized. Findings were synthesized to illustrate what information acoustic telemetry has provided for marine turtle ecology thus far. To assess the applicability of acoustic telemetry in eliciting high-priority information on marine turtle ecology and conservation, we reviewed three publications that employed expert opinions to identify top research priorities and key questions: Hamann et al. (2010), Hays et al. (2016), and Wildermann et al. (2018). These studies focused on marine turtles on a global scale, marine megafauna movement ecology, and immature marine turtles, respectively. After removing similarities, we critically reviewed the questions presented in these publications to assess whether acoustic telemetry could address key topics of importance for further marine turtle research.

The 48 studies that met our criteria for inclusion in this review (Supplementary Table 1) were published between January 1983 and January 2021. The number of studies published per year has increased, with an evident shift from active to passive tracking methods (Figure 1). While 63% of the studies utilized active tracking methods, these studies generally had smaller sample sizes (avg: 16 individuals; range: 2–94) than passive studies (avg: 28; range: 3–89). Therefore, of 870 individuals tracked with acoustic telemetry, 66% were tracked passively. Studies were found to be globally distributed with most (46%) conducted in the Caribbean and Gulf of Mexico, followed by 31% in the Pacific, 13% in the Indian Ocean, and 10% in the North Atlantic (Figure 2).

All marine turtle species except the olive ridley (Lepidochelys olivacea) have been tracked acoustically, with most studies focusing on green (Chelonia mydas) turtles (Figure 3). Green and hawksbill turtles (Eretmochelys imbricata) are the only species that have been tracked with both active and passive telemetry (Figure 4A). Individuals from all life stages have been tracked, although the vast majority have been juveniles (45%) and hatchlings (36%; Figure 4B). Transmitters were most often (78%) applied to the dorsal posterior marginal scutes (3% to the ventral posterior marginal scutes) of juvenile and adult individuals. Attachment methods included wiring or bolting the transmitter through the marginal scutes (34%), using an epoxy, putty, or glue to affix the transmitter (25%), or a combination of both methods (36%). For hatchlings, acoustic transmitters were attached to the plastron with adhesive (71%) or attached via a tether (29%).

Overall study periods ranged from 4 days to 8 years, with longer studies typically tagging new individuals throughout the study or replacing individuals’ transmitters periodically to increase tracking time. On average, individual tracking durations were less than 1 day (range: 5 min to 3.5 days) for active studies and 186 days (range: 7 min to 1,414 days) for passive studies. Short tracking durations were mostly those of hatchlings that traversed the study site quickly during their dispersal. Transmitter retention was largely unknown since in many cases (72%) it could not be discerned whether the transmitter was lost, the turtle never returned to the study area, or tracking ceased due to the end of the study. There were 55 known instances of tags becoming dislodged or lost from an individual, of which the average tag retention was approximately 255 days (range: 1–1,414 days).

Acoustic telemetry has been used to obtain information on a variety of topics related to marine turtle ecology and conservation. Most studies (83%) to date have focused on the spatial distribution of juvenile, sub-adult, and adult marine turtles and have addressed questions related to their home range and site fidelity, movement, vertical space use, and the drivers that influence these (Figure 5). Additionally, acoustic telemetry has been used to explore hatchling dispersal, assess spatial proximity and response to threats, and monitor turtle health and physiological parameters (Figure 5). Several of these topics fall within the research priorities identified by experts in the field, with acoustic telemetry directly or indirectly addressing 60% of key questions identified by Hamann et al. (2010), Hays et al. (2016), and Wildermann et al. (2018) (Supplementary Table 2).

While acoustic telemetry has been utilized by a relatively small number of marine turtle studies, the technology has provided a wealth of information regarding marine turtle ecology, particularly regarding spatial distribution and movement within study sites. Here, we detail the contribution of acoustic telemetry to our knowledge of marine turtle ecology and explore both commonly used approaches as well as those less routinely used. We discuss the application of acoustic telemetry toward marine turtle conservation, while also acknowledging its limitations.

Future marine turtle acoustic telemetry studies can build upon the approaches already established to develop novel ways to answer outstanding research questions. For example, further use of active tracking to recapture individuals can increase studies on physiological parameters over time (Wibbels et al., 1990; Southwood et al., 2003, 2006). Subsequent sampling of individuals can also inform epibiont colonization rates (Silver-Gorges et al., 2021), as well as provide additional data for growth rate curves (Patrício et al., 2014). Further, active tracking can be used to retrieve additional bio-logging equipment and sensors from individuals, allowing incorporation of in situ environmental data or video footage into studies (Harcourt et al., 2019; Jeantet et al., 2020; Smulders et al., 2021). Improved demographic information, including survivorship and timing of ontogenetic habitat use shifts, could be achieved with increases in acoustic studies investigating long-term home ranges and site fidelity (Hamann et al., 2010; Wildermann et al., 2018). Additionally, further studies analyzing predation and storm response could inform survivorship rates. Acoustic arrays placed offshore of known nesting beaches could provide insight into the movement patterns of internesting females (Addison et al., 2002). While only 10% of current acoustic telemetry studies analyzed the spatial overlap of marine turtles with threats, this approach can be expanded upon to further conduct threat assessments of chemical spills, pollution (e.g., plastic, light, noise) and a variety of human activities (e.g., boating, snorkeling, fishing) (Hamann et al., 2010; Sequeira et al., 2019).

As the field of acoustic telemetry grows, it will be important to look to other taxa to learn new approaches. For example, fisheries and elasmobranch studies have utilized tightly gridded arrays to triangulate locations of individuals for more fine-scale location data and have analyzed social interactions among individuals (Armansin et al., 2016; Binder et al., 2018; Becker et al., 2020). While differences in life histories, morphology, and behavior may deem some approaches inapplicable to marine turtles, there is much to gain from reviewing studies from fields where acoustic telemetry has been more widely utilized, such as fisheries (see reviews: Heupel and Webber, 2012; Donaldson et al., 2014; Hussey et al., 2015; Matley et al., 2021b). Currently, one of the main limitations to marine turtle acoustic studies is that transmitters are applied externally, which reduces the retention time compared to transmitters surgically implanted in fish and shark species (Smith et al., 2019; Smukall et al., 2019; Kennedy et al., 2020). Although marine turtle transmitter retention varies by species, the longest recorded duration (hawksbill) was just under 4 years (Chevis et al., 2017; Smith et al., 2019). Further research is needed to identify the effect of transmitter application techniques on tag retention rates. Meanwhile, active telemetry can be used to recapture individuals and proactively replace transmitters that are loose or near the end of their battery life in order to increase tracking durations (Selby et al., 2019).

Despite its limitations, acoustic telemetry’s capacity for answering important research questions is growing due to its advantages in providing spatial information on hard-to-monitor species and with the proliferation of acoustic tracking networks (Bangley et al., 2020; Matley et al., 2021b). With data-sharing within networks, acoustic studies can potentially span coastal regions and even ocean basins and inform migration paths and long-distance movements of marine species (Guttridge et al., 2017; Young et al., 2020). Networks, such as the OTN, are dedicating resources toward establishing standardization of metadata and analysis protocols and are working with manufacturers to resolve differences between transmitter models and encourage open decoding protocols (Whoriskey, 2015; Bangley et al., 2020; Reubens et al., 2021). New technologies are being developed to aid in data retrieval, such as autonomous underwater vehicles (AUVs) that can retrieve data from receivers, as well as systems that can transmit detection data to the surface with a modem and hydrophone. Advancements are also being made in utilizing marine organisms and AUVs as roving receiver platforms to increase detection probabilities (Whoriskey, 2015; Davis et al., 2018; Bangley et al., 2020; Ennasr et al., 2020). A proposed framework for standardization of data across all biotelemetry technologies will increase compatibility of data and improve biotelemetry’s ability to inform spatial distribution of marine species (Sequeira et al., 2021).

The combination of these advances provides opportunities for the exploration of a wider scope of topics related to marine species ecology and conservation. Data sharing can lead to studies that track multiple species to identify global hotspots for biodiversity and examine both intra-and interspecies interactions to understand social behavior among individuals, as well as predator-prey relationships. The advancement of multispecies studies can lead to ecological rather than species-level approaches to conservation and management (Lea et al., 2016; Brodie et al., 2018; Bangley et al., 2020).

The original contributions presented in the study are included in the article/Supplementary Material, further inquiries can be directed to the corresponding author/s.

MF developed the initial concept for this review and provided critical edits to the final manuscript. EH designed and conducted the systematic search, extracted and analyzed the data, and wrote the original manuscript. Both authors contributed to the article and approved the submitted version.

The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

All claims expressed in this article are solely those of the authors and do not necessarily represent those of their affiliated organizations, or those of the publisher, the editors and the reviewers. Any product that may be evaluated in this article, or claim that may be made by its manufacturer, is not guaranteed or endorsed by the publisher.