Satellite tags an essential tool with risks for shark science

The preservation of sharks, which has been of utmost importance in regards to the marine ecosystem as a whole due to their recently declining populations, has recently been aided by the development of new satellite tracking technologies.

Acquiring policy-accommodating material puts pressure on marine researchers to better understand the habits and migratory patterns of threatened or endangered shark species. In this way, satellite tagging technology has aided these researchers in a vast amount of knowledge that would otherwise be inaccessible due to the difficulty of observing marine animals. Furthermore, highly pelagic species’ migratory patterns would be impossible to understand without such technologies.

Modern tags have greatly improved upon the initial functionality of those from 70 years prior, however each option still comes with a host of caveats. Some tags offer the ability to measure salinity, depth, and ambient temperatures along with greatly improved tracking capabilities. These tags are most often clamped onto the sharks’ dorsal fins and are built to degrade or detach after a set period of time.

Controversy has surrounded the tagging process and made headlines due to the posed risks of interfering with sharks’ daily habits with an invasive procedure, which has further complicated a practice already plagued with high-cost barriers and steadily improving but still imprecise data collection. Despite the wealth of new knowledge that satellite tags have provided about shark habits and migration patterns, there is much to still be improved on.

Analysis

2.1 Comparison of Modern Tags

2.1.1 PSATs

Modern PSATs (Pop-up Satellite Archival Tags) use a combination of satellite transmission and archival data in order to transmit information. They store data on temperature, depth, and light. Data is transmitted when they resurface, which can occur if they detach from their host shark and float to the surface or remain attached when the shark swims to the surface of the water. The best method of data collection is via recovery of the tag since the full recorded data history can be downloaded from the tag’s storage.

PSATs are secured onto a shark via tether when its dorsal fin is exposed above water after being lured to a boat. One of its greatest benefits over SPOT tagging is that it can be attached while the shark is still underwater. An anchor is held in place by applicator pins, often in conjunction with a swobbit pole to secure the tether through the shark’s fin with minimal interaction between the researchers and shark. Most tags are built with the attachment mechanisms made from magnesium, which corrodes over time in ocean water and allows the tags to fall off naturally, assuming no complications occur.

The three types of PSATs currently marketed for use on sharks are the Survivorship PAT, MiniPAT, and Mark Report PAT. The two latter tags have an advertised maximum usage time of 800 days but are generally used for a much shorter span of time, with an ambiguous detachment time that can occur randomly based on how secure the attachment is and how volatile a shark’s movement patterns are.

2.1.2 SPOT Tags

Shark species have generally been ideal for habit tracking via SPOT (Smart Position Only Tracking/Smart Position and Temperature Tracking) tags due to their tendency to resurface. SPOT tags are secured to the dorsal fin as well, but the antenna is secured at the highest point it can safely reach above the fin. Once the tag is above water, it transmits data to Argos satellites that are then sent to the researcher who requested or administered the tag. Most SPOT tags are accurate within roughly 350 meters, which is far more precise and consistent than PSATs. Unfortunately, SPOT tags record fewer data types (either location and temperature or location only) and require a more invasive form of attachment to remain secure.

Unlike PSATs that are secured via a dart-like mechanism, SPOT tags must be bolted on and risk severe degradation and deformation of the shark fin when they do not properly detach after a set period of time.

2.1.3 Other Tags

Recently Business Card Tags (BCTs) allow researchers to view the interactions between tagged individuals based on communication between 2 BCT devices.

A newly released technology called Fastloc, which has been incorporated into SPOT tags as SPOT-F, allows for extreme GPS location precision with less than a second required for transmission. Unfortunately, this device is double the price of most tags. Coupled with the already risky investment of purchasing tags that could fail, this is a less viable option for the majority of shark research (excluding well-funded case studies for individual sharks).

2.2 Applications

2.2.1 Discoveries

The unique ability for PSATs to track depth opened several new possibilities for researching shark habits. For example, a 2009 study utilizing PSATs found that whale sharks often dive under 1km.[3] Rough temperature and location estimates were also tracked, but the two tags used both detached prematurely. This highlights the difficult decision that researchers must make between PSAT and SPOT tagging. Neither option provides perfect data but they can sacrifice reliability and accuracy for more overall information received when using a PSAT.

Recent studies have still benefited from the improved accuracy of new satellite tags. In 2014, researchers used data collected from 18 tiger sharks tagged using a combination of SPOT and PAT tags off the eastern Australia coast to determine their partial migration habits. The data were represented as a density chart showing the areas in which the sharks most commonly traversed. A strong correlation was found between water temperature and migration patterns, which was aided by data from the SPOT-tagged sharks.

Using a combination of SPOT and PAT functionality provides a strong dataset for study but increases costs and provides fractured data between the sharks tracked. Researchers also noted a drop in accuracy for the SPOT tags that remained deployed for longer than 300 days. The sharks tagged were also primarily juvenile, which are at greater risk for fin damage or stunted growth due to tagging despite the researchers noting minimal damage on their subjects.

2.2.2 Large-Scale Analysis

The quintessential application of modern tracking tags is an analysis of data from 4306 tags that were used to track a range of different migratory species, primarily within the California Current large marine ecosystem [2]. Notably, the authors found that predators like sharks would actively swim toward regions in which they were physiologically advantaged despite an abundance of their natural prey existing opposite to their direction of travel. Unique adaptations between shark species that inhabit different regions manifested as notable migratory patterns between these species.

The authors confirmed that removal of sharks within these ecosystems has a significant role. Populations of predators such as the porbeagle shark and bluefin tuna have rapidly declined in the Atlantic, causing their respective food chains to suffer a detrimental impact and chain reaction of declining populations. These declines were primarily attributed to commercial fishing and bycatch.

Trackers allow for a more thorough understanding of shark behavior and population density within different regions, oftentimes based on location tracking data alone. In addition, the authors noted that there is very little research on population assessments for marine predators.

Discussion

3.1 Drawbacks

3.1.1 Lack of Accessibility

The difficulty of conducting population assessments or large-scale studies on sharks, as mentioned in Section 2.2.2, revolves around three primary limiting factors: variation, price, and legality.

The vast selection of satellite-based tagging options now available for shark research has become the newest hurdle but will likely be overcome naturally given enough time. Since tags are decided upon based on the type of research being conducted, they fit their respective niches very well. This means that the data from short-term studies such as those deploying PSAT tags could not be used in an analysis of yearlong shark habitation or migration. Data from any particular study is not broad enough to provide information on a given group of sharks in the future without the need for researchers to re-tag sharks in that area using different trackers.

Price is a primary concern for researchers and ensures that any proposed shark studies are gate-kept by the financial inaccessibility of these devices. Tags have been reported as costing anywhere from $1000 to more than $6000 depending on features, so despite the introduction of improved tags like the SPOT-F these tags are unlikely to be utilized in most studies.

Researchers must carefully consider the functions they need and consider whether the added capabilities of a more expensive tag outweighs its cost-of-entry. Complications also arise with the risky recovery process, discussed more extensively in Sections 3.1.2 and 3.1.5. The high cost compounds the issue of accuracy, since limiting the number of sharks studied for any one study results in data that can be skewed by habit and pattern differences between individual sharks. 15-20 sharks studied within a relatively small time frame cannot accurately represent their species as a whole.

Considering the risks that sharks are exposed to throughout the tagging process, the restrictions that have been placed on researchers in order to ensure ethical study and observation of sharks has been beneficial in terms of shark conservation. Permits are required for tag-based studies, researchers must be adequately educated on proper tag application, and certain marine protected areas restrict the number of intervention researchers can perform.

3.1.2 Capture

The ethics of shark tagging have come into question due to the invasive procedures required for tag attachment. Catch-and-release is inherently stressful for sharks and poses significant risks if complications occur at some point

throughout the process. Researchers will oftentimes need to capture for reasons other than tagging, such as taking measurements or aiding an injured shark. Unfortunately SPOT tagging ensures that this process must be done regardless. Sharks are most commonly captured using gill nets, which can damage or even kill sharks if used improperly.

Sharks lack nociceptors, so they do not conventionally perceive pain based on the current understanding of sensory receptor function. A more significant cause for concern is stress, which has been observed as a result of tagging and capture.[1] Blood lactic acid levels were increased as a result of any human-induced deviation from the sharks’ natural movements, including capture and reeling. Some species exhibited less lactic acid buildup than others, but given the high mortality risk as a result of stress-induced lactic acid buildup this should ideally be avoided under any circumstances possible. Stress also impacts sharks’ habits; Sharks generally respond to stress negatively by displaying atypical behaviors and increased aggression, which can also affect long- term mortality rate as the frequency of shark tagging increases.

3.1.3 Tagging Damage

There are conflicting studies on the long term impact of tag attachment. This method of data collection has been controversial due to both public concerns about the damage caused by tag attachment as well as the invasive practice of catching sharks for tagging. A 2011 study tracked the effects of SPOT tagging on 15 white sharks in order to shed more light on the repercussions involved in shark tagging.[7] 8 of the sharks had minimal damage with the tags properly detached from their fins. One shark had its SPOT tag still attached after 822 days, resulting in severe deformation of its dorsal fin.

They assured that the only damage that could occur when a tag properly detaches is cosmetic and harmless toward the well being of the shark, but concluded that there could be structural damage from SPOT tags remaining on juvenile white sharks remaining attached for a period of longer than 12 months.

Severe fin damage due to SPOT tag attachment.[7]

3.1.4 Inaccuracy of Devices

Given that PSATs estimate distance traveled based on light geolocation alone, they can occasionally produce wildly inaccurate data. If researchers wish to determine salinity, depth, and other factors along with precise location data they would not be able to do so with a PAT or SPOT tag alone. The effectiveness of PSATs is also severely reduced for less migratory species of sharks because light-level data analysis requires long distances of travel for efficient measurements.

In a 2008 study published by Fisheries Research, researchers highlighted the fact that modern sensors do not use the simplistic and less invasive form of attachment employed by PSATs due to the fact that the erratic motion of tethered sensors distorts location data. [4]

Tags use a wet-dry sensor in order to determine whether data can be transmitted to an Argos satellite. Position accuracy estimates are also provided with this data based on the number of messages received at a given location, which is impeded by both a shark’s tendency to resurface (or lack thereof) and the likelihood of an Argos satellite being within range.

It is up to the discretion of the tag users to either take more plentiful but less accurate data or more accurate data with fewer data points. Location data is categorized with an accuracy of 3, 2, 1, 0, A, B, or Z in order from most to least accurate. Despite the SPOT tags’ greatest location range being within 250 meters, this error margin is only available with LC 3 data. For researchers studying tiger shark movements in a 180-day span in 2014, this meant being restricted to the 500m error margin of LC 3 to LC 1.[6]

3.1.5 Data Retrieval

Researchers who deploy PSATs must take into account the timing at which they attach the tags so they can properly recover the data. This also requires a prediction of where the sharks may be at a given time for retrieval as well as luck since there is no guarantee the tag will remain secured for the full deployment period.[9]

PSAT tags can transmit data to an Argos satellite at most once every four hours, which is also limited by the system’s bandwidth. This requires the recovery of the tags in order to access the full log of collected data within the onboard memory storage. Researchers who opt for PSATs take a financial risk since these tags can and often will fail or detach, at which point the data may be permanently lost. To combat this, PSATs are constructed to float to the surface once detaching and have a built-in pinger so the tracker can be recovered via boat.

In the event of a stuck PSAT or SPOT tag, data retrieval requires manual recovery of the tag from the shark. Given the financial and ethical constraints of traveling out to the water with a crew and recapturing a shark, these $1000+ investments are equivalent to a one-time-use disposable.

Dr. Neil Hammerschlag of the University of Miami conducted a review of shark tags noting that the overall rate of failure for tags, mostly SPOT and PAT, was roughly 10%.[5] This dropped to 7.2% between the studies from 2007-2010 and has likely declined further, in part due to greater knowledge and ethical understanding of shark research. Tags are more advanced, attached more securely, and often coated with anti-fouling paint to reduce potential damage to both the shark fin and tag.

3.2 Proposed Improvements

3.2.1 Attachment and Detachment

As Section 3.1.3 highlights, SPOT tags in particular need a redesign and future designs of tags should improve upon their attachment techniques in order to minimize damage to sharks.

While tethering stands as a less invasive technique, it is reserved for tags that do not use direct location tracking due to the noise from their movement. Utilizing a device that automates the bolting process instead of requiring a full capture and release would lessen the stress on sharks from capturing them.

Bolting is effective at attachment but not detachment, so at the moment the progress of SPOT tag designs are hindered by the pace at which technological advances provide smaller circuits and circuit components.

A newly designed tag should take into account the corrosive properties of magnesium (or alternative materials used) and find a more reliable method of detachment without sacrificing too much of the time specificity of detachment offered.

3.2.2 Fouling

Anti-fouling paints can reduce the susceptibil- ity of tags to degrade over time by deterring algal growth and the proliferation of other mi- croorganisms on a shark’s dorsal fin, which in turn can decrease the risk of damage to the fin as well. Fouling of a tag not only irritates the skin near its point of attachment, but it can also interfere with the detachment process and increase drag when the shark is swimming due to the added weight of the biofoul. Despite this, anti-fouling is not mandatory and does not come pre-packaged with tags. It is clear that satellite tags, particularly SPOT tags, should come pre-applied with an anti-fouling agent.

3.2.3 Design

The devices offered for shark tagging are primarily used with a wide range of species and their design, therefore, caters more to minimizing the overall size of the casing so that it can fit all of the components. A design that sits nearly flush with the dorsal fin by incorporating numerous smaller batteries as opposed to three or four larger ones and increasing the diameter of the device while reducing its protrusion would be farther more suitable for sharks. This would decrease the drag and reduce the risk of the tag interfering with sharks’ stealthy predatory habits.

3.2.4 Standardization

The greatest potential improvement, and the least likely one, would be the standardization of satellite tracking tags. The inclusion of various

data types on a single tag that could be used for all modern shark tagging studies as opposed to a wide range of different tags with different capabilities and price points would both expedite the manufacturing process and ensure that future studies have enough data from prior ones so that additional shark tagging no longer becomes warranted.

[1]  Gallagher AJ et al. “Physiological stress response, reflex impairment, and survival of five sympatric shark species follow- ing experimental capture and release”. In: Marine Ecology Progress Series 496 (2014), pp. 207–218. DOI: 10.3354/meps10490. URL: http://www.int-res.com/abstracts/ meps/v496/p207-218.

[2]  B. A. Block et al. “Tracking apex marine predator movements in a dynamic ocean”. In: Nature 475.7354 (2011), pp. 86–90. ISSN: 1476-4687. DOI: 10.1038/nature10082. URL: https://doi.org/10.1038/nature10082.