Position Tracking

Without Light

Method of Tracking Monkfish Argos ID #133449 without Light

April 11, 2015 (by Marco Flagg)

Monkfish tagged with SeaTag-MOD using a two-point attachment that aligns tag along longitudinal axis 

Credit: Pictures and tag data in this application note courtesy Biopol

General Method

SeaTag normally uses light observations (noon time detection) to determine the tagged animal's longitude, and magnetic field intensity to determine latitude. The data for monkfish Argos ID #133449 tagged by Biopol shows it staying at depths beyond light measurements.

In the absence of light, the general tracking method is to use all available measurements (depth, magnetic field, acceleration, temperature) as 'breadcrumbs' excluding areas that are not consistent with some measurements, and finding a corridor that is consistent with all measurements. These data must be viewed in the context of environmental parameters such as bathymetry and the magnetic field, and the known behavioral characteristics and capabilities of the tagged animal (pelagic or benthic, maximum realistic travel distance per day, etc.).

The result of this data analysis is generally not a definitive track, but rather a plausible track or migration area that is consistent with the available measurements.

It should be noticed that the method of tracking animals in the deep ocean beyond the reach of light and making use of magnetic anomalies as waypoint markers is clearly in its infancy or exploratory stage. The reader is encouraged to explore in other directions or using other methods or data sources to advance this fascinating field.

Principal Tools

  1. Plotting of both the daily summary data and sensor snapshot data in Excel

  2. Google Earth used as the mapping tool and to provide approximate bathymetry data

  3. International Geomagentic Reference Field (IGRF) overlay for Google Earth: link

  4. World Digital Magnetic Anomaly Map (WDMAM) overlay for Google Earth: link

  5. Marine magnetic survey data from the WDMAM project: link

  6. NOAA Geomagnetic field calculator to obtain IGRF predicted total magnetic field values for the tagging and popup positions: link

  7. KML to CSV converter for plotting WDMAM marine survey data: link

Step 1: Tagging and Pop-Up Position

The most definitive pieces of information are the tagging and pop-up positions and dates shown in figure 1. The tagging position is recorded at the tagging location, and the pop-up position is obtained from Argos with the first positions of the tag after pop-up. It is important to inspect the Argos position track before deciding on a pop-up position, because first positions may be of poor quality. Look at both the Argos position quality class indicator (B, A, 0, 1, 2, 3 from worst to best), and the consistency of position fixes on subsequent passes to find a reliable pop-up location.

Figure 1: Tagging position obtained by GPS, and pop-up position and date from Argos. The area shown here is from about 63.0 deg N, 25.0 deg W to 64.2 deg N; 20.8 deg W

Step 2: Depth Measurements and Bathymetry

Figure 2 shows the momentary depth readings of the fish from the MODSN2 raw sensor data packets (blue) and the maximum daily depth (brown) from the MODDAILY daily summaries packets. (Since only a fraction of the raw observations are obtained by satellite, there are some discrepancies).

Figure 2: Momentary depth (blue) from MODSN2 sensor snapshot packets, and maximum observed daily depth from MODDAILY daily summary packets. The monkfish is a benthic animal, and so the maximum daily depth is assumed to be the sea floor.

The monkfish is a benthic or sea floor animal, and the assumption is that the deepest depth in each day reflects the bottom depth at the fish's location. For the majority of the migration, the depth is between 250m and 350m. This constrains the available migratory region as shown in figure 3. Note that the depth record in figure 2 starts with shallow depth, including 50m on the first day at liberty on 11/26, but already reaching 287m depth 3 days later on 11/29. The shortest path to this depth is shown by the white arrow in figure 3.

Figure 3: Depth data shows fish reaching 250m-350m depth range after three days, and spending much of the migration in that depth range. The green polygon outlines the 250m-350m depth range. The white error is the shortest path the fish could have used to reach this depth zone. Google Earth does not provide depth contours, but the depth at any location can be obtained by mouse-over. The southern edge of the polygon is the approximate 350m contour line, while the northern edge is 250m.

Step 3: Magnetic Readings, IGRF and magnetic sensing bias adjustment

IGRF is the International Geomagnetic Reference Field, which models the main magnetic field of the earth. For tracking purposes, we use the total field intensity. The field intensity generally grows stronger towards the poles, however it is irregular. Best tracking accuracy is obtained where the gradient is steep, i.e. the field grows strongly over a short distance. For example, in the western North Atlantic it is about 10 nano Tesla per nautical mile. Near Iceland however the gradient is relatively shallow at about 4 nano Tesla per nautical mile (Figure 4). This means that the magnetic measurements translate to a wide migration corridor.

Figure 4: The earth's main magnetic field as modeled by IGRF. Field intensity is used for position estimates, and a steeper gradient such as around the east coast of the US (~ 10nT / nautical mile) results in better position estimates. Around Iceland, the gradient is weaker (~4nT / nautical mile), and so position estimates will exhibit a larger error. The red patch over Iceland shows the north-south extend of migration allowed by IGRF within the magnetic observations of the tag from 51800nT to 52600nT (the patch is limited east-west here, but per magnetic measurements alone the monkfish could be anywhere along that corridor of field intensity around the globe).

Figure 5 shows the magnetic field measurements. As will be shown later, the steep slopes and extreme minima and maxima in early December are magnetic anomalies. Excluding these, the daily averages are from about 51800nT to 52600nT.

Figure 5: Magnetic field intensity observed by the tag. Momentary measurements (blue) and reported daily averages (brown)

Figure 6: Possible swath of migration with IGRF predicted field values of 51800nT to 52600nT matching the daily average observations of the tag

Magnetic observation bias compensation

For SeaTag-MOD calibrated to 'standard' as opposed to 'precision' levels (which was not yet available at the time of this tagging), the tag magnetometer readings generally exhibit some bias that must be compensated for. The method is to compare the IGRF predicted magnetic field intensity values to the tag's observed magnetic field intensity values for the known tagging and pop-up locations. As with the Argos pop-up location positions, it is again important to compare these daily summary reports to adjacent days to make sure the observations aren't significant outliers, such as due to strong anomalies. A method of error estimation is to compare the offset predicted for the tagging location and the pop-up location. Use the NOAA geomagnetic field calculator to obtain IGRF predicted intensity readings. The plots in figures 4-6 are based on subtracting an estimated offset of 1519 nT.


DATE 1/24/2014 11/26/2013
LATTITUDE 63.798 63.310
LONGITUDE -24.617 -22.566
OBSERVATION DATE 1/16/2014 11/29/2013 AVERAGE (nT)
OFFSET 1699 1340 1519.5
Table 1: Magnetic observation bias compensation
Step 4: Magnetic Anomalies

In addition to the earth's main field represented by the IGRF, the tag will also sense local anomalies. In many cases, these anomalies appear small enough that they can be ignored. But, around Iceland as well as in some other areas such as for example the Galapagos rift zone, anomalies can be significant. Further, as anomalies originate with magnetized rock in the earth's crust and as these bodies of rock may be close to or even at the sea floor, an anomaly's strength can grow dramatically with depth and can change quickly over a short horizontal distance [1] [2].

Figure 7 shows the magnetic measurements of the tag during the period of strong disturbances from about 12/7/2013 to 12/14/2014. Some extreme readings are cropped here, with peak-to-peak amplitude about 15000nT. This plot shows how quickly field readings can change over time due to anomalies, implying these changes occur over a short travel distance of the tagged animal. (In contrast, the earth's main field as modeled by IGRF grows about 50nT in intensity from the surface to the tags crush depth at 2000m. Its horizontal gradient exhibits a maximum of about 10nT per nautical mile)

Figure 7: Magnetic measurements while fish is transitioning strong anomalies

As published marine survey data [1] from offshore the Iceland west coast shows (figure 8), magnetic anomalies can exhibit a gradient of over 6000nT / km even at the sea surface, explaining the strong disturbances seen by the tag.

Figure 8: Profiles of major magnetic anomalies from a marine survey offshore western Iceland [1]. Vertical scale 1000nT per bar. Horizontal scale indicated by 1km horizontal bar to the left of each anomaly profile.

While strong anomalies such as present around Iceland make animal tracking using IGRF main field data imprecise (see constraints in figure 6), they can also be used to find a plausible path of migration. A good starting point for this exploration is the Word Digital Magnetic Anomaly Map (WDMAM) overlay for Google Earth. This map represents field intensity as color coding from dark blue ( ⇐ -100nT anomaly) to purple (>= +100nT anomaly) at a reference altitude of 5km above mean seal level.

Figure 9 shows WDMAM for the area between the tagging and pop-up location. Markers indicate locations along a plausible track that might support the anomaly signature from figures 5 and 7. The track starts out with generally falling intensity, then a mix of extreme highs and lows around 12/11-12/14, followed by another trough through about 12/21.

Figure 9: WDMAM anomaly map for the region between the tagging and pop-up positions, with markers indicating a plausible track supporting the magnetic field intensity measurements of figure 5.

The next question is if not only the magnetic field intensity trends but also the amplitude seen by the tag is supported by magnetic data. WDMAM here is not useful because its reference altitude of 5km is too far above the sea bed, and magnetic field intensity declines rapidly with distance from the magnetic substrate.

Marine survey data, taken by vessels towing magnetometers, is a better source. This is also available from the WDMAM project, although in form of a large text .XYZ file. The processing method was to split that file into portions supported by Excel (1048576 rows maximum), convert to .CSV, then sort and extract the available data for the region around Iceland. Finally, using the KMLCSV converter, the data was converted to.KML for Google Earth viewing. Figure 10 includes this overlay. Marine survey anomaly measurements at the surface in the region of the markers are roughly from -500nT to 500nT. This is best compared to the tags daily average readings, as the fish resting on the sea bed close to magnetized rock can be expected to cause much stronger anomaly amplitudes. The amplitude between the lowest and highest daily average reports is about 1000nT, that is the same as for the marine survey surface values in the area. So, the overall magnetic field intensity readings are plausible for a transition of the marked area.

Figure 10: WDMAM with anomaly readings from marine survey data. Sea level field intensity data is a better reference for anomaly readings from the tag and can be used to judg e if the field intensity variations observed by the tag are plausible for a proposed track.

Step 5: Judging a Plausible Track

Our principle 'breadcrumbs' that may be used to construct a plausible track in this case are the bathymetry and the magnetic field readings. Such a plausible track may be constructed as follows (see figure 11):

WP1: From the start of track at the tagging location on 11/26, the magnetic field strength quickly drops and the fish reaches a depth of 287m within just 3 days on 11/29. The shortest track to 287m is in a south-southwesterly direction, after 56km. This path also leads to a negative magnetic anomaly, meeting both conditions.

WP2: From 12/7 to 12/9 we see extreme magnetic field intensity changes, a condition met if the fish is crossing the Reykjanes Ridge, the area of greatest magnetic disturbance. The water is also more shallow here, consistent with the dip in maximum depth observed in that time period (figure 2).

WP2A: This is an alternate waypoint consideration to WP2. The marine survey data shows strong magnetic anomalies. But, published data [4] shows that in this area there is at least 400m of sediment on top of the magnetic base rock. The strong spikes seen in figure 7 indicate that the fish was close to the base rock, and thus WP2A is withdrawn from consideration.

WP3: The depth sensor places the fish in a depth range of roughly 300-350m from 12/10 to 1/9, requiring a move to the south and tracing the shelf south of the Reykjanes ridge in a southwesterly direction. Outer shelves are described by NOAA HCD as a monkfish habitat [3].

Pop-Up: Depth data shows the maximum depth declining to the final depth of 250m between 1/9 and 1/10. This indicates that the fish was now moving north toward the pop-up location.

This move north implies a second crossing of the Reykjanes Ridge, yet no strong magnetic disturbance is seen at this time in the tag data. WDMAM and the associated marine survey data points indicate a gap without a positive anomaly in this area. But, the data is sparse. A better survey data set [5][6][7] might help.

Figure 11: Proposed monkfish track consistent with depth and magnetic field measurements

Why did this tag Pop-Up?

The tag was programmed to pop-up and report on 11/26/2014 but in fact popped up on 1/24/2014. Why the early popup? The reason in this case is plain. The tag status code, reported in each engineering packet (SDPT_MODEN3) after pop-up decodes to 'Constant Depth Release Occurred' (CDR). CDR is designed to allow a tag to pop-off if a fish has died. The tag configuration file shows that CDR was programmed for release if the depth sensor indicates the fish has been a 20m depth range for 7 days. The detail depth plot (figure 12) shows the fish indeed stayed in a very tight depth range until pop-off.

Figure 12: Detail depth plot for the period before pop-up. Pop-up was caused by the tag staying within a 20m band for 7 days, prompting the programmed CDR release. The plot also resolves the tidal amplitude at the site and shows the fish briefly rose a few meters above the sea floor a few times during this final period.

This leads to the question if the fish in fact died. The depth data show the fish briefly rising above the sea floor a few times during these last few days, which suggests it was still active.

To study this further, figure 13 plots the 3-axis magnetic data, 3-axis acceleration data, depth and temperature for the entire deployment to see if there are any difference in the measurements after the fish comes to rest as compared to before that time. A clear change occurs on 1/17/2014, six days after the fish first is seen resting in the depth plot. At that time, the temperature drops abruptly by about 0.6 deg C, the biggest such change seen in the entire migration. The acceleration measurements, which reflect primarily changes in the orientation of the tag (and depending on the tag mounting method orientation changes of the fish), also exhibit different characteristics. The Z-axis reading, which is the longitudinal axis of the tag, starts rising already on 1/11/2014 when the fish first rests. The value ultimately reaches about 0.5G, indicating the antenna is up at an angle of about 45 degrees from vertical. Steady overall readings with small but frequent wiggle on the three acceleration axes cease are replaced with more steady point-to-point readings, but also now with big spikes. That indicates that the tag is experiencing less movement but sometimes its orientation changes significantly.

Figure 14 zooms in for some more detail. We see that the final depth spikes occur about a day before the temperature drop starts, and that this temperature drop coincides with a change in the acceleration measurements from steady with small frequent spikes, to periods of relative stability interrupted by sudden, significant orientation changes.

A starting hypothesis or interpretation of the data: The fish in fact died around 1/17/2014, and perhaps the final depth spikes a day earlier indicate it was attacked by a predator. The temperature drop on 1/17/2014 coincides with a distinct change in the characteristics of the accelerometer readings. Thus the temperature drop is probably not due to environmental changes, but perhaps reflects a lowering of the body temperature of the dying animal (assuming here that these fish have a body temperature somewhat above ambient). The lack of the frequent small wiggles in the acceleration measurements indicate the that fish is no longer moving, while the large changes in the profile followed by periods of relatively steady state may indicate that the fish is no longer actively controlling its attitude.

Figure 13: Depth (top), temperature, acceleration and magnetic field (bottom) for entire time on animal

Figure 14: Depth (top), temperature, acceleration and magnetic field (bottom) for final ten day on animal

Summary and Possibilities for Improved Analysis and Tag Use

The standard for this migration analysis was merely to find a plausible track that is consistent with the tag's various observations and the known environmental conditions, particular the bathymetry in the area, and the magnetic anomalies to the extent that they are known. But, significant uncertainty about this track remains. Here are a few practical ideas for improvements of this particular analysis, and for extending the tag use and analysis to a larger number of animals.

  1. Using the tag as a compass: The tag's two-point attachment aligns the tag along longitudinal axis of the animal. This provides the possibility of using the tag's magnetometer and accelerometer measurements to obtain compass data. Such data could provide the heading of the fish and thereby support or reject a 'plausible path'. A caution here is that the high inclination of the magnetic field of about 75 degrees will require that the gravity plane is computed with the tags accelerometer to avoid drastic apparent heading shifts due to tag orientation uncertainty.

  2. Improved tag magnetic calibration: Already available for SeaTag-MOD is a higher precision magnetometer calibration. Tag #133449 calibration was for gain, offset and tag magnetic field along the X, Y and Z axes. The new high-precision calibration method also compensates for non-orthogonality of the three sensors and the temperature coefficient. The resulting reduction of bias and greater accuracy improve the confidence of the magnetic measurements. High-precision magnetometer calibration is an option for SeaTag-MOD, but is generally recommended for tracking beyond the reach of light and for small-scale migrations.

  3. Deeper light sensing: Although generally in darkness, the tag detected some light around noon from 12/5/2013 to 12/10/2013, being at depths from 84m to 171m. This wasn't enough for longitude sensing, which is currently limited to depths with light levels of about >= 1% of surface sunlight. Desert Star is currently working on enhancing light based longitude detection to a level of 0.001% to 0.01% of surface sunlight. Based on the data from this track, this may have enabled longitude detection on some days.

  4. Using SeaTag-LOT to obtain pop-up positions and boost sample size: This small tag costs approx. 1/3 of the SeaTag-MOD. The most definitive position data are pop-up positions, and by deploying a number of these tags and sequencing pop-up dates, the overall migratory range can be mapped out. This data may then be used to confirm the position tracks computed from SeaTag-MOD data.


[1]: Marine Magnetic Surveys off the West Coast of Iceland. Leo Kristjansson, Soc. Sci. Isl., Greinar V, 1976

[2]: Magnetic and Petrologic Variations along the Galapagos Spreading Centerand Their Relationship to the Galapagos Melting Anomaly. Roger N. Anderson et. al., Geological Society of America Bulletion, 1975;86 

[3]: NOAA: Essential Fish Habitat Description Monkfish. link

[4]: A Marine Magnetic Survey Off Southern Iceland. Leo Kristjansson, University of Iceland, 1976

[5]: Reykjanes Ridge Marine Magnetic Survey, R. Hey et al., Geochemistry, Geophysics, Geosystems vol. 11 2010, doi: 10.1029/2009CG002865

[6]: Reykjanes Ridge Marine Magnetic Survey, J.C. Sempere et al., Marine Geophysical Researches 12, 215-234, 1990

[7]: Maps of the Arctic and North Atlantic Oceans compiled by W.R. Roest et al., published as Geological Survey of Canada Open Files 3125a,b, 3280, 3281 and 3282 in 1996.


This report required significant knowledge of the characteristics of the magnetic anomalies around Iceland. A thank you and acknowledgement go to Leo Kristjansson for his comments and suggestions that altered the 'plausible path' of the monkfish, as well as pointing me to the insightful references cited above.