Novel CTD tag establishes shark fins as ocean observing platforms

Abstract

Animal-borne tags are effective instruments for collecting ocean data and can be used to fill spatial gaps in the observing network. We deployed the first conductivity, temperature, and depth (CTD) satellite tags on the dorsal fin of salmon sharks (Lamna ditropis) to demonstrate the potential of sharks to monitor essential ocean variables and oceanographic features in the Gulf of Alaska. Over 1360 km and 36 days in the summer of 2015, the salmon shark collected 56 geolocated, temperature-salinity profiles. The shark swam through a plume of anomalously salty water that originated from the "Blob" and encountered several mesoscale eddies, whose subsurface properties were altered by the marine heatwave. We demonstrate that salmon sharks have the potential to serve as submesoscale-resolving oceanographic platforms and substantially increase the spatial coverage of observations in the Gulf of Alaska.

Figure 1

Design of “twin” CTD-SRDL fin tag. (A) “Twin” CTD-SRDL fin tag (B) deployed on the dorsal fin of a salmon shark. The CTD sensor is located in the package placed closer to the body of the shark while the satellite transmitter and its antenna are in the package placed near the tip of the dorsal fin.

Figure 2

Trajectory of salmon shark equipped with CTD-SRDL fin tag in the Gulf of Alaska between August 14 and September 18, 2015. (A) Locations of shark-collected temperature-salinity profiles (circles colored by date) and other ARGOS location estimates (black dots) taken by the tag. Also shown are locations of profiles from the World Ocean Database (black circles) over this period. White triangle with black outline denotes the tagging location. (B) Mean absolute dynamic topography over the 36-day CTD-SRDL fin tag deployment with ARGOS location estimates and locations of shark-collected temperature-salinity profiles colored by whether the shark is in an anticyclonic eddy (ACE, red), cyclonic eddy (CE, blue) or outside of eddy (black) is also showed.

Figure 3

Water properties and anomalies along the trajectory of the salmon shark. (A) Temperature-salinity diagram of shark-collected (B) conservative temperature (Θ, °C) and (C) absolute salinity (S_A, g/kg) profiles. Profiles are colored by date. Temperature-salinity characteristics of water masses in the Gulf of Alaska are indicated with boxes: Pacific Subarctic Upper Water (PSUW, 3.0 to 15.0 °C, 32.6 to 33.6 ‰, solid black line) and Pacific Subarctic Intermediate Water (PSIW, 5.0 to 12.0 °C, 33.8 to 34.3 ‰, dashed black line). Depth (m) by along-track distance (km) section of (D) conservative temperature and (F) absolute salinity anomalies with (E) eddy polarity (red = anticyclone, blue = cyclone).

Figure 4

Comparison between shark-collected temperature-salinity profiles and co-located Argo profiling float. Linear regression of shark-collected and Argo profiling float (A) conservative temperature (Θ, °C) and (B) absolute salinity (S_A, g/kg) data co-located in time (± 1 day) and space (± 0.75° latitude and longitude). The locations of co-located shark (pink and purple) and Argo (black) (C and F) Θ and (D and G) S_A profiles are shown on (E) the map.

Figure 5

Ability of shark-collected profiles to resolve submesoscale flows. Histograms showing the (A) spatial and (D) temporal resolution of shark-collected temperature-salinity profiles (black dots) from the CTD-SRDL fin tag in the Gulf of Alaska shown in (B). (C) A heatmap showing the relationship between distance and temporal resolution for data shown in panels (A) and (D). (E) through (H) are the same panels as (A) through (D) but for temperature-depth profiles from “double tagged” sharks with recovered PAT tags. Color in the heatmap represents the number of profiles. The red lines show the spatial (full-depth Rossby deformation radius; R_{d_full}) and temporal cutoff for submesoscale estimates, respectively. The 50th percentile of the data (black dashed lines) and modal value (black dots) are also shown. Data in all panels is cut to 100 km and 30 h.

Figure 6

Potential geographic coverage of shark-collected temperature-salinity profiles compared to those in the World Ocean Database between 2002 and 2019 in the Gulf of Alaska. (A) Tracks of SPOT-tagged salmon sharks colored by deployment year. Each dot represents a potential profile every 17.2 h. (B) Minimum number of temperature-salinity profiles that could have been collected if all SPOT-tagged salmon sharks were instrumented with CTD-SRDL fin tags and (C) the number of temperature-salinity profiles in the World Ocean Database (WOD) on days when salmon sharks were present in the Gulf of Alaska. (D) Difference between the number of profiles in the WOD and the minimum number of shark-collected profiles, with (E) regions where there would be more profiles in the WOD shown in black and more shark-collected profiles in white. Bins are 0.5° latitude and 0.5° longitude.

Figure 7

Potential temporal coverage of shark-collected profiles compared to those in World Ocean Database between 2002 and 2019 in the Gulf of Alaska. Minimum number of temperature-salinity profiles that could have been collected if all SPOT-tagged salmon sharks were instrumented with CTD-SRDL fin tags (blue) by (A) year and (B) month. Also shown are the number of temperature-salinity profiles in the World Ocean Database on days when salmon sharks were present in the Gulf of Alaska (red) as well as the number of SPOT-tagged (dashed green line and circles) sharks.