Dramatic effect of pop-up satellite tags on eel swimming

The journey of the European eel to the spawning area in the Sargasso Sea is still a mystery. Several trials have been carried out to follow migrating eels with pop-up satellite tags (PSATs), without much success. As eels are very efficient swimmers, tags likely interfere with their high swimming efficiency. Here we report a more than twofold increase in swimming cost caused by a regular small satellite tag. The impact was determined at a range of swimming speeds with and without tag in a 2-m swimming tunnel. These results help to explain why the previous use of PSATs to identify spawning sites in the Sargasso Sea was thus far unsuccessful

Geolocation of eels may be possible using depth data alone.

<p>Based on eel descent and ascent timings in the morning and evening, sunrise and sunset times were estimated and used for calculating latitude (A) and longitude (B) positions of ten wild eels. Actual eels’ position is shown by red triangle. Eel positions estimated by average and equation methods (see <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0121801#sec008" target="_blank">Results and Discussion</a>) are shown by yellow and black circles, respectively.</p

Vertical movement profiles of 11 wild eels.

<p>The colors and contours indicate the water temperature. No temperature data was available for gray area. See <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0121801#pone.0121801.t001" target="_blank">Table 1</a> for individual information. Midnight (inverted closed triangle).</p

Four locations (A-D) for collecting the Japanese eels (<i>Anguilla japonica</i>) that were used for tracking, and eleven locations (1–11) where the eels were released and tracked.

<p>A: Tone River (Chiba Prefecture), B: Naka River (Tokyo), C: Mikawa Bay (Aichi Prefecture), D: Amikake River (Kagoshima Prefecture). 1–6: tropical to subtropical area (TS). 7–11: temperate area around Kuroshio Current (KC). See Tables <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0121801#pone.0121801.t001" target="_blank">1</a> and <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0121801#pone.0121801.t002" target="_blank">2</a> for details.</p

Vertical movement profiles (black lines) of five wild eels and lunar cycle during nighttime (18:00–4:00) in TS area.

<p>A, WE2999 (July 10–11). B, WE3001 (July 13–14). C-E, WE3002 (July 17–20). F-K, WE4264 (August 8–14). L-N, WE4265 (August 16–19). Moon phase is shown in the black quadrangle and is indicated by the number. Sunset (inverted closed triangle) and moon set (inverted open triangle).</p

Vertical movement profiles (black lines) of five wild eels and lunar cycle during nighttime (17:00–5:00) in KC area.

<p>A-E, WE6285 (November 29—December 4). F, WE6288 (December 5–6). G-L, WE6289 (December 8–14). M, WE6287 (December 16–17). N, WE6286 (December 18–19). Moon phase is shown in the black quadrangle and is indicated by the number. Sunset (inverted closed triangle), moon rise (open triangle) and moon set (inverted open triangle).</p

Time difference between descent timing and sunrise (A) and between ascent timing and sunset (B) plotted against migration depth at descent or ascent timing.

<p>Regression line is shown by a solid line. Dotted lines indicate 55 min difference in dawn and 0 min in dusk. Eels start descent approximately one hour before sunrise, corresponding to nautical dawn, in which individuals swimming at a shallower layer start descent earlier than those swimming at a deeper layer. Ascent in the evening is closely associated with sunset, in which individuals swimming at a shallower layer start ascent later than those swimming at a deeper layer. Observations in TS and KC areas are shown by open and closed circles, respectively.</p

Vertical movement profiles of 10 wild eels in the morning (4:00–7:00) and evening (16:00–19:00) in the last day of tracking.

<p>WE2999 (July 12), WE3001 (July 14), WE3002 (July 20), WE4264 (August 14), WE4265 (August 19), WE6285 (December 3), WE6288 (December 6), WE6289 (December 15), WE6287 (December 17), and WE6286 (December 19). Black and red lines indicate eels’ migration depth and light intensity, respectively. Visually determined starting points of descent in the morning (inverted closed triangle) and ascent in the evening (open triangle). Vertical dotted line indicates sunrise or sunset.</p

Vertical movements of eels during daytime (7:00–16:00) showing a reverse trajectory between migration depth (black lines) and light intensity (red lines) with a significantly positive correlation.

<p>A, WE4264 (August 14) (r = 0.814, P<0.01). B, WE6289 (December 11) (r = 0.874, P<0.01).</p