Research ArticleBEHAVIORAL ECOLOGY

Empirical observations of the spawning migration of European eels: The long and dangerous road to the Sargasso Sea

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Science Advances  05 Oct 2016:
Vol. 2, no. 10, e1501694
DOI: 10.1126/sciadv.1501694
  • Fig. 1 Reconstructed migrations of European eels.

    Reconstructed migrations or end positions of 87 eels that migrated into the Atlantic Ocean relative to the assumed spawning area in the Sargasso Sea (hatched area). Release positions are shown as green squares, whereas end positions are shown by crosses. Dashed lines show the most direct (great-circle) routes to the spawning area from the Celtic Sea, Baltic Sea, and Mediterranean Sea. The shaded gray lines shows the approximated routes of Atlantic and Mediterranean migrations used for extraction of temperature data shown in Fig. 4.

  • Fig. 2 Migration speeds.

    (A) Frequency distribution of observed migration speeds of tagged eels. Migration speed was calculated from the distance taken over either the fully reconstructed migration path (n = 38) or the distance between release and pop-up (n = 49) when full reconstruction was not possible. Labels show the midpoints of the bins. (B) As for (A), but expressed in body lengths per second. The migration speeds of eels that reached the ocean are shown in white, whereas the speeds of those eels that did not reach the ocean are shown in black. (C) Comparison of eel travel speeds reported here and in the literature (citations are given within or above the data). Box plots show the median value as a horizontal line and the mean as a cross. To enable comparison, swimming speeds observed in swim tunnel studies have been adjusted downward by 5 km day−1 to account for the average current speed that eels are likely to experience in the northeast Atlantic (45).

  • Fig. 3 Vertical migration behavior.

    (A) Example vertical migrations of individuals measured over a 6-day period. Charts show an individual (#49559) released from Ireland that was migrating in the mid-Atlantic Ocean, an individual released from Sweden (#118814) that was migrating in the Norwegian Sea, and an individual from southern France (#133979) that was migrating in the eastern Atlantic Ocean. Depth and temperature data collected at between 15- and 30-min intervals are shown interpolated to a 1-min resolution. The color scales show temperature in degrees Celsius and vary between data sets in relation to eel location. (B) Vertical displacement histograms for the oceanic portion of migration for the same eels shown in (A). Because the average depth of eels increases considerably over the course of their migration, the depth measurements for each day were normalized to the average daily depth and separated into day/night (black/gray) periods. Negative values represent depths shallower than the mean.

  • Fig. 4 Depth and temperature experience of eels during the westward migration.

    (A) Depth of eels during the oceanic migration. (B) Temperature experience. Data from eels migrating in the Atlantic Ocean are shown on the left, and the Mediterranean Sea on the right. The data are shown as a violin plot. Black violin symbols indicate the depths or temperatures occupied at nighttime, whereas gray symbols indicate the depths or temperatures occupied during the day. Maximal and minimal values for each degree longitude are indicated by the upper and lower limits of each violin symbol, whereas the width of the violin symbols shows the kernel density distribution of observations at that value. Circles within each violin represent the median value for each longitudinal bin. Colored contour lines in (A) indicate thermal structure of the water column within hydrographic sections (shown in Fig. 1) centered on the general oceanic migration paths of eels in the Atlantic Ocean and the Mediterranean Sea. Data are included for all longitude bins where there were more than 20 days of data.

  • Fig. 5 Larval growth rates, timing of spawning, and timing of escapement of European eels.

    (A) Length measurements of Anguilla anguilla leptocephali belonging to the first year cohort. Each symbol represents the mean of length measurements of leptocephali sampled on each day of the calendar year. The weighted least-squares regression line between day of year and length is represented by a solid black line (length = 0.1401 × day − 0.94). (B) Frequency of spawning of European eels based on back calculation of growth rates. Timing is shown for three different larval instantaneous mortality rates (0, black; 2, dark gray; 3.6, light gray). (C) Timing of escapement of silver eels as a proportion of total measured escapement in 19 catchments between 70° and 43°N within Europe. Each colored line represents a catchment, with the thick black line denoting the average across all catchments. Lines are colored from red to purple according to the timing of peak escapement, whereas the legend is ordered by latitudinal position (north to south). The transparent gray shading shows the start of the spawning period to peak spawning. Colored lines at the top of the chart show the time between peak escapement and peak spawning. Sources of data are given in tables S5 and S6 (66, 93107).

  • Table 1 Migration metrics for eels that escaped the coast and entered the Atlantic Ocean.

    Values are average ±1 SD of the mean. Figures in brackets show the maximum values observed. Two different measures of distance and migration speed were calculated, one representing the entire migration and one that took into account only the distance and speed during the oceanic portion of the migration (eel occupies water depth >200 m). A table providing details for all eel migrations is provided in table S3b.

    Location
    (n)
    Duration
    (days)
    Full distance
    (km)
    Speed
    (km day−1)
    Oceanic distance (km)Oceanic speed (km day−1)
    Celtic Sea (8)106.8 ± 77.0 (273)2716.8 ± 1985.1 (6982.5)26.3 ± 7.2 (42.5)2277.3 ± 2066 (6709)24.4 ± 7.0 (33.8)
    Mediterranean (3)157.1 ± 43.2 (183)1774.3 ± 434.7 (2245)11.5 ± 1.9 (12.9)1360.6 ± 498 (1905)14.5 ± 7.2 (21.8)
    North Sea (5)27.8 ± 9.3 (34)533.6 ± 269.8 (794)21.4 ± 5.9 (29.1)
    Baltic Sea (22)103.5 ± 42.8 (157)1768.2 ± 623.6 (2887)18.1 ± 5.5 (29.0)1574.6 ± 602.2 (2638)22.7 ± 8.3 (35.4)
    Summary (38)98.5 ± 57.1 (273)1805.9 ± 1176.2 (6982.5)19.8 ± 6.9 (42.5)1725.5 ± 1136.4 (6709)22.4 ± 8.1 (35.4)
  • Table 2 The fate of eels released from the European coast.
    Bay of BiscayCeltic
    Sea
    MediterraneanNorth SeaBaltic SeaTotal
    Pop-off
      Premature1702212
      Scheduled02311723
    Predation
      Strong evidence of predation21351223
      Suspected pelagic predation4900114
      Suspected benthic predation050005
      Caught by fisherman000100
    Unknown080019
    Total744852387
  • Table 3 The phenology and arrival success of eels leaving European catchments.

    The proportion of eels arriving by peak spawning and 90% spawning was calculated using observed swimming speeds. The required average travel speed for 50% of eels to arrive by peak spawning was modeled by incrementing the observed speeds at intervals (1 km day−1) and recalculating. The percentage of eels arriving in time for the second spawning opportunity was calculated from observed migration speeds and represents the percentage arriving after the first spawning period has ended, and by 90% of the next. NA, not applicable.

    Departure
    (day of year)
    Time to peak
    spawning (day)
    Peak
    spawning
    90%
    spawning
    Required speed
    (km day−1)
    Second
    spawning
    All catchments28813051245.965
    Imsa, Norway28113741246.366
    Dee, Scotland246172122343.161
    Lower Bann, Ireland276142102133.865
    Windermere, U.K.29312581636.365
    Schwentine River2831353744.160
    Burrishoole, Ireland33682921NA66
    Shannon, Ireland3071112936.374
    Oir, France35563312NA71
    Fremur, France3091091544.171
    Loire, France317101312NA71
    Fume morte, Spain3121063645.160
    Nive, France293125212NA71
    Gudena, Denmark2681503745.168
    Ulla, Spain2471711021NA67
    Halselva, Norway22319541238.662
    ICES 41G2, Sweden2961222741.364
    ICES 40G4, Sweden25816039NA62
    ICES 42G6, Sweden29312551245.160
    ICES 45G6, Sweden3338571541.358
    Warnow, German20920951137.859

Supplementary Materials

  • Supplementary material for this article is available at http://advances.sciencemag.org/cgi/content/full/2/10/e1501694/DC1

    fig. S1. Pop-off or recovery positions of DSTs and PSATs.

    fig. S2a. Migration and end points of eels released from the Baltic Sea.

    fig. S2b. Migration and end points of eels released from Ireland.

    fig. S2c. Migration and end points of eels released from western France.

    fig. S2d. Migration and end points of eels released from Germany.

    fig. S2e. Migration and end points of eels released from the Mediterranean coast.

    fig. S3a. Example of an eel (PSAT #89310, released from Ireland) being preyed upon by a (assumed) pelagic fish.

    fig. S3b. Example of an eel (PSAT #49644, released from Ireland) being preyed upon by an endothermic fish.

    fig. S3c. Example of an eel (PSAT #101445, released from Ireland) being preyed upon by a coastal pelagic predator.

    fig. S3d. Example of an eel (PSAT #83156, released from Ireland) being preyed upon by a deep living fish.

    fig. S3e. Example of an eel (PSAT #49559, released from Ireland) being preyed upon by a marine mammal.

    fig. S3f. Example of an eel (PSAT #133984, released from southern France) being preyed upon by an unknown endotherm.

    fig. S4. Example of daytime thermal experience of eels during migration along the Norwegian trench/deep in the Norwegian Sea.

    fig. S5. Depth and temperature time series from PSAT #83140.

    fig. S6. Relative frequency distribution of swimming speed.

    fig. S7. Electronic tag types and attachment technique.

    fig. S8. Principle and validation of behavioral geolocation.

    fig. S9. Analysis of the total number of length-measured European eel leptocephali in the ICES database.

    table S1. Details of reconstructed migrations of eels that reached the ocean.

    table S2. Assessment of effect of tag and release country on speed, migration duration, and migration distance of eels.

    table S3. The speed of European eels.

    table S4a. The fate of eels that reached oceanic waters.

    table S4b. The fate of all released eels for which data sets were recovered.

    table S5. Summary of literature used to assess escapement date.

    table S6. Silver eel escapement timing in the River Gudena, Denmark.

    table S7. Release locations and numbers of eels released during the study.

    table S8a. Summary metrics of tagged eels.

    table S8b. Summary metrics of eels for which migratory data were recovered (±1 SD).

    table S9. Metrics of all eels tagged (n = 707).

  • Supplementary Materials

    This PDF file includes:

    • fig. S1. Pop-off or recovery positions of DSTs and PSATs.
    • fig. S2a. Migration and end points of eels released from the Baltic Sea.
    • fig. S2b. Migration and end points of eels released from Ireland.
    • fig. S2c. Migration and end points of eels released from western France.
    • fig. S2d. Migration and end points of eels released from Germany.
    • fig. S2e. Migration and end points of eels released from the Mediterranean coast.
    • fig. S3a. Example of an eel (PSAT #89310, released from Ireland) being preyed upon by a (assumed) pelagic fish.
    • fig. S3b. Example of an eel (PSAT #49644, released from Ireland) being preyed upon by an endothermic fish.
    • fig. S3c. Example of an eel (PSAT #101445, released from Ireland) being preyed upon by a coastal pelagic predator.
    • fig. S3d. Example of an eel (PSAT #83156, released from Ireland) being preyed upon by a deep living fish.
    • fig. S3e. Example of an eel (PSAT #49559, released from Ireland) being preyed upon by a marine mammal.
    • fig. S3f. Example of an eel (PSAT #133984, released from southern France) being preyed upon by an unknown endotherm.
    • fig. S4. Example of daytime thermal experience of eels during migration along the Norwegian trench/deep in the Norwegian Sea.
    • fig. S5. Depth and temperature time series from PSAT #83140.
    • fig. S6. Relative frequency distribution of swimming speed.
    • fig. S7. Electronic tag types and attachment technique.
    • fig. S8. Principle and validation of behavioral geolocation.
    • fig. S9. Analysis of the total number of length-measured European eel leptocephali in the ICES database.
    • table S1. Details of reconstructed migrations of eels that reached the ocean.
    • table S2. Assessment of effect of tag and release country on speed, migration duration, and migration distance of eels.
    • table S3. The speed of European eels.
    • table S4a. The fate of eels that reached oceanic waters.
    • table S4b. The fate of all released eels for which data sets were recovered.
    • table S5. Summary of literature used to assess escapement date.
    • table S6. Silver eel escapement timing in the River Gudena, Denmark.
    • table S7. Release locations and numbers of eels released during the study.
    • table S8a. Summary metrics of tagged eels.
    • table S8b. Summary metrics of eels for which migratory data were recovered (±1 SD).
    • Legend for table S9

    Download PDF

    Other Supplementary Material for this manuscript includes the following:

    • table S9 (Microsoft Excel format). Metrics of all eels tagged (n = 707).

    Files in this Data Supplement: