The dynamical impact of mesoscale eddies on migration of Japanese eel larvae

Author: 2017-06-12

Papers

by Yu-Lin Chang[1][2],
in collaboration with Yasumasa Miyazawa[3], Mélanie Béguer-Pon[4]

In this study[5], we explore the dynamical role of mesoscale eddies on fish larvae migration using the example of Subtropical Counter Current eddies and the migration of Japanese eel larvae in the western North Pacific Ocean. An idealized experiment is conducted to isolate the effects of eddies, and use a three-dimensional particle-tracking method to simulate virtual eel larvae (v-larvae) migration, including both horizontal and vertical swimming behaviors (Fig. 1).

Figure1

Fig. 1: An example of the 3D trajectories of v-larvae in the warm eddy in Exp1 (swimming speed=0.01 m s−1). The black curve is the horizontal projection of the migration path. Red dot marks the initial position.

 

The impact of eddies strongly depends on the swimming speed of v-larvae relative to the eddy speed. Eddies accelerate the movement of v-larvae that swim slower than the propagation speed of the eddy, whereas faster-swimming v-larvae are dragged by eddies (Fig. 2).

figure2

Fig. 2: Distributions of v-larvae on day 0 (black, gray), day 100 (red, pink), and day 200 (blue, cyan) for swimming speeds of (left) 0.01 m s−1 and (right) 0.06 m s−1, under conditions of (a, b) no eddy, (c, d) warm eddy, and (e, f) cold eddy in the case of the idealized experiment. Green contours are the SSHA, showing the positions of eddies. Dark and light colors indicate v-larvae originating from the inner and outer core of the eddy, respectively.

 

A modified stream function that incorporates biological swimming ability explains the non-uniform trapping of v-larvae in mesoscale eddies. A high swimming speed and/or a small eddy rotation speed results in a weak trapping capacity (Fig. 3). Simulations of v-larvae migration in realistic cases of eddy fields indicate that the abundance of eddies significantly affects the duration of larval migration, with the effects being largely dependent on the larvae swimming speed. We noted a negative relationship between the observed annual eel recruitment index in Taiwan and the eddy index subtropical countercurrent (STCC) region, which suggests a potentially important role of mesoscale eddies in eel larvae migration.

Figure3

Fig. 3. Stream functions in the idealized experiment for (a) warm and (b) cold eddies for v-larvae swimming speeds of (black) 0 m s−1, (red) 0.01 m s−1, and (green) 0.06 m s−1. (c) Time-varying U/cef with various swimming speeds (labeled numbers) of v-larvae from the inner core of the eddy. Triangles and circles mark the times when the first and 50% of v-larvae move out of the inner core of the eddy, respectively.

  1. [1] Institute of Marine Environmental Sciences and Technology, National Taiwan Normal University↩
  2. [2] Update Aug 1, 2017: Dr. Chang Joined JAMSTEC from Aug 1, 2017. ↩
  3. [3] Application Laboratory, Japan Agency for Marine-Earth Science and Technology↩
  4. [4] Department of Oceanography, Dalhousie University↩
  5. [5] Chang YL, Miyazawa Y, Beguer-Pon M (2017) The dynamical impact of mesoscale eddies on migration of Japanese eel larvae PLoS One 12:e0172501 doi:10.1371/journal.pone.0172501↩