you're reading...

Biological oceanography

The Declining Japanese Eel Population: Is Ocean Circulation to Blame?

Source: Chang, Y.-L. K.; Miyazawa, Y.; Miller, M. J.; Tsukamoto, K. Potential impact of ocean circulation on the declining Japanese eel catches. Scientific Reports 2018, 8 (1), 5496. DOI: 10.1038/s41598-018-23820-6


Whether you first heard of it on the T.V. show Friends, or actually had the good fortune of dining on the sweet-savory dish unagi, many around the world are familiar with Anguilla japonica, commonly known as the Japanese eel (Figure 1). But populations of Anguilla japonica have been declining in recent decades (Figure 2), to the point that the International Union for Conservation Efforts (IUCN) has included it on its Red List of Endangered Species. Overfishing is recognized as the chief cause of the decline in Japanese eel populations, but there remains another largely unexplored factor that could have a significant impact: ocean circulation.


Figure 1. A mature Japanese eel. Source: opencage via Wikimedia Commons.


Figure 2. Annual eel catches measured in tons in Japan (red) and Taiwan (blue). Figure reprinted from Nature Scientific Reports, Potential impact of ocean circulation on the declining Japanese eel catches, Yu-Lin K. Chang, Yasumasa Miyazawa, Michael J. Miller & Katsumi Tsukamoto (2018).


The movements of the sea are central to the Anguilla japonica life cycle. These eels cover a remarkable geographical distance during their lifetimes; to spawn, they travel thousands of miles offshore to an area near Guam. Powerful ocean currents then transport the newly hatched eel larvae northward. Think of the current-riding turtles from Finding Nemo, but replace them with tiny flat translucent eel larvae (Figure 3). In this way the eels reach the fresher waters where they mature. But because the young creatures rely on ocean currents, a change in water movement patterns has the potential to interfere with this important step in their life cycles.


Figure 3. Ocean eel larva. Source: Kils at the English language Wikipedia, CC BY-SA 3.0, https://commons.wikimedia.org/w/index.php?curid=2242707


Off the coast of Japan, it is the North Equatorial Current (NEC) that carries the eel larvae into the Kuroshio Current to continue their journey to join the rest of the mature eel population (Figure 4). But as the Japanese eel population, hugely important in agricultural and fishing industries, has declined in recent decades, some have suspected that a disruption to this transport process might play a role. To investigate, Chang and his colleagues turned to ocean model simulations involving virtual eel larvae, referred to as v-larvae.


Figure 4. Map of spawning area (light red oval) and maturing areas (marked in green) of the Japanese eel. The North Equatorial Current transports the larvae northward after bifurcating into the south-flowing Mindanao Current. Source: Aoyama J, Watanabe S, Miller MJ, Mochioka N, Otake T, Yoshinaga T, et al. (2014) Spawning Sites of the Japanese Eel in Relation to Oceanographic Structure and the West Mariana Ridge. PLoS ONE 9(2): e88759. https://doi.org/10.1371/journal.pone.0088759


The researchers used a particle-tracking method in an ocean physics model to represent the eel larvae being whisked across the ocean basin. They used ocean physics data between 1993 and 2013 to simulate how virtual larvae (or v-larvae) would be transported from their spawning site by the currents. Two abilities were bestowed upon the thousands of virtual larvae: first, a day-to-night vertical migration swimming behavior, in which they swim down from the surface water during the day to avoid predators. They were also granted the ability to swim horizontally, as one might expect these juvenile eels to do. The researcher tracked these v-larvae for eight months, and measured how many unique larvae visited eight different swaths of the ocean (Figure 5), referred to as visitation frequency. They ran the model using data on ocean circulation spanning 20 years with the hopes of finding out how changing ocean physics over this time frame might have impacted the eel larvae distribution.


Figure 5. Map of regions of interest in the model study. The spawning region is denoted by the yellow box near the bottom right corner. The numbers indicate the index of each region that the researchers measured v-larvae visitation frequency (the number of unique v-larvae that visited each region). Figure reprinted from Nature Scientific Reports, Potential impact of ocean circulation on the declining Japanese eel catches, Yu-Lin K. Chang, Yasumasa Miyazawa, Michael J. Miller & Katsumi Tsukamoto (2018).


After setting loose the v-larvae and counting their visitation frequency to each region, the researchers compiled a collection of graphs displaying visitation frequency over the years (Figure 6). They found that more of the v-larvae were being pushed south where the NEC splits into the north-flowing Kuroshio and south-flowing Minandao, affirming the notion that changing circulation patterns were hindering the eel larvae from reaching the region where they could mature.


Figure 6. Visitation frequency of the v-larvae between 1993 and 2013 in the model simulations by Chang et al. Figure reprinted from Nature Scientific Reports, Potential impact of ocean circulation on the declining Japanese eel catches, Yu-Lin K. Chang, Yasumasa Miyazawa, Michael J. Miller & Katsumi Tsukamoto (2018).


Honing in on the ocean physics, the next question Chang and his colleagues explored was exactly what changes were occurring in the local circulation that could be responsible for this shift in where the v-larvae were ending up. Studies have shown that the westward flow of the NEC has been weakening over the past two decades, while a below-the-surface southward flow of water around 14˚ N latitude was strengthening. The weaker NEC would lengthen the travel time of the v-larvae, perhaps increasingly subjecting them to the southward pull of the subsurface current, preventing the larvae from making their connection to the Kuroshio.

The researchers speculated that the weakening of the NEC is likely caused by changing wind patterns. Powerful large-scale winds blowing across the ocean pulls water around at the surface, enforcing the structure of the currents. But long-term changes in wind strengths might disrupt the usual pattern of circulation, accounting for the weakening of the NEC’s northward flow to the Kuroshio. By similar mechanisms, a reduction in the strength of the Kuroshio might also be to blame for the reduction in v-larvae reaching their northern destinations.

The reason why the winds are changing – and further, the ocean circulation pattern is changing – is still unclear. While some have attributed it to human-caused climate change, the forcings remain complex and ambiguous. Chang and his colleague recognize that the 20-year period they examined for this study is not long enough to draw conclusions about long-term climate changes. In addition, the contributions of a number of other unexplored factors to the decline in Japanese eel populations, including changes in food source for the larvae, changes in fishing efforts, and changes in regional salinity levels (which eels are acutely attuned to) due to shifts in rainfall patterns, require more research to untangle.

This study reinforces the idea that recent changes in ocean circulation may partially explain the recent drop in Japanese eel populations. While the Japanese eel populations were the first to show declines, they are not alone in their disappearance; world eel species have shown alarming declines in recent decades. Conservation efforts for Anguilla japonica and eel species around the planet requires an understanding of how changes in the ocean-atmosphere system forcing changes in ocean circulation impacts the fate of these creatures.



No comments yet.

Post a Comment


  • by oceanbites 3 months ago
    Happy Earth Day! Take some time today to do something for the planet and appreciate the ocean, which covers 71% of the Earth’s surface.  #EarthDay   #OceanAppreciation   #Oceanbites   #CoastalVibes   #CoastalRI 
  • by oceanbites 4 months ago
    Not all outdoor science is fieldwork. Some of the best days in the lab can be setting up experiments, especially when you get to do it outdoors. It’s an exciting mix of problem solving, precision, preparation, and teamwork. Here is
  • by oceanbites 5 months ago
    Being on a research cruise is a unique experience with the open water, 12-hour working shifts, and close quarters, but there are some familiar practices too. Here Diana is filtering seawater to gather chlorophyll for analysis, the same process on
  • by oceanbites 6 months ago
    This week for  #WriterWednesday  on  #oceanbites  we are featuring Hannah Collins  @hannahh_irene  Hannah works with marine suspension feeding bivalves and microplastics, investigating whether ingesting microplastics causes changes to the gut microbial community or gut tissues. She hopes to keep working
  • by oceanbites 6 months ago
    Leveling up - did you know that crabs have a larval phase? These are both porcelain crabs, but the one on the right is the earlier stage. It’s massive spine makes it both difficult to eat and quite conspicuous in
  • by oceanbites 7 months ago
    This week for  #WriterWednesday  on  #Oceanbites  we are featuring Cierra Braga. Cierra works ultraviolet c (UVC) to discover how this light can be used to combat biofouling, or the growth of living things, on the hulls of ships. Here, you
  • by oceanbites 7 months ago
    This week for  #WriterWednesday  at  #Oceanbites  we are featuring Elena Gadoutsis  @haysailor  These photos feature her “favorite marine research so far: From surveying tropical coral reefs, photographing dolphins and whales, and growing my own algae to expose it to different
  • by oceanbites 8 months ago
    This week for  #WriterWednesday  on Oceanbites we are featuring Eliza Oldach. According to Ellie, “I study coastal communities, and try to understand the policies and decisions and interactions and adaptations that communities use to navigate an ever-changing world. Most of
  • by oceanbites 8 months ago
    This week for  #WriterWednesday  at  #Oceanbites  we are featuring Jiwoon Park with a little photographic help from Ryan Tabata at the University of Hawaii. When asked about her research, Jiwoon wrote “Just like we need vitamins and minerals to stay
  • by oceanbites 8 months ago
    This week for  #WriterWednesday  on  #Oceanbites  we are featuring  @riley_henning  According to Riley, ”I am interested in studying small things that make a big impact in the ocean. Right now for my master's research at the University of San Diego,
  • by oceanbites 8 months ago
    This week for  #WriterWednesday  at  #Oceanbites  we are featuring Gabby Stedman. Gabby is interested in interested in understanding how many species of small-bodied animals there are in the deep-sea and where they live so we can better protect them from
  • by oceanbites 8 months ago
    This week for  #WriterWednesday  at  #Oceanbites  we are featuring Shawn Wang! Shawn is “an oceanographer that studies ocean conditions of the past. I use everything from microfossils to complex computer models to understand how climate has changed in the past
  • by oceanbites 9 months ago
    Today we are highlighting some of our awesome new authors for  #WriterWednesday  Today we have Daniel Speer! He says, “I am driven to investigate the interface of biology, chemistry, and physics, asking questions about how organisms or biological systems respond
  • by oceanbites 9 months ago
    Here at Oceanbites we love long-term datasets. So much happens in the ocean that sometimes it can be hard to tell if a trend is a part of a natural cycle or actually an anomaly, but as we gather more
  • by oceanbites 10 months ago
    Have you ever seen a lobster molt? Because lobsters have exoskeletons, every time they grow they have to climb out of their old shell, leaving them soft and vulnerable for a few days until their new shell hardens. Young, small
  • by oceanbites 11 months ago
    A lot of zooplankton are translucent, making it much easier to hide from predators. This juvenile mantis shrimp was almost impossible to spot floating in the water, but under a dissecting scope it’s features really come into view. See the
  • by oceanbites 11 months ago
    This is a clump of Dead Man’s Fingers, scientific name Codium fragile. It’s native to the Pacific Ocean and is invasive where I found it on the east coast of the US. It’s a bit velvety, and the coolest thing
  • by oceanbites 11 months ago
    You’ve probably heard of jellyfish, but have you heard of salps? These gelatinous sea creatures band together to form long chains, but they can also fall apart and will wash up onshore like tiny gemstones that squish. Have you seen
  • by oceanbites 12 months ago
    Check out what’s happening on a cool summer research cruise! On the  #neslter  summer transect cruise, we deployed a tow sled called the In Situ Icthyoplankton Imaging System. This can take pictures of gelatinous zooplankton (like jellyfish) that would be
  • by oceanbites 1 year ago
    Did you know horseshoe crabs have more than just two eyes? In these juveniles you can see another set in the middle of the shell. Check out our website to learn about some awesome horseshoe crab research.  #oceanbites   #plankton   #horseshoecrabs 
WP2Social Auto Publish Powered By : XYZScripts.com