Fish larvae float across national borders, binding the world’s oceans in a single network


Larval black sea bass, an important commercial species along the US Atlantic coast.
NOAA Fisheries/Ehren Habeck

Nandini Ramesh, University of California, Berkeley; James Rising, London School of Economics and Political Science, and Kimberly Oremus, University of Delaware

Fish populations are declining around the world, and many countries are trying to conserve them by regulating their fishing industries. However, controlling fishing locally may not do enough to strengthen fish populations. Often one nation’s fish stocks depend on the spawning grounds of a neighboring country, where fish release eggs and sperm into the water and larvae hatch from fertilized eggs.

We do research on oceans, climate and fisheries. In a recent study, we showed that global fisheries are even more tightly connected than previously understood. The world’s coastal marine fisheries form a single network, thanks to the drift of larvae along ocean currents.

This suggests that country-by-country fishery management may be fundamentally insufficient. If a fish species that provides food to one country should decline, the amount of fish spawn, or eggs and larvae, riding the ocean currents from there to other countries would also decline dramatically, resulting in further loss of fish elsewhere.

Many countries live with this risk, although they may not realize it. To manage fisheries effectively, nations must understand where the fish in their territories originate.

Ocean currents affect the speed at which fish eggs and larvae drift and vary through the year. This map shows surface current speeds for January: yellow = fastest, dark blue = slowest. Each country’s territory is highlighted with red dots during the month of maximum spawning activity in that country. In each territory, a different number of species spawn in each month of the year. The red dots appear in the month during which the largest number of species spawn in that territory.

Crossing national borders

Fish don’t recognize political boundaries, and regularly travel internationally. Scientists have tracked adult fish movements using electronic tags, and have shown that a few species migrate over long distances.

Countries and territories have negotiated agreements to ensure sustainable sharing of migratory fish. One such agreement joins several nations in the Western and Central Pacific Fisheries Commission to ensure that the territories fish cross share them sustainably.

But fish eggs and larvae are much harder to follow. Many species lay eggs in large numbers that float near the ocean surface. When they hatch, larvae measure a few millimeters long and continue to drift as plankton until they grow large enough to swim. During these stages of the life cycle, ocean currents sweep fish spawn across international boundaries.

Simulating the journeys of eggs and larvae

Like weather on land, the pattern of ocean currents varies with the seasons and can be predicted. These currents are typically sluggish, traveling about an inch per second, or less than 0.1 miles per hour.

There are a few exceptions: Currents along the eastern coasts of continents, like the Gulf Stream in North America or the Kuroshio in Asia, and along the equator can be significantly faster, reaching speeds of 2 miles per hour. Even a gentle current of 0.1 miles per hour can carry spawn 40 miles over a month, and some species can float for several months.

Government and academic scientists use a vast network of satellites, moored instruments and floating buoys to monitor these surface flows. Using this information, we performed a computer simulation of where drifting particles would be carried over time. Scientists have used this type of simulation to study the spread of marine plastic pollution and predict where debris from plane crashes at sea could have washed ashore.

Different fish species spawn in different seasons, and a single species may spawn in several months at different locations. We matched the seasons and locations of spawning for over 700 species with ocean current data, and simulated where their spawn would drift. Then, using records of where those species have been fished, and information about how suitable conditions are for each species in different regions, we deduced what fraction of the fish caught in each country arrived from other countries because of ocean currents.

A small-world network

Scientists and policymakers can learn a lot by studying these international connections. Each species that floats across international boundaries during its plankton stage represents a linkage between countries. These linkages span the world in a dense, interconnected network.

Each color represents a region in the network of fish larvae connections. This map shows the strongest 467 connections among a total of 2,059 that the authors modeled.
Nandini Ramesh, James Rising and Kimberly Oremus, CC BY-ND

At a global level, this network of connections has an important property: It is a small-world network. Small-world networks connect regions that are far apart to each other by just a few steps along the network. The concept is rooted in social scientist Stanley Milgram’s 1960s experiments with social networks, which found that it was possible for a letter to reach almost any total stranger by passing through six or fewer hands. Milgram’s work was popularized in the 1990 play “Six Degrees of Separation.”

Among fisheries, the world seems even smaller: We found that the average number of degrees of separation among fisheries is five. This means that local problems can become global risks.

For example, imagine that a fishery collapses in the middle of the Mediterranean. If the population in one spawning region collapses, it could quickly put pressure on neighboring fisheries dependent upon it. If fishers in those neighboring countries overfish the remaining population or shift to other species, the disturbance can grow. Within just a few years, a fisheries disturbance could travel around the world.

We assessed how countries would be affected in terms of food security, employment and gross domestic product if they were to lose access to fish spawn from other territories. The most affected countries cluster in the Caribbean, the western Pacific, Northern Europe and West Africa. These hotspots correspond to the network’s most clustered areas, because the effects of these flows of fish spawn are most pronounced where many coastal countries lie in close proximity.

International flows of fish eggs and larvae affect countries’ total catch, food security, jobs and economies.
Nandini Ramesh, James Rising and Kimberly Oremus, CC BY-ND

Thinking globally about fisheries

Because the world’s fisheries are so interconnected, only international cooperation that takes flows of fish spawn into account can effectively manage them. Aside from egg and larvae connections, fisheries are linked by movements of adult fish and through agreements among countries allowing them to fish in each other’s waters.

All of this suggests that fishery management is best conducted at a large, international scale. Proposals for doing this include defining Large Marine Ecosystems to be jointly managed and creating networks of Marine Protected Areas that safeguard a variety of critical habitats. Ideas like these, and careful study of interdependence between national fisheries, are crucial to sustainable use of the oceans’ living resources.

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Nandini Ramesh, Postdoctoral Researcher in Earth and Planetary Science, University of California, Berkeley; James Rising, Assistant Professorial Research Fellow, London School of Economics and Political Science, and Kimberly Oremus, Assistant Professor of Marine Policy, University of Delaware

This article is republished from The Conversation under a Creative Commons license. Read the original article.

What can you learn from studying an animal’s scat?



A bear leaving its calling card.
Dean Harvey/Flickr, CC BY

Verity Mathis, University of Florida

Curious Kids is a series for children of all ages. If you have a question you’d like an expert to answer, send it to curiouskidsus@theconversation.com.


What can you learn from studying an animal’s scat? – Cora, age 9, Brookline, Massachusetts


Everybody poops. There are even whole books written about it. And we can learn a lot about animals from what they leave behind.

Scientists study animal poop, also called scat, to learn about the hidden lives of animals. We can find scat in the wild and know what type of animal left it based on its shape, size and contents.

I study mammals, so I know that a pile of brown pellet-shaped scat that’s about the size of chocolate-covered raisins could be a sign that there are white-tailed deer in the area. Bigger, tube-shaped scat with hair and bones in it might be from a coyote.

The Smithsonian National Zoo uses scat to assess lions’ health.

Scat can tell us a lot about an animal’s diet, habits and movement, so scientists like to study it both in nature and in the lab. Outdoors, scat can identify what animals are present in an area. Then researchers take it to a lab, dry it out and dissect it for clues about the animal’s diet.

Some mammal poop is full of seeds, which shows that the animal eats fruit or berries. Or it might contain bones and fur, which scientists can identify to learn what species that animal is eating.

Animal scat also contains DNA – molecules inside the cells of organisms that carry genetic information. Extracting DNA from scat is a non-invasive way to study animals, since scientists don’t need to handle the animals to learn about them.

DNA from scat can tell scientists about the genetic health of a species, who is occupying what territory, and the relationships of groups of animals in a particular area. For example, DNA from the scat of rare Bengal tigers in India helped scientists estimate how many tigers were in an area, see where individual animals were traveling and better understand their genetic relationships.

Studying animal scat can also support conservation. Some researchers have trained dogs to sniff out the scat of endangered species, such as the blunt-nosed leopard lizard, which is found only in a few grasslands in central California. By locating an endangered animal’s scat, scientists can estimate how many of that species are left in an area, analyze its diet and do DNA testing without having to disturb it.

It’s not hard to find scat if you know where to look. Some mammals, such as coyotes and bobcats, like to poop in the middle of trails or trail crossings. Others, like porcupines, do their business at the bases of trees. Guidebooks and websites can tell you what kinds of scat you’re likely to find in your area.

It is important never to pick up scat with bare hands, since you don’t know what kind of diseases might be present. But you can use a stick to look at it and see if you can figure out what the animal was eating, or take pictures and look in a guide to identify the creature that left it behind.


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And since curiosity has no age limit – adults, let us know what you’re wondering, too. We won’t be able to answer every question, but we will do our best.The Conversation

Verity Mathis, Mammal Collections Manager, Florida Museum of Natural History, University of Florida

This article is republished from The Conversation under a Creative Commons license. Read the original article.