ASK A SCIENTIST2020-05-20T10:14:34+00:00


Here are the questions we have received so far. If after reading them you are still wondering about something, scroll down to the bottom of the page and send us your question by filling out the form. We will answer you and also post your question here.



What is the Smallest Fish?


Juan Andrés, age 5, asked us: What is the smallest fish?

Drawing about the smallest fish

Aina and Anna, from Planet Tuna, answered: There’s fierce competition out there for being the smallest fish. According to the Australian Museum, there are three species vying for the World’s Smallest Fish title. One of them is Paedocypris progenetica, a little freshwater species from Sumatra that reaches a maximum length of 10.3 mm. The second contender, Photocorynus spiniceps, doesn’t count because only the male is tiny (6.2mm). He is parasitic on the much larger female (46mm). The third is Schindleria brevipinguis. Australian scientists claim this little fish from the Great Barrier Reef, growing to a maximum length of 10 mm, is the smallest. They base this statement on the fact that it’s not only the shortest, but also the lightest fish of all. That would make Schindleria brevipinguis the winner of the smallest fish category.

What kinds of tunas are there in the Atlantic Ocean?


Ibon from Bilbao wanted to know: What kinds of tunas are there in the Atlantic Ocean?

Chart about kinds of tuna that live in the Atlantic Ocean

Patricia, from Planet Tuna, answered: The tuna species we find in the Mediterranean are also present in the Atlantic. They are listed here. In addition, among the Atlantic tunas you can also find tropical species that are not present in the Mediterranean, such as yellowfin and bigeye. The International Commission for the Conservation of the Atlantic Tunas (ICCAT), the organization in charge of tuna management in the Atlantic Ocean, has a list of 104 species of tunas, similar fishes, and sharks which you can access on their website, under the Species section. We will be posting information about some of them on Planet Tuna.

What is the life cycle of the yellowfin tuna and its migration routes?


Claudio, from A Coruña, Spain, sent us the following question: Can you tell me about the life cycle of the yellowfin tuna and its migration routes?

Patricia, from Planet Tuna, answered: It took us longer than we expected to answer this question, because we had to check with the experts and read research papers to realize that there’s still a lot we don’t know about the ecology of this species. For example, some studies show that there’s one single population of yellowfin tuna, which breeds on both sides of the Atlantic Ocean; others suggest the presence of two separate, large yellowfin populations in the Atlantic –one Eastern and one Western. Right now, yellowfins are managed as a single population, which means that much more research has to be done.

Scientific illustration Thnnus Albacares or Yellowfin Tuna

Let’s start with what we do know. The yellowfin tuna’s life cycle is one of the best-known among the different tuna species, because the Inter-American Tropical Tuna Commission has a laboratory in Achotines (Panama) where experts have been researching its development for years. The life cycle of all tunas is very similar during the egg and larval stages, so you can read our article about the life cycle of the Atlantic bluefin tuna: up until the juvenile stage, both species develop very much the same way. In fact, their eggs are hard to tell apart at first glance, and the only visible difference between the larvae is their pigmentation. The two species differ a lot more in their adult stage. Yellowfins are smaller as adults than Altantic bluefins (although they can be up to two meters long). They start breeding earlier, when they are 2 years old, whereas Atlantic bluefins reach their age of reproduction at 4-6 years. They breed for several months in a row, in tropical waters –in the Gulf of Mexico, the southern Caribbean, off the coast of Senegal, and in the Gulf of Guinea.

Habitat and reproduction areas for Thunnus Albacares or Yellowfin Tuna

As far as the yellowfin’s migration routes are concerned, our colleague Miguel Cabanellas, a scientist at the Spanish Institute of Oceanography, tells us that we know much less about this topic than we do about its life cycle. Little is known about the yellowfin’s migratory patterns. What seems to be sure is that these tunas travel in search of the best areas for feeding –for example, places that are rich in nutrients– or for breeding, which requires, among other things, waters with the right temperature for the eggs and the larvae to survive. The main spawning grounds of the Western population are the Gulf of Mexico (May through July) along with the southern Caribbean (October through March). The same occurs with the areas where the Eastern population spawns: Senegal (April through June) and the Gulf of Guinea (October through March). This last area (the Gulf of Guinea) is not only one of the main spawning grounds, but also a key area for recruitment, which is the moment when, after having reached a certain size or for other reasons, the fish start to be fished for the first time. Juveniles that hatched in this area migrate from north to south along the African coast in search of warmer waters and better food supplies, to then disperse across the entire tropical Atlantic when they reach adulthood. In fact, some studies suggest that the adults migrate across the ocean and that there may be a central area of the Atlantic (30ºW) where adults from both populations gather to reproduce.

Kitchens, L. L. (2017). Origin and Population Connectivity of Yellowfin Tuna (Thunnus albacares) in the Atlantic Ocean (Doctoral dissertation, Texas A&M University).

Margulies, D., Scholey, V. P., Wexler, J. B., & Stein, M. S. (2016). Research on the reproductive biology and early life history of yellowfin tuna Thunnus albacares in Panama. In Advances in tuna aquaculture (pp. 77-114). Academic Press.

Which is the tuna’s natural predator?


A group of students from the Lycée Français in Mallorca asked us:
Which is the tuna’s natural predator?

Patricia from Planet Tuna answered: When we think of tuna, we usually think of big fish at the top of the food chain. But we have to remember that they hatch from a 1-millimeter egg. So their predators change a lot throughout their life cycle. When they’re only a few days old, during the egg and larva stages, their predators are usually other fish larvae, or even young of their own species, and invertebrates such as jellyfish. As they grow, they can swim faster and get away from smaller predators. At that point, their predators are other fish, but when they reach adulthood, only large predators are able to feed on tuna: other, larger tuna and similar species, large sharks, and killer whales. And, of course, humans.

Do tuna sleep? And if so, how?


A group of students from the Lycée Français in Mallorca asked us:
Do tuna sleep? And if so, how?

Patricia and Hannah from Planet Tuna answered: First of all we would have to define what we consider sleeping. As humans, we associate sleeping with closing our eyelids, not moving our bodies, and, in many cases, lying down. When it’s dark out, our pineal gland releases melatonin and our body recognizes that it’s time to “shut down the neurons.” If we were to measure the brain’s electrical activity with an EEG when we’re asleep, we would identify a different pattern from the one we would see if we were awake.

Tuna don’t have eyelids, so their eyes are always open. That, in and of itself, doesn’t necessarily mean they’re not asleep: we can’t close our ears, for example, and that doesn’t stop us from sleeping. Tuna can’t ever stop swimming, because they need the oxygen they get from filtering water while they move. If you look into a tank with tuna larvae or juveniles at night, you see they move more slowly, but although we’ve tried, we haven’t yet managed to measure their swimming speed in the dark, nor have we been able to record them without any light. And it appears that the brain activity we associate with sleep in mammals is different from that of fish. There are indications that tuna release melatonin, but we’re not yet sure about the relationship between their daily rhythm and that secretion. So for now all we can say is that, although there’s no scientific evidence of their sleeping, they probably enter some similar state in which they rest and save energy.